Choose 2 of the 4 case studies from below and answer questions for the case study.

20200616143155_pg._1_282__cwna_guide_to_wireless_lans__3rd_ed_.pdf

Home >Business & Finance homework help >Management homework help >Choose 2 of the 4 case studies from below and answer questions for the case study.

By Mark Ciampa, Ph.D.

CWNA Guide to Wireless LANs Third Edition

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CWNA Guide to Wireless LANs, Third Edition

Mark Ciampa

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Brief Contents

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

CHAPTER 1 The World of Wireless . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

CHAPTER 2 Wireless Local Area Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

CHAPTER 3 Radio Frequency Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

CHAPTER 4 Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

CHAPTER 5 Physical Layer Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

CHAPTER 6 Media Access Control Layer Standards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

CHAPTER 7 WLAN Management and Architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

CHAPTER 8 Conducting a Site Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277

CHAPTER 9 Wireless LAN Security Vulnerabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

CHAPTER 10 Implementing Wireless LAN Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353

CHAPTER 11 Managing a Wireless LAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391

CHAPTER 12 Wireless Network Troubleshooting and Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425

CHAPTER 13 Other Wireless Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461

APPENDIX A CWNA Certification Exam Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495

APPENDIX B URLs for Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505

APPENDIX C Wireless Web Sites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509

GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525

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Table of Contents

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

CHAPTER 1 The World of Wireless . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Wireless Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Education . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Business. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Industry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Travel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Public Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Health Care. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Wireless Advantages and Disadvantages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Types of Wireless Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Wireless Personal Area Network (WPAN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Wireless Local Area Network (WLAN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Wireless Metropolitan Area Network (WMAN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Wireless Wide Area Network (WWAN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Comparison of Wireless Networks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Wireless Standards Organizations and Regulatory Agencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 International Telecommunication Union Radio Communication Sector (ITU-R) . . . . . . . . . . . . . . . . . . . . 21 Federal Communications Commission (FCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 International Organization for Standardization (ISO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Institute of Electrical and Electronics Engineers (IEEE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Wi-Fi Alliance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Certified Wireless Network Administrator (CWNA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Case Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

CHAPTER 2 Wireless Local Area Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Understanding Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 The Need for Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Sources of Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Types of Wireless LANs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 IEEE 802.11-2007 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 IEEE 802.11n-2009 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

WLAN Client Hardware and Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Wireless Client Network Interface Card . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Client Utility Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

WLAN Infrastructure Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Access Points (APs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

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WLAN Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Gateways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Power over Ethernet (PoE) Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Case Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

CHAPTER 3 Radio Frequency Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Principles of Radio Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 What Are Electromagnetic Waves? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Electromagnetic Wave Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 The Electromagnetic Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Radio Frequency Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Analog Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Digital Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

RF Signal Strength Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Milliwatt (mW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Decibel Milliwatt (dBm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Receive Signal Strength Indicator (RSSI). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 Percentage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Signal-to-Noise Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

Radio Frequency Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Propagation Behaviors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Impact of Behaviors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

Case Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

CHAPTER 4 Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

Antenna Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 What Is an Antenna? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Antenna Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

Types of Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 Omnidirectional Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 Semidirectional Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 Highly-Directional Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

Antenna Coverage Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Azimuth and Elevation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Beamwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Fresnel Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

Multiple-Input Multiple-Output (MIMO). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 What Is MIMO? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 MIMO Signal Processing Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

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Antenna Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 Antenna Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 Measuring Antenna Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

Case Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

CHAPTER 5 Physical Layer Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

Wireless Modulation Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 Narrowband Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 Spread-Spectrum Transmissions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 Comparison of Modulation Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

IEEE 802.11 Physical Layer Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 IEEE 802.11b Physical Layer Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 IEEE 802.11a Physical Layer Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 IEEE 802.11g Physical Layer Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 IEEE 802.11n Physical Layer Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

Case Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194

CHAPTER 6 Media Access Control Layer Standards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

WLAN Service Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 Basic Service Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 Extended Service Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 Independent Basic Service Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

802.11 Media Access Control Layer Frame Formats and Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 MAC Frame Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 MAC Frame Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

MAC Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Discovering the WLAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Joining the WLAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 Transmitting on the WLAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230

Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

Case Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238

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CHAPTER 7 WLAN Management and Architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

Autonomous Access Point Architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 Network Connectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 Feature Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 Advantages and Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

Controller-Based Architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248 Access Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 Wireless LAN Controllers (WLCs). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252

Other Architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 WLAN Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 Cooperative Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 Cloud Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 Wireless Mesh Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

Multiple-Channel Architecture vs. Single-Channel Architecture Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 Multiple-Channel Architecture (MCA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 Single-Channel Architecture (SCA). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

Wireless Network Management Systems (WNMS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258

Power Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 Basic Power Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Enhanced Power Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260

Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269

Case Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274

CHAPTER 8 Conducting a Site Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277

What Is a Site Survey? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 Purpose of a Site Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 When to Perform a Site Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 Types of Site Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282

Site Survey Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 Wireless Device Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 Specialized Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 Voice over WiFi (VoWiFi) Tools and Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

Procedures for Performing a Site Survey. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294 Gathering Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294 Performing the Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 Creating the Site Survey Report. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301

Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

Case Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314

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Table of Contents ix

CHAPTER 9 Wireless LAN Security Vulnerabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

Principles of Information Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 What Is Information Security? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 Challenges of Information Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

Wireless Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324 Enterprise Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324 Mobile User Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 Home Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

Legacy IEEE 802.11 Security Protections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331 Access Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331 Wired Equivalent Privacy (WEP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332 Authentication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335

Vulnerabilities of IEEE 802.11 Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 Authentication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 Address Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 WEP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337

Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342

Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345

Case Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351

CHAPTER 10 Implementing Wireless LAN Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353

Transitional Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 WEP2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 Dynamic WEP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 Wi-Fi Protected Access (WPA). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357

IEEE 802.11i/WPA2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361 Encryption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362 Authentication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364

Wireless Intrusion Detection and Prevention Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 Wireless Security Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371

Other Wireless Security Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373 Virtual Private Network (VPN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373 Secure Device Management Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374 Wi-Fi Protected Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 Role-Based Access Control (RBAC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376 Rogue AP Discovery Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377

Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381

Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384

Case Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389

Table of Contents ix

CHAPTER 11 Managing a Wireless LAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391

Procedural Security Defenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 Managing Risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394 Defenses Against Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398

Monitoring the Wireless Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 WLAN Monitoring Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 Standard Network Monitoring Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410

Maintaining the Wireless Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412 Upgrade Firmware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412 RF Site Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414

Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418

Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

Case Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423

CHAPTER 12 Wireless Network Troubleshooting and Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425

Troubleshooting a Wireless Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427 RF Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428 WLAN Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433 Wireless Device Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437

WLAN Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445 Channel Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445 Access Point Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447 Wireless Device Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 448

Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451

Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454

Case Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458

CHAPTER 13 Other Wireless Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461

Wireless Personal Area Networks (IEEE 802.15) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 Bluetooth (802.15.1-2005) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 Ultra-Wideband (802.15.3c-2009) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468 Low Rate Technologies (802.15.4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469 Body Area Networks (802.15.6) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472 Visible Light Communications (802.15.7) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473

Wireless Metropolitan Area Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476 Free Space Optics (FSO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476 Broadband Radio Service (BRS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

Wireless Wide Area Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 WiMAX (802.16) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 Long Term Evolution (LTE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479

x Table of Contents

IEEE 802.11ac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480

Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485

Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489

Case Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492

APPENDIX A CWNA Certification Exam Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495

APPENDIX B URLs for Hands-On Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505

APPENDIX C Wireless Web Sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509

Wireless Standards Organizations and Regulatory Agencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509

Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510

Wireless Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510

Security Organizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510

Wireless Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511

Other . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511

GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525

Table of Contents xi

Introduction

It is difficult to think of a technology over the last decade with a greater impact on our lives than wireless data communications. Because it’s no longer necessary to remain connected by cable to a network, users are free to surf the Web, check e-mail, download electronic books, or watch videos from virtually anywhere. Free wireless Internet connections are available in coffee shops and restaurants across the country. Students use wireless data services on their school’s campus in order to access instructional material as well as remain connected to friends. Travelers can have wireless access while waiting in airports, traveling on airplanes and trains, and working in their hotel rooms. At work, employees can access remote data during meetings and in conference rooms, thus significantly increasing their productivity. Wireless has also spurred the growth of many other new technologies, such as portable tablet devices. Although wireless voice communication started the revolution in the 1990s, wireless data communica- tions are the driving force in the twenty-first century. It has truly become a wireless world.

Statistics confirm how widespread wireless technology has become. Each year, hundreds of millions of wireless data devices are sold. Virtually all laptop, netbook, and tablet computers have wireless data capabilities as standard equipment. Since 2007, the number of locations where wireless data services are available has increased 40 percent annually. According to some estimates, by 2014 there will be 1.4 billion devices shipped annually that support wire- less data standards, and these devices will transmit the amount of data traffic equal to almost one billion DVDs.i By the end of 2011 one quarter of all households around the world, or 439 million households, were using wireless data technology, with South Korea leading the way with over 80 percent of its households using wireless (the United States was eighth with 61 percent). It is estimated that by 2016 over 800 million households will have wireless data

xiii Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

technology installed.ii Considering that wireless local area networks were not even available until 2000, this makes their widespread installation that much more amazing.

With wireless local area networks (WLANs) becoming so commonplace it is important that today’s network managers and system administrators understand their complexities. The CWNA Guide to Wireless LANs, Third Edition provides the information you need to manage a wireless network. This book takes a comprehensive view of planning, deploying, securing, and troubleshooting wireless networks. It examines the technology that makes wireless net- works work and offers practical tools, tips, and techniques. CWNA Guide to Wireless LANs, Third Edition helps you understand and use the latest wireless network technologies.

The CWNA Guide to Wireless LANs, Third Edition also prepares you to take the Certified Wire- less Network Administrator (CWNA) examination. This certification, administered by CWNP, Inc., is the leading vendor-neutral WLAN certification and is considered one of the fastest- growing certifications today. Based on the latest exam objectives, the CWNA Guide to Wireless LANs, Third Edition will equip you with the knowledge and skills necessary for taking this exam.

Intended Audience This book is designed to meet the needs of students and professionals who want to master wireless data networks. A basic knowledge of computers and networks is all that is required to use this book. Those seeking to pass the CWNA certification exam will find the text’s approach and content especially helpful because all the exam objectives are covered (see Appendix A). (For more information on CWNA certification, visit CWNP’s Web site at www .cwnp.com.) Yet the CWNA Guide to Wireless LANs, Third Edition is much more than an exam “prep book.” This textbook helps you learn wireless data technology in the context of wireless technologies and data networks. It will help you establish a solid foundation for becoming a WLAN professional and prepare you for taking the CWNP exam.

The book’s pedagogical features are designed to provide a truly interactive learning experience to help prepare you for the challenges of WLANs. In addition to the information presented in the text, each chapter includes Hands-On Projects that guide you through implementing practical wireless hardware, software, and network configurations step by step. Each chapter also contains case studies that place you in the role of problem solver, requiring you to apply concepts presented in the chapter to achieve successful solutions.

Chapter Descriptions Here is a summary of the topics covered in each chapter of this book:

Chapter 1, “The Word of Wireless,” begins by examining how wireless applications are used across education, business, industry, travel, public safety and health care. This chapter also discusses the advantages and disadvantages of wireless technologies, the different types of wireless networks, and the wireless standards organizations and regulatory agencies.

Chapter 2, “Wireless Local Area Networks,” examines standards and the different types of wireless LANs. It also looks at WLAN client hardware and software and WLAN infra- structure devices.

xiv Introduction

Chapter 3, “Radio Frequency Fundamentals,” explores the principles behind radio fre- quency (RF), including RF modulation, RF signal strength measurements, and RF behaviors.

Chapter 4, “Antennas,” gives an in-depth look at antennas. It covers basic antenna concepts, the different types of antennas, antenna coverage patterns, Multiple-Input Multiple-Output antenna concepts, and antenna installation.

Chapter 5, “Physical Layer Standards,” discusses wireless LAN functions at the lowest layer of the OSI reference model, the Physical layer. It begins by exploring the different wireless modulation schemes, followed by a look at each of the IEEE WLAN standards and how they are implemented at the Physical layer.

Chapter 6, “Media Access Control Layer Standards,” explores the three types of WLAN configurations and also looks in detail at the IEEE 802.11 media access control layer standard that implements specific WLAN features.

Chapter 7, “Wireless LAN Management and Architectures,” discusses the three general categories of WLAN architectures and the differences between single- and multiple-channel architecture network models. It also examines how to manage these architectures through a wireless network management system and how WLANs handle power management.

Chapter 8, “Conducting a Site Survey,” explores the necessary steps for locating wireless equipment by performing a site survey. This chapter first discusses what a site survey is and the different types of surveys followed by an exploration of the tools that are used to conduct the survey. Finally, it covers how to gather the necessary data and conduct a survey.

Chapter 9, “Wireless LAN Security Vulnerabilities,” looks at wireless security and vulner- abilities. It begins by briefly reviewing security in general before exploring the types of attacks against a WLAN. This chapter also examines the basic IEEE 802.11 security protections and the vulnerabilities in those protection mechanisms.

Chapter 10, “Implementing Wireless LAN Security,” discusses how to make a WLAN secure by explaining the different transitional security solutions before discussing the secure features of wireless authentication and encryption. It also looks at wireless intrusion detec- tion and prevention systems along with other wireless security defenses.

Chapter 11, “Managing a Wireless LAN,” explores some of the tasks involved in WLAN management. It first looks at procedural security defenses. Next, the steps for monitoring the network’s performance are examined. Finally, the steps in maintaining a WLAN are discussed.

Chapter 12, “Wireless Network Troubleshooting and Optimization,” looks at how to troubleshoot WLANs by locating and correcting wireless network problems. It also discusses the various ways in which a WLAN can be optimized for peak performance.

Chapter 13, “Other Wireless Networks,” discusses other wireless technologies and net- works that are used today. It also covers the next IEEE WLAN standard, IEEE 802.11ac.

Appendix A, “CWNA Certification Examination Objectives,” provides a complete listing of the latest CWNA certification exam objectives and shows the chapters and headings in the book that cover material associated with each objective.

Appendix B, “Downloads and Tools for Hands-On Projects,” lists the Web sites used in the chapter Hands-On Projects.

Introduction xv

Appendix C, “Wireless LAN Web Sites,” offers a listing of several important Web sites that contain wireless information.

Features To aid you in fully understanding computer and network security, this book includes many features designed to enhance your learning experience.

● Maps to CWNA Objectives. The material in this text covers all of the CWNA exam objectives. Throughout the chapters, references list the specific objective being covered.

● Chapter Objectives. Each chapter begins with a detailed list of the concepts to be mastered within that chapter. This list provides you with both a quick reference to the chapter’s contents and a useful study aid.

● Real World Wireless. Each chapter opens with a vignette of an actual wireless imple- mentation that helps to introduce the material covered in that chapter.

● Illustrations and Tables. Numerous illustrations of wireless technologies help you visualize wireless elements, theories, and concepts. In addition, the many tables provide details and comparisons of practical and theoretical information.

● Chapter Summaries. Each chapter’s text is followed by a summary of the concepts introduced in that chapter. These summaries provide a helpful way to review the ideas covered in each chapter.

● Key Terms. All of the terms in each chapter that were introduced with bold text are gathered in a Key Terms list, with definitions, at the end of the chapter, providing additional review and highlighting key concepts.

● Review Questions. The end-of-chapter assessment begins with a set of review questions that reinforce the ideas introduced in each chapter. These questions help you evaluate and apply the material you have learned. Answering these questions will ensure that you have mastered the important concepts and provide valuable practice for taking CWNA exam.

● Hands-On Projects. Although it is important to understand the theory behind wireless networks, nothing can improve upon real-world experience. To this end, each chapter provides several Hands-On Projects aimed at providing you with practical wireless LAN experience. These projects use the Windows 7 operating system, as well as software downloaded from the Internet.

● Case Projects. Located at the end of each chapter are several Case Projects. In these extensive exercises, you implement the skills and knowledge gained in the chapter through real design and implementation scenarios.

New to This Edition ● Fully maps to the latest CWNA PW0-105 exam objectives. ● Updated information on the latest wireless technologies. ● Expanded in-depth coverage of topics such as radio frequency fundamentals, antennas,

wireless LAN architectures, conducting site surveys, wireless security, and others.

xvi Introduction

● New material on using wireless in a Windows 8 environment, types of wireless attacks, wireless intrusion detection and prevention systems, new technologies used in IEEE 802.11n, and coverage of IEEE 802.11ac.

● Additional Hands-On Projects in each chapter covering some of the latest wireless software.

● More Case Projects in each chapter.

Text and Graphic Conventions Wherever appropriate, icons throughout the text alert you to additional materials. The icons used in this textbook are described below.

The Note icon draws your attention to additional helpful material related to the subject being described.

Tips based on the author’s experience provide extra information about how to attack a problem or what to do in real-world situations.

Each Hands-On activity in this book is preceded by the Hands-On icon and a description of the exercise that follows.

Case Project icons mark Case Projects, which are scenario-based assign- ments. In these extensive case examples, you are asked to implement indepen- dently what you have learned.

The CWNA Certification The CWNA certification, which is administered by CWNP, Inc., is the leading vendor-neutral WLAN certification and is considered one of the fastest-growing certifications today. Here are the domains covered on the latest CWNA PW0-105 exam:

Domain % of Examination

Radio Frequency (RF) Technologies 21%

IEEE 802.11 Regulations and Standards 17%

IEEE 802.11 Protocols and Devices 17%

IEEE 802.11 Network Implementation 20%

IEEE 802.11 Network Security 10%

IEEE 802.11 RF Site Surveying 15%

Introduction xvii

Instructor’s Materials

Instructor Resources CD (ISBN: 9781133132189) A wide array of instruc- tor’s materials is provided with this book. The following supplemental materials are avail- able for use in a classroom setting. All the supplements available with this book are provided to the instructor on a single CD-ROM and online at the textbook’s Web site.

Electronic Instructor’s Manual. The Instructor’s Manual that accompanies this textbook includes the following items: additional instructional material to assist in class preparation, including suggestions for lecture topics, tips on setting up a lab for the Hands-On Projects, and solutions to all end-of-chapter materials.

ExamView Test Bank. This Windows-based testing software helps instructors design and administer tests and pre-tests. In addition to generating tests that can be printed and admin- istered, this full-featured program has an online testing component that allows students to take tests at the computer and have their exams automatically graded.

PowerPoint Presentations. This book comes with a set of Microsoft PowerPoint slides for each chapter. These slides are meant to be used as a teaching aid for classroom pre- sentations, to be made available to students on the network for chapter review, or to be printed for classroom distribution. Instructors are also at liberty to add their own slides for other topics introduced.

Figure Files. All of the figures and tables in the book are reproduced on the Instructor Resources CD. Similar to PowerPoint presentations, these are included as a teaching aid for classroom presentation, to make available to students for review, or to be printed for classroom distribution.

CourseMate CWNA Guide to Wireless LANs, Third Edition offers CourseMate, a complement to your textbook. CourseMate includes the following:

● An interactive eBook, with highlighting, note taking, and search capabilities. ● Interactive learning tools, including Quizzes, Flash cards, PowerPoint slides, Glossary

and more! ● Engagement Tracker, a first-of-its-kind tool that monitors student engagement in the

course.

Go to login.cengage.com to access these resources.

CourseMate Printed Access Code (ISBN: 9781133132226)

CourseMate Instant Access Code (ISBN: 9781133132233)

Please visit login.cengage.com and log in to access instructor-specific resources.

To access additional course materials, please visit www.cengagebrain.com. At the CengageBrain.com home page, search for the ISBN of your title (from the back cover of your book) using the search box at the top of the page. This will take you to the product page where these resources can be found.

Additional materials designed especially for you might be available for your course online. Go to www.cengage.com/coursetechnology and search for this book title periodically for more details.

xviii Introduction

About the Author Mark Ciampa is Assistant Professor of Computer Information Systems at Western Kentucky University in Bowling Green, Kentucky. Previously, he served as Associate Professor and Director of Academic Computing for 20 years at Volunteer State Community College in Gallatin, Tennessee. Dr. Ciampa has worked in the IT industry as a computer consultant for the U.S. Postal Service, the Tennessee Municipal Technical Advisory Service, and the University of Tennessee. He is also the author of many Cengage/Course Technology textbooks, includ- ing Guide to Wireless Communications, Security+ Guide to Network Security Fundamen- tals, Fourth Edition, Security Awareness: Applying Practical Security In Your World, CWNA Guide to Wireless LANs, Second Edition, and Introduction to Healthcare Informa- tion Technology. He holds a Ph.D. in digital communications systems from Indiana State University.

Acknowledgments A large team of dedicated professionals contributed to the creation of this book. I am honored to be part of such an outstanding group of professionals, and to everyone on the team I extend my sincere thanks. A special word of thanks goes to Acquisitions Editor Nick Lombardi for giving me the opportunity to work on this project and for being patient with its many delays. Also thanks to Senior Product Manager Michelle Cannistraci, who was very supportive and helped keep this project on track, and to John Freitas, Senior Technical Editor, for carefully reviewing the book and identifying many corrections. A special word of thanks goes to Developmental Editor Ann Shaffer. Ann did a great job of making sugges- tions, uncovering errors, and providing valuable feedback. And a big “Thank You” to the team of peer reviewers who evaluated each chapter and provided very helpful suggestions and contributions: Karl Dietrich, Lansing Community College; Kim Doane, Mott Community College; Jim Drennan, Pensacola State College; Scott Miller, Yavapai College; Keith Price, Western Iowa Tech Community College; Beau Sanders, Greenville Technical College; and Carol Tilden, Kaplan University.

Finally, I want to thank my wonderful wife, Susan. Her loving and patient support helped see me through another project. I could not have written this book without her.

Dedication To Braden, Mia, Abby, and Gabe.

To the User This book should be read in sequence, from beginning to end. Each chapter builds upon those that precede it to provide a solid understanding of networking security fundamentals. The book may also be used to prepare for CWNP’s CWNA certification exam. Appendix A pinpoints the chapters and sections in which specific exam objectives are located.

Introduction xix

Hardware and Software Requirements Following are the hardware and software requirements needed to perform the end-of-chapter Hands-On Projects.

● A computer connected to a wireless network ● Microsoft Windows 7 ● An Internet connection and Web browser ● Microsoft Office

Specialized Requirements The needs for specialized requirements were kept to a minimum. The following chapter features specialized hardware:

● Chapter 6: 4GB USB flash drive or blank DVD (this can also be used in Chapters 9 and 10)

● Chapter 13: A computer that either has built-in Bluetooth technology or a Bluetooth USB adapter and a Bluetooth device (Bluetooth mouse, keyboard, or smartphone)

Free Downloadable Software Requirements Free, downloadable software is required for the Hands-On Projects in the following chapters. Appendix B lists the Web sites where these can be downloaded.

Chapter 1:

● Network Meter

Chapter 2:

● Virtual Router ● Connectify

Chapter 3:

● Vistumbler ● inSSIDer

Chapter 4:

● Xirrus Wi-Fi Monitor

Chapter 5:

● Xirrus Wi-Fi Inspector

Chapter 6:

● Unetbootin ● BackTrack

Chapter 8:

● Vistumbler

xx Introduction

Chapter 9:

● SMAC ● Unetbootin ● BackTrack

Chapter 10:

● Vistumbler ● PuTTY ● Unetbootin ● BackTrack

Chapter 13:

● BluetoothView ● Blueauditor

Notes

i. John Cox, “Wi-Fi client surge forces new look at WLAN designs,” Network World, Jun 20, 2011.

ii. Jia Wu, “A quarter of households worldwide now have wireless home networks,” Strategy Analytics, http://www.strategyanalytics.com/default.aspx?mod=pressreleaseviewer &a0=5193.

Introduction xxi

chapter1

The World of Wireless

After completing this chapter you should be able to:

• List different wireless data applications • Explain the advantages and disadvantages of wireless technologies • List the four types of wireless networks • Explain the roles of the different standards organizations • Describe the CWNA certification

1 Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

A disruptive technology is a radical technology or innovation that fills a new role that an existing device or technology could not. Examples of disruptive technologies, along with those that that they replaced in the last 150 years, include: steamships (which replaced sailing ships), telephones (which replaced telegraphs), automobiles (which replaced horses), word

After a turbulent start, wireless Internet access for airline travelers is now taking off. In 2003, Boeing, the world’s largest manufacturer of commercial airplanes, created the wireless technology subsidiary Connexion to roll out passenger in-flight Internet service for both wired and wireless access. After trials on several airlines, Boeing announced plans to retrofit over 4,000 airplanes with servers, wireless equipment, and antennas. Boeing estimated that 30 percent of passengers would purchase this service, but by 2006 Boeing discontinued the service, explaining that the number of airlines that signed up with Connexion was insufficient.

After the worldwide economic downturn in 2007, airlines searched for innovative ways to both increase revenue and attract more passengers, particularly business tra- velers. U.S. airlines began charging fees on checked baggage, thereby reaping addi- tional revenue of almost $8 billion annually. They also returned to the idea of adding wireless Internet access to their airplanes. In the spring of 2009, AirTran Airways announced plans to equip its Boeing jets with in-flight wireless Internet and to charge passengers a fee for its use. Other U.S. airlines announced similar wireless services. A typical passenger jet requires 125 pounds of equipment and special fiber- optic cable to give the plane wireless capabilities, which can be installed in a single overnight installation at a cost of about $100,000. As of mid-2010, all major airlines had either installed wireless on selected flights or announced plans to do so.

When a wireless-equipped plane reaches an altitude of 10,000 feet, a passenger can turn on his laptop, tablet, or smartphone. On his first attempt to view a Web page, the plane’s wireless network displays a page asking him to enter credit card information to pay for the wireless Internet access. The costs for laptops vary, with typical fees ranging from $12.95 for a three-hour flight to $5.00 for shorter flights. Filters prevent passengers from viewing pornographic Web sites or making Internet-based phone calls. Connection speeds are around 3 Mbps (million bits per second).

Implementation of this next generation of airline wireless Internet access has gone smoothly, with one exception: on long flights, passengers who did not have a fully- charged battery on their device found themselves without an electrical outlet to recharge the battery during the flight. Some airlines have acknowledged the need to solve this electrical power problem, but concede it’s not likely to happen anytime soon.

Real World Wireless

2 Chapter 1 The World of Wireless

1 processors (which replaced typewriters), and the Internet (which, increasingly, is replacing libraries). These disruptive technologies have had a profound impact on society, altering the way people live, work, and play.

Today, another disruptive technology is changing our world: wireless. Thanks to wireless data communications, it’s no longer necessary to remain tethered by cable to a network in order to surf the Web, check e-mail, or access inventory records. Wireless has made mobility possible to a degree rarely even imagined before: users can access the same resources standing on a street corner or walking across a college campus as they can while sitting at a desk. Although wireless voice communication started the revolution in the 1990s, wireless data communica- tions are the driving force in the 21st century.

Wireless data networks are found virtually everywhere. Travelers can have wireless access while waiting in airports, traveling on airplanes and trains, and working in their hotel rooms. At work, businesses have found that employees who have wireless access to data during meetings and in conference rooms can significantly increase their productivity. Free wireless Internet connections are available in restaurants across the country, and in some arenas and stadiums fans can even order concessions wirelessly and have them delivered to their seats. There is hardly a sector of the economy that has not been dramatically affected by wireless data technology.

Statistics confirm this. Since 2007, the number of locations where wireless data services are available has increased 40 percent annually. Each year, hundreds of millions of wireless data devices are sold (up from 22 million in 2003 and almost zero in 1999). Virtually all laptop computers sold today have wireless data capabilities as standard equipment. By 2014, the amount of data traffic traveling the mobile network will be equal to about a billion DVDs. Ours is truly a wireless world.

In this chapter you will first explore some of the contemporary uses for wireless data commu- nications. Next, you will look at the advantages and disadvantages of wireless, and also explore the four types of wireless networks. After looking at wireless standards organizations and regulatory agencies, you will then learn about the Certified Wireless Network Administrator (CWNA) certification.

Wireless Applications

C W N A

4.1.1. Identify technology roles for which WLAN technology is appro- priate and describe implementation of WLAN technology in those roles.

Wireless data communications are found across all sectors of the economy. However, several sectors use wireless more extensively than others. These include education, business, industry, travel, public safety, and health care.

Education Wireless data communications are an ideal technology for colleges and schools. In fact, educational institutions were among the earliest adopters of wireless technology because of

Wireless Applications 3

the advantages it offers to people engaged in teaching and learning. A teacher who creates a classroom presentation on her laptop computer in her office can carry that computer with her into the classroom, where it will connect automatically to the campus network. This frees the teacher from spending limited class time wrestling with connecting cables to often hard-to-find network jacks. Many classroom projection systems have built-in wireless capa- bilities, allowing the teacher to transmit her presentation to the projector without any cable connections. And in settings where students bring their own wireless devices to class, teachers can immediately send handouts directly to students sitting in the classroom.

Students also benefit from the freedom that wireless data connections offer. No longer must students go to a specific computer lab or to the library in order to access the school’s com- puter network; instead, they can connect to the network wirelessly from anywhere on cam- pus. As they move to different classrooms in different buildings, they can remain connected to the network. Most schools publish maps, like the one shown in Figure 1-1, which show the location of wireless coverage areas in buildings across campus.

To Villas Performing

Arts Center

Vista East

Current Coverage

Wireless Coverage

Cam bri

dg e A

ven ue

Fo rsy

the Blv

SPACZ

Tokay Cir

cle

Ryan Circle

Cam

bridge A ve

n u e

Engineering

M o n o n a

W ay

Memorial Union

Classroom Building

Biochemistry

Theatre

Library

Engr. I Business Admin. I

Business Admin II

Education

Aquatic Center

Student Health Services

Figure 1-1 Campus wireless access

4 Chapter 1 The World of Wireless

1 Wireless data technology even translates into a cost savings for schools. Traditional class- rooms can become fully accessible computer labs without the added expense of installing wir- ing or purchasing student computers. And schools can reduce the number of open computer labs (those without classes scheduled to use them) since all students can access network resources at anytime and from any location on campus.

Business Wireless data technologies have also transformed the way business is conducted. Before the days of wireless, meetings were often held in conference rooms, where participants were unable to access the data stored on their office computers that they needed to make important deci- sions. This meant it was often impossible to finalize critical decisions at a meeting; important decisions were therefore delayed, with valuable time used up later by extensive follow-up e-mails and voice mail messages. Conference rooms that did have wired connections rarely had enough connections for all participants, and if someone forgot to bring a cable (or lacked a cable that was long enough), connecting to the network was impossible.

The introduction of wireless data access in conference rooms provides all employees with a mobile office that offers immediate access to the data that they need while away from their desks. Employees no longer have to compete for an available wired connection or carry long cables with them. Instead, using wireless devices, everyone can be a productive member of the discussion.

Wireless data connectivity can result in substantial cost savings for an organization. A study by Cisco of over 25,000 of its employees showed that significant savings were realized when employees used the wireless data network.i Examples include time saved in moving to other rooms within a building or between buildings on campus sites, time saved before and during meetings, improved collaboration and access to information during meetings and impromptu working groups, and improved response time to customers. The average increase in produc- tivity was 86 minutes per day, or the equivalent of 315 additional hours per user per year. For Cisco this equaled a productivity increase of over $24,500 annually per employee.

This same study by Cisco showed that 43 percent of its employees used a wireless network as their primary connection method and 80 percent said that wireless was critical or highly useful to their job.

Large business organizations are not the only beneficiaries of wireless data communications. A small office/home office (SOHO), which typically has ten or fewer employees, can likewise benefit from the advantages of wireless. The fact that cabling is not required for each com- puter, printer, scanner, or other device can result in cost savings. To further reduce costs, all employees can share a single Internet connection. Wireless technologies allow businesses to even create an office in a space where a wired infrastructure does not exist. This means new space can be made accessible immediately without long waits for an expensive cabling infra- structure to be installed.

Industry The possibilities for wireless data transmissions in industry are limitless. Some interesting examples are found in the fields of construction, warehouse management, and manufacturing.

Wireless Applications 5

Construction When constructing a new building or creating an addition to an existing structure, it is often necessary to complete one part of the construction process before the next can begin. For example, it is necessary to pour the concrete footings for a new building before constructing the walls. A single delay in such a chain of events can idle large numbers of construction employees or force rescheduling, sometimes on short notice.

Another complication arises when construction employees have to travel to several job sites on the same day. This makes paperwork management a difficult task. Pay sheets must be manually filled out by different foremen and dropped off at the office late at night. The next day, payroll clerks have to wrestle with scrawled or illegible notes, sometimes having to contact multiple foremen for clarification. Construction professionals are resolving these problems with wireless data technology. Information from the job site, such as a tardy sub- contractor or a problem with materials, can be instantly relayed back to the main office so that workers can be routed to other sites to prevent idle time. When foremen enter timesheet information on their mobile wireless tablets, it is immediately transmitted to the main office in the construction trailer located on the job site.

Construction equipment such as bulldozers and earth graders are fitted with wireless devices in order to turn them into smart machines capable of precise positioning using a global positioning system (GPS). The GPS is composed of 27 earth-orbiting satellites (some satellites are spares in case one fails), each of which circles the globe twice a day at a height of 12,000 miles (19,300 km, or kilometers). It was originally developed by the U.S. military in the late 1970s as a navigation system, but was later opened to civilian use. Because the system can identify the precise location of a GPS receiver, heavy equipment so fitted can receive the accurate location information that is essential in modern construction. The exact location where the bulldozer should begin shoveling can be transmitted to a terminal on the bulldozer, as shown in Figure 1-2. Some systems even display a color-coded map to guide the operator.

The same bulldozer can also be connected to a wireless data system that tracks the company’s heavy equipment inventory, including the location of each bulldozer on the construction site. Meanwhile, the bulldozer’s engine diagnostic system can send wireless reminders about upcoming maintenance tasks, such as oil changes, to the maintenance team.

Warehouse Management Managing a warehouse stocked with inventory can be a nightmare. New products arrive continuously and must be inventoried and stored. When products are shipped out of the warehouse they must be located, transferred to the correct loading dock, and, finally, placed on the right truck. A mistake in any one of these steps can result in a warehouse stocked with products that cannot be located, customers receiving the wrong items, or a store running out of goods to sell.

Wireless technology has become essential for warehouse operations. Forklift trucks in a warehouse are typically outfitted with wireless equipment, while employees wear portable wireless inventory devices (like that shown in Figure 1-3) to scan inventory bar codes. All of this equipment is connected to a wireless data network. Warehouse management sys- tem (WMS) software is used to manage all warehouse activities, from receiving through

6 Chapter 1 The World of Wireless

shipping. Because the WMS is tied into the front office computer system, managers have ready access to up-to-the-minute statistics.

Today new wireless technologies are making warehouse management even more efficient. Pallet loads arriving from other locations come fitted with tiny radio frequency identification

Figure 1-2 GPS on bulldozer

Figure 1-3 Portable wireless inventory device

Courtesy of Hewlett-Packard Company

Wireless Applications 7

(RFID) tags. RFID tags emit a wireless data signal containing an identification number that is linked to information such as product code dates or expiration dates, originating plants, lines of manufacture, and so on, in the WMS. As shown in Figure 1-4, an RFID tag can eas- ily be affixed to the underside of a printed label.

As pallets arrive, a forklift truck operator picks up a load and drives it over an RFID pad reader on the floor, as illustrated in Figure 1-5. The pad reader receives the RFID signal,

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ABABAB acementplaceace

a nmmas as a

• Serial No. :

• 0 1 2 3 4 5 6

Figure 1-4 RFID tag

Figure 1-5 RFID pad reader

8 Chapter 1 The World of Wireless

1 and sends data about the contents of the pallet to the WMS, which designates a storage location for the pallet, updates the inventory database, and relays the information back to the forklift truck operator via the wireless data network. The forklift operator then trans- ports the pallet to the correct storage location in the warehouse. Additional RFID tags attached to the front of every storage rack (that is, to each face rack) serve to identify each storage location. The wireless signals from these tags allow the forklift operator to confirm that he has arrived at the correct location before depositing the load.

Some forklift trucks even have sensors to help identify what the fork- lift is doing by monitoring the position of its load rest (a load rest is on the front of the forklift where the pallet of merchandise is held). If the load rest is moving vertically, it is assumed that the forklift is transporting a load to another location or is traveling to pick one

up. Yet if the load rest is moving horizontally, it is assumed that the forklift is about to pick up or deliver a pallet and the RFID reader can then look for the face rack location tag and a corresponding pallet tag. The system can also determine if the pallet should be placed in the location just read or if the operator has removed the pallet from that location.

In the front office, orders are received and entered into the computer that connects to the wireless data network in the warehouse. The WMS software manages order picking (that is, the sequence in which separate orders are filled), balances workloads, and selects the sequence individual items of an order to be gathered for lift truck operators. The dock con- trol module then releases orders for picking. A forklift operator locates the correct storage location, receives the RFID tag signal, and then ferries it to the shipping dock to be loaded onto a truck.

Manufacturing In a manufacturing environment wireless data communications are also extensively used. Much like warehouse management, RFID tags are read with portable wire- less devices that are connected to the wireless data network to manage raw materials as they come in and inventory as it goes out.

Wireless data networks are also used in the manufacturing process itself. In one manufactur- ing plant, when additional parts are needed on the production line, workers at the facility press call buttons, located on wireless tags mounted adjacent to parts containers, to request more stock. The battery-powered tags transmit the request over the facility’s wireless data network. Because no cables have to be installed for this system, the production line can be quickly reconfigured to accommodate special production orders or changes in the manufacturing process.

Travel Because traveling is all about mobility, the travel industry uses wireless technologies exten- sively. Airplanes are not the only form of transportation to provide wireless data communica- tions for passengers. The San Francisco Bay Area Rapid Transit (BART) trains are outfitted with wireless data transmission capabilities for Internet access on trains moving 81 miles per hour (128 kilometers per hour); subscriptions can be purchased by day, month, or year. In Boston, the Massachusetts Bay Transportation Authority (MBTA) offers free wireless data service on all of its 13 commuter rail lines. The Washington State Ferry system provides wireless Internet data access to ferry passengers on its 15 boats near Seattle.

Wireless Applications 9

Most transportation terminals provide wireless data communications for passengers to use as they wait for their carrier. Some airport terminals charge a fee while others provide the service for free. A growing trend is to provide free wireless access but force the users to watch a 30-second video advertisement before they can

connect. The ad revenue reaped by airports generally matches or exceeds the amount previ- ously generated from paid services.

Wireless technology, however, goes beyond serving just the passenger. Airlines use wireless technology to communicate with the aircraft that are parked or taxiing on the ground. This allows the airline to automatically upload and install software updates for onboard compu- ters or the electronics (called avionics) system before the next flight. Airlines are also using wireless technology for flight maintenance information. Aircraft maintenance personnel with a wireless laptop can have immediate access to an online database that stores important information about each type of aircraft and maintenance procedures. This information is much more current than that found in printed manuals.

New automobiles are equipped with wireless data communications. Within the car itself, wireless connectivity allows passengers to synchronize their cellular phone address books with the car’s installed hard drive or control their portable electronic devices (like MP3 players) using the radio and steering wheel controls. The car can also be connected wirelessly to the Internet, allowing passengers using laptops or tablet computers to surf the Web. Wireless technology is also used on automobiles to monitor tire pressure, unlock doors and trunks, and even eliminate the need to use a key to start the engine.

A newly developed wireless technology is poised to dramatically change the way drivers use their cars. Known as vehicle-to-vehicle (V2V) communications, it uses both GPS and wireless technology to create a network that allows cars to communicate with one another. Such a system can alert a driver that a car ten vehicles ahead has suddenly braked or that another car is about to run a red light in an intersection ahead, giving drivers enough time to avert an accident. The V2V system will also allow cars to constantly communicate with roadside units connected to a traffic management center. This means cars caught in a traffic jam can send signals to alert approaching vehicles to exit the road and find a better route.

The V2V system actually is closer to implementation than it may seem. Much of the technology is already present in today’s modern vehicles, and adding additional capabilities is relatively inexpensive. The major cost is building smart intersections, with the wireless infrastructure required to receive data from cars and then relay this data to other cars.

Public Safety Vehicles owned by public safety organizations, such as police and fire departments, are moving away from older telecommunications systems to newer wireless networks. To make this possible, wireless data equipment is usually installed on city-owned buildings or on sites frequented by public safety vehicles, such as municipal fuel pumps where police cars are refueled. Through this equipment, data, such as building floor plans, photographs of criminal suspects, maps, and other information, can be quickly downloaded to the portable computer installed in a vehicle’s front seat. Officers on patrol, or firefighters at a fire, can then quickly access this information as needed.

10 Chapter 1 The World of Wireless

1 Health Care Administering medication in a hospital is a significant challenge for the health-care industry. It is estimated that incorrectly dispensed medication results in hundreds of thousands of med- ical emergencies annually. Typically, printouts listing medications to be administered are posted at the medication storage area. Whenever a nurse dispenses a medication, she crosses it off the list and initials the change. However, the elapsed time between when a doctor issues an order for a new or changed medication and the time that an updated printout is posted in the medication storage area means that a patient could get an unnecessary and potentially dangerous dose of medication before the new medication order can be processed.

Another drawback to the paper printout system is that it entails duplicate documentation. A nurse first has to check the prescription printout to determine the medication to be given. Then she has to document on paper that she actually gave the medication to the patient. Later, she has to enter that same data on a computer.

By contrast, wireless data point-of-care computer systems allow medical staff to access and update patient records immediately. Many hospitals use laptop computers on mobile carts or hand-held tablet computers with barcode scanners or RFID readers and a wireless connection. Health-care professionals can document a patient’s medication administration immediately in the computer as they move from room to room without reconnecting and dis- connecting cables. Nurses first identify themselves to the computer system by scanning their own personal bar-coded ID badge or RFID tag. The patient’s bar-coded armband or RFID tag is then scanned and all medications that are currently due for that particular patient are displayed on the screen. The medications to be administered are sealed in RFID-tagged pouches or bar-coded bottles. Nurses scan this barcode or RFID tag before opening the package. An alert immediately appears on the screen if the wrong medication or an incorrect amount is identified. After administering the medication to the patient, the nurse indicates through the wireless network that the medication has been given, essentially electronically signing the distribution form. A hard copy can be printed out as needed.

The system immediately verifies that medication is being administered to the correct patient in the correct dosage, which eliminates potential errors and documentation inefficiencies. Also, because the documentation process takes place at the bedside where care is delivered, the accuracy of the documentation is improved. Another advantage is that the system gives all hospital personnel real-time access to the latest medication and patient status information.

Wireless technology is also used in other medical areas besides health-care administration. For example, a new technique for diagnosing digestive problems involves a tiny camera and a wireless transmitter in the form of a pill that a patient swallows. As the pill works its way through the digestive system, it records two images each second and transmits those images to a receiving device worn on the patient’s belt. At the end of eight hours the device is retrieved and the doctor can then view the downloaded pictures, all obtained without performing invasive surgery. A video pill is illustrated in Figure 1-6.

Health care outside of the hospital also benefits from wireless technology. A pilot project in Massachusetts helps patients manage their hypertension, diabetes, and weight by having them take their blood pressure twice per week with a digital monitor that is attached to a computer connected to the Internet. This data is sent to the center’s main database, where it is automatically compared with the patient’s history accumulated over several months to

Wireless Applications 11

determine his progress and alert physicians if additional actions need to be taken. The only drawback to this system is that the patient has to use the specific computer to which the digital monitor is attached. To provide more flexibility, the program is now looking at incorporating wireless technology so that patients can send in their information from virtually anywhere.

Researchers are also working on “smart slippers” that monitor the acceleration and pressure in a patient’s walk. The wireless sensor can not only alert caregivers when a patient falls but even can identify unsteady steps that could mean disorientation or sickness.

Wireless Advantages and Disadvantages

C W N A

4.1.1. Identify technology roles for which WLAN technology is appro- priate and describe implementation of WLAN technology in those roles.

Every technology comes with its own set of advantages and disadvantages. Wireless data networks are no exception.

Advantages There are several advantages to using wireless technology. These include mobility, increased access, improved and extended connectivity, and easier and less expensive deployment.

Mobility One of the primary advantages of wireless technology is the ability to move without being connected to a network with a cable. This mobility enables individuals to use a wireless

Figure 1-6 Video pill

PillCam® SB 2 courtesy of Given Imaging Ltd.

12 Chapter 1 The World of Wireless

1 device like a tablet, laptop computer, or smartphone that remains in contact with the network no matter where the user may roam, as long as they stay within range of that device’s network.

In today’s business world, an increasingly mobile workforce is important. Many employees spend large portions of their time away from the office. These workers, equipped with laptop or tablet computers, rely on wireless data networks to connect to information on the company network. Many occupations that require workers to be mobile, such as field repair technicians or inventory clerks, find that wireless technology is essential to their work.

Another characteristic of the business world of today is flatter organizations—that is, orga- nizations structured around teams that cross functional and organizational boundaries. As a result, employees are involved extensively in team meetings that occur away from their desks, yet still require immediate access to network resources. Wireless networks are again the solution to the problem. They give team-based workers the ability to access the network resources they need while collaborating in a team environment.

Access Wireless data communications can also be used to provide access to a network where previously none existed. A hotspot is a specific geographic location that is served by a wireless data system and provides network access to mobile users. Hotspots are typically located in areas in which many users can take advantage of the service, such as college campuses, libraries, conventions centers, airports, and hotels.

Although difficult to determine precisely, by one estimate there are over 562,000 free and pay hotspot locations in 142 countries around the world. The United Kingdom has the most hotspots, with 143,000, compared to second-place China’s 102,000. The U.S. cities with the most hotspot locations are listed in Table 1-1.

Several city and county local governments have funded and deployed wireless data commu- nications hotspots, primarily in downtown areas, parks and recreation areas, and other high-traffic areas. Known as municipal networks, these hotspots offer several advantages to citizens and local economies:

● Areas with municipal networks can become more attractive to businesses, especially high-tech or research companies that need communication.

City Number of Hotspots

New York 3,520

San Francisco 850

Chicago 811

Houston 696

Los Angeles 547

Seattle 546

Atlanta 495

Austin 494

Table 1-1 U.S. cities with the most hotspots

Wireless Advantages and Disadvantages 13

● Businesses can recruit new employees from the area without the individuals needing to relocate since they can do their work using the local municipal network.

● Local police, firefighters, and municipal workers can use the municipal wireless net- work to access information such as images from security cameras, blueprints, and criminal records.

● Municipal networks provide high speed Internet access for free or at a cost less than Internet Service Providers (ISPs) who serve the area.

Municipal networks allow police to show witnesses to a crime a “virtual lineup” by downloading images of suspects through the wireless network.

However, municipal networks have experienced problems. Several large cities that embarked on ambitious municipal networks, such as Philadelphia and San Francisco, have had to scale back or even suspend their plans. Existing ISPs have objected to unfair competition from governments using tax funding, and some private companies who contracted with local governments to install wireless data networks have been forced to pull out of the projects because their costs in building the network had significantly increased and the number of projected users who would sign up for the service could not be accurately determined.

Despite other cities scaling back, New York City is increasing its wire- less presence. Free wireless service will be provided in 20 city parks by 2016. The city has also partnered with a company to provide a fast (up to 26 Mbps) network that covers seven square miles of Manhattan.

Connectivity Wireless data networks can also be used to provide improved service, to extend the reach of networks into areas that were difficult or even impossible to serve, and to provide a less expensive alternative to wired connections.

One of the problems that plague both home and office users who need to access the Internet is the last mile connection. This refers to the connection that begins at a fast Internet service provider, goes through the local neighborhood, and ends at the home or office. Whereas the connections that make up the nation’s data transmission infrastructure are very fast and well established, the last mile connection that links these high-speed transmission lines to the home or office are much slower and not universally available. These slow last mile connections are bottlenecks for users: any high-speed traffic must be throttled back to the slow speed of the last-mile connection, thus reducing the overall connection speed.

A solution to the last mile connection problem is a wireless ISP. A wireless ISP provides wireless data access directly to the home or office instead of a cable or Digital Subscriber Line (DSL) connection. High-speed wireless ISPs can provide in some instances speeds up to 7 Mbps. In addition, this wireless service is not restricted to the home: it can be available while visiting other offices in the area or even traveling in a car.

Organizations typically have multiple buildings, or even entire campuses with multiple build- ings, that all need to be connected into a single data network via a backhaul connection. A backhaul connection is an organization’s internal infrastructure connection between two or more remote locations.

14 Chapter 1 The World of Wireless

1 Until recently, organizations typically used trunk-based leased lines, leased from a local car- rier, to connect multiple locations. However, installing these special high-speed circuits is a very expensive and lengthy process. Current costs to run a fiber optic cable can range as high as $200,000 per mile (1.6 km), with almost 85 percent of that amount spent on to dig- ging trenches and installation. Street trenching and digging are not only expensive but they also can cause major traffic inconvenience, displace trees, and sometimes even mar or destroy historical areas. Because of these disruptions, some cities are considering a morato- rium on street trenching. And since laying fiber optic cable can be so complex—involving the acquisitions of right-of-ways, moving existing buried utilities, and burying fiber optic cable—it is not uncommon for fiber optic installations to take six to eight months or longer, even between two buildings in close proximity.

In some cities, the cost of laying a fiber optic cable can run as high as $3 million per mile.

As an alternative, wireless data networks can be used for building-to-building connections, eliminating the costs associated with leasing lines or installing fiber optic cables. Wireless networks can also be used to extend an existing network to remote sites.

The wireless equipment that is used for connecting buildings and sites is covered in Chapter 2.

Deployment Many buildings were constructed long before personal computers and networks were even imagined. Installing network cabling in these older buildings, which often have thick masonry walls and plaster ceilings, can be difficult and costly. Also, older buildings often contain asbestos which, according to today’s regulations, has to be completely removed before any major cabling infrastructure can be installed. Adding to the difficulties associated with older buildings, local or national landmark regulations often restrict the modification of facilities with historical value.

In these instances, deploying a wireless network is the ideal solution because the need to run cables is eliminated. Historic buildings can be preserved, dangerous asbestos is left untouched, and difficult drilling is avoided. Deploying a wireless network is also cheaper than laying cable. With cable connections costing $1 to $3 per connection, a wireless net- work can easily pay for itself by eliminating the need for a network connection in each office. Choosing wireless also means the days or even weeks it can take to pull wires through the ceiling and then drop cables down walls to network outlets are no longer an issue. Finally, deploying a wireless network is far less detrimental to productivity than instal- ling cable because, with wireless, employees do not have to struggle to do their work in the midst of the disruption caused by cable installation.

Wireless networks also make it easier for any office—in either an old or new building—to be modified with new cubicles or furniture. The floor plan for a remodeled office does not, first and foremost, have to accommodate the location of existing computer jacks. Instead, the focus can be on creating the most effective work environment for the

Wireless Advantages and Disadvantages 15

employees. The computer can be connected to the network no matter where it is placed in the cubicle.

Wireless networks can also improve network reliability. One of the common sources of network problems is network cable failure. Moisture from a leak during a thunderstorm or a coffee spill can erode metallic conductors on a network connection. A user who shifts the computer on his or her desk can break one or more of the wires in a patch cable. A cable splice that is done incorrectly can cause problems that result in intermittent errors that are very difficult to identify. Using wireless networks eliminates these types of cable failures and increases the overall reliability of the network.

Table 1-2 lists the advantages of wireless data networks.

Disadvantages The many advantages of wireless data networks technology should not mask the dis- advantages and concerns. These include security, radio signal interference, range of coverage, and slower speeds.

Security A wireless signal is not confined to a cable as in a traditional network; instead, it is broadcast in the open, making security for wireless networks a prime concern. Wireless security presents a number of unique challenges, including:

● Unauthorized users can access the network. Because a wireless signal is not confined to the four walls of the building, an unauthorized user can often pick up the signal outside the building’s security perimeter. It is possible for an intruder to be lurking in the parking lot with a wireless laptop computer to intercept the signals and access the network. Once on the network, the intruder could read sensitive documents or infect the entire network with malicious software.

● Attackers can eavesdrop on transmissions. In a wired network, an attacker would have to gain access to the interior of the building to reach the network cabling infrastructure before he could access the network to eavesdrop on data being transmitted. With a wireless network, if that information is not properly protected an attacker only has to pick up the wireless signal in order to see what is being transmitted.

● Employees can easily compromise network security. An employee could purchase inexpensive wireless equipment and then secretly bring it into the office in order to provide personal wireless access. However, this could defeat the existing network security because it allows an attacker to pick up the employee’s illicit wireless signal and enter the network by bypassing the organization’s security.

Advantage Example

Mobility Worker can read e-mail while traveling

Access User can access Internet at a restaurant

Connectivity Building-to-building network can be created at significant cost savings

Deployment Older building can easily have network capacity created without major renovation

Table 1-2 Advantages of wireless data network

16 Chapter 1 The World of Wireless

1 ● Attackers can easily crack legacy wireless security. Early types of wireless networks

had basic security features that were easy for attackers to defeat. Although current products have more robust security features, legacy products with these weak security features are still in use. Users are then sometimes mistaken in their belief that, because the security features are turned on, they are protected.

Wireless security is covered in Chapters 9 and 10.

Radio Signal Interference Wireless devices operate using radio signals, creating the potential for two types of signal interference. Signals from other devices can disrupt wireless network transmissions, or the wireless network device may itself be the source of interference for other devices. Several different types of devices transmit a radio signal that could inter- fere with a wireless data network, either by causing errors or by completely preventing transmissions. These devices include microwave ovens, elevator motors, photocopying machines, certain types of outdoor lighting systems, theft protection devices, and cordless telephones.

Range of Coverage The area in which a wireless network signal radiates is limited. Some wireless signals only have a range of 10 feet (33 meters), while other signals can extend over 350 feet (107 meters). Although this may be sufficient for a home or classroom, this area of coverage is very limited for many applications.

Slow Speed Standard wired LANs can transmit at 1 billion bits per second—1,000 Mbps, or 1 Gbps (gigabit per second)—and even faster networks operating at 10 times that speed are now becoming common. Older wireless data transmissions are far slower than that, generally ranging less than 50 Mbps. Although new wireless technologies can push speeds above 600 Mbps, this is still relatively slow for tasks such as downloading large video files.

Another speed-related problem has to do with the fact that a packet moving through a wire- less network is slower than a packet moving through a wired network. This delay, known as latency, can be problematic for time-sensitive communications such as voice or video transmissions.

When wireless data networks were first introduced, the potential health risks of the radio frequency energy emitted by such net- works were considered another disadvantage. At the time, it was well documented that high levels of this energy can produce bio- logical damage through heating effects (this is how a microwave

oven is able to cook food). Some experts feared that the energy emitted by wireless networks would have similar effects. However, most wireless devices emit very low levels of energy; these levels are now considered nonsignificant and do not appear to have health consequences.

Table 1-3 lists the disadvantages of wireless data networks.

Wireless Advantages and Disadvantages 17

Types of Wireless Networks

C W N A

2.1.1. Identify some of the uses for spread spectrum technologies.

The different types of wireless networks can be classified into four broad categories: wireless personal area networks, wireless local area networks, wireless metropolitan area networks, and wireless wide area networks. In the following sections, you will read how a fictional char- acter named Braden uses these types of networks during a typical work day.

Wireless Personal Area Network (WPAN) A wireless personal area network (WPAN) is a wireless network designed for hand-held and portable devices at slow transmission speeds and in close proximity to other devices. The maximum distance between devices is generally 33 feet (10 meters) with speeds of only 1 Mbps.

As Braden drives to his office, he needs to make a phone call to confirm his morning appointment. Braden’s car is equipped with hands-free cellular calling based on the Bluetooth wireless standard. Bluetooth is the name given to a WPAN technology that uses short-range transmissions. Origi- nally designed in 1994 by the cellular telephone company Ericsson as a way to replace wires with radio-based technology, Bluetooth has moved beyond that original design. Bluetooth tech- nology enables users to connect wirelessly to a wide range of computing and telecommunica- tions devices. It provides for rapid on-the-fly ad hoc connections between devices. Bluetooth is designed for notebook and tablet computers, cellular smartphones, and other portable devices. One of the advantages of Bluetooth is its low power consumption.

Bluetooth is named after the tenth-century Danish King Harald “Bluetooth” Gormsson, who was responsible for unifying Scandinavia.

Braden first turns on his Bluetooth-enabled cellular smartphone to pair it with the Bluetooth capabilities in his car. He presses the Talk button on the steering wheel and

Disadvantage Example

Security Attacker can read sensitive information by picking up wireless signal outside of building

Radio signal interference Intermittent errors occur on wireless network due to interference

Range of coverage User cannot access network outside of home

Slow speed Wireless device times out while trying to download large e-mail attachment

Table 1-3 Disadvantages of wireless data network

18 Chapter 1 The World of Wireless

1 says aloud the name of the person he’s calling, Mia. His cell phone dials the number that is stored in the car’s phonebook. The dialing and caller information, as well as signal and battery strength, appear on the car’s instrument panel. Once Mia answers the call, Braden can talk hands-free. The Bluetooth system also works with the car’s navigation system, so Braden can auto-dial phone numbers associated with local points of interest, such as hotels and restaurants.

Wireless Local Area Network (WLAN) A wireless local area network (WLAN) is designed to replace or supplement a wired local area network (LAN). Devices such as laptop computers, smartphones, printers, and game consoles that are within 350 feet (107 meters) of each other or a centrally located connection device can send and receive information at transmission speeds that typically range up to 600 Mbps.

On his way to the customer’s office Braden stops at a local coffee shop. As he sits at a table outside on the patio, he opens his laptop computer to go through his presentation again. Because his laptop has WLAN capabilities, he can wirelessly connect to the coffee shop’s cus- tomer WLAN. He attaches his presentation to an e-mail and sends it to his coworker, Abby, for her to review at her office. Braden also has a WLAN at home, where he can connect the stereo system and television to the home computer network. This allows him to view pictures stored on his computer on the larger TV screen, stream movies through his wireless network to his TV, and to play stored MP3 music through his audio system.

Wireless Metropolitan Area Network (WMAN) A wireless metropolitan area network (WMAN) is designed for devices in a broader area of coverage or at higher speeds. Generally a WMAN coverage area is from several city blocks to an entire small city.

Braden drives to the customer’s manufacturing plant, which is located in an industrial park at the edge of the city. As Braden sets up his laptop for his presentation, he asks Mia, the plant manager, if she could share her company’s latest sales figures for a product. Mia pulls her tablet computer from her purse and taps on it to access the company’s network at the main office building almost two miles away. The two buildings are connected with a WMAN tech- nology that uses light impulses to send and receive data. These low-powered beams, which do not harm the human eye, are transmitted by transceivers that are mounted high indoors and aimed at each other to provide a clear transmission path. Mia retrieves the information and reads the latest sales figures to Braden.

Wireless Wide Area Network (WWAN) A wireless wide area network (WWAN) is a wireless data network that extends beyond the range of a WMAN. It can encompass multiple states, regions, or countries; in fact WWAN can be a world-wide wireless data network.

After Mia sees Braden’s presentation in the conference room, she decides to show him a new production facility on the plant floor. As they walk through the factory, she asks a question that Braden cannot answer without accessing a recent e-mail. Braden can still connect to his company’s network through the Internet using a 4G (Fourth Generation) cellular wireless data network technology known as Long Term Evolution (LTE). Braden opens his backpack

Types of Wireless Networks 19

and connects a small device to his laptop computer, an LTE modem, which looks like an oversized Universal Serial Bus (USB) flash drive, as shown in Figure 1-7. This device allows him to connect to his office because it can provide wireless access several miles away from the transmission point at speeds up to 30 Mbps. Because Braden is within this range, he can easily retrieve the e-mail.

Comparison of Wireless Networks Although the distinctions between the four types of wireless networks—WPAN, WLAN, WMAN, and WWAN—can sometimes blur, they are generally distinguished by transmis- sion speeds and the maximum distances they cover. Figure 1-8 illustrates these differences. Table 1-4 lists the characteristics of these types of wireless networks.

Wireless Standards Organizations and Regulatory Agencies

C W N A

2.3.1. Define the roles of the following organizations in providing direction, cohesion, and accountability within the WLAN industry.

2.2.6. Understand the IEEE standard creation and ratification process and identify IEEE standard naming conventions.

Several different organizations provide direction, standards, and accountability in wireless technology. These include the International Telecommunication Union Radio Communication Sector (ITU-R), the U.S. Federal Communications Commission (FCC), the International Orga- nization for Standardization (ISO), the Institute of Electrical and Electronics Engineers (IEEE), and the Wi-Fi Alliance.

Figure 1-7 LTE modem

Courtesy of Bell Canada

20 Chapter 1 The World of Wireless

International Telecommunication Union Radio Communication Sector (ITU-R) The International Telecommunication Union Radio Communication Sector (ITU-R) is a divi- sion of the International Telecommunication Union (ITU) and is responsible for the global man- agement of the radio frequency spectrum. Its mission is to “ensure the rational, equitable, efficient and economical use of the radio frequency spectrum by all radiocommunication services.”

ITU-R also develops and manages space-related assignment for satel- lites by locating suitable orbital slots.

WWAN

WLAN

WPAN

WMAN

Figure 1-8 Coverage areas of wireless networks

Wireless Type Example Speed Distance

Wireless personal area network (WPAN) Bluetooth 1 Mbps 33 feet (10 meters)

Wireless local area network (WLAN) Wireless LAN 600 Mbps 350 feet (107 meters)

Wireless metropolitan area network (WMAN) Low-powered light beams 100 Mbps 35 miles (56 km)

Wireless wide area network (WWAN) LTE 30 Mbps World-wide

Table 1-4 Characteristics wireless networks

Wireless Standards Organizations and Regulatory Agencies 21

The ITU-R, which works on a global level, manages the international radio frequency spectrum and develops standards for wireless communications systems to ensure the most effective possible use of the spectrum. The ITU-R’s system for managing the international spectrum is based on the ITU-R’s regulatory procedures for frequency coordination, notifica- tion and registration.

Federal Communications Commission (FCC) In the United States, the organization that controls and regulates wireless transmissions for use by citizens is the Federal Communications Commission (FCC). The FCC serves as the primary regulatory agency for wireless communications in the United States and its territorial possessions. The FCC is an independent government agency that is directly responsible to Congress, established by the Communications Act of 1934 and charged with regulating interstate and international communications by radio, television, wire, satellite, and cable.

In order to preserve its independence, the FCC is directed by five commissioners who are appointed by the President and confirmed by the Senate for five-year terms. Only three commissioners may be members of the same political party, and none of them can have a financial interest in any FCC-related business.

The FCC’s responsibilities are broad. In addition to developing and implementing regulatory programs, they also process applications for licenses and other filings, analyze complaints, conduct investigations, and take part in congressional hearings. They also represent the United States in negotiations with other nations about telecommunications issues.

The FCC plays an important role in wireless communications. It regulates radio and televi- sion broadcast stations as well as cable and satellite stations, and also oversees cellular tele- phones, pagers, and two-way radios. The FCC regulates the use of radio frequencies to fulfill the communications needs of businesses, local and state governments, public safety service providers, aircraft and ship operators, and individuals. The FCC is also charged with regulat- ing the radio frequency spectrum within the United States.

International Organization for Standardization (ISO) The International Organization for Standardization (ISO) is an international body that sets industrial and commercial standards. Composed of representatives from national standards organizations, the ISO is officially not a government entity. However, because of treaties or existing national standards, the decisions of the ISO usually become law in all nations.

The ISO says that their work is to “make a positive difference to the world we live in.”

The ISO identifies needs in business and then develops standards to address those needs. The goal of the ISO is to make the development, manufacturing, and supply of products and services more efficient, safer, and cleaner. The ISO also works to make trade between countries easier and fairer.

22 Chapter 1 The World of Wireless

1 Institute of Electrical and Electronics Engineers (IEEE) In the field of computer networking and wireless communications, the most widely known and influential organization is the Institute of Electrical and Electronics Engineers (IEEE). The IEEE and its predecessor organizations date back to 1884. The IEEE is one of the lead- ing developers of global standards in a broad range of industries such as energy, biomedical and health care, and transportation. It is currently involved in developing and revising over 800 standards. Some of these standards apply to circuits and devices, communication and information technology, control and automation, electromagnetics, geoscience, ocean technol- ogy and remote sensing, instrumentation and measurement and testing, optics, power and energy, and signal processing.

The IEEE is best known for its work in establishing standards for computer networks. In the early 1980s, the IEEE began work on Project 802, which focused on developing computer net- work architecture standards. Project 802 quickly expanded into several different categories of network technology, such as 802.3 (Ethernet), 802.5 (Token Ring), and 802.15.1 (Bluetooth).

The IEEE standard for WLANs is typically referred to as IEEE 802.11. The full name for this standard is as follows: IEEE 802.11-2007 IEEE Standard for Information technology— Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. Amendments, modifications, and changes to the standard are designated by a letter (or two) appended to “IEEE 802.11.” For example, some variations on the standard include IEEE 802.11g, IEEE 802.11n, and IEEE 802.11af.

The IEEE group responsible for creating and overseeing a specific standard is a committee called a working group (WG) and is identified by the standard for which they are responsible, such as the IEEE 802.11 WG. Within the WGs are various subgroups. These include ad-hoc groups, study groups (SGs), and task groups (TGs). The TGs are responsible for specific amendments and are also designated by the amendment letter, such as the IEEE 802.11 TGa.

The creation and ratification process of IEEE standards and their amendments is a lengthy process. First, an SG, composed of interested persons, defines the purpose of the amendment and creates a charter. The charter identifies the problem the proposed amendment is designed to address and explains how to identify when the problem has been solved. The charter is then presented to the entire WG. Because the WG only meets once each quarter, creating the charter by the SG usually takes at least two quarters (six months). If the SG charter is accepted by the WG, a TG is sanctioned; the TG must then either fulfill the charter that results in an amendment or a recommendation, or disband if no solution can be identified.

The TG begins its work by making a formal call for proposals. Each proposal is presented, reviewed, and voted upon before the actual amendment writing process can begin. (The writ- ing process itself can take anywhere from six months to several years. Once the TG finishes writing the amendment, the amendment is circulated to the entire 802.11 WG voting mem- bership for comments and subsequent voting. This commentary and voting process is done for as many iterations, or rounds, as necessary, until all comments are resolved and an 80-percent favorable vote is reached. This voting process can take at least six more months and up to several years.) Once the amendment has been approved by the 802.11 WG, it is passed on to the entire IEEE 802 committee. Upon approval from that committee, the amendment becomes ratified. After ratification, the amendment must be officially published and distributed to the general public. Figure 1-9 illustrates the IEEE ratification process.

Wireless Standards Organizations and Regulatory Agencies 23

Developing a standard or amendment is a very time-consuming process. Under the best of conditions, where there is already general agreement with little discussion, the standards process takes a mini- mum of 18 months, while a controversial proposal can take several years to be approved. Because the 802.11 WG meetings are held

quarterly and attended by over 1,000 participants, of whom 70 percent have earned voting status, someone has said that producing an IEEE-ratified amendment is like writing a book with 700 authors!

The IEEE also uses its own set of terms to define the different documents used in the process. These terms are listed in Table 1-5.

There is no standard or TG named “802.11x.” Instead, “802.11x” is used informally to designate a current or future 802.11 amendment in those cases where further precision is not necessary.

Wi-Fi Alliance In 1999, a consortium of wireless equipment manufacturers and software providers was formed to promote wireless network technology. This group was initially known as the Wireless Ethernet Compatibility Alliance (WECA) and had three goals:

● To encourage wireless manufacturers to use the IEEE WLAN technologies in their wireless networking products

● To promote and market these technologies to consumers in the home, SOHO settings, and in large enterprise businesses and organizations

Charter

TGSG WG IEEE 802.11 General Public

6 Months- Several Years

6 Months 6 Months- Several Years

3 Months

Group

Document

Time

Amendment Approve

Amendment Amendment Published

Amendment Ratified

Figure 1-9 IEEE ratification process

Name Definition Comments

Drafts A preliminary standard or amendment proposal Created by WG

Standards The current standard Denoted by IEEE 802.11-published date

Ratified amendments Final document distributed to the public Approved by WG and 802.11

Supplements An addition to an existing standard Not common

Recommended practices Interpretations of standards Designated by uppercase letter (802.11F, 802.11T)

Table 1-5 IEEE documents

24 Chapter 1 The World of Wireless

1 ● To test and certify that wireless products adhere to the IEEE standards to ensure

product interoperability

In October 2002 the WECA organization changed its name to Wi-Fi (Wireless Fidelity) Alliance, which reflected the name of the certification that it uses (Wi-Fi) to verify that a product follows IEEE standards. Wireless devices are sometimes generically called Wi-Fi, while in reality only products that have passed Wi-Fi Alliance testing are allowed to refer to their products as Wi-Fi Certified, which is a registered trademark.

Certified Wireless Network Administrator (CWNA) The goal of the Certified Wireless Network Professional (CWNP) organization is to educate professionals in the technology of enterprise WLAN products. This can help these profes- sionals to manage a wireless LAN infrastructure—regardless of which vendor’s products are used—and maintain a wireless network that is cost-effective, reliable, and secure.

In order to provide this level of education, the CWNP offers multiple vendor-neutral enterprise wireless LAN certifications. The Certified Wireless Network Administrator (CWNA) certifica- tion is the foundation level wireless LAN certification for the CWNP program. Individuals who hold the CWNA certificate have demonstrated the necessary skills to successfully administer enterprise-class wireless LANs.

Chapter Summary ■ Wireless data communications are found across all sectors of the economy. Wireless

data communication is an ideal technology for colleges and schools. Instructors can use wireless to facilitate lectures in the classroom, while students can use wireless to access the school’s network from virtually any location on campus. Schools can save funds by installing wireless networks and reducing the number of traditional computer labs that are needed. Businesses use wireless technologies to increase employee productivity. A wired conference room allows all participants to have access to their data while away from their desks and create a mobile office environment. Even small office/home office (SOHO) businesses can reduce costs using wireless technologies.

■ The construction industry relies heavily on wireless data technologies, from managing pay sheets and rescheduling workers to precisely positioning earth movers and bulldozers on the construction site. Warehouse management has become a critical element in today’s operations. Both forklifts and employees are fitted with portable wireless systems for tracking the flow of inventory into and out of the warehouse. Coupled with radio frequency identification (RFID) tags, warehouse wireless data systems help control inventories. Manufacturing plants use wireless data networks for managing raw materials, finished inventory, and in the manufacturing process itself.

■ The travel industry uses wireless communications for providing Internet access to passengers, installing software updates on airplanes, and accessing the latest mainte- nance information. Many public safety departments use wireless data networks for downloading building plans or even photographs of criminal suspects. Hospitals and health-care organizations use wireless data networks to control the administration of pharmaceuticals to patients.

Chapter Summary 25

■ There are many advantages to wireless technology. One of the primary advantages is the user’s capability to move about without being connected by a cable, thus enabling access from almost any location inside or out of a building. Wireless data communica- tions can also be used to provide access to a network where previously none existed. A hotspot is a specific geographic location that is served by a wireless data system and provides network access to mobile users; hotspots are found in a variety of settings. A municipal network is a hotspot or series of hotspots funded by local governments. Another advantage of wireless networks is improved connectivity; they can be used to provide improved service, to extend the reach of networks into areas that were diffi- cult or even impossible to serve, or to provide a less expensive alternative to wired connections. Because installing a wired network cabling system can be costly and often difficult in older structures, wireless networks eliminate the need for extensive drilling or retrofitting older buildings.

■ One of the disadvantages of wireless data networks is security. Because a wireless signal is not confined to a cable, attackers can pick up the signal outside of the build- ing, read sensitive transmissions, and inject malware into an otherwise secure network. Another disadvantage is radio signal interference: because wireless devices share the same frequency spectrums they can interfere with the transmission of each other. The coverage range of wireless signals can be limited, and the slow speed of the networks in comparison to wired networks can also be problematic.

■ There are four basic types of wireless networks. A wireless personal area network (WPAN) is a wireless network designed for hand-held and portable devices at slow transmission speeds and in close proximity to each other. A wireless local area network (WLAN) is designed to replace or supplement a wired LAN, whereas a wireless metropolitan area network (WMAN) is designed for devices in a broader area of coverage (from several city blocks to an entire city) or at higher speeds. A wireless wide area network (WWAN) can encompass multiple states, regions, or countries.

■ Several different organizations provide direction, standards, and accountability in wireless technology. These include the International Telecommunication Union Radio Communication Sector (ITU-R), the U.S. Federal Communications Commission (FCC), the International Organization for Standardization (ISO), the Institute of Electrical and Electronics Engineers (IEEE), and the Wi-Fi Alliance.

■ The Certified Wireless Network Administrator (CWNA) certification is the foundation level wireless LAN certification for the CWNP program. Individuals who hold the CWNA certificate have demonstrated the necessary skills to successfully administer enterprise-class wireless LANs.

Key Terms 4G (Fourth Generation) A cellular wireless data network with average download speeds of 4 Mbps. backhaul connection An organization’s internal infrastructure connection between two or more remote locations. Bluetooth A WPAN technology that uses short-range transmissions.

26 Chapter 1 The World of Wireless

1 Certified Wireless Network Administrator (CWNA) A certification that is the foundation level wireless LAN certification for the CWNP program. Federal Communications Commission (FCC) A body that serves as the primary regulatory agency for wireless communications in the United States and its territorial possessions. global positioning system (GPS) A system of earth-orbiting satellites used as a navigation system. hotspot A specific geographic location that is served by a wireless data system and provides network access to mobile users. Institute of Electrical and Electronics Engineers (IEEE) An organization best known for its work in establishing standards for computer networks. International Organization for Standardization (ISO) An international body that sets industrial and commercial standards. International Telecommunication Union Radio Communication Sector (ITU-R) A division of the International Telecommunication Union (ITU) that is responsible for the global management of the radio frequency spectrum. last mile connection The connection that begins at a fast Internet service provider, goes through the local neighborhood, and ends at the home or office. latency The time lapse between when a packet is sent on a network and when it is received. Long Term Evolution (LTE) A wireless metropolitan area network technology that can provide access from up to 10 miles (15 km) in distance. municipal network A hotspot funded by city, county, or other local governments. radio frequency identification (RFID) A wireless technology that emits a wireless data signal over a short range. small office/home office (SOHO) A business setting that typically has ten or fewer employees. task group (TG) An IEEE subgroup that is responsible for fulfilling a charter that results in an amendment or a recommendation, or that disbands if no solution can be identified. trunk-based leased lines Special high-speed circuits leased from a local carrier that can be used to connect remote sites of a business. warehouse management system (WMS) Software that can manage all activities in a warehouse, from receiving through shipping. Wi-Fi (Wireless Fidelity) Alliance An organization that verifies that a product follows IEEE standards. wireless ISP An Internet Service Provider that makes wireless data access available directly to the home or office. wireless local area network (WLAN) A wireless network designed to replace or supplement a wired local area network. wireless metropolitan area network (WMAN) A wireless network is designed for devices in a broader area of coverage than a WLAN or at higher speeds. wireless personal area network (WPAN) A wireless network designed for hand-held and portable devices at slow transmission speeds and in close proximity. wireless wide area network (WWAN) A wireless data network that can encompass multiple states, regions, or countries. working group (WG) An IEEE committee that is responsible for creating and overseeing a specific standard.

Key Terms 27

Review Questions 1. Each of the following is an advantage of using wireless data technology in education

except:

a. instructors can carry their laptops into the classroom without connecting cables.

b. presentations can be transmitted wirelessly to projector systems.

c. students can access the school network from any location.

d. costs are increased by schools that use wireless technologies.

2. Which of the following is an advantage of wireless technology for a business?

a. Employees need a laptop or tablet computer.

b. Wireless data access in conference rooms provides employees with immediate access to the data that they need while away from their desk.

c. The expense of a wireless network exceeds that of a wired network.

d. Wireless networks are always faster than wired networks.

3. Each of the following is true about the global positioning system (GPS) except:

a. it was originally developed by the U.S. military in the late 1970s as a navigation system.

b. it is used to provide precise location information.

c. a license must be obtained before using it.

d. devices communicate with earth-orbiting satellites in order to determine their location.

4. In a warehouse management system radio frequency identification (RFID) tags .

a. emit a wireless data signal that contains an identification number

b. are only used when inventory leaves the warehouse

c. replace printed labels

d. are only found on storage racks and not on pallets

5. A is a specific geographic location that is served by a wireless data system.

a. JWire

b. wireless tag

c. Wi-Fi Community (WFC)

d. hotspot

6. Which of the following is not an advantage of a municipal network?

a. Areas with municipal networks can become more attractive to businesses.

b. Local municipal employees can use the network to access information.

c. ISPs no longer have to provide Internet access.

d. Municipal networks provide high speed Internet access for free or at a reduced cost.

28 Chapter 1 The World of Wireless

1 7. The refers to the connection that begins at a fast Internet service

provider, goes through the local neighborhood, and ends at the home or office.

a. last mile connection

b. first mile connection

c. backhaul connection

d. splice

8. are special high-speed circuits leased from a local carrier that can be used to connect remote sites of a business.

a. Trunk-based leased lines

b. WISPs

c. Fronthaul connections

d. Fiber merge networks (FMN)

9. Each of the following is a limitation of wired networks that wireless networks over- come except:

a. a user who shifts the computer on his or her desk may break one or more of the wires in a patch cable.

b. a cable splice that is done incorrectly can cause problems that result in intermittent errors that are very difficult to identify.

c. moisture can erode metallic conductors on a network connection.

d. wired networks require that all equipment be IEEE certified.

10. Which of the following is not a disadvantage of a wireless data network?

a. mobility

b. security

c. interference

d. coverage range

11. In a wireless personal area network (WPAN), the maximum distance between devices is generally .

a. 33 feet

b. 175 feet

c. 3 city blocks

d. There is no maximum distance for a WPAN

12. Which of the following is true regarding Bluetooth?

a. It is a WWAN technology.

b. It was originally designed to replace wires with radio-based technology.

c. It cannot support ad hoc connections.

d. It consumes a large amount of power.

Review Questions 29

13. Typical transmissions speeds for WLANs do not exceed Mbps.

a. 11

b. 54

c. 108

d. 600

14. An example of a WMAN technology is .

a. WLAN

b. LTE

c. 2G

d. RFID

15. is a cellular wireless data network technology through which a user can access the Internet.

a. 4G

b. OFDM

c. FFRD

d. KRG

16. Which of the following wireless networks has a maximum speed of 350 Mbps and a coverage area of 350 feet?

a. WLAN

b. WPAN

c. WMAN

d. WWAN

17. The is responsible for the global management of the radio frequency spectrum.

a. International Standards Organization (ISO)

b. Federal Communications Commission (FCC)

c. International Telecommunication Union Radio Communication Sector (ITU-R)

d. Wireless Ethernet Compatability Alliance (WECA)

18. is the primary regulatory agency for wireless communications in the United States and its territorial possessions.

a. The Federal Communications Commission (FCC)

b. Wi-Fi

c. The Institute of Electrical and Electronics Engineers (IEEE)

d. RCID

30 Chapter 1 The World of Wireless

1 19. The is an international body that sets industrial and commercial stan-

dards and is composed of representatives from national standards organizations.

a. International Organization for Standardization (ISO)

b. Federal Communications Commission (FCC)

c. Wireless Ethernet Compatability Alliance (WECA)

d. Occupational Standards Institute (OSI)

20. The Wi-Fi Alliance .

a. certifies that wireless products adhere to the IEEE standards to ensure product interoperability

b. has been replaced by the ISO

c. is a subgroup of the IEEE

d. only works with large enterprise businesses

Hands-On Projects

Project 1-1: Locating Area Hotspots with Hotspot- Locations Different Internet tools are available to locate both free and fee-based hot- spots. In this project you use one of those tools to find hotspots in your area.

1. Use your Web browser to go to www.hotspot-locations.com.

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions or the location on the Web page has changed then open a search engine and search for “Hotspot-locations”.

2. Under Search Wireless Hotspot Database enter your location information under Country, State, and City or Zip.

3. Be sure that Operator and Type are both are set to All. Click Search.

4. All hotspots registered with Hotspot-Locations are displayed under Hotspots found. If no hotspots appear, change the location to a large city that is close to you or with which you are familiar and search again.

You can also click All cities under Search Tip and manually select a country, and then a city, that has a hotspot location registered with Hotspot-Locations.

5. Under Hotspot name, click one of the links to a hotspot. What type of information appears? Would this be useful in locating and connecting to that hotspot?

6. Click show on map. A map now appears with the location of this hotspot.

Hands-On Projects 31

Depending on how the hotspot was registered, the exact location may not be displayed and you may need to enter additional informa- tion, such as the city or state of the area in which you are searching.

7. Using the toolbar, pan to the left or right to move the map focus to the region in which you live.

8. Use the toolbar’s zoom in and zoom out buttons to “drill down” to the city in which you live.

9. Close the browser window that contains the map and return to the Hotspot-Locations search results. Locate more hotspots by entering different location information and viewing them on the map.

10. How would you rate Hotspot-Locations in terms of ease of use, accuracy, options, etc.?

11. Leave this browser window open for the next project.

Project 1-2: Locating Area Hotspots with Hotspotr In this project you use a different online tool to find hotspots in your area.

1. Open a new tab on your Web browser that is still open from the previous project and go to hotspotr.com/wifi.

It is not unusual for Web sites to change. If the URL above no longer functions then open a search engine and search for “hotspotr.com/wifi”.

2. Instead of allowing you to click on a map, the default search asks you to enter a loca- tion as the focal point of the search. Enter an address as the starting point.

3. Click go.

4. Compare your results with the results you received using Hotspot-Locations.

5. Between Hotspot-Locations and Hotspotr, which tool has more hotspots listed? Why is there a difference?

6. Now compare the options available on Hotspotr with those available on Hotspot- Locations. Which are more comprehensive?

7. How would you rate Hotspotr in terms of ease of use, accuracy, options, etc.? Which would you recommend to another wireless user?

8. Close your Web browser.

Project 1-3: Installing Network Meter Gadget How do you locate a WLAN hotspot without information from using one of the online hotspot locaters? One solution is to use a wireless signal meter that shows when you have moved into a hotspot coverage area and also tells you the strength of the signal. In this project you will download and install a pro- gram to find a wireless signal.

32 Chapter 1 The World of Wireless

1 1. Use your Web browser to go to addgadget.com/network_meter

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions then open a search engine and search for “Network Meter”.

2. Under Download locate the link that contains the latest version of Network Meter and click on it.

3. Follow the instructions to install this gadget on your computer.

4. The gadget now appears on your Windows sidebar. To enlarge the size of the gadget, hover your mouse pointer over the gadget until a wrench icon appears and then click it.

5. Click the Display tab.

6. Click the Size list arrow and select 400%.

7. Click OK.

8. Click on the title of the gadget to display the flyout features. Note that it gives detailed information about your wireless network. Which details do you find helpful?

9. Click Speed Test. What does Network Meter display?

10. Close all windows.

If you choose to keep Network Meter on your computer you may want to change the Size back to 100. If you want to remove this gadget, open the Windows Control Panel and click Programs and then Uninstall a gadget.

Case Projects

Case Project 1-1: New Wireless Careers The widespread use of wireless data networks technology has created new job opportunities. What are these new wireless careers? How do they differ from existing network careers? How are they similar? Using the Internet and print

resources, research the types of wireless jobs that are in demand. Create a table that lists the job title, the necessary education and/or certification and experience for the position, and the approximate starting salary.

Case Project 1-2: Wireless Technology in Other Economic Sectors Wireless data communications is used extensively in education, business, industry, travel, public safety, and health care. What other areas of the economy, other than those listed in the chapter, strongly depend upon wireless technology today? Research another area, such as finance, logistics, military, etc., and research how this sector is using wireless technology. Give several examples of its use in that area. Write a one-page paper on your findings.

Case Project 1-3: Municipal Networks Select a municipality—if possible choose one that is close to where you live—that has imple- mented a municipal wireless network. What was the rationale for its implementation? How

Case Projects 33

has it been used? What are the plans for the future? Now compare that network with a different network that the governing municipality has been forced to alter or even abandon. What happened? Why did this other network not succeed while the former has been success- ful? What suggestions would you make for a municipality that wanted to create a wireless network? Write a one-page paper on your findings.

Case Project 1-4: IEEE and Wi-Fi Using the Internet, compare the objectives and work of the IEEE and Wi-Fi. What are the goals of each group? How are they different? How are they the same? Do these two organizations cooperating, and if so, in what way? Create a one-page document of your research.

Case Project 1-5: Nautilus IT Consulting Nautilus IT Consulting (NITC) is a computer technology business that helps organizations develop IT solutions. To accommodate its growing customer base, NITC often looks to outside consultants for assistance on specific projects. NITC has asked you to help with new customers.

Luv Those Grandchildren! (LTG!) is a regional children’s clothing store that carries chil- dren’s clothing. As a pilot program, LTG! wants to develop a wireless in-house inventory management system for its store in the new upscale Indian Lake outdoor mall to replace their manual system. As boxes of clothes are received at the Indian Lake store from the distri- bution warehouse, they must be entered into the store’s inventory. (Because the storage area for the retail stores is small, no forklifts are used.) Items for sale are then placed on the correct shelf or rack in the store for customers to purchase. When a purchase is made, that item is sub- tracted from the existing electronic inventory. A report produced every Monday and Thursday informs the staff which items require restocking. Every Friday, the staff conducts a manual inventory on all unsold items unsold. The manager then compares the manual inventory totals the electronic inventory, and works with her staff to reconcile any discrepancies. NITC has been asked to assist LTG! in this pilot program, and NITC has turned to you for assistance.

1. Create a PowerPoint presentation of eight or more slides that covers the basics of your design for the new wireless inventory system. Include the advantages and disadvantages, as well as the specific wireless technologies that you would implement.

2. After your successful presentation, NITC decides to encourage LTG! to expand the wireless pilot program to include using a WMAN to connect the Indian Lake store to the distribution warehouse located 12 miles away. The company management believes this will help cut costs. Create a one-page memo addressed to LTG! Management that lists the advantages and disadvantages of a WMAN. In the memo, recommend a specific wireless technol- ogy and explain why. Use the Internet to research both costs for implementing the system and any recurring monthly costs, and include those in your memo.

Note

i. “How Cisco WLAN Became Primary Corporate User Network.” Accessed July 22, 2011, http://www.cisco.com/web/about/ciscoitatwork/downloads/ciscoitatwork/pdf/Cisco_ IT_Case_Study_WLAN_Benefits.pdf.

34 Chapter 1 The World of Wireless

chapter2

Wireless Local Area Networks

After completing this chapter you should be able to:

• Explain the need for and sources of wireless networking standards • Describe the features of the IEEE 802.11a/b/g/n WLANs • List the different types of client hardware and software • Describe the different functions of infrastructure devices

35 Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

When wireless technologies were first introduced in the early 2000s, the health-care industry was reluctant to implement them because of concerns that the signals from wireless data networks would interfere with sensitive medical devices. This was proven to not be an issue. Today, wireless has been embraced by hospitals and health-care providers as a means to reduce costs and improve services. Hospitals rou- tinely use wireless data point-of-care computer systems that allow medical staff to access and update patient records from virtually any location.

The medical profession is now moving beyond using wireless as an information technology (IT) tool and is implementing wireless for direct patient care. One hospital chain runs a patient-monitoring application at its 23 medical facilities. Using a wire- less local area network (WLAN), data from infusion pumps (portable devices that are used to administer medications intravenously) is transmitted wirelessly to the patient information system. This system can remotely monitor how much of a drug the patient has received and compare it against standards for the correct dosage. If an incorrect amount is being delivered, an alarm is activated. This wireless application costs $3 million to implement.

Another use of wireless in the health-care field involves wireless cardiac- monitoring tools. In the past, a patient was attached by cables to a recording device that typically recorded only two or three days’ worth of heart data. Now wireless sen- sors, attached to a patient’s chest, can transmit remote data (called telemetry) in real time. This data can include heart rhythms, body temperature, and blood-oxygen levels. The wireless system can detect any heart irregularities and immediately alert cardiac technicians. The wireless sensors can even indicate body position, such as whether the patient is standing or lying down. The data is transmitted through a spe- cial Bluetooth device connected to a cellular phone. For up to 21 days, these wireless sensors capture data and continuously send it to a call center. Having three weeks’ worth of data, instead of only three days’ worth, gives physicians a much broader window of the patient’s heart rhythm and can improve diagnostic accuracy.

As helpful as these wireless devices are, the rapid proliferation of wireless medical devices has also caused some problems. As hospitals and medical centers deploy wire- less medical telemetry services, patient monitoring systems, two-way radios, cellular networks, paging, public safety radio and security communications, as well as WLAN visitor hotspots, the wireless airwaves are becoming crowded and can interfere with each other. To help hospitals simplify their multiple wireless systems, several consulting companies are now available to assist in simplifying the wireless networks’ installation, maintenance, control, and management to harmonize them into a more universal health-care wireless network.

Real World Wireless

36 Chapter 2 Wireless Local Area Networks

As you read in Chapter 1, wireless networks can be divided into four broad categories: wireless personal area networks (WPANs), wireless local area networks (WLANs), wireless metropolitan area networks (WMANs), and wireless wide area networks (WWANs). Argu- ably, the catalyst for today’s wireless data revolution is WLANs. These networks are designed to replace or supplement a wired local area network (LAN). Devices such as laptop compu- ters, smartphones, printers, and game consoles that are within 350 feet (107 meters) of each other or a centrally located connection device can send and receive information at transmis- sion speeds that typically range from 1 Mbps (million bits per second) up to 600 Mbps.

In this chapter you will be introduced to WLANs in detail. First you will learn about the importance of standards and then explore the various WLAN standards. Next, you will inves- tigate the various devices that allow a wireless client to be part of a wireless network. Finally, you will study the various wireless devices that make up a WLAN infrastructure.

Understanding Standards A standard may be thought of as a model that is used for comparison. Much of the world today is based on standards. The size of a sheet of notebook paper, the shape of a light bulb socket, and the physical dimensions of a DVD are all specified by recognized standards. Stan- dards make it easier for users to purchase and use a wide variety of products. Because all DVDs are the same size, a user can purchase a disc from virtually anywhere and be confident that it will function in her DVD player at home. Standards continue to be needed today, par- ticularly in the field of IT. There are also different sources for standards.

The Need for Standards In the early years of the microcomputer industry, there were virtually no standards; vendors developed their own equipment based on what they considered to be most useful. This resulted in a fragmented market. Users were generally reluctant to purchase hardware or software knowing that it might only function on a single device and could not be inter- changeable. However, when computers entered the mainstream and were more widely uti- lized, standards became much more important.

It is argued that because IT is a complex field, it is particularly depen- dent on standards to stabilize technology, which in turn encourages investment and growth. Because the size of a DVD is standardized, multiple vendors can willingly manufacture and market DVD players, knowing that they have a chance to capture a share of the market and profit from the venture.

Standards have many advantages. These include:

● Interoperability. Standards ensure that devices from one vendor will function with those from other vendors. Devices that are not based on standards often cannot inter- operate with similar devices from other vendors.

● Competition. Standards serve to create competition. If a vendor creates a new device without regard to current standards, then it automatically owns the specifications for the device; the vendor might even take out a patent on the device. This makes it virtu- ally impossible for another vendor to produce the same device; thus, competition

Understanding Standards 37

among multiple vendors selling the same device is impossible. From the point of view of the consumer, standards are desirable because they encourage competition. Any vendor can create a device based on a recognized standard. In order to compete, vendors will add additional features to their products, thus increasing the overall value for users.

● Lower costs. Competition results in lower costs for both users and manufacturers. When several vendors make similar products based on the same standards, they com- pete against each other on the price, which in turn makes the product less expensive for users. Competition also results in lower costs for manufacturers. Because standards have been established, manufacturers do not need to invest large amounts of capital in research and development. This reduces start-up costs as well as the amount of time required to bring a product to market. Also, manufacturing to standards encourages manufacturers to deploy mass-production techniques and economies of scale to keep production costs low, with savings that in turn are passed on to users.

● Protection. Standards help protect the user’s investment in equipment. It is not uncommon for a proprietary vendor to phase out a product line, leaving a business that purchased the equipment with two choices: continue to use the now-obsolete system with escalating costs for supplies and technical support, or discard the legacy system and buy a new system. Both choices are costly. Standards, however, can help create a migration path for equipment upgrades. Newer standards are generally backward compatible or at least provide a means of migrating to equipment based on the newer standards at a minimal cost.

Sources of Standards There are essentially three sources of standards:

● De facto standards. De facto (from Latin meaning from the fact) standards are not actually standards. Rather, they are common practices that the industry follows for var- ious reasons (such as ease of use, tradition, or what the majority of users choose to do). For the most part, de facto standards are established by success in the marketplace.

An example of a de facto standard is the Microsoft Windows operat- ing system for most personal desktop computers. Because the major- ity of users have elected to run Windows on their computers it is considered the standard desktop operating system.

● De jure standards. De jure (from Latin meaning from the law) standards are official standards. De jure standards are those that are controlled by an organization or body that has been entrusted with that task.

De facto standards sometimes become de jure standards after being approved by a committee. Ethernet is one example of a de facto standard that later became a de jure standard.

● Consortia-created standards. One of the complaints against de jure standards is the amount of time it takes for a standard to be completed. In reaction to this, consortia sometimes take on the task of creating standards. Consortia are usually industry-sponsored organizations that want to promote a specific technology. The goal of consortia is to develop a standard that promotes a specific technology in a short period of time.

38 Chapter 2 Wireless Local Area Networks

One of the limitations of consortia is that membership is not always be open to everyone who wants to participate.

Types of Wireless LANs

C W N A

2.2.5. Supported data rates for each IEEE 802.11-2007 PHY.

C W N A

2.1.4. Identify and apply the concepts which make up the functional- ity of spread spectrum technology.

One reason WLANs have been so successful is that, from the outset, they have been based on de jure standards. If WLAN technology had been based on de facto standards, different ven- dors would have developed competing wireless technologies. They would then have been forced to battle in the marketplace to win the hearts and minds of consumers. The resulting confusion could have easily alienated consumers, while the need for each vendor to invest in its own research and development would certainly have slowed the spread of wireless. Instead, the de jure standards for WLAN helped to fuel the rapid growth of WLANs.

WLAN standards are set by the Institute of Electrical and Electronics Engineers (IEEE). There currently is one wireless LAN standard, IEEE 802.11-2007, and one significant amendment, 802.11n-2009.

The Institute of Electrical and Electronics Engineers (IEEE) is covered in Chapter 1.

IEEE 802.11-2007 Since the late 1990s, the IEEE has approved four standards for wireless LANs—IEEE 802.11, 802.11b, 802.11a, and 802.11g—along with several amendments (such as IEEE 802.11d, IEEE 802.11h, and so on). In order to reduce the confusion of this alphabet soup of standards, amendments, and the year in which it was ratified, in 2007 the IEEE combined the standards and amendments into a single standard officially known as IEEE 802.11-2007, or the IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. This document officially retires all previous standards (such as IEEE 802.11b-1999) and combines them into this single comprehensive document. Specifically it includes the following:

● IEEE 802.11-1999 ● IEEE 802.11a-1999

Types of Wireless LANs 39

● IEEE 802.11b-1999 ● IEEE 802.11d-2001 ● IEEE 802.11g-2003 ● IEEE 802.11h-2003 ● IEEE 802.11i-2004 ● IEEE 802.11j-2004 ● IEEE 802.11e-2005

In addition, this single standard specifies technical corrections and clarifications to the origi- nal 802.11 standard, as well as enhancements for improved security, vendor-specific exten- sions, and interpretations.

The IEEE 802.11-2007 document, which is 1,232 pages long, is avail- able as a free download from the IEEE Web site.

The four standards for wireless LANs are IEEE 802.11, 802.11b, 802.11a, and 802.11g.

Although the IEEE has officially retired the 802.11, 802.11b, 802.11a, and 802.11g standards and rolled them into the IEEE 802.11-2007 standard, it is still common to refer to them individually.

IEEE 802.11 In 1990 the IEEE formed a working group (WG) to develop a standard for WLANs operating at 1 and 2 Mbps. The WG recommended several different proposals before developing a draft. This draft went through seven different revisions that took seven years to complete. On June 26, 1997 the IEEE approved the final draft known as IEEE 802.11. The IEEE 802.11 standard specified that wireless transmissions could occur in one of two ways—via infrared light or radio waves.

One of the transmission options is through using infrared light. All the different types of light that travel from the sun to the earth make up the light spectrum, which is composed of both visible and invisible light. Although invisible, infrared light is next to visible light on the light spectrum and shares many of the same characteristics.

Some of the other energies of the spectrum that are invisible to the human eye include X-rays, ultraviolet rays, and microwaves.

Infrared transmissions send data by the intensity of the infrared light wave instead of turn- ing the light off and on. To transmit a 1 an emitter (a device that transmits a signal) increases the intensity of the current and sends a pulse using infrared light. On the receiving

40 Chapter 2 Wireless Local Area Networks

end a detector (a device that receives a signal) senses the higher intensity pulse of light and produces a proportional electrical current.

Infrared transmissions can be either directed or diffused. A directed transmission requires that the emitter and detector be directly aimed at one another in a line of sight (LoS) path, as shown in Figure 2-1. A diffused transmission relies on reflected light. The emitters on diffused transmis- sions have a wide-focused beam instead of a narrow beam and are pointed at the room’s ceiling, which serves as the reflection point. When the emitter transmits an infrared signal, the signal bounces off the ceiling, expanding (called diffusing) to filling the room. The detectors—pointed at the same reflection point—can then pick up the reflected signal, as shown in Figure 2-2.

Infrared wireless systems have significant limitations, which are listed in Table 2-1. Because of these limitations, 802.11 infrared WLAN systems were never widely adopted.

Emitter

Line of sight

PC Emitter

Detector

Figure 2-1 Directed infrared transmission

Detector

Ceiling reflection point

Wide angle

Emitter

Figure 2-2 Diffused infrared transmission

Types of Wireless LANs 41

The other form of transmission approved in the IEEE 802.11 standard is radio waves. Unlike infrared light, radio waves can penetrate objects like walls and allow the wireless user to be mobile. In addition, radio waves travel longer distances and can be used outdoors as well as indoors. Finally, radio waves can travel at much higher speeds than infrared transmissions. The use of radio waves in transmissions has become the preferred method for wireless LANs.

How radio waves actually carry data signals is covered in Chapter 3.

IEEE 802.11b Although a speed of 2 Mbps was considered adequate when work on the 802.11 standard was begun in 1990, by the time the standard was completed in 1997, a 2 Mbps wireless network proved to be too slow. The IEEE body revisited the 802.11 standard shortly after it was released to determine what changes could be made to increase the speed. In September 1999 a new IEEE 802.11b amendment was added to the standard, which added two higher speeds, 5.5 Mbps and 11 Mbps, to the original 802.11 standard of 1 Mbps and 2 Mbps.

The 802.11b standard supports wireless devices (officially known as a station or STA) that are up to 350 feet (107 meters) apart. However, as wireless devices are moved farther apart, the rate at which the data is transmitted between devices will decrease. This is because radio waves decrease in power over distance, much like the sound of the human voice: someone standing 3 feet (1 meter) away from a person who is speaking at a normal volume would typically be able to hear the sound very clearly, whereas a person 100 feet (30 meters) away would likely have difficulty hearing. When a mobile wireless device moves farther away from the transmitter, the 802.11b standard specifies that the device decrease its rates to the next lower acceptable level (from 11 Mbps down to 5.5, 2, or 1 Mbps) instead of completely dropping the connection. This allows distant devices to remain connected, albeit at slower speeds.

Limitation Explanation

Lack of mobility Directed infrared wireless systems require an obstruction-free line of sight path between the emitter and the detector. This makes it unusable for mobile applications, in which the alignment between the emitter and the detector must be continuously adjusted.

Limited range A directed infrared system, which requires a line-of-sight path, cannot be placed in an environment in which an obstruction could interfere with the infrared beam. Due to the angle of deflection, diffused infrared can only cover a range of about 50 feet (15 meters).

Confined to indoor use

Bright sunlight can affect an infrared signal, making wireless infrared LANs unreliable outdoors.

Slow transmission speed

Diffused infrared can send data at speeds no higher than 4 Mbps because the wide angle of the beam loses energy as it reflects.

Table 2-1 Limitations of infrared wireless systems

42 Chapter 2 Wireless Local Area Networks

Other factors that have an impact on the speed of transmissions include the number of wire- less devices in the network and the type of obstructions between devices. Generally, two terms are used for measuring wireless network speeds:

● Data rate. The data rate is the theoretical maximum rated speed of a network. For example, the data rate for IEEE 802.11b is 11 Mbps. However, the data rate is only theoretical. Due to a variety of factors, a network rarely achieves its stated data rate.

● Throughput. Throughput is the measure of how much actual data can be sent per unit of time across a network. Throughput is often used to measure the amount of data actually sent across a network in a real world setting. If two 802.11 devices are 30 feet (10 meters) apart, the throughput may only be 5.5 Mbps.

IEEE 802.11a At the same time the IEEE created the 802.11b standard, it also issued another standard with even higher speeds. The IEEE 802.11a standard specifies a maximum rated speed of 54 Mbps and also supports 48, 36, 24, 18, 12, 9, and 6 Mbps transmissions using a different set of radio wave frequencies than 802.11b. Even though the 802.11a and 802.11b specifications were published at the same time by the IEEE in 1999, 802.11b pro- ducts started to appear almost immediately, while 802.11a products did not arrive until late 2001. The 802.11a products came to the market later because of technical issues and because of the high cost of developing products for the standard. Devices based on the 802.11a standard cannot use complementary metal oxide semiconductor (CMOS), which is the semiconductor used in 802.11b WLANs. Instead, they must use a compound such as gallium arsenide (GaAs) or silicon germanium (SiGe). These semiconductors are more expensive and require more capital investment and time to develop and manufacture.

Although the 802.11a standard achieves higher speed, the trade-off is that devices cannot be as far apart as with the 802.11b standard. In a 802.11a wireless network, devices can typi- cally be no more than 100 feet (30 meters) apart.

IEEE 802.11g The success of the IEEE 802.11b standard prompted the IEEE to re-examine the 802.11b and 802.11a standards to determine if a third intermediate standard could be developed. This "best of both worlds" approach would preserve the stable and widely accepted features of 802.11b, but increase the data transfer rates to those similar to 802.11a. The IEEE formed a task group (TG) to explore this possibility; by late 2001 a draft standard, known as IEEE 802.11g, was proposed. This standard was formally ratified in 2003.

The IEEE 802.11g draft was a compromise based on input from several different chip (microprocessor) manufacturers, who had a major stake in the outcome. Although most major commercial wireless networking product vendors will build and sell products based upon whatever standard is approved, the stakes are much higher for the chip manu-

facturers. These businesses must make huge monetary investments in designing, sampling, and manufacturing the silicon chips used in the wireless network products. They must then try to sell their chips to product vendors that design and build commercial products based on those chips.

The 802.11g standard supports a maximum data speed of 54 Mbps, with lesser speeds of 48, 36, 24, 18, 12, 9, and 6 Mbps. The standard also specifies that devices operate in the same radio frequency as IEEE 802.11b and not the frequency used by 802.11a. This gives

Types of Wireless LANs 43

the 802.11g standard the ability to support devices that are farther apart with higher speeds. Like 802.11b, 802.11g can support devices that are up to 350 feet (107 meters) apart.

Because 802.11g shares many of the same features as 802.11b, most WLAN equipment allows both 802.11g and 802.11b wireless devices to function together in the same 802.11g wireless network. This is done by configuring the 802.11g network settings to different “modes” based on the different IEEE standards. The configuration options for using devices from these two standards in the same wireless network are listed in Table 2-2.

IEEE 802.11n-2009 In September 2004, the IEEE formed Task Group n (TGn) to begin work on a dramatically new WLAN standard that would significantly increase the speed, range, and reliability of wireless local area networks. Known as IEEE 802.11n-2009 (or 802.11n), this standard was intended to usher in the next generation of WLAN technology. The final 802.11n standard, based on 802.11n Draft 9, was ratified on September 11, 2009 exactly five years to the day after TGn first started its work. The official name of the standard is 802.11n-2009 IEEE Standard for Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements Part 11: Wire- less LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications Amend- ment 5: Enhancements for Higher Throughput.

The 802.11n standard has four significant improvements over previous standards:

● Speed. A data rate of up to 600 Mbps ● Coverage area. Double the indoor range and triple the outdoor range ● Interference. Uses different frequencies to reduce interference ● Security. Requires the strongest level of wireless security

TGn initially evaluated 62 different proposals for the wireless tech- nology that would form the basis of 802.11n.

Although it was originally estimated that 802.11n would be ratified by 2006, the process took much longer than first anticipated. This delay caused vendors to introduce products that were not strictly based on the IEEE standards. From 2004 until 2007, several vendors introduced “Pre-n” devices. These were based on each vendor’s implementation of the

Mode Explanation

G-only Only 802.11g devices are recognized and 802.11b devices are ignored.

B-only Only 802.11b devices are recognized and 802.11g devices are ignored.

Mixed mode Although both 802.11b and 802.11g devices can function together on the same wireless network, the presence of any 802.11b device will cause the network to decrease its data rate to only 802.11b speeds.

Table 2-2 IEEE 802.11g configuration options

44 Chapter 2 Wireless Local Area Networks

proposed standards and were often incompatible with other vendor’s products. In June 2007, the Wi-Fi Alliance began certifying vendor products based on Draft 2 of 802.11n. It was anticipated that products based on the final 802.11n standard would be backward compati- ble with Draft 2.0 devices. The Wi-Fi Alliance certified over 500 of these products including more than 80 enterprise products in just two years.

The official Wi-Fi statement regarding Draft 2.0 devices is, “Wi-Fi cer- tified n products must be backward compatible [with Draft 2.0 devices] … However, keep in mind that Wi-Fi certified 802.11n draft 2.0 devices may not include some of the advanced features included in Wi-Fi certified n products.”

WLAN Client Hardware and Software

C W N A

3.3.2. Describe the purpose of WLAN client devices and explain how to install, configure, secure, and manage them.

There are different types of hardware and software that can be used on the wireless client so that the device can use the wireless network. These can be divided into wireless client network interface cards and the client utility software to support the hardware.

Wireless Client Network Interface Card The hardware that a desktop or laptop client computer needs to send and receive data on a wired network is called a network interface card (NIC) or client network adapter. Early NICs were separate cards that had one edge connected to the expansion slot of the computer’s bus (the subsystem for transferring data between the system’s components), while the RJ-45 connection on the other end provided access for a cable connection, as illustrated in Figure 2-3.

Figure 2-3 Network interface card for wired network

WLAN Client Hardware and Software 45

Today’s computers typically have NIC components built directly into the motherboard, so that only the RJ-45 connection is externally exposed. The cable connects the NIC to the net- work, thus establishing the link between the computer and the wired network.

A wireless client network interface card adapter performs the same functions as a wired NIC with one major exception: there is no external cable RJ-45 connection. In its place is an antenna (sometimes embedded into the adapter) designed to send and receive signals through the airwaves.

Wireless NICs come in a variety of shapes and styles. These can be categorized into wireless NIC devices for desktop computers and for portable devices.

Today wireless NICs are no longer found only in computing devices like desktops, laptops, and tablets. One vendor offers an external hard drive with a built-in wireless NIC so that multimedia files such as movies, music, and photos can be stored and then streamed to and from computers. This is particularly useful for tablets that may have limited storage space.

Cards for Desktops On desktop computers, early wireless NICs, like the one shown in Figure 2-4, plugged into an internal expansion slot inside the computer. These have generally been replaced with external wireless NICs that plug into the Universal Serial Bus (USB) port. Such a USB NIC can be either a stand-alone device connected by a USB cable (Figure 2-5) or a key fob (Figure 2-6).

Figure 2-4 Internal wireless NIC

46 Chapter 2 Wireless Local Area Networks

Figure 2-5 Standalone USB wireless NIC

Figure 2-6 Key fob wireless NIC

WLAN Client Hardware and Software 47

The advantage of a stand-alone USB device over a key fob is that the stand-alone USB device can be repositioned to improve reception. However, some USB key fobs provide the best of both worlds: they can be inserted directly into the computer’s USB port or into a small stand that is connected by a cable to the USB port. This allows for the stand (and key fob) to be moved to improve reception.

More recently, as wireless networks have become the standard for network communication, more desktop computers are shipping with wireless NICs as standard equipment along with a wired NIC. This allows the desktop device to connect to either the wired network or to a wireless network.

Cards for Portable Devices Unlike tablet devices that generally only have an internal wireless NIC as standard equipment, portable laptop computers often can support wireless NICs in different form factors (that is, in different sizes and shapes). These form factors include large form factor devices, small form factor devices, and internal devices.

Large Form Factor Cards A credit card-sized peripheral that slides into a slot on a laptop computer can add additional functionality to the laptop, much like a card can be inserted into the bus expansion slot on a desktop computer. Originally these cards were known as PCMCIA (Personal Computer Memory Card International Association) cards, and later the name was changed to PC Card.

“PCMCIA” refers to a now-dissolved nonprofit trade association and standards body that promoted the technology.

The PC Card standard defines three form factors for three types of PC Cards. All three card types are the same length and width and use the same 68-pin connector. The cards only differ in their thickness. Table 2-3 lists the dimensions and typical uses of different PC Cards.

Laptops in the 1990s usually were configured with two PC Card Type II slots with no barrier in between them. This allowed for the installa- tion of either two Type II cards or one Type III card. Today laptops often have a single Type II slot, while smaller netbooks or tablets have no PC Card slots at all.

PC Card Type Length Width Thickness Typical Uses

Type I 85.6 mm 54 mm 3.3 mm Memory

Type II 85.6 mm 54 mm 5.0 mm Input/Output devices

Type III 85.6 mm 54 mm 10.5 mm Rotating mass storage devices

Table 2-3 PC Card form factors

48 Chapter 2 Wireless Local Area Networks

An enhanced type of PC Card is the CardBus. CardBus is a 32-bit bus in the PC Card form factor. CardBus also includes a bus mastering feature, which allows a controller on the bus to talk to other devices or memory without going through the CPU.

A notch on the left front of a CardBus device prevents it from being inserted into a slot that can only accept PC Cards. Most new slots are compatible with both CardBus and the PC Card devices.

Today PC Card and CardBus devices are being replaced by ExpressCard technology. ExpressCard is designed to deliver higher-performance modular expansion in a smaller size. There are two standard ExpressCard form factors: the ExpressCard/34 module (34 mm � 75 mm) and the ExpressCard/54 module (54 mm � 75 mm). Both formats are 5 mm thick (the same as the Type II PC Card) yet 10.6 mm shorter than a PC Card.

Wireless NICs for laptops are available as Type II PC Cards, CardBus, and ExpressCards. Figure 2-7 illustrates an ExpressCard wireless NIC.

Small Form Factor Cards CompactFlash (CF) is small form factor (43 � 36 � 3.3 mm for Type I, and 43 � 36 � 5 mm for Type II) that is generally used as a mass storage device format for portable electronic devices. CF was used as a storage medium for digital cameras for several years, although it is being replaced by smaller cards. CF wireless NICs were primarily designed for use in personal digital assistant (PDA) devices. However, as PDAs have been replaced by smartphones, CF wireless NICs are no longer as popular as they once were.

The portion of the CF device containing the antenna protrudes from the PDA for improved reception.

Similar to CF, a Secure Digital (SD) card is another small form factor (32 mm � 24 mm � 2.1 mm) that started as a portable storage device for digital cameras and PDAs. A variation of an SD card is a Secure Digital Input Output (SDIO) device, which is a combination of an

Figure 2-7 ExpressCard wireless NIC

WLAN Client Hardware and Software 49

SD card and an input/output (I/O) device such as a wireless NIC. Like CF, SDIO devices were primarily designed for use in PDA devices.

A variation of the SDIO is an SD card that is a combination of a wire- less NIC and storage. Once inserted into a digital camera, this type of SD card can wirelessly transmit pictures across the network to a desk- top or laptop’s hard disk drive or to a wireless printer.

Internal Cards Desktop computers typically have one or more Peripheral Component Interconnect (PCI) expansion slot inside the computer to make it possible to add devices to the system. PCI expansion slots are being replaced with PCI Express (PCI-e). PCI-e has a high-speed point-to-point serial bus replacement of the older shared parallel PCI bus architecture.

Because of their smaller size, laptop computers do not have full-size PCI or PCI-e slots. Instead, they have Mini-PCI slots or Mini-PCI-e slots that accept Mini-PCI or Mini-PCI-e cards or cards that are half the length of a Mini-PCI-e card (called a Half Mini PCIe card). Most laptop computers today come with a wireless Mini-PCI or Mini-PCI-e network inter- face card installed. These can be exposed by removing the access panels on the underside of the laptop, as shown in Figure 2-8. Tablet computers have a similar type of hardware. Some vendors have enhanced the slot by embedding an antenna in the case of the laptop that sur- rounds the screen. When a wireless NIC Mini-PCI or Mini-PCI-e card is used, it automati- cally activates the antenna to improve the reception of the wireless signal.

Mini-PCI was specifically developed for integrating communications peripherals such as modems and NICs onto a laptop computer.

Figure 2-8 Mini-PCI card in laptop

50 Chapter 2 Wireless Local Area Networks

Client Utility Software The software that interfaces between the wireless NIC and the computer can be part of the operating system or a separate third-party utility program. Microsoft Windows XP first intro- duced the Wireless Zero Configuration (WZC) service. WZC is a wireless connection manage- ment utility that operates as a Windows service and interacts with the client hardware NIC drivers. WZC automatically determines which wireless network to connect to based on default settings and preferences set by the user. This tool had security weaknesses and limited function- ality. In Microsoft Vista and Microsoft Windows 7, it was replaced with WLAN AutoConfig.

Manufacturer third-party utility programs are generally included with the purchase of a wire- less NIC client and are often specific to that device. Other client utility software programs are generic and work with wireless NICs from different vendors.

WLAN Infrastructure Devices

C W N A

3.3.1. Identify the purpose of WLAN infrastructure devices and describe how to install, configure, secure, and manage them.

4.3.1. IEEE 802.3-2005, Clause 33 (formerly IEEE 802.3af).

4.3.2. Powering HT (802.11n) devices.

Whereas the hardware and software for a wireless device is essentially limited to the wireless network interface card adapter and the client utility software, there are a number of wireless hardware devices that can be used to create the wireless network infrastructure. These include access points, WLAN bridges, gateways, and Power over Ethernet devices.

Access Points (APs) An access point (AP), shown in Figure 2-9, consists of three major parts:

● An antenna and a radio transmitter/receiver to send and receive wireless signals ● Special bridging software to interface wireless devices to other devices ● A wired network interface that allows it to connect by cable to a standard wired network

An AP has two basic functions. First, it acts as the base station for the wireless network. Any device with a wireless NIC transmits its signal to an AP, which can then redirect the signal, if necessary, to other wireless devices. The second function of an AP is to act as a bridge between the wireless and wired networks. The AP can be connected to the wired network by a cable, allowing all the wireless devices to access through the AP to the wired network (and vice versa), as shown in Figure 2-10.

The number of wireless clients that a single AP can support varies. In theory, some types of APs can support over 100 wireless clients. How- ever, because the radio signal is shared among users, generally one AP for a maximum of 40 wireless users may be acceptable if they are per- forming basic e-mail, light Web surfing, and occasionally transferring

small-sized files. If the users are performing more intense network access and transferring large files, a smaller ratio is necessary.

WLAN Infrastructure Devices 51

Acess point

PCFile server

Wired network

Laptop Laptop

Figure 2-10 AP connected to wireless network

Figure 2-9 Access point (AP)

52 Chapter 2 Wireless Local Area Networks

There are three categories of APs: autonomous access points, lightweight access points, and mesh router access points.

Autonomous Access Points Standard APs are known as autonomous access points. These devices are considered autonomous, or independent, because they are separate from other network devices and even other autonomous access points. Autonomous access points have the intelligence required to manage wireless authentication, encryption, and other func- tions for the wireless client devices that they serve. Because everything is self-contained in these single devices they are also called fat access points.

Lightweight Access Points Although autonomous access points are adequate for a home or a small office home office (SOHO) setting in which there may be one or two APs, what happens in a large enterprise or college campus where there can be hundreds of APs? In this case autonomous access points are not a viable option. Because each AP is autonomous, a single wireless network configuration change would require that they be reconfigured individ- ually, which can take an extended period of time and manpower to complete.

Lightweight access points, also called thin access points, can be a better option. A light- weight access point does not contain the management and configuration functions found in autonomous access points; instead, these features are contained in a central device known as a wireless LAN controller (WLC), or wireless switch. The WLC is the single device that can be configured and then these settings are automatically distributed to all lightweight access points (a remote office WLAN controller is used to manage multiple WLCs at remote sites from a central location). Lightweight access points with a WLC are shown in Figure 2-11.

Lightweight access points only have simplified radios for wireless communication between devices and a media converter for accessing the wired network.

Besides centralized management, lightweight access points provide other advantages over autonomous access points. As wireless client devices move through a WLAN with multiple

Wired network Lightweight

access point (no management

or configuration)

Enterprise WLAN controller (management and configuration

performed here)

Lightweight access point

(no management or configuration)

Laptop Laptop Laptop Laptop

Figure 2-11 Lightweight access point with enterprise WLAN controller

WLAN Infrastructure Devices 53

autonomous access points, a lengthy handoff procedure occurs during which one autono- mous access point transfers authentication information to another. Slow handoffs can be unacceptable on WLAN systems using time-dependent communication, such as voice or video. With lightweight access points, however, the time for this handoff procedure is reduced because all authentications are performed in the WLC. Another advantage of WLCs are the tools that many of them provide for monitoring the environment and provid- ing information regarding the best locations for APs, wireless configuration settings, and power settings.

Yet lightweight access points have some disadvantages. WLCs still do not provide true con- vergence (integration) of the wired and wireless networks, but only ease some of the man- agement burdens of WLANs. In addition, these devices are proprietary, which means all the lightweight access points and WLCs on a network must be from the same vendor in order to function cohesively.

Mesh Access Points Consider a college that wants to provide wireless access to stu- dents throughout the entire campus. Installing APs inside buildings makes it possible to take advantage of the existing campus’s wired network infrastructure. Any AP installed inside a building can be connected to the wired network. However, what about locating APs outside of buildings to provide wireless access in outdoor areas? This would require a costly installation of cables to each AP that would involve digging trenches, burying net- work cable in conduit, and connecting a cable individually to each AP. Because each AP must individually be connected to the wired network infrastructure it can result in high costs when providing coverage in areas where a wired connection is not readily available.

A solution is to use a mesh access point, which does not have to be individually connected by a cable to the existing wired network. Instead, each mesh access point communicates wirelessly with the next closest mesh access point. Dozens—or even hundreds—of mesh access points can communicate between themselves to create a wireless mesh network (WMN). Only one mesh access point must be physically connected to the wired network; all the other mesh access points transparently connect, or hop, through each other to reach the mesh access point with the wired connection. This arrangement allows for multiple inter- connected paths through which the signal can reach the mesh access point that is connected to the wired network. Figure 2-12 illustrates a WMN.

Because mesh access points function in a similar manner to routers in directing traffic along the best traffic path, these devices are sometimes called wireless mesh routers or mesh access points/routers. A WMN that connects mesh access points for the purpose of sharing an Internet connection is also known as a backhaul wireless mesh network. These mesh net- works provide alternative data paths for the backside connection to the Internet.

The original IEEE 802.11 standard included an option for a wireless distribution system (WDS) that interconnected two APs wirelessly. However, it was never widely implemented.

Another type of wireless mesh network is an ad hoc wireless mesh network. Instead of mesh access points communicating with each other, wireless client devices like laptop computers act as the relay station for signals to and from the AP, with the signal hopping through the wireless clients.

54 Chapter 2 Wireless Local Area Networks

There are several advantages to using mesh access points to create a WMN:

● Low-cost installation. The cost for installing a WMN is reduced because only a single mesh AP must be physically connected to the wired network.

● Large coverage area. A WMN can cover large areas with multiple mesh access points. ● Easy-to-change coverage area. The coverage area of a WMN can be easily expanded

or contracted by adding or reducing the number of mesh access points. ● Can be installed in areas without wired infrastructure. Warehouses, outdoor concert

venues, and even trains can have wireless access without a wired network. ● Self-configuring WMN. Once a mesh access point is added to the WMN, the network

automatically adds the new AP to the existing mesh without needing any special adjustments.

Laptop Laptop

Wireless mesh network (WMN)

Mesh access points

Wired network

File server PC

Figure 2-12 Wireless mesh network

WLAN Infrastructure Devices 55

● Self-healing WMN. If a mesh access point fails, the other mesh access points can take advantage of the remaining network paths to automatically bypass the failed node. This is possible because there are multiple paths in the network.

● Fast installation. WMNs can be installed quickly. WMNs have been used extensively in the aftermath of the hurricanes and other natural disasters to provide immediate communication facilities for rescue personnel.

Currently no standards exist for WMNs. The IEEE 802.11 TG subgroup “s” started work on developing standards in 2003 and is still working on the project. In the interim, over 70 competing WMN routing protocols are available. These protocols differ over:

● Algorithm. The routing algorithm should attempt to ensure that the data takes the most appropriate (fastest) route to its destination. The various protocols all use differ- ent routing algorithms.

● Management data vs. transmit data. Wireless mesh routers in a WMN receive two types of data from other devices: management data and transmit data. In order to implement dynamic routing capabilities, each wireless mesh router must constantly communicate its own routing information to every other wireless mesh router within its area. This information is known as management data. Devices also con- tinually receive data from client devices that they need to pass on. This informa- tion is known as transmit data. A wireless mesh access point must quickly deter- mine what it should do with the data it receives. It needs to keep management data for reference and not pass it on. When it receives transmit data, it needs to use the information about nearby access points contained in the management data to pass the transmit data on to its neighbor. Different protocols implement these techni- ques differently.

● Number of radios. Unlike some WLANs (such as 802.11a/b/g) in which only one radio is used, many wireless mesh networks use multiple radios. This helps to separate the tasks of managing management data and transmit data.

The competing WMN routing protocols range from well-known pro- tocols such as OSPF (Open Shortest Path First) to lesser-known protocols such as BATMAN (Better Approach to Mobile Adhoc Networking).

WLAN Bridges A bridge is a device that is used to connect two network segments, even if those segments use different types of physical media (such as wired and wireless connections). There are two types of wireless LAN bridges. A wireless workgroup bridge is used to connect a wired net- work segment to a wireless network segment, as illustrated in Figure 2-13. The wired devices connect through the wireless workgroup bridge to an access point, which then connects to the wired backbone network. A wireless workgroup bridge only supports the wired devices and not any other wireless devices (that is, it does not function as an access point).

Wireless bridges are also available to connect a single wired device to the wireless network. These are popular for consumer home users. For example, a wireless bridge can be attached to a standard laser printer to turn it into a wireless networked printer.

56 Chapter 2 Wireless Local Area Networks

Whereas a wireless workgroup bridge connects wireless devices on a network segment that are in relatively close proximity (a workgroup) such as in a room or on the floor of an office building, a remote wireless bridge connects two or more networks together that are sepa- rated by a longer distance. Remote wireless bridges are commonly used to connect networks (either wired or wireless networks) between different buildings. Remote wireless bridges have significant differences from APs or wireless workgroup bridges. These differences include:

● Increased power. Remote wireless bridges transmit at higher power than APs. This enables them to transmit over longer distances.

● Directional antenna. While an AP has an antenna that sends out its signal in all direc- tions, a remote wireless bridge generally uses a directional antenna to focus the trans- mission in a single path, significantly increasing the distance that it can transmit.

● Special software. A remote wireless bridge contains special software for transmitting and receiving signals. For example, this software can enable a wireless bridge to avoid interference by selecting the clearest transmission channel.

Remote wireless bridges support two types of connections, point-to-point and point- to-multipoint. In a point-to-point (PtP) configuration, two buildings are connected, as shown in Figure 2-14. In a point-to-multipoint (PtMP) configuration, multiple buildings are connected, as shown in Figure 2-15.

Wired network backbone

Access point

Wireless workgroup bridge

Network switch

File server

Laptop PC PC Laptop PC PC

Figure 2-13 Wireless workgroup bridge

WLAN Infrastructure Devices 57

A remote wireless bridge can function in one of four different modes:

● Root mode. In root mode, a remote wireless bridge can communicate only with other bridges that are not in root mode. When in root mode, the remote wireless bridge is called a root bridge. There must be one (and only one) root bridge in a PtP or PtMP configuration, and it cannot communicate with another root bridge or a wireless client.

● Nonroot mode. In nonroot mode, a remote wireless bridge can transmit only to a bridge that is in root mode. Although some remote wireless bridges in nonroot mode can also be configured as an AP to simultaneously communicate with a remote wire- less root bridge and wireless clients, this is discouraged for security purposes.

Figure 2-15 Point-to-multipoint remote wireless bridge

Figure 2-14 Point-to-point remote wireless bridge

58 Chapter 2 Wireless Local Area Networks

● Repeater mode. In order to enhance the signal strength between buildings, another remote wireless bridge may be positioned between two other bridges. This bridge then functions in repeater mode.

● Access point mode. In access point mode, a remote wireless bridge functions as a nor- mal AP only and does not communicate with other remote wireless bridges; instead, it only communicates with wireless client devices such as laptop computers.

Remote wireless bridges are a cost-effective alternative to expensive leased wired options for connecting remote buildings. Remote wireless bridges can connect sites such as satellite offices, remote campus settings, or temporary office locations when the sites are separated by obstacles such as bodies of water, freeways, or railroads that make using a wired connec- tion impractical or very expensive.

The speed of remote wireless bridges and the distance over which they can transmit depend primarily on the number of obstacles in the trans- mission path. A path with many obstacles is called a non-line-of-sight, or non-LoS path. One with few obstacles is called a near-LoS path, and one with no obstacles is called a line-of-site, or LoS path. In a

non-LoS setting, the buildings can be up to 6 miles (10 km) apart, in near-LoS they can be up to 25 miles (40 km) apart, and in a LoS setting they can be up to 124 miles (200 km) apart. The data speeds can vary between 7 Mbps to 150 Mbps depending upon the distance and obstacles.

Gateways A gateway is a network device that acts as an entrance to another network. There are two types of gateways in wireless networks, Enterprise Encryption Gateways and residential WLAN gateways.

Enterprise Encryption Gateway (EEG) An Enterprise Encryption Gateway (EEG) provides encryption and authentication services for a wireless network. An EEG typically resides between the wireless network and the wired network and serves as the entry point to the wired network, as illustrated in Figure 2-16. The purpose of an EEG is to relieve an AP from the burden of encryption and authentication. Traffic leaving the backbone travels through the EEG where it is encrypted before going out to the WLAN. The EEG also serves the reverse function, decrypting traffic as it arrives from the WLAN. The EEG can even authenticate devices on the WLAN and approve or deny their access into the wireless network.

EEGs have largely been replaced by WLCs using lightweight access points.

Residential WLAN Gateway A single wireless hardware network device for SOHO or home use typically combines multiple features into a single hardware device. These fea- tures often include those of an AP, firewall, router, dynamic host configuration protocol (DHCP) server, and other features. Strictly speaking these devices are residential WLAN

WLAN Infrastructure Devices 59

gateways as they are the entry point into the wireless network from the Internet. However, most vendors instead choose to label their products as “wireless broadband routers.”

Recent developments in hardware devices that serve as residential WLAN gateways are com- plemented by new software functions that simplify the management and functionality of these devices. For example, Windows 7 added two significant wireless functions to its oper- ating system. One function, Windows Connect Now, or WCN, is a solution for home net- working and SOHOs. (WCN is not intended for enterprises.) A Windows 7 computer can scan the airwaves for a newly installed WCN capable device, such as an AP. When the device is detected, the user can simply enter a personal identification number (PIN) from the hardware device, as shown in Figure 2-17. The user can then take advantage of a Windows Wizard that steps through the process of creating the wireless network configurations—including security—and that can also transmit these settings wirelessly back to the AP to configure it. These configuration settings can even be stored on a USB flash drive and used to easily config- ure other computers without the need to enter the information manually.

In addition to APs, WCN allows a range of mobile and embedded devices to exchange settings with one another.

The second wireless function added to Microsoft Windows 7 is the wireless Hosted Net- work. This feature has two parts: the virtualization of the physical wireless NIC into

Wired network backbone

Enterprise Encryption Gateway (EEG)

Access points

Laptop PC

File server

Figure 2-16 Enterprise Encryption Gateway (EEG)

60 Chapter 2 Wireless Local Area Networks

multiple virtual wireless NICs (called Virtual WiFi) and a software-based wireless access point (SoftAP) that uses a designated virtual wireless NIC. The wireless Hosted Network allows users to extend the functionality of their portable laptop computer. For example, a user could set up her computer to create a wireless network so that other users can quickly share documents wirelessly between multiple computers. Another function allows a laptop’s network connection to be shared by other computers and devices. For example, a user could connect her computer to the Internet and then turn her computer into an AP that shares the Internet connection with other wireless laptop devices, much like a hardware AP.

A network created via Windows 7’s wireless Hosted Network is not the same as an ad hoc wireless mesh network, in which wireless cli- ent devices relay signals from one laptop to another. Instead, all of the mobile devices in a wireless Hosted Network all connect directly back to the laptop running the Virtual WiFi and SoftAP.

Power over Ethernet (PoE) Devices Access points are typically mounted on a ceiling or a similar area high off the ground to reduce interference from surrounding objects. However, electrical power outlets are generally not found in these locations. In such cases Power over Ethernet (PoE) can be used. Instead of receiving power directly from an alternating current (AC) electrical outlet, direct current (DC) power is delivered to the AP through the unused wires in a standard unshielded twisted pair (UTP) Ether- net cable that connects the AP to the wired network. This eliminates the need for installing electrical wiring and makes mounting APs more flexible. The current PoE standard is IEEE 802.3at-2009. Prior to this standard, different vendors offered their own proprietary standards.

Figure 2-17 WCN request for pin

WLAN Infrastructure Devices 61

The total amount of continuous power that can be sent to each device using PoE is 12.95 watts. Wireless LAN access points typically consume 3.5 to 10 watts.

There are two common approaches to providing PoE, which are illustrated in Figure 2-18. These include:

● PoE-enabled Ethernet switch. A PoE-enabled Ethernet switch can contain embedded PoE technology, called power sourcing equipment, or PSE, that provides both electrical power and data. Nothing more must be done other than connecting the device to the switch with an Ethernet cable. End devices that can support PoE send the switch an authenticated PoE signature that indicates that they do support this technology, which helps prevent damage to other equipment. Once the switch knows that the device is PoE compliant it sends power along one pair of unused wires in the cable. In addition, the switch will discontinue the power when the PoE device is disconnected.

Wired network backbone

PoE-enabled

Ethernet switch Standard switch

Data only on

Ethernet data cable

Power added to Ethernet cable

PoE injector

Access point

Power and data

supplied through

Ethernet data cable

Access point

File server

Laptop

Electrical outlet

Laptop

Figure 2-18 Power over Ethernet

62 Chapter 2 Wireless Local Area Networks

● PoE injectors. A PoE injector is a small, inexpensive device that can inject power into an Ethernet cable. These injectors can be endspan devices (such as a network switch enabled to provide power on each port) or a midspan device, which is connected in- line to each end device and adds power to the line. Using PoE injectors, a standard, non–PoE-enabled Ethernet switch can be used to supply the data while the PoE injec- tor provides the power. Positioned between the switch and the end device, the PoE injector is connected to both the switch and end device. Typically PoE injectors can provide power to a single cable (single port PoE injectors) or to multiple cables simul- taneously (multiport PoE injectors).

Chapter Summary ■ Standards are particularly important today and provide a variety of benefits. In the

field of IT, standards play a vital role by ensuring interoperability, increased competi- tion, lower costs, and protection in investments. There are three types of standards: de facto (which are common practices established by the marketplace), de jure (standards controlled by an entity entrusted with that task), and consortia-based standards (industry-sponsored organizations).

■ There currently are four types of wireless LANs based on the IEEE 802.11 standard. 802.11b networks transmit at 11 Mbps over a distance of up to 350 feet (107 meters). 802.11a standard devices transmit at up to 54 Mbps but only up to 30 meters (100 feet). A compromise between the two, the 802.11g, can transmit at 54 Mbps up to 115 meters (375 feet). The 802.11n standard, ratified in 2009, can transmit as fast as 600 Mbps with significant increases in coverage area, reduced interference, and man- datory security.

■ Wireless LAN client devices are in many respects similar to those found in a wired network. The difference is that, instead of a wired connection, wireless devices use an antenna or other means to send and receive signals. A wireless network interface card performs the same function as a wired NIC in that it receives signals from the net- work. There are a variety of different types of wireless NICs: USB stand-alone or key fob devices, CardBus, PC Card, ExpressCard, PCI, Mini-PCI, Mini-PCI-e, Compact- Flash, and SDIO cards. The software that interfaces between the wireless NIC and the computer can be part of the operating system or a separate third-party utility program.

■ An access point (AP) serves as both the base station for the wireless network and as a bridge to connect the wireless network with the wired network. The range of an access point and the number of wireless clients that it can support varies. Standard APs have all of the intelligence required for wireless authentication, encryption, and manage- ment. A lightweight access point does not contain the management and configuration functions that are found in autonomous access points; instead, these features are con- tained in a central device known as a wireless LAN controller (WLC).

■ A mesh access point does not have to be individually connected by a cable to the existing wired network. Instead, each mesh access point communicates wirelessly with the next closest mesh access point, creating a wireless mesh network (WMN). Because mesh access points function in a similar manner to routers in directing traffic along the best traffic path, these devices are sometimes called wireless mesh routers. A WMN

Chapter Summary 63

that connects mesh access points for the purpose of sharing an Internet connection is also known as a backhaul wireless mesh network.

■ A wireless workgroup bridge is used to connect a wired network segment and a wireless network segment that are in relatively close proximity to each other, such as within a room or the floor of a building. A remote wireless bridge connects two or more net- works that are separated by a longer distance. This device is commonly used to connect networks (either wired or wireless networks) that are situated in different buildings. A remote wireless bridge can connect two buildings via a point-to-point (PtP) configura- tion or it can connect multiple buildings via a point-to-multipoint (PtMP) configuration.

■ A gateway is a device that acts as an entrance to another network. There are two types of gateways in wireless networks. An Enterprise Encryption Gateway (EEG) provides encryption and authentication services for a wireless network. It typically resides between a wireless network and a wired network and serves as the entry point to the wired network. A single wireless hardware network device for SOHO or home use that typically combines multiple features into a single hardware device is called a resi- dential WLAN gateway.

■ Power over Ethernet (IEEE 802.3af) technology allows an AP to be positioned in almost any location because electrical current is supplied through the Ethernet cable.

Key Terms access point (AP) A device that connects wireless devices to each other and to a wired network. access point mode A mode of a wireless bridge that causes the bridge to function as a standard AP only. In access point mode, a wireless bridge and does not communicate with other remote wireless bridges but only with wireless client devices. ad hoc wireless mesh network A network in which wireless client devices act as the relay station for signals to and from the AP. autonomous access point A device that is separate from other network devices including other autonomous access points and that contains all the intelligence required for wireless authentication, encryption, and management. backhaul wireless mesh network A wireless mesh network (WMN) that connects mesh access points for the purpose of sharing an Internet connection. bridge A device that is used to connect two network segments together, even if those segments use different types of physical media. bus On a computer, the subsystem for transferring data between the system’s components. bus mastering A technology that allows a controller on the bus to talk to other devices or memory without going through the CPU. CardBus A 32-bit bus in the PC Card form factor. client network adapter A device that connects a computer to a wired network. CompactFlash (CF) A small form factor that is generally used as a mass storage device format for portable electronic devices. consortia Industry-sponsored organizations that want to promote a specific technology. Consortia often take on the task of creating standards for specific technologies.

64 Chapter 2 Wireless Local Area Networks

data rate The theoretical maximum rated speed of a network. de facto standards Standards that are common practices that the industry follows for various reasons such as ease of use or tradition. de jure standards Standards that are official standards controlled by an organization or body that has been entrusted with that task. detector A device that receives a signal. diffused transmission An infrared wireless transmission that relies on reflected light. directed transmission An infrared wireless transmission that requires that the emitter and detector be directly aimed at one another. emitter A device that transmits a signal and is used in an IEEE 802.11 infrared network. endspan device A Power over Ethernet (PoE) device that injects power through a network device like a switch to provide power on each port. Enterprise Encryption Gateway (EEG) A device that provides encryption and authentication services for a wireless network. ExpressCard A type of expansion card designed to deliver higher-performance modular expansion in a small size. fat access points Devices that are separate from other network devices and even other (autonomous) access points that have all of the “intelligence” for wireless authentication, encryption, and management contained within the AP itself. form factor A term used to refer to the size and shape of a device. gateway A network device that acts as an entrance to another network. Half Mini PCIe card A PCI-e card that is half the length of a Mini-PCI-e card. IEEE 802.11 The first wireless LAN standard with a speed of 1 and 2 Mbps. IEEE 802.11-2007 The official document of all of the IEEE 802.11 standards and amendments. IEEE 802.11a A wireless LAN standard that specifies a speed of 54 Mbps and uses a different set of radio wave frequencies than 802.11b. IEEE 802.11b A wireless LAN standard with a maximum speed of 11 Mbps. IEEE 802.11g A wireless LAN standard that supports a speed of 54 Mbps and uses the same set of radio wave frequencies as 802.11b. IEEE 802.11n-2009 A wireless LAN standard that supports a speed of up to 600 Mbps while also increasing the area of coverage. infrared light An invisible light that can be used for wireless transmissions. light spectrum All visible and invisible light. lightweight access points An access point that does not contain the management and configuration functions that are found in autonomous access points. Line of sight (LoS) Term used to refer to a setting in which an emitter is aimed directly at a transmitter with no intervening obstacles. mesh access point An access point that communicates wirelessly with the next closest mesh access point. midspan device A Power over Ethernet (PoE) device that is connected in-line to each end device and adds power to the line. Mini-PCI A connector in a laptop computer used to expand the laptop’s capabilities. Mini-PCI-e A smaller version of a Mini-PCI connector.

Key Terms 65

multiport PoE injectors A PoE injector that can provide power to multiple cables simultaneously. network interface card (NIC) A device used to connect a computer to a network. nonroot mode A mode of a wireless bridge in which the bridge can transmit only to a wireless bridge that is in root mode. PC Card A type of expansion card used in a laptop computer, also known as a PCMCIA card. PCI Express (PCI-e) An expansion slot that contains a high-speed point-to-point serial bus. This technology has replaced the older shared parallel PCI bus architecture. PCMCIA (Personal Computer Memory Card International Association) cards A type of expansion card used in a laptop computer. Peripheral Component Interconnect (PCI) Expansion slots inside the computer that allow devices to be added to the system. PoE injector A small and inexpensive device that can inject power into an Ethernet cable. PoE-enabled Ethernet switch A device that can contain embedded PoE technology that provides both electrical power and data. point-to-multipoint (PtMP) A remote wireless bridge configuration in which multiple buildings are connected. point-to-point (PtP) A remote wireless bridge configuration in which two buildings are connected. Power over Ethernet (PoE) A technology that sends direct current (DC) power to an AP through the unused wires in a standard unshielded twisted pair (UTP) Ethernet cable. power sourcing equipment (PSE) A PoE device that provides data and electrical power via embedded PoE technology. remote office WLAN controller A device used to remotely manage multiple enterprise WLAN controllers from a central location. remote wireless bridge A device that connects two or more networks that are separated by a longer distance. repeater mode A mode of a wireless bridge that allows the bridge to extend the distance between buildings. residential WLAN gateway A single wireless hardware network device for SOHO or home use that typically combines multiple features into a single hardware device. RJ-45 connection A connector on a network interface card used to connect the card to a wired network using a cable. root bridge Term used to refer to a wireless bridge operating in root mode. A root bridge can communicate only with other wireless bridges that are not in root mode. root mode A mode of a remote wireless bridge in which the bridge can communicate only with other bridges that are not in root mode. Secure Digital (SD) A small form factor that was originally used as a format for portable storage devices for digital cameras and PDAs. Secure Digital Input Output (SDIO) A combination of an SD card and an input/output (I/O) device such as a wireless NIC. single port PoE injectors A PoE injector that can provide power to a single cable.

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SoftAP A software-based wireless access point that uses a designated virtual wireless NIC. standard A model that is used for comparison. station (STA) A wireless device. thin access points An access point that does not contain the management and configuration functions that are found in autonomous access points. throughput The measure of how much actual data can be sent per unit of time across a network. Virtual WiFi Term used to refer to the virtualization of the physical wireless NIC into multiple virtual wireless NICs. Windows Connect Now (WCN) A feature of Microsoft Windows 7 for connecting wireless devices for home networking and SOHOs. wireless client network interface card adapter A device that connects a wireless device to a wireless network. Wireless LAN controller (WLC) A device that can be configured with a wireless network’s settings, after which the settings are automatically distributed to all lightweight access points on the network. wireless mesh network (WMN) A network of wireless mesh access points that communicate between themselves. wireless mesh routers A mesh access point that functions similar to routers in directing traffic along the best traffic path. wireless switch A device that contains the management and configuration functions for a lightweight access point. wireless workgroup bridge A device used to connect a wired network segment to a wireless network segment. Wireless Zero Configuration (WZC) A wireless connection management utility that operates as a Windows service and interacts with the client hardware NIC drivers. WLAN Autoconfig A Microsoft Windows 7 and Vista wireless connection management utility that operates as a Windows service and interacts with the client hardware NIC drivers default settings.

Review Questions 1. Each of the following is an advantage of standards except:

a. interoperability.

b. protection.

c. more competition.

d. increased costs.

2. Standards that set by an official body are called:

a. de jure.

b. de facto.

c. de rigueur.

d. de consortia.

Review Questions 67

3. The current IEEE WLAN standard is .

a. IEEE 802.11h

b. IEEE 802.11-2009

c. OSI/IEEE 802.1x

d. IEEE PHY 2008-802.11

4. IEEE 802.11 specified that either radio frequency waves or could be used in a WLAN.

a. circuit packet transfer (CPT)

b. low-frequency emission signals (LFES)

c. ultraviolet light

d. infrared light

5. A(n) transmission requires that the emitter and detector be directly aimed at one another.

a. reflected

b. diffused

c. directed

d. indirected

6. Each of the following is a data rate of IEEE 802.11b except:

a. 54 Mbps.

b. 1 Mbps.

c. 5.5 Mbps.

d. 11 Mbps.

7. A mobile wireless device is official known as a .

a. Mobile Transfer Detector (MTD)

b. port

c. module

d. station

8. What happens when a mobile wireless devices moves away from the transmitter in a WLAN?

a. Nothing; the maximum data rates remain the same no matter how far away the device is located.

b. The connection is dropped after the device moves away more than 10 feet (30 meters).

c. All authentications must be renegotiated every 3 feet (9 meters).

d. The device decreases its data rates to the next lower acceptable data level.

68 Chapter 2 Wireless Local Area Networks

9. is the theoretical maximum rated speed of a network.

a. Data rate

b. Throughput

c. Bit/byte rate

d. Packet Calculation Level (PCL)

10. What is the maximum data speed of an IEEE 802.11a WLAN?

a. 48 Mbps

b. 64 Mbps

c. 68 Mbps

d. 54 Mbps

11. An IEEE 802.11g network can transmit at the same maximum speed as .

a. IEEE 802.11b

b. IEEE 802.11a

c. IEEE 802.11r

d. IEEE 802.11x

12. Each of the following is an improvement of IEEE 802.11n over previous WLAN standards except:

a. speed.

b. security.

c. coverage area.

d. form factor.

13. is a 32-bit bus in the PC Card form factor.

a. Type IV

b. PCCMIA

c. EDO

d. CardBus

14. is a combination of an SD card and an input/output (I/O) device such as a wireless NIC.

a. Secure Digital Input Output (SDIO)

b. CF-PCA

c. NIC-CIA

d. ExpressCard

15. Which of the following would not be found on a laptop computer?

a. Mini-PCMCIA

b. Mini-PCI

c. Mini-PCI-e

d. Type II slot

Review Questions 69

16. An access point consists of each of the following except:

a. an antenna and a radio transmitter/receiver to send and receive wireless signals.

b. special bridging software to interface wireless devices to other devices.

c. a wired network interface that allows it to connect by cable to a standard wired network.

d. an infrared transmitter/emitter.

17. A(n) does not contain the management and configuration functions.

a. lightweight access point

b. fat access point

c. autonomous access point

d. switch access point

18. A(n) does not have to be individually connected by a cable to the existing wired network but instead each device communicates wirelessly with the next closest one.

a. backhaul access point

b. thin access point

c. fat access point

d. mesh access point

19. A wireless workgroup bridge is used to connect:

a. a wired network segment with a wireless network segment.

b. a wireless network segment with another wireless network segment.

c. a gateway and a bridge.

d. a router and a switch.

e. mirror slack.

20. A(n) provides encryption and authentication services for a wireless network and resides between the wireless network and the wired network to serve as the entry point to the wired network.

a. remote authentication server

b. access point

c. mesh hub router

d. Enterprise Encryption Gateway (EEG)

Hands-On Projects

Project 2-1: Investigating Microsoft Windows 7 WLAN AutoConfig The WLAN AutoConfig service in Windows 7 is used to discover, connect to, and disconnect from an access point. In this project you will activate the ser- vice, if necessary, and view network connection information.

70 Chapter 2 Wireless Local Area Networks

For this project you will need a computer running Microsoft Win- dows 7 that has a wireless NIC and can access a wireless LAN.

1. The first step is to ensure that the WLAN AutoConfig service (known as WLANSVC) is active. In Microsoft Windows 7, click Start and then click Control Panel.

2. Click System and Security, and then click Administrative Tools.

3. In the Administrative Tools window, double-click Services to display the Services window.

4. Scroll down and then double-click WLAN AutoConfig to open the WLAN AutoConfig Properties (Local Computer) dialog box.

5. Click the General tab, if necessary, and then, make sure Started is displayed to the right of Service status. If it is not, click the Start button to launch the service.

6. To the right of Startup type select Automatic if it is not already selected.

7. Click OK to return to the Services window.

8. Close the Services window and then close the Administrative Tools window to return to the Control Panel.

9. In left pane of the Control Panel window, click Network and Internet.

10. Click Network and Sharing Center.

11. Under Connect or disconnect click Wireless Network Connection. This opens the Wireless Network Connection Status dialog box, as shown in Figure 2-19.

Figure 2-19 Wireless Network Connection Status dialog box

Hands-On Projects 71

12. Click the Details button to view the information about this wireless connection. On a piece of paper, make a note of the value listed to the right of Description, which is the type of wireless NIC on this computer. Also record the SSID value, which is the name of the net- work. Click Close to return to the Wireless Network Connection Status dialog box.

13. Click the Wireless Properties button to view the configuration properties of the current WLAN to which you are connected.

14. Click the Security tab.

15. If the Network security key box displays several black dots, security has been config- ured on your computer for the WLAN. If that is the case, record the Security type and Encryption type.

16. Click the Show characters box to add a check.

17. Record the Network security key.

18. Uncheck the Show characters box.

19. Click the Connection tab.

20. Make sure the Connect automatically when this network is in range box is unselected. Click OK. Note that you may now be disconnected from your WLAN.

21. Close all open windows.

Project 2-2: Connect to a WLAN Manually and Automatically In this project you manually configure a connection to a WLAN, and then have Windows 7 configure the same connection automatically.

For this project you will need a computer running Microsoft Windows 7 that has a wireless NIC and can access a wireless LAN.

1. Right-click the WLAN icon in the system tray and then click Open Network and Sharing Center.

2. To view all of the wireless profiles, click Manage wireless networks.

3. All of the wireless profiles for this computer are now displayed. Click the network name you recorded in Project 2-1.

4. Now you will erase this profile. Click Remove and then click Yes.

5. Next you will manually recreate this profile. Click the Back button to return to the Network and Sharing Center, and then, under Change your networking settings click Set up a new connection or network.

6. Click Manually connect to a wireless network.

7. Click Next to display the dialog box shown in Figure 2-20.

8. Enter the Network name, Security type, Encryption type, and Security key information recorded in Project 2-1. Click Next.

9. Click Close.

10. To view the profile you just created, click Manage wireless settings.

72 Chapter 2 Wireless Local Area Networks

11. All of the wireless profiles for this computer are now displayed. Double-click the profile to display its properties.

12. Be sure that Connect automatically when this network is in range is unselected. Click OK. You will now be unconnected from this WLAN.

13. Next you will erase this manual profile you just created and have Windows 7 automati- cally create the profile for you.

14. Click the network name you recorded in Project 2-1.

15. Click Remove and then click Yes.

16. Click the WLAN icon in the system tray, and locate the network name in the list of available networks.

17. Click the network name.

18. Click the Connect automatically box to select it.

19. Click Connect.

20. Enter the security information if you are requested to do so.

21. Verify that you are connected to the wireless network by opening your Web browser and pointing to www.course.com.

22. Close all windows.

Project 2-3: Installing and Using Virtual Router The wireless Hosted Network function in Microsoft Windows 7 makes it possible to virtualize the physical wireless NIC into multiple virtual wireless NICs (Virtual WiFi). It also includes a software-based wireless access point (SoftAP) that uses a

Figure 2-20 Manually connect to a wireless network dialog box

Hands-On Projects 73

designated virtual wireless NIC. These features allow a laptop’s single network connection to be shared by other computers and devices. In this project, you will download and install the Virtual Router application to set up a virtual AP.

For this project you will need a computer running Microsoft Windows 7 that has a wireless NIC and can access a wireless LAN. Note that Windows 7 Starter Edition cannot be used for this project. You will also need a sec- ond wireless device such as another computer or a smartphone.

1. Use your Web browser to go to virtualrouter.codeplex.com.

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions then open a search engine and search for “Virtual Router”.

2. Click Documentation.

3. Under Which WiFi Devices are Supported? click Supported Devices. Use the information retrieved from Step 12 of Project 2-1 to verify that your NIC is supported by Virtual Router.

4. Click Downloads.

5. Click the current version of Virtual Router. Follow the prompts to download and install Virtual Router on your computer.

6. If the Virtual Router program does not launch after the installation is complete, click Start and Virtual Router Manager. You should now see the Virtual Router Manager setup window as shown in Figure 2-21.

7. Verify that the Network Name (SSID) box displays the name VirtualRouter.

Figure 2-21 Virtual Router Manager dialog box

74 Chapter 2 Wireless Local Area Networks

8. In the Password box, enter a password that is at least 15 characters long.

9. If necessary, click the Shared Connection list arrow, click Wireless Network Connec- tions, and then click Start Virtual Router.

10. Click the WLAN icon in the system tray. You should see Virtual Router listed as one of the Wireless Network Connections. If it is not listed it may be necessary to restart the computer.

11. Click Open Network and Sharing Center.

12. Click Wireless Network Connection 2.

13. Click Details. How does this information differ from the details for the physical NIC? Close all windows. Now you will try to access Virtual Router from another wireless device, such as another computer. (The following steps assume you are accessing the Virtual Router from a second computer, but you could try accessing it from a smartphone instead. If you are using a smartphone, make a note of the wireless network it is currently connected to, use the phone’s built-in utility to connect to Virtual Router, and then continue this project starting at Step 19.)

14. On the second computer, click the WLAN icon in the system tray and make a note of the wireless network the computer is currently connected to.

15. Click the network name VirtualRouter in the list of available networks.

16. Select the Connect automatically box.

17. Click Connect.

18. Enter the security information if you are requested to do so.

19. Verify that you are connected to the wireless network by opening your Web browser and going to www.course.com.

20. Return to the first computer and open the Virtual Router Manager if it is not already open. The second computer should appear under Peers Connected (1).

21. Return to the second computer (or to the smartphone) and reconnect to the AP it was connected to before you began this project.

22. On the first computer click Stop Virtual Router.

23. Close all windows.

Project 2-4: Installing and Using Connectify Another application similar to Virtual Router is Connectify. In this project you will download and install the Connectify application, and then use it to set up a virtual AP.

For this project you will need a computer running Microsoft Windows 7 that has a wireless NIC and can access a wireless LAN. Note that Windows 7 Starter Edition cannot be used for this project. You will also need a second wireless device such as another computer or a smartphone.

1. Use your Web browser to go to connectify.me.

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions then open a search engine and search for “Connectify”.

Hands-On Projects 75

2. Click Support and then click Supported Cards & Devices. Use the information retrieved from Step 12 of Project 2-1 to verify sure that your NIC is supported by Connectify.

3. Click Download. Follow the prompts to download and then install Connectify on your computer. You may need to reboot your computer following the installation.

4. If the Connectify program does not launch after the installation is complete, click Start and Connectify.

5. If the Connectify Hotspot Setup Wizard does not appear, click the Easy Setup Wizard button to display the wizard shown in Figure 2-22.

6. Accept the Network Name Connectify-me.

7. Under Password enter a password that is at least 15 characters.

8. Under Internet to Share select Wi-Fi.

9. Click Start Hotspot.

10. Click the WLAN icon in the system tray. Connectify-me should appear in the list of wireless network connections.

11. Click Open Network and Sharing Center.

12. Click Wireless Network Connection 2.

Figure 2-22 Connectify Hotspot Setup Wizard dialog box

76 Chapter 2 Wireless Local Area Networks

13. Click Details. How does this information differ from the details for the physical NIC and Virtual Router? Now you will try to access Connectify from another wireless device, such as another computer. (The following steps assume you are accessing Con- nectify from a second computer, but you could try accessing it from a smartphone instead. If you are using a smartphone, make a note of the wireless network it is cur- rently connected to, use the phone’s built-in utility to connect to Connectify, and then continue this project starting at Step 19.)

14. On the second computer, click the WLAN icon in the system tray and make a note of the wireless network it is currently connected to.

15. Click the network name Connectify-Me in the list of available networks.

16. Click the Connect automatically box to select it.

17. Click Connect.

18. Enter the security information if you are requested to do so.

19. Verify that you are connected to the wireless network by opening your Web browser and going to www.course.com.

20. Return to the first computer and click the Connectify icon in the system tray. The second computer (or the smartphone) should appear on the Clients tab.

21. Which application, Virtual Router or Connectify, do you think is better? Why?

22. From the second computer (or the smartphone) reconnect to the AP it was connected to before you started this project.

23. On the first device click Stop Hotspot.

24. Close all open windows.

Case Projects

Case Project 2-1: Pricing Wireless NICs Suppose that you are asked to equip a desktop with a wireless NIC. Which type of wireless network adapter would you want to purchase? Using the Internet, research different types of wireless client adapters for a desktop. Create a table or chart that shows the advantages and disadvantages of each

type all along with the cost from at least three different online vendors. Which would you choose for a desktop? Why?

Case Project 2-2: Comparing Access Points Using the Internet, identify five different APs, each from a different manufacturer. Create a table that lists each AP, its features, and costs. Which would you choose for home use? Which would you choose for a SOHO of 25 employees? Why?

Case Project 2-3: Standards Do standards stifle IT or promote it? Contact three IT professionals and ask what they think about standards. After you understand each person’s position, ask him or her to discuss the opposing position. Write a one-page paper on your findings.

Case Projects 77

Case Project 2-4: Wireless Mesh Networks Using the Internet, research at least three different real-world implementations of wireless mesh networks. Why were they installed? What were their advantages? What were their dis- advantages? Summarize your research in a one-page document.

Case Project 2-5: Nautilus IT Consulting Nautilus IT Consulting (NITC) is a computer technology business that assists organizations in developing IT solutions. NITC has asked you to help with new customers. A local construction company, RDC Construction (RDC), wants to install a wireless LAN in both its showroom as well as in its warehouse, which is located half a mile (.8 kilometer) away in a remote area. Because of a slowdown in construction, RDC is very cost-conscious and needs an economical solution to provide wireless connectivity both within the two buildings and between the buildings as well.

1. Create a PowerPoint presentation of eight or more slides that covers the advantages and disadvantages of each of the IEEE 802.11 WLANs. In addition, include additional slides that outline how the two sites could be connected with a wireless solution. Conclude your presentation with your recommendations for RDC.

2. After your presentation, RDC announces that it is interested in your solutions, yet is concerned that their warehouse has limited electrical outlets. Create a one-page memo to RDC that lists the advantages of PoE and include costs for both midspan and end- point PSEs. Use the Internet to research costs for implementing PoE, and include those in your memo.

78 Chapter 2 Wireless Local Area Networks

chapter3

Radio Frequency Fundamentals

After completing this chapter you should be able to:

• Explain the basic principles of radio frequency transmissions • Describe the different types of analog and digital modulation • List the units of measurement for radio frequency transmissions • Describe how radio frequency waves behave and the impact of these behaviors on transmissions

79 Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

At first glance sailboats and wireless local area networks (WLANs) may seem to have very little in common. That is, unless you’re talking about the world’s most presti- gious sailboat racing event, in which case WLANs played a crucial rule in the recent victory by the United States.

The America’s Cup regatta is a series of races between two yachts. This event attracts top sailors and yacht designers because of its international prestige and his- tory, dating as far back as 1857. The 2010 America’s Cup was held off the coast of Valencia, Spain in February 2010 between the defending champion Alinghi 5 and the challenger, an American multihull yacht named USA-17 from the BMW Oracle Racing team. For this 33rd America’s Cup race, the competing teams could not agree on a set of rules, so only the most basic guidelines were established: the boats could be as long as 90 feet (27 meters) and the course would be 20 miles (32 kilometers) out windward and the same distance back. Within those parameters, the ship could be designed to run as fast as possible. And the USA-17 designers took full advantage of this freedom.

The USA-17 was considered a technical marvel. Its wing sail was over 223 feet (68 meters) high, or almost 20 stories, and barely fit under the Golden Gate Bridge in San Francisco. The wing was made of an aeronautical fabric that was stretched over a carbon fiber frame, giving it a three-dimensional shape similar to that of an airplane wing. This wing had a fixed leading edge and an adjustable trailing edge, which allowed the USA-17 crew to continuously change the shape of the sail during the course of a race in order to adjust for changes in the wind. With this advanced wing and other improvements, such as encasing the hull with special drag-resistant materials, the USA-17 could reach speeds up to 28 knots (32 miles per hour).

In order to take advantage of the ability to make constant adjustments to the sail dur- ing the race, the USA-17 was fitted with 250 sensors to collect raw data. This data included pressure sensors on the wing, angle sensors on the adjustable trailing edge of the wing sail to monitor the effectiveness of each adjustment, and strain sensors on the mast and wing to allow for maximum thrust without overextending the wing. Some sen- sors were making measurements up to 10 times per second (in one hour of sailing almost 90 million data points were collected). Yet gathering the data during the race was just one step. Somehow that data had to be provided to the crew members while the race was in progress so that they could monitor the ship and make further adjustments.

That’s where WLANs came in. The helmsman, or person who steered the ship, wore a backpack that contained a laptop computer. The USA-17’s onboard computer, located in the main hull, would wirelessly send information to the helmsman’s laptop, which would in turn wirelessly transmit it to both a wrist-mounted personal digital assistant (PDA) as well as a “heads-up” display in the goggles that he wore. This display allowed

Real World Wireless

80 Chapter 3 Radio Frequency Fundamentals

Troubleshooting is a critical skill for any information technology (IT) professional. Whether it’s diagnosing a personal computer that will not boot or determining why a program fails to successfully launch, knowing how to isolate, identify, and resolve hardware, software, or tele- communications problems is essential.

A good troubleshooting skill set is particularly important when working with wireless net- works. Whereas with a wired network the data is confined to a cable that can be tested fairly easily for any problems, that is not the case with wireless LANs. Different types of unexpected problems can arise when a wireless signal travels through the open airwaves. Are specific objects blocking the signal? Is the wireless device too far away? Are there conflicts with other wireless devices? The list goes on and on.

Good troubleshooting skills for WLANs start with an understanding of the fundamentals of wireless transmissions. In this chapter, you will explore the basics of how wireless technology works. You will begin by looking at the principles behind radio frequency (RF) transmissions. Next you will study how data can be sent through wireless transmissions, and then you will see how transmissions can be measured. Finally, you will learn about the behavior of radio frequency transmissions and how this behavior affects WLANs.

Principles of Radio Frequency

C W N A

1.3.1. Identify RF signal characteristics, the applications of basic RF antenna concepts, and the implementation of solutions that require RF antennas.

Understanding the principles of RF transmissions is important for both troubleshooting wire- less LANs as well as creating a context for understanding wireless terminology. This knowl- edge includes an understanding of what electromagnetic waves are, their characteristics, and the electromagnetic spectrum.

Of the six categories of CWNA exam objectives, Radio Frequency (RF) Technologies has the highest weight of 21 percent.

him to see both graphical and numeric data through the goggles while never taking his eyes off the sea and the boat. Each crew member was also equipped with his own wrist-mounted wireless PDA that contained customized data for his specific job on the ship, such as the load balance on a particular rope or the current aerodynamic perfor- mance of the wing sail. In addition, a second ship, known as the performance tender, accompanied the USA-17 while it was on the water. The tender served in part as a floating datacenter and was also connected to the USA-17 by a wireless LAN.

While it’s virtually impossible to predict what new technologies will impact the next America’s Cup race, it’s safe to say that wireless will again play a key role.

Principles of Radio Frequency 81

What Are Electromagnetic Waves? Suppose you were to stand next to a campfire at night. You could see the light from the fire, feel its heat, and hear the crackling of the logs. The light, heat, and sound from the burning logs actually move to you through space as waves. The sound waves travel through the air as vibrations: a vibration of an object causes the air around the object to shake and move out in waves towards the human eardrum, which in turn vibrates. By contrast, light and heat waves travel through space in a different way. These utilize a special form of energy known as electro- magnetic waves. An electromagnetic wave consists of an electric field (electro) and a magnetic field (magnetic) that are perpendicular, or at right angles, to each other, as shown in Figure 3-1. These waves, which require no special medium for movement, travel freely through space in all directions at the speed of light, or 186,000 miles (300,000 kilometers) per second.

Some seventeenth-century scientists theorized that there was a spe- cial medium in space called the ether through which light and heat travelled. This was later proven to be incorrect. The network type Ethernet was named for the ether that was proposed in these early theories.

Electromagnetic Wave Characteristics Suppose that you pick up a garden hose to put out the campfire. If you move your hand up and down the water will create what look like waves that also move up and down, as shown in Figure 3-2. Suppose you were to start with your hand level at your waist while holding the garden hose and then bring it up, then down and finally back to your waist where you started. You would have completed what could be called one complete cycle. If you were to do that same movement repeatedly, you would continue to create waves and complete cycles as long as the water was turned on.

This movement of water from a garden hose is similar to some of the characteristics of elec- tromagnetic waves:

● Continuous. An electromagnetic wave is a continuous wave; it does not repeatedly start and stop.

● Cycle. Whenever an electromagnetic wave completes its repetitive movement and returns back to the starting point, it has finished one cycle.

Magnetic field

Electric field

Direction

Figure 3-1 Electromagnetic wave

82 Chapter 3 Radio Frequency Fundamentals

Cycles are illustrated by an up-and-down wave called an oscillating signal or a sine wave. This is illustrated in Figure 3-3. Notice that the wave starts at zero (that is, at the x-axis), and then moves up to the maximum voltage (þ), then down to the minimum voltage (�), and finally returns back to its starting point (zero) before beginning the cycle over again.

The cycling nature of an electromagnetic wave produces an alternating current (AC) because it flows between positive (1) and negative (2). AC is the type of current that runs to the electrical outlets in a house. Direct current (DC), which is found in batteries, flows only from one terminal (1) to the other (2) and does not alternate.

All electromagnetic waves, whether carrying light, heat, or something else, share the same four characteristics: wavelength, frequency, amplitude, and phase.

Wavelength With the garden hose still in your hand, if you keep your hand at waist level while you move the end of the hose up and down quickly, you will create short waves, as shown in Figure 3-4. Yet if you move your hand slowly you will create long waves, as seen in Figure 3-5. Thus, the distance between the waves’ peaks can be either short or long, depending on how you move the garden hose.

Figure 3-2 Garden hose waves

V o lt a g e

Cycle 1 Cycle 2 Cycle 3

Figure 3-3 Sine wave

Principles of Radio Frequency 83

Electromagnetic waves can likewise have different distances between the peaks. This dis- tance, called the wavelength, is shown in Figure 3-6. The wavelength, represented by the Greek symbol � (lambda), is typically measured from the peaks (crests) of the wave; how- ever, they can be measured from the valleys (troughs) or anywhere in between, as long as it is at the same point in each cycle.

Some electromagnetic waves have very long wavelengths while others are very short. For exam- ple, the wavelength of the radio station AM 1120 is 878 feet (267 meters), while the wavelength of an X-ray used by a physician to spot a broken bone is measured in one billionth of a meter.

Fast movement

Figure 3-4 Fast movement creates short waves

Slow movement

Figure 3-5 Slow movement creates long waves

Wavelength

V o

lt a

g e

Figure 3-6 Wavelength

84 Chapter 3 Radio Frequency Fundamentals

Although wavelength measurements are most often stated in metric units instead of imperial units (feet and inches), in this book both units are given where possible for comparison purposes.

Frequency Consider again what would happen if you kept your hand at your waist but moved the garden hose up and down slowly? And what would happen if you moved your hand very rapidly in the up and down motion? Besides creating long waves and short waves, another difference would be that moving your hand slowly would result in fewer waves, while moving it rapidly would result in more waves being created.

Electromagnetic waves behave similarly. The rate at which an event occurs, like moving the garden hose either slow or fast, will result in a different number of electromagnetic waves being created over the same time period. This creates an electromagnetic wave’s frequency. Frequency is the number of times that an event occurs within a specific period of time— that is, the number of times that a wave completes a cycle within a given amount time. A lower frequency has fewer cycles than a higher frequency, as illustrated in Figure 3-7.

How frequently an event occurs is its frequency.

Although electromagnetic frequencies are based on the number of cycles per second, the term hertz (Hz) is instead used as the unit of measurement. An event that occurs one time per second is equivalent to 1 Hz. Because of the high number of cycles, metric abbreviations are used when

V o lt a g e

Higher frequency

V o lt a g e

Lower frequency

Figure 3-7 Lower and higher frequencies

Principles of Radio Frequency 85

referring to frequencies. For example, a wave measured as 710,000 Hz (710,000 cycles per sec- ond) would more properly be listed as 710 KHz. The hertz abbreviations are listed in Table 3-1.

The relationship between wavelength and frequency is sometimes called an inverse relation- ship (the effect of one reverses that of the other, and vice versa). Using the speed of light as a constant (c), the formula for wavelength is � ¼ c/f, while the formula for frequency is f ¼ c/�. This means that the higher the frequency, the shorter the wavelength will be, and the longer the wavelength, the lower the frequency.

Amplitude Suppose there is a problem with the power behind the water coming from the garden hose (perhaps an electric water pump is faulty). If a weak amount of water power runs through the garden hose the peaks and valleys would not be as high and deep as if a strong amount of water power was gushing through the hose to produce tall peaks and deep valleys.

In electromagnetic wave terminology, amplitude refers to the magnitude of the change of the wave and is measured by how high or how deep the wave is as shown in Figure 3-8. Whereas the wavelength is the distance from the peak of one wave to the next, amplitude is how high (or deep) the waves are.

Amplitude can either be measured at a wave’s peak or valley, since they are the same.

Amplitude is essentially a measure of the strength of an electromagnetic wave’s signal. Although the signal strength may weaken, this does not impact the wavelength or

Name Abbreviation Definition

Hertz Hz 1 cycle per second

Kilohertz KHz 1,000 cycles per second

Megahertz MHz 1,000,000 cycles per second

Gigahertz GHz 1,000,000,000 cycles per second

Table 3-1 Hertz abbreviations

Wavelength

Amplitude

V o lt a g e

Figure 3-8 Amplitude

86 Chapter 3 Radio Frequency Fundamentals

frequency of the signal as it does the amplitude. Figure 3-9 illustrates a weak and a strong signal.

Different types of transmissions require different signal strengths. For example, the AM radio station 1120 may transmit at an extremely strong power of 50,000 units of power, while a WLAN AP only uses one one-thousandth of the same unit.

The units of power for transmitting and receiving are covered later in this chapter.

Phase Pretend that you pick up a second garden hose so you now have one in each hand. With your right hand you hold the first hose at your waist and move it first up and then down to create waves. At the same time with your left hand you hold the second hose at your shoulder level and move it first down and then up (just the opposite of the other hose) to create waves. Because the two waves have two different starting points—one at your waist starting up and one at your shoulder level starting down—the peaks and valleys would not match, as illustrated in Figure 3-10.

This example illustrates the concept of phase. The term phase refers to the relationship between at least two signals that share the same frequency yet have different starting points (and thus dif- ferent peaks and valleys). Two signals that have the same peaks and valleys are called in phase; if the peaks and valleys do not match they are out of phase. If two signals are the complete

Strong amplitude

Weak amplitude

V o lt a g e

Figure 3-9 Strong and weak amplitude

Shoulder level

Waist level

Figure 3-10 Different starting points

Principles of Radio Frequency 87

opposite of each other (the peak of one signal matches the valley of the other signal) the first sig- nal is in phase while the second signal is 180 degrees out of phase, as shown in Figure 3-11.

The Electromagnetic Spectrum All the different types of electromagnetic waves make up the electromagnetic spectrum, which is the range of all electromagnetic radiation. These waves may be categorized by their frequency, wavelength, or the energy needed to produce the wave. Figure 3-12 shows the cat- egories of waves in the electromagnetic spectrum. The properties of these categories are fur- ther defined in Table 3-2.

The electromagnetic spectrum is further subdivided into 450 different sections or bands. Table 3-3 lists some common bands by their frequencies (number of cycles per second). The United States is obligated to comply with the international spectrum allocations established by interna- tional governing bodies. However, the U.S. domestic spectrum uses may differ from international allocations if these domestic uses do not conflict with international regulations or agreements.

V o lt a g e

V o lt a g

180 degrees out

of phase signal

In-phase signal

Figure 3-11 In phase and 180 degrees out of phase

Name of wave Radio

103

Buildings Humans Butterflies Needle Point Protozoans Molecules Atoms Atomic Nuclei

104 108 1012 1015 1016 1018 1020

1022 1025 1028 10210 102120.531026

Microwave Infrared Visible Ultraviolet X-ray Gamma ray

Wavelength

(in meters)

Approximate Scale

of Wavelength

Frequency (Hz)

Frequency (wavelength)

Figure 3-12 Electromagnetic spectrum

88 Chapter 3 Radio Frequency Fundamentals

Frequencies of common devices include garage door openers (315 MHz), alarm systems (40 MHz), baby room monitors (49 MHz), radio-controlled airplanes (72 MHz), radio-controlled cars (75 MHz), wildlife tracking collars (215 MHz–220 MHz), and global positioning system or GPS (1.227 GHz and 1.575 GHz).

A license is normally required from the Federal Communications Commission (FCC) to send and receive via a specific frequency. However, there is a notable exception known as the license-exempt spectrum or unlicensed bands. Unlicensed bands are parts of the radio spec- trum that are available nationwide to all users without requiring a license (devices that use these bands can be either fixed or mobile). However, the FCC does impose power limits on devices using the unregulated bands, which in effect reduces their range.

The FCC says that it created the unlicensed bands to “foster the development of a broad range of new devices, stimulate the growth of new industries, and promote the ability of U.S. manufacturers to compete globally by enabling them to develop unlicensed digital products for the world market.”

Name Description How They Are Used

Radio Radio waves have the longest wavelengths in the electromagnetic spectrum, ranging from longer than a soccer field to as short as the length of a soccer ball.

Radio waves can carry music to car radios, signals for television, and voice and data for cellular phones.

Microwave Microwaves have wavelengths that can be measured in centimeters; longer microwaves of almost one foot in length are used to heat food in a microwave oven.

Microwaves are good for transmitting information over distances because microwave energy can penetrate haze, light rain and snow, clouds, and smoke.

Infrared Infrared light has a range of wavelengths, from “near infrared” light with wavelengths that are microscopic to “far infrared” with wavelengths about the size of a pin head.

Far infrared waves are thermal, which means they can carry heat; for example special lamps that emit thermal infrared waves are often used in fast food restaurants, while near infrared waves are used by television remote controls.

Visible Visible light waves are the only electromagnetic waves that can be seen by humans and appear as the colors of the rainbow.

Each color has a different wavelength, with red the longest wavelength and violet the shortest wavelength.

Ultraviolet Ultraviolet (UV) light, which has shorter wavelengths than visible light, are invisible to the human eye, although some insects can see it.

The sun emits light at all the different wavelengths in electromagnetic spectrum, but ultraviolet waves are responsible for causing sunburns.

X-ray X-rays have smaller wavelengths and thus higher energy than ultraviolet waves; usually X-rays are referred to in terms of their energy rather than wavelength.

The earth’s atmosphere is thick enough that virtually no X-rays are able to penetrate from outer space all the way to the earth’s surface.

Gamma ray Gamma rays have the smallest wavelengths and the most energy of any other wave in the electromagnetic spectrum.

Gamma rays are generated by radioactive atoms and in nuclear explosions and can be used to kill cancerous cells.

Table 3-2 Electromagnetic spectrum properties

Principles of Radio Frequency 89

Table 3-4 lists the unlicensed bands, two of which are allocated for WLANs. One of these bands is the Industrial, Scientific and Medical (ISM) band, which was approved by the FCC in 1985. Another unlicensed band used for WLANs is the Unlicensed National Information

Band Frequency Common Uses

Very Low Frequency (VLF) 10 KHz to 30 KHz Maritime ship-to-shore

Low Frequency (LF) 30 KHz to 300 KHz Aircraft beaconing signals

Medium Frequency (MF) 300 KHz to 3 MHz AM radio

High Frequency (HF) 3 MHz to 30 MHz Shortwave radio, CB radio

Very High Frequency (VHF)

30 MHz to 144 MHz 144 MHz to 174 MHz 174 MHz to 328.6 MHz

TV stations 2–6, FM radio Taxi radios TV stations 7–13

Ultra High Frequency (UHF)

328.6 MHz to 806 MHz 806 MHz to 960 MHz 960 MHz to 2.3 GHz 2.3 GHz to 2.9 GHz

Public safety Cellular telephones Air traffic control radar Wireless LANs

Super High Frequency (SHF)

2.9 GHz to 30 GHz Wireless LANs

Extremely High Frequency (EHF)

30 GHz and above Radio astronomy

Table 3-3 Common radio frequency bands

Unlicensed Band Frequency Total Bandwidth Common Uses

Industrial, Scientific and Medical (ISM)

902–928 MHz 2.4–2.4835 GHz 5.725–5.85 GHz

234.5 MHz Cordless phones, WLANs, Wireless Public Branch Exchanges

Unlicensed Personal Communications Services

1910–1930 MHz 2390–2400 MHz

30 MHz Wireless Public Branch Exchanges

Unlicensed National Information Infrastructure (UNII)

5.15–5.25 GHz 5.25–5.35 GHz 5.725–5.825 GHz

300 MHz WLANs, Wireless Public Branch Exchanges, campus applications, long outdoor links

Millimeter Wave 59–64 GHz 5 GHz Home networking applications

Table 3-4 Unlicensed bands

90 Chapter 3 Radio Frequency Fundamentals

Infrastructure (UNII or U-NII), approved in 1996. The UNII band is intended for devices that provide short-range, high-speed wireless digital communications.

Unlicensed bands can also pose some difficulties. Because they are not regulated and licensed, different devices from different vendors may attempt to use the same frequency. This conflict can cause the signals from different devices to interfere with each other and prevent them from functioning properly. This means that the performance of devices using unregulated bands may not always be consistent and predictable.

WLANs use either the ISM 2.4–2.4835 GHz or the UNII 5.725–5.825 GHz band. The wave- length of a 2.45 GHz wave is 4.8 inches (12 centimeters), while the wavelength for a 5.775 GHz wave is only 2 inches (5 centimeters).

A WLAN transmitting at 2.45 GHz is sending an RF signal of 2,450,000,000 cycles each second.

Radio Frequency Modulation By itself an electromagnetic wave cannot carry any useful information. In order for the wave to transmit information it must be modified. This modification is called modulation or keying, and an electromagnetic wave that has been modified for the purpose of conveying information is a carrier (sometimes called a carrier wave or carrier signal).

The practice of using modulation and carriers for communication are not new. As far back as Ancient China smoke signals were used to transmit messages, where the smoke from a fire was interrupted (modulated) by a wet blanket in order to convey a special dispatch (carrier).

Because all transmissions can be digitized and broken down into a series of bits (0 and 1), the modulations of electromagnetic waves essentially involves manipulating the wave to represent either a 0 or 1. There are three modulations that can be made to a wave to enable it to carry these bits: amplitude, frequency, or phase. Modulations can be performed on either analog or digital transmissions.

Although WLAN transmissions are digital, knowing about analog modulation helps in the understanding of digital modulation.

Analog Modulation An analog signal is a continuous signal with no breaks in it, which means that no individual element of an analog signal can be uniquely identified from another element of the signal. Audio, video, and even light are all examples of analog signals.

Radio Frequency Modulation 91

Three types of modulations can be performed on analog signals. In the first type, known as amplitude modulation (AM), the amplitude (height) of the wave is changed so that a higher wave represents a 1 bit and a lower wave represents a 0 bit. Figure 3-13 illustrates the letter A (ASCII 65 or 01000001) being transmitted by amplitude modulation.

Amplitude modulation, most frequently used by broadcast radio sta- tions, is often susceptible to interference from outside sources such as lightning and is not generally used for data transmissions.

Whereas AM varies the amplitude (height) of the signal, frequency modulation (FM) changes the frequency (number of waves). FM uses more cycles to represent a 1 bit and fewer cycles to repre- sent a 0 bit. Figure 3-14 illustrates the letter A (ASCII 65 or 01000001) being transmitted by FM.

10 0 0 0 10 0

Figure 3-13 Amplitude modulation (AM)

10 0 0 0 100

Figure 3-14 Frequency modulation (FM)

92 Chapter 3 Radio Frequency Fundamentals

Like amplitude modulation, frequency modulation is often used by broadcast radio stations. Yet unlike AM, FM is not as susceptible to interference from outside sources.

Whereas AM changes the amplitude and FM changes the frequency, phase modulation (PM) changes the starting point of the cycle. This change takes place only when the bits being transmitted change from a 1 bit to a 0 bit or vice versa: the change in starting point indicates that a different bit is now being sent. Figure 3-15 illustrates the letter A (ASCII 65 or 01000001) being transmitted by phase modulation.

Although radio broadcasts use either AM or FM, broadcast television uses all three types of modulation: AM for video, FM for sound, and PM for color information.

Digital Modulation Consider again holding a garden hose. As you move your hand up and down, the water cre- ates continuous waves as long as the water is turned on. But what if you placed your thumb over the end of the garden hose for a second and then removed it? Water would stop flowing (while your thumb was over the hose) and then would squirt out (when you moved your thumb). This on-off activity is similar to a digital signal. A digital signal consists of data that is discrete or separate, as opposed to an analog signal, which is continuous. A digital sig- nal has numerous starts and stops throughout the signal stream.

Because computers operate using digital signals (binary code is dis- crete, thus it is digital), when analog data, such as a video image or an audio sound, needs to be processed it must be first converted from analog data into a digital format.

1 00 0 0 10 0

Figure 3-15 Phase modulation (PM)

Radio Frequency Modulation 93

Although analog modulation could be used for data communications, almost all wireless sys- tems use digital modulation. Digital modulation holds several advantages over analog modulation:

● It makes better use of the available spectrum. ● It requires less power to transmit digital modulation than analog modulation. ● It performs better when there is interference from other signals. ● The error-correcting techniques are more compatible with other digital systems.

In an analog system, the carrier signal is continuous, and amplitude, frequency, and phase changes also occur continuously. With a digital system, on the other hand, the changes are in distinct or discrete steps using binary signals.

Digital modulation, like analog modulation, uses three types of modulation: amplitude, fre- quency, or phase. Amplitude shift keying (ASK) is a binary modulation technique similar to amplitude modulation in that the height of the carrier can be changed to represent a 1 bit or a 0 bit. However, instead of both a 1 bit and a 0 bit having a carrier signal as with amplitude modulation, the ASK 1 bit has a carrier signal (positive voltage) while a 0 bit has no signal (zero voltage). Figure 3-16 illustrates the letter A (ASCII 65 or 01000001) being transmitted by ASK.

Digital (binary) modulation is still shown as a standard sine wave.

Similar to frequency modulation, frequency shift keying (FSK) is a binary modulation tech- nique that changes the frequency of the carrier signal. Because it is sending a binary signal, the carrier signal starts and stops. Figure 3-17 illustrates the letter A (ASCII 65 or 01000001) being transmitted by FSK.

Phase shift keying (PSK) is a binary modulation technique similar to phase modulation. The difference is that the PSK signal starts and stops because it is a binary signal. Figure 3-18 illustrates the letter A (ASCII 65 or 01000001) being transmitted by PSK.

10 100 0 0 0

Figure 3-16 Amplitude shift keying (ASK)

94 Chapter 3 Radio Frequency Fundamentals

Note that the figures illustrating the six modulation types for analog and digital transmissions would actually have multiple cycles for each bit. For the sake of clarity, these figures only show one cycle per transmitted bit where appropriate.

RF Signal Strength Measurements

C W N A

1.2.1. Understand and apply the basic components of RF mathematics.

It is often necessary to measure the RF signal strength that is being received by a mobile device. Four units of measurement are used to represent RF signal strength: mW (milliwatts), dBm (decibel

10 0 0 0 100

Figure 3-17 Frequency shift keying (FSK)

1 00 0 0 10 0 1

Figure 3-18 Phase shift keying (PSK)

RF Signal Strength Measurements 95

milliwatts), RSSI (Receive Signal Strength Indicator), and a percentage measurement. Although all of these measurements are related to each other, some are more closely related than others. It is possible to convert from one unit to another, yet such a conversion sacrifices some accuracy.

Milliwatt (mW) Consider the garden hose once again. The pressure on the water line (typically 80 pounds per square inch or 552 kilopascals) is analogous to the voltage (V) in an electrical circuit, which is measured in volts. The water flow, typically 2.5 gallons (9.5 liters) per minute is analogous to the current (I) in an electrical circuit, which is measured in amperes or amps. Increasing the diameter of the garden hose from half an inch to one inch (1.2 cm to 2.5 cm) would cause more water to flow. When talking about an electrical circuit, opposition to the flow of the current is referred to as resistance (R). Resistance is measured in ohms and the total amount of resistance is called the impedance. In electrical terms, voltage is equal to current times resistance, or V ¼ I � R. Electrical power (P), which is measured in watts (W), is volt- age multiplied by current, or P ¼ V � I. These electrical terms are summarized in Table 3-5.

The formula for voltage (V 5 I×R) was first proposed in 1827 by German physicist George Ohm and is called Ohm’s Law.

A watt is a basic unit of power of 1 amp of current that flows at 1 volt. A milliwatt (mW) is one thousandth of a watt of power.

The power that most WLANs transmit is less than 200 mW.

Decibel Milliwatt (dBm) Most linear scales, like that of the watt, have a reference that is fixed at zero, or the absence of what is being measured. The speedometer in a car is an example of a linear scale: when the

Electrical term Abbreviation Description Garden Hose Analogy

Unit of Measurement

Voltage V Electrical pressure on wire

Water pressure Volts

Current I Rate of electrical flow

Water flow rate Amperes (amps)

Resistance R Impedance of electrical flow

Diameter of hose Ohms

Electrical power P Amount of energy

Total amount of water coming out of hose

Watts

Table 3-5 Electrical terminology

96 Chapter 3 Radio Frequency Fundamentals

needle is pointing to zero or the display reads zero, the car is not moving. In the same way, RF power can be measured on the linear scale by the number of mWs that are being transmitted.

However, measuring by mWs using a linear scale is not always an accurate measurement. This is in part due to the fact that as the RF signal strength fades it does not do so in a linear manner. For example, suppose that a user measures an access point’s signal strength when he is 3 feet (1 meter) away from the access point. If the user then moved twice as far away (6 feet or 2 meters) the signal would decrease not by a factor of two but instead by a factor of four. An alternative way of measuring RF power is needed besides the linear mW.

RF signal strength fades inversely as the square of the distance.

A second way to measure RF power is to use a relative scale. In a relative scale the reference point is the measurement itself, rather than being fixed at zero. A relative scale can be used as a comparison between two values; the difference can reveal the gain or loss in relation to the whole. Although the relative scale measurement is not as precise as the linear scale, it gives a better picture of the loss or gain. It is common to use a logarithm to express the relative rela- tionship of the measurement to the whole. A logarithm is the exponent to which the number 10 must be raised to reach a given value. For example, the logarithm (or log) of 1,000 is 3 (10³ ¼ 1,000) because the log is always the exponent. RF power gain and loss on a relative scale are measured in decibels (dB) instead of mW. This is because gain and loss are relative concepts and a decibel is a relative measurement.

The reference point that relates the logarithmic relative decibel (dB) scale to the linear milli- watt scale is known as the decibel milliwatt (dBm). This is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). This reference point specifies that 1 mW ¼ 0 dBm and is a measurement of absolute power or “raw signal strength.” A comparison of dBm levels and watts is listed in Table 3-6.

dBm Level Watts of Power

0 1.0 mW

1 1.3 mW

2 1.6 mW

3 2.0 mW

4 2.5 mW

5 3.2 mW

10 10 mW

20 100 mW

30 1.0 W

45 32 W

Table 3-6 dBm levels and watts

RF Signal Strength Measurements 97

Typically WLAN vendors provide information regarding the signal strength needed for a wireless device to transmit. This is given in units of negative dBm. For example, the maxi- mum signal reading might be �30 dBm, the minimum reading might be �85 dBm, and the recommended might be �70 dBm or higher. The reason why dBm is expressed in negative values is because in logarithms the value indicated represents an exponent (a value of �2 represents 10 to the �2 power, which equals 0.01), so a negative dBm means that a negative exponent is being applied in the power calculations. Since dBm equals 1 mW of power, a �10 dBm is equal to 0.1 mW, a �20 dBm is 0.01 mW, etc. Even though the dBm number is negative, these are small but positive numbers on a logarithmic scale.

Because dB and mW use different scales (linear vs. relative), a conversion is necessary when moving between the two. The conversion formula is log (mW) � 10 ¼ dBm. For example, converting from mW to dBm is as follows:

● 100 mW ¼ 20 dBm (log of 100 is 2 or 102 ¼ 100) ● 50 mW ¼ 1.69 dBm (log of 50 is 1.698 or 101.698 ¼ 50) ● 25 mW ¼ 13.9 dBm (log of 25 is 1.397 or 101.397 ¼ 25) ● 13 mW ¼ 11.1 dBm (log of 13 is 1.113 or 101.113 ¼ 13) ● .5 mW ¼ �3.01 dBm (log of .5 is �0.3010 or 10�0.3010 ¼ .5) ● .25 mW ¼ �6.02 dBm (log of .25 is �0.602 or 10�0.602 ¼ .25) ● .13 mW ¼ �8.86 dBm (log of .13 is �0.886 or 10�0.886 ¼ .13)

Notice that, in the preceding examples, each time the actual mW power level decreases by half, the dBm measurement goes down by about 3 dBm. This leads to a shortcut for calculat- ing the increase or decrease of these RF values is known as the 10’s and 3’s Rules of RF Math. The rules are:

● �3 dB – A loss of 3 decibels means that half of the power in mW has been lost ● þ 3 dB – A gain of 3 decibels means that the power has been doubled in mW ● �10 dB – A loss of 10 decibels means that 90 percent of the power has been lost in mW ● þ10 dB – A gain of 10 decibels indicates a tenfold increase in mW

Table 3-7 summarizes these rules.

As a general guideline, a decrease of 3 dBm yields roughly half the original value and an increase of 3 dBm yields roughly twice the original value

The Certified Wireless Network Administrator (CWNA) exam does not require you to perform logarithmic calculations, but it does require you understand the concepts of the 10’s and 3’s Rules of RF Math.

Receive Signal Strength Indicator (RSSI) It is no surprise that users generally find the dBm unit difficult to understand. For example, it may be hard to explain why a negative dBm is acceptable or that a �100 dBm is equivalent to 0.0000000001 mW. For this reason, other RF signal strength measurements have been used.

98 Chapter 3 Radio Frequency Fundamentals

The IEEE 802.11 standard defines a mechanism by which RF signal strength energy can be measured by the circuitry on a wireless network interface card adapter (wireless NIC). This is known as the Receive Signal Strength Indicator (RSSI). This value was intended for inter- nal use by the wireless NIC. For example, a wireless NIC in a laptop computer can check the RSSI value to determine if it is clear to send its transmission or if the user is roaming beyond the range of a particular AP.

Some client applications display the RSSI to the user as an indication of RF signal strength. However, the RSSI should not be relied upon as a valid indicator. This is for three reasons. First, RSSI was not intended to be used in this way. The IEEE standard states, “The RSSI is intended to be used in a relative manner. Absolute accuracy of the RSSI reading is not specified.” Second, each vendor may implement RSSI differently. This is because the RSSI is actually a numeric integer value between the range of 0 and whatever number the vendor chooses (as long as it is no more than 255). For example, Cisco uses the range 0–100, Atheros uses 0–60, while a third vendor uses 0–31. This means that there is no specified accuracy to the RSSI reading that requires the RSSI value to correlate to a specific mW or dBm. Finally, all pos- sible energy levels (mW or dBm values) may not be represented by the integer set of RSSI values.

Percentage To avoid the inaccuracies of using RSSI as a basis for reporting RF signal strength, many wireless clients represent signal strength as a percentage. The percentage represents the RSSI for a particular packet divided by the maximum RSSI value, and then multiplied by 100. For example, because Atheros uses the RSSI range 0-60, an RSSI of 30 would mean that the signal strength is 50 percent (Cisco’s RSSI of 50 would indicate the same percentage since it uses a range of 0–100).

One well-known client software application fails to accurately com- pute both the RSSI and the percentage. It does not even display the actual RSSI value; instead, it arbitrarily computes its own value based on “signal quality [as] a percentage value between 0 and 100, where 0 equals −100 dBm and 100 equals −50 dBm” and then erro- neously calls it the RSSI.

Rule Explanation

Percentage of Power Lost/ Gained

Current Power Level Example

�3 dB Half the watt value

50% lost Half of original 100 mW – 3 dB ¼ 50 mW

þ3 db Double the watt value

100% gained Double the original

10 mW þ 3 dB ¼ 20 mW

�10 dB Decrease watt value to one tenth of original

90% lost One-tenth of original

300 mW – 10 dB ¼ 30 mW

þ10 dB Increase the watt value by 10-fold

1,000% gained Ten times the original

10 mW þ 10 dB ¼ 100 mW

Table 3-7 The 10’s and 3’s Rules of RF Math

RF Signal Strength Measurements 99

Using a percentage for signal strength provides a reasonable measurement for comparisons, even though different vendor’s NICs were used to make the measurements. The percentage measurement allows compensation for the integer nature of the RSSI.

Some client utilities only display bars to indicate signal strength without any indication of what the bars represent, much like that of a cell phone.

Signal-to-Noise Ratio Noise is defined as unwanted interference that impacts an RF signal. The signal-to-noise ratio (SNR) is a ratio of the desired signal to undesired signal (noise) in the average power level of a transmission. SNR values are given as dB, which is the difference between the two logarithmic values of signal level minus the noise level. For example, a signal level of �53 dBm measured near an AP with a typical noise level of �90 dBm results in a 37 dB SNR. For a WLAN noise is typically interference from other RF signals, such as from a cordless telephone or microwave oven. In addition, the distance between devices also reduces the sig- nal level. That’s because the lower the power of the signal the more difficult it is to distin- guish it from noise (similar to trying to hear a whisper while standing at a rock concert).

As a general rule, the following SNR values relate to overall WLAN performance and the number of bars that are typically displayed:

● Over 40 dB SNR—Excellent signal (5 bars) ● 25 dB–40 dB SNR—Very good signal (3–4 bars) ● 15 dB–25 dB SNR—Low signal (2 bars) ● 10 dB–15 dB SNR—Very low signal (1 bar) ● Less than 10 dB—No signal (0 bar)

Radio Frequency Behavior

C W N A

1.1.1. Define and explain the basic concepts of RF behavior.

The behavior of a radio frequency signal has a significant impact upon the speed of the trans- mission and the distance that be achieved between the devices. These RF behaviors can be classified as propagation behaviors. Different behaviors can impact the signal by either taking power away from the signal or adding to it.

Propagation Behaviors A common misconception is that an RF signal that goes out from an antenna is a single sig- nal that takes a direct straight path to the receiver. However, this is incorrect in two ways. First, there is not just one RF signal that reaches the receiver. Along with the primary signal,

100 Chapter 3 Radio Frequency Fundamentals

multiple copies of that signal may reach the receiver, all at different times. This phenome- non is known as multipath. Second, because the signals radiate out in many directions, they may not always take a straight path to the receiver. The signal may “bounce” off of walls and other objects in the area. The way in which the signal travels is known as wave propagation. The incorrect and correct views of wave propagation and multipath are illus- trated in Figure 3-19.

There are several different behaviors that the wave will take, depending upon the objects and even the materials that the surrounding objects are made of (that is, depending on their com- position). These behaviors include absorption, reflection, scattering, refraction, and diffraction.

Absorption Certain types of materials can absorb the RF signal. This is known as absorption. The types of materials that will absorb an RF signal include concrete, wood, and asphalt. Absorption is illustrated in Figure 3-20.

Laptop

Incorrect

Laptop

Correct

Figure 3-19 Incorrect and correct views of wave propagation and multipath

Radio Frequency Behavior 101

Reflection Reflection is the opposite of absorption. Instead of the signal being soaked up, it is bounced back. Reflection generally is caused by objects that are very large (in relation to the size of the wavelength of the signal, or the distance between successive amplitude peaks) and relatively smooth, such as walls, buildings, and the surface of the earth. Also, objects that are made out of metal will reflect a signal. These can include metal roofs, metal walls, and elevator shafts. A signal is generally weaker after it is reflected. Reflection is seen in Figure 3-21.

Scattering Whereas reflection is caused by large and smooth objects, scattering is caused by small objects or rough surfaces. Objects that can cause scattering include foliage, rocks, and sand. Scattering can also occur when the RF signal comes in contact with elements in the air, such as rain or heavy dust particles. Scattering is illustrated in Figure 3-22.

Refraction Over a long distance, an RF signal might move through a variety of atmo- spheric conditions. For example, it might start out in a relatively transparent condition, such as in bright sunshine, then go through a much denser condition, such as cold damp air. When an RF signal moves from one medium to another of a different density the signal actually bends instead of traveling in a straight line. This bending behavior is known as refraction and is seen in Figure 3-23.

RF signal Absorption

Figure 3-20 Absorption

RF signal

Reflection

Figure 3-21 Reflection

102 Chapter 3 Radio Frequency Fundamentals

Refraction is the reason why a swimming pool appears deeper than it actually is. When you look into a pool, the light from the bottom is refracted away from the perpendicular because the index of refrac- tion in air is less than in water.

RF signal

Scattering

Figure 3-22 Scattering

RF signal

Refraction

Figure 3-23 Refraction

Radio Frequency Behavior 103

Diffraction Unlike refraction, in which the medium through which the signal passes causes the RF signal to bend, diffraction is caused by an object in the path of the transmis- sion signal. Diffraction occurs when an object with rough surfaces is in the path of the RF signal and causes it to bend. Diffraction is seen in Figure 3-24.

Impact of Behaviors As to be expected, wave propagation (absorption, reflection, scattering, refraction, and dif- fraction) and multipath can all have an impact upon the RF signal reaching its destination. This impact can be either a negative loss or a positive gain.

Loss There are two primary phenomena that can result in the loss of an RF signal. These are Free Space Path Loss and Delay Spread.

Free Space Path Loss (FSPL) As the RF signal move away from the sending source it spreads out. The further the signal spreads out, the weaker it becomes. Attenuation is the loss of signal strength. For an RF signal this loss results in a decrease in amplitude of the sig- nal. Free space path loss (FSPL) is the “natural” loss of signal strength through space and is not the loss impacted by absorption, reflection, scattering, refraction, or diffraction. FSPL represents the single greatest source of power loss in a wireless system. Table 3-8 illustrates the approximate free space path loss for 802.11b/g WLANs.

The decrease in signal strength due to free space path loss is inversely proportional to the distance traveled and proportional to the wavelength of the signal.

RF signal

Diffraction

Figure 3-24 Diffraction

Distance in Feet Distance in Meters Loss in dB

328 100 80

820 250 88

1,312 400 92

2,460 750 97

3,280 1,000 100

Table 3-8 FSPL for IEEE 802.11b/g WLANs

104 Chapter 3 Radio Frequency Fundamentals

Delay Spread Due to reflection, scattering, refraction, or diffraction, multiple “copies” of the RF signal may reach the receiver all at different times (multipath). Although the difference between the signals can be measured in units as small as a billionth of a second (nanosecond), these brief time differences can still affect reception. These time differences are known as delay spread. When a device receives the primary signal along with multiple copies, these copies are “added” to the primary signal. The three negative effects of delay spread are:

● Downfade. A delayed RF signal that is out-of-phase with the primary signal can cause decreased signal amplitude at the receiver known as downfade. This is because the delayed out-of-phase signal is “added” to the primary signal, which results in an over- all decrease.

● Corruption. When an out-of-phase delayed signal that has become attenuated is added to the primary signal, it may weaken the amplitude to the point that the receiving device cannot properly interpret the transmission. This is known as corruption.

● Nulling. If the delayed RF signal arrives 180 degrees out of phase it will completely cancel the primary RF signal. This is called nulling.

Amplification Not all delay spread is negative. Surprisingly it can also be positive. The strengthening of the signal is called a gain and is defined as the positive difference in ampli- tude between two signals. Gain is achieved by an amplification of the signal.

Sometimes “gain” is used synonymously with “amplification.” However, gain is technically the measure of amplification.

Amplification in delay spread can occur if the copies arrive virtually at the same time as the primary signal and are in phase (or just slightly out of phase). The end result is that the sig- nal is actually strengthened when added together. This phenomenon is called upfade.

Chapter Summary ■ A special form of energy known as an electromagnetic wave carries elements through

the universe. All electromagnetic waves share the same four characteristics: (1) electro- magnetic waves can have different distances between the peaks and distance is called the wavelength; (2) waves vary by the number of cycles created each second and this is known as the wave’s frequency; (3) for an electromagnetic wave the amplitude is the magnitude of the change of the wave and is measured by how high or how deep the wave is; and (4) the wave’s phase is the relationship between at least two signals that share the same frequency yet have different starting points.

■ All the different types of electromagnetic waves make up the electromagnetic spectrum, which is the range of all electromagnetic radiation. These waves may be categorized by their frequency, wavelength, or the energy needed to produce the wave. The electro- magnetic spectrum is further subdivided into 450 different sections or bands. Although a license is normally required to send and receive using a specific frequency, this is not the case for the license-exempt spectrum or unlicensed bands, two of which are used

Chapter Summary 105

for WLANs: Industrial, Scientific and Medical (ISM) band and the Unlicensed National Information Infrastructure (UNII or U-NII).

■ By itself an electromagnetic wave cannot carry any useful information; instead, to transmit information it must be modified (called modulation or keying). Because all transmissions can be digitized and broken down into a series of bits (0 and 1), the modulation of electromagnetic waves essentially involves manipulating the wave to represent either a 0 or 1. There are three modulations that can be made to a wave to enable it to carry these bits: amplitude, frequency, or phase, and these modulations can be performed on either analog or digital transmissions. For analog transmissions, amplitude modulation (AM) varies the height of the wave so that a higher wave represents a 1 bit while a lower wave represents a 0 bit. Frequency modulation (FM) changes the frequency (number of waves) so more cycles to represent a 1 bit and fewer cycles to represent a 0 bit , while phase modulation (PM) changes the starting point of the cycle (the change in starting point indicates that a different bit is now being sent).

■ Although analog modulation could be used for data communications, almost all wire- less systems use digital modulation. There are several advantages of digital modulation over analog modulation, and digital modulation, like analog modulation, uses three types of modulation: amplitude, frequency, or phase. Amplitude shift keying (ASK) is a binary modulation technique similar to amplitude modulation, in that the height of the carrier can be changed to represent a 1 bit or a 0 bit; yet instead of both a 1 bit and a 0 bit having a carrier signal as with amplitude modulation, the ASK 1 bit has a carrier signal (positive voltage) while a 0 bit has no signal (zero voltage). Frequency shift key- ing (FSK) is a binary modulation technique that changes the frequency of the carrier signal, and phase shift keying (PSK) is a binary modulation technique that changes the starting point of the cycle.

■ It is often necessary to measure the RF signal strength that is being received by a mobile device. Four basic measurements are used for WLANs. A watt is a basic unit of power of 1 amp of current that flows at 1 volt, and milliwatt (mW) is one thousandth of a watt of power. RF power gain and loss on a relative scale are measured in deci- bels (dB) instead of mW. The reference point that relates the logarithmic relative deci- bel (dB) scale to the linear milliwatt scale is known as the decibel milliwatt (dBm). A shortcut for calculating the increase or decrease of RF between mW and dBm is the 10’s and 3’s Rules of RF Math. The Receive Signal Strength Indicator (RSSI) is a value that was intended for internal use by the wireless NIC, yet some client applications display the RSSI to the user as an indication of RF signal strength. However, the RSSI should not be relied upon as a valid indicator. To avoid the inaccuracies of using RSSI as a basis for reporting RF signal strength, many wireless clients represent signal strength as a percentage.

■ The behavior of the radio frequency signal has a significant impact upon the speed of the transmission and the distance that be achieved between devices, and these RF behaviors can be classified as propagation behaviors. Along with the primary signal, multiple “copies” of that signal may reach the receiver, all at different times, and this is known as multipath. Also, because the signals radiate out in many directions, they may not always take a straight path to the receiver but instead may “bounce” off of walls and other objects in the area. The way in which the signal travels is known as wave propagation. Certain types of materials can absorb the RF signal (absorption),

106 Chapter 3 Radio Frequency Fundamentals

while other materials bounce the wave back (reflection). Whereas reflection is caused by large and smooth objects, scattering is caused by small objects or rough surfaces. Over a long distance, an RF signal may move through different atmospheric condi- tions. Known as diffraction, this phenomenon occurs when an object with rough sur- faces is in the path of the RF signal and causes it to bend.

■ This impact of propagation behaviors can be either a negative loss or a positive gain. As the RF signal moves away from the sending source, it spreads out and weakens, resulting in attenuation (a decrease in amplitude of the signal). Free space path loss (FSPL) is the “natural” loss of signal strength through space. Due to reflection, scat- tering, refraction, or diffraction, multiple “copies” of the RF signal may reach the receiver all at different times (multipath). Although the difference between the signals can be measured in units as small as a billionth of a second, this difference can affect reception. These time differences are known as delay spread. Delay spread can either result in a loss or a gain in signal strength.

Key Terms 10’s and 3’s Rules of RF Math A shortcut for calculating the increase or decrease of RF values. 180 degrees out of phase Term used to describe two electromagnetic signals that are the complete opposite of each other. absorption The RF propagation behavior in which an RF signal is assimilated into a material. amperes (amps) The measure of the flow of electrical current. amplification An increase in a signal’s strength to achieve gain. amplitude The magnitude of the change of a wave; measured by how high or how deep the wave is. amplitude modulation (AM) A modification of an analog electromagnetic wave that changes the amplitude (height) of the wave. amplitude shift keying (ASK) A modification of a digital electromagnetic wave that changes the amplitude (height) of the wave. analog signal A continuous signal with no “breaks” in it. attenuation Loss of signal strength that results in a decrease in the signal’s amplitude. bands The 450 different sections of the electromagnetic spectrum. carrier A modified electromagnetic wave that is used to transmit information. Also known as a carrier wave or a carrier signal. carrier signal See carrier. carrier wave See carrier. corruption The loss in a signal that occurs when a delayed multipath signal is significantly out-of-phase with the primary signal. current (I) The flow of electrical energy. cycle The repetitive movement of an electromagnetic wave that returns back to its starting point.

Key Terms 107

decibel milliwatt (dBm) The power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). decibels (dB) The measure used to determine RF power gain and loss on a relative scale. delay spread The difference in time of multipath signals that reach the receiver. diffraction The RF propagation behavior in which an RF signal bends in response to striking a rough surface. digital signal A signal that consists of data that is discrete or separate. downfade The signal loss that occurs when a delayed multipath signal is out-of-phase with the primary signal. electromagnetic spectrum The range of all the different types of electromagnetic waves. electromagnetic wave A special form of energy that transmits heat and light. Free space path loss (FSPL) The “natural” loss of signal strength that occurs as a signal travels through space. frequency The number of times that a wave completes a cycle within a given amount time. frequency modulation (FM) A modification of an analog electromagnetic wave that changes the frequency (number of waves). frequency shift keying (FSK) A modification of a digital electromagnetic wave that changes the frequency (number of waves). gain The positive difference in amplitude between two signals. hertz (Hz) The unit of measurement for electromagnetic frequencies. impedance The total amount of resistance to the flow of electrical current. Industrial, Scientific and Medical (ISM) An unlicensed band used for WLANs. in phase Two electromagnetic signals that have the same peaks and valleys. keying See modulation. license-exempt spectrum Parts of the radio spectrum that are available nationwide to all users without requiring a license. milliwatt (mW) One thousandth of a watt of power. modulation The modification of an electromagnetic wave to transmit information; also called keying. multipath The phenomena in which multiple copies of a signal reach the receiver at different times. nanosecond One billionth of a second. noise Unwanted interference that impacts an RF signal. nulling The cancellation of a signal that occurs when a delayed multipath signal is 180 degrees out of phase with the primary signal. ohms The measure of the restriction of the flow of current. oscillating signal The visual representation of up-and-down electrical waves. out of phase Term used to describe two electromagnetic signals with peaks and valleys that do not match. phase The relationship between at least two signals that share the same frequency yet have different starting points. phase modulation (PM) A modification of an analog electromagnetic wave that changes the starting point of the wave.

108 Chapter 3 Radio Frequency Fundamentals

phase shift keying (PSK) A modification of a digital electromagnetic wave that changes the starting point of the wave. Receive Signal Strength Indicator (RSSI) A value intended for internal use by the wireless NIC. reflection The RF propagation behavior in which an RF signal bounces back after striking a material. refraction The RF propagation behavior in which an RF signal bends due to a change in atmospheric condition. resistance (R) Measure of the restriction of the flow of electrical current. scattering The RF propagation behavior in which an RF signal bounces off small objects, such as raindrops. signal-to-noise ratio (SNR) A ratio of the desired signal to undesired signal in the average power level of a transmission. sine wave See oscillating signal. unlicensed bands Parts of the radio spectrum that are available nationwide to all users without a license. Unlicensed National Information Infrastructure (UNII or U-NII) An unlicensed band used for WLANs. upfade The gain in a signal that occurs when the delayed multipath signal arrives at the same time as and is in phase with the primary signal. voltage (V) Electrical pressure on a wire. volts The measure of electrical pressure on a wire. watts (W) A basic unit of power of 1 amp of current that flows at 1 volt. wavelength The distance between peaks in an electromagnetic wave. wave propagation The way in which an electromagnetic signal travels.

Review Questions 1. Cycles are illustrated by an up-and-down wave called a(n) .

a. oscillating signal

b. cyclical repeater

c. AC-to-DC converter

d. amplitude

2. The distance between peaks of an electromagnetic wave is called the .

a. hertz

b. frequency

c. crest

d. wavelength

Review Questions 109

3. A million cycles per second is a .

a. GHz

b. MHz

c. KHz

d. wHz

4. A weakening electromagnetic signal does not impact which two characteristics?

a. wavelength and frequency

b. phase and amplitude

c. frequency and phase

d. amplitude and volts

5. Two signals that have the same peaks and valleys are called .

a. reflected

b. 180 degrees out of phase

c. out of phase

d. in phase

6. Each of the following is a category of the electromagnetic spectrum except:

a. SIM

b. X-ray

c. microwave

d. radio

7. The number of unlicensed bands that are used for WLANs is .

a. 2

b. 3

c. 4

d. 5

8. Each of the following is a name for the electromagnetic signal that has been modified to carry information except:

a. carrier signal.

b. carrier wave.

c. carrier.

d. carrier keying.

9. The modification of the height of a digital signal is called .

a. amplitude modulation (AM)

b. frequency modulation (FM)

110 Chapter 3 Radio Frequency Fundamentals

c. phase modulation (PM)

d. amplitude shift keying

10. A is a basic unit of power of 1 amp of current that flows at 1 volt.

a. V

b. W

c. R

d. P

11. The power that most WLANs transmit is less than .

a. 2.45 GHz

b. 200 mW

c. −120 dBm

d. 65 V

12. The reference point that relates the logarithmic relative decibel (dB) scale to the linear milliwatt scale is known as the .

a. decibel milliwatt (dBm)

b. decibel (dB)

c. milliwatt carrier (mWc)

d. R scale

13. Which of the following is a typical recommended signal strength for a WLAN?

a. 99 dB

b. 0.01 mW

c. 32 W

d. −70 dBm

14. A loss of 3 decibels means that percent of the power in mW has been lost.

a. 25

b. 50

c. 75

d. 90

15. Each of the following is a reason why RSSI should not be used as a true indicator of signal strength except:

a. RSSI is not a linear scale.

b. all possible mW or dBm values may not be represented by the RSSI values.

c. each vendor may implement RSSI differently.

d. RSSI was not intended to be used in this way.

Review Questions 111

16. The materials concrete, wood, and asphalt that “soak up” an RF signal can all cause .

a. reflection

b. refraction

c. scattering

d. absorption

17. What is the difference between refraction and diffraction?

a. With refraction, the medium through which the signal passes causes the signal to bend, while diffraction is caused by an object in the path of the transmission.

b. There is no difference between them.

c. Refraction causes signals to be reflected while diffraction causes signals to be absorbed by materials.

d. Diffraction is caused by rocks and sand while refraction is caused by foliage.

18. Free space path loss (FSPL) is caused by .

a. scattering

b. reflection

c. deflection

d. the loss of signal strength as the signal travels

19. A is one billionth of a second.

a. picosecond

b. millisecond

c. microsecond

d. nanosecond

20. occurs when a multipath signal arrives 180 degrees out of phase.

a. Downfade

b. Corruption

c. Mirroring

d. Nulling

Hands-On Projects

Project 3-1: Using the Microsoft Windows 7 Netsh Utility The Windows Netsh utility for wireless local area network (WLAN) provides the means to configure wireless connectivity and security settings using a com- mand line instead of a graphical user interface (GUI). Benefits of the wireless

Netsh interface include easier wireless deployment as an alternative to Group Policy, the ability for administrators to configure clients to support multiple security options, and the ability to block undesirable networks. In this project you will explore some of the Netsh commands.

112 Chapter 3 Radio Frequency Fundamentals

For this project you will need a computer running Microsoft Windows 7 that has a wireless NIC and can access a wireless LAN.

1. In Microsoft Windows 7, click Start, click All Programs, and then click Accessories.

2. Right-click Command Prompt and then click Run as administrator. This opens the Administrator Command Prompt window in elevated privilege mode.

3. Type netsh and press Enter. The command prompt changes to netsh>.

4. Type wlan and press Enter. The command prompt changes to netsh wlan>.

5. Type show drivers and press Enter to display the wireless NIC driver information. It may be necessary to scroll back towards the top to see all of the information.

6. Next, to view the WLAN interfaces for this computer, type show interfaces and press Enter. Record the SSID value and the name of the profile.

7. To look at the global wireless settings for this computer, type show settings and press Enter.

8. Display all of the available networks to this computer. Type show networks and press Enter.

9. Windows creates a profile for each network that you connect to. To display these pro- files, type show profiles and press Enter. If you see a profile of a network that you no longer use, type delete profile name 5 profile-name, where profile- name is the name of the unused profile, and then press Enter.

10. To disconnect from your current WLAN, type disconnect and press Enter. Note the message you receive, and observe the status in your system tray.

11. Reconnect to your network by typing connect name 5 profile-name ssid 5 ssid- name, where profile-name is the profile name you recorded in Step 6 and ssid- name is the SSID you recorded in Step 6. Press Enter.

12. Now you will use Netsh to block another network. Type show networks and press Enter. Select a network in the list to block. Make sure you are not currently connected to that network, and then record its SSID.

13. Type add filter permission 5 block ssid 5 ssid-name networktype 5 infrastructure, where ssid-name is the SSID you recorded in Step 12. Press Enter.

14. Type show networks and press Enter. Does the network that you blocked in Step 13 appear in the list?

15. To display the blocked network (without allowing access to it), type set blockednetworks display 5 show and press Enter.

16. Type show networks and press Enter. Does the network that you blocked above appear in the list?

17. Click the wireless icon in your system tray. Does the network appear in this list?

Hands-On Projects 113

18. Click again the wireless icon in your system tray. What appears next to the name of the blocked network? Click the network name. What does it say?

19. Now re-enable access to the blocked network by typing delete filter permis- sion 5 block ssid 5 ssid-name networktype 5 infrastructure, where ssid- name is the SSID you recorded in Step 12. Press Enter.

20. Type Exit and press Enter to exit the Netsh utility.

21. Type Exit again and press Enter to close the Administrator Command Prompt window.

Project 3-2: Installing and Using Vistumbler A variety of applications for displaying RF signal strength. In this project you will download and install one such application, Vistumbler.

For this project you will need a computer running Microsoft Windows 7 that has a wireless NIC and can access a wireless LAN.

1. Use your Web browser to go to www.vistumbler.net.

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions then open a search and search for “Vistumbler”.

2. Click Download EXE: Vistumbler vXX (where XX is the latest version).

3. Follow the prompts to download and install Vistumbler on your computer.

4. If the program does not launch after the installation is complete, click Start and then click Vistumbler.

5. Expand the window to full screen.

6. Click Scan APs. If no networks appear, click Interface and then select the appropriate wireless NIC interface.

7. Note the columns Signal and High Signal. Why does the Signal column change?

8. Click View.

9. Click Show Signal dB (Estimated). The columns Signal and High Signal now provide the estimated db. How does it compare to the percentage values?

10. Click Graph 1.

11. Click one of the APs displayed at the bottom of the screen. Allow Vistumbler to accu- mulate data over several minutes. What information is displayed on this graph?

12. Click Graph 2.

114 Chapter 3 Radio Frequency Fundamentals

13. Click another one of the APs displayed at the bottom of the screen. Allow Vistumbler to accumulate data over several minutes. What information is displayed on this graph? How is this different from the previous graph?

14. Click No Graph to return to the previous screen.

15. Leave Vistumbler running for the next project.

Project 3-3: Comparing Vistumbler Information In this project you will compare information accumulated through Vistumbler with Windows 7 Netsh.

1. Return to the Vistumbler window, which you left open at the end of Project 3-2.

2. In the left pane, click the plus sign next to Network Type and then click the plus sign next to Infrastructure.

3. Expand the tree next to the network you are currently attached to. How does this infor- mation compare with the Netsh information from Project 3-1?

4. Note that the default is to display signal strength by percentage. Now we will compare this percentage with the one calculated by Microsoft Windows.

5. Click Start, click All Programs, and then click Accessories.

6. Right-click Command Prompt and then click Run as administrator. This opens the Administrator Command Prompt window in elevated privilege mode.

7. Type netsh and press Enter. The command prompt changes to netsh>.

8. Type wlan and press Enter. The command prompt changes to netsh wlan>.

9. Next view the WLAN interfaces for this computer. Type show interfaces and press Enter. Note the percentage, and compare that with the percentage from Vistumbler. Are they the same?

10. Type Exit and press Enter to exit Netsh.

11. Type Exit again and press Enter to close the Administrator Command Prompt window.

12. Stop Vistumbler by clicking Stop and then File and Exit. If you are asked if you want to save data, click No.

Project 3-4: Installing and Using inSSIDer Another application similar to Vistumbler is inSSIDer. In this project you will download and install the inSSIDer application.

For this project you will need a computer running Microsoft Windows 7 that has a wireless NIC and can access a wireless LAN.

1. Use your Web browser to go to www.metageek.net/support/downloads.

Hands-On Projects 115

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions then open a search engine and search for “inSSIDer”.

2. Click inSSIDer.

3. Click Download. Follow the prompts to download and install the application on your computer.

4. Click Start, click All Programs, click the MetaGeek folder, and then click inSSIDer.

5. From the drop down menu select the appropriate WLAN interface.

6. Click the Start button if insider does not automatically begin scanning.

7. Click the 2.4 GHz Channels tab in the lower portion of the screen. This displays the dB in real time. Now click the Time Graph tab, which displays the dB signal strength as a line graph over time. Compare the two graphs.

8. The inSSIDer application can scan both 2.4 GHz as well as 5.0 GHz. Click the 5 GHz Channels tab to determine if there are any 5 GHz networks in your area. If not, switch back to 2.4 GHz.

9. Select a specific network by clicking it. What happens to the graphs when you select a network?

10. Notice that “RSSI” is displayed in the table at the top of the application. However, the values displayed are identical to the dB values in the graph. Do you think this is actually the RSSI or is it dB? Why? (Hint: Consider if the numbers are negative or positive.)

The source code for inSSIDer says, “On some systems, the bssEntry.RSSI value seems to be inaccurate, so we will base the RSSI value on the link quality”.

11. Compare Vistumbler with inSSIDer. Which gives more detailed information? Which do you prefer? Which application would you recommend to a friend? Why?

12. Close all windows.

Case Projects

Case Project 3-1: Interference on WLANs Unlicensed bands are parts of the radio spectrum that are available nationwide to all users without requiring a license. However, a drawback is that, because there are no licenses, there can be interference between devices. Use the

Internet to identify a list of devices that can interference with WLANs in either the ISM or UNII bands.

116 Chapter 3 Radio Frequency Fundamentals

Case Project 3-2: Impact of Propagation Behaviors Absorption, reflection, scattering, refraction, and diffraction can all have an impact on RF signal strength. Using the Internet, identify at least three objects or types of materials in each of the five categories that can impact RF signals. Next, research how much the impact is in terms of loss (either dBm or mW).

Case Project 3-3: Attenuation Attenuation is not limited to wireless LANs, but can also affect wired LANs as well. Research the types of attenuation that can impact wired networks. How can attenuation be measured on a wired network compared to a wireless network? Which is more difficult to uncover? Why? Write a one-page paper on your findings.

Case Project 3-4: Graphing Digital Modulation Select the ASCII value of a letter from M-Z and create ASK, FSK, and PSK sine graphs showing the transmission of that letter.

Case Project 3-5: Nautilus IT Consulting Nautilus IT Consulting (NITC), a computer technology business that assists organizations in developing IT solutions, has contacted you to help them with their customers.

Washington Heating and Cooling (WHC) has recently moved into a new facility—an older renovated warehouse—and contracted with NITC to install a wireless LAN. However, because of the materials in the older building and other factors, the reception has been spotty. NITC told WHC that additional access points are now needed, but WHC has said that they think NITC simply did a poor installation job to begin with and now wants to add additional equipment beyond the original agreement. WHC is refusing to make the final payment to NITC for work already completed. NITC has asked you to come in as an arbitrator to work through the situation.

1. Create a PowerPoint presentation of eight or more slides that covers the different prop- agation behaviors and their impact upon RF signal strength. Because you will be addres- sing WHC’s senior management, your presentation should not be too technical.

2. After your presentation, WHC had a third-party perform an analysis that showed mW and dBm values. Because of the trust relationship you established with WHC, you’ve been asked to explain those values to WHC management. Create a one-page summary of mW and dBm values that would help WHC management understand those figures.

Case Projects 117

chapter4

Antennas

After completing this chapter you should be able to:

• Explain the different concepts that relate to antennas • List the types of antennas • Describe the antenna coverage patterns • Explain MIMO • List the different antenna measurements

119 Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

To many users, antennas are just one of life’s great mysteries. They know from experience that any antenna is better than having no antenna, and that the higher the antenna is located, the better the reception will be. However, that is virtually all most people know about these strange-looking devices.

Like most cities, San Francisco wrestles with automobile parking. Drivers endlessly circle city blocks—adding more traffic to the roads and more carbon emissions to the air—as they watch and wait for a parking place on the street to open, hoping that they are in the right place at the right time to pull in. Yet after they find a space, only 45 percent of the drivers actually put money into the meter, which results in an annual loss in city revenue of $2,400 per meter. Now, all of that is starting to change, thanks to the help of wireless networks.

San Francisco has contracted with Streetline Networks, a company that sells parking-control systems, to help manage its parking problems. Low-power magnetic sensors about three inches wide are embedded in the street to detect both moving traffic and cars parked in a metered parking spot. Wireless devices are also installed on the parking meters themselves. Data from the sensors and meters are wirelessly transmitted to devices on top of streetlights and traffic-signal boxes to form a wire- less mesh network that sends the information back to the San Francisco parking authorities. This data can be used to identify real-time parking trends, make immedi- ate site-specific parking policies, and create a more focused enforcement effort.

For example, if a particular street has a high volume of traffic and no available parking spots, the San Francisco parking authorities will raise the meter rates up to $6 an hour (the wireless meters can be reprogramed remotely to instantly reflect the new prices). The goal is to encourage drivers to stop circling the block looking for a parking place and instead go to a less-expensive municipal garage or find another neighborhood in which to park. In addition, the system also sends expired-meter data to smartphones carried by police officers, who can go directly to a parking spot that has a car with an expired meter to write a citation (instead of checking each meter to see if time has expired).

San Francisco started this project with a pilot study in which the wireless sensors were placed along one curb mile of about 200 on-street parking spaces. The pilot study identified and ticketed so many drivers who had not “fed the meter” that the system paid for itself within the first two months of operation. The city is now planning to buy 8,000 sensors. It is estimated that San Francisco could generate $40 million in new revenue annually from parking citations, while at the same time improving overall parking performance. Other cities are also looking at installing the wireless Streetline technology.

Real World Wireless

120 Chapter 4 Antennas

Yet the antenna is arguably one of the most important parts of a wireless radio frequency (RF) network. Antennas play a vital role in both sending and receiving signals: A properly positioned and functioning antenna can make all the difference between a wireless LAN oper- ating at peak efficiency or a network that nobody can use.

In this chapter you will learn about antennas. You will start by examining what an antenna is, how it works, and how it can be measured. Then, you will look at the different types of antennas, as well as the various coverage patterns they create. Next, the new technology of using multiple antennas, known as multiple-input multiple-output (MIMO), will be explored. Finally, you will examine the installation of antennas, the types of required equipment, and how the signal can be measured.

Antenna Concepts

C W N A

1.2.1. Understand and apply the basic components of RF mathematics.

1.3.1. Identify RF signal characteristics, the applications of basic RF antenna concepts, and the implementation of solutions that require RF antennas.

1.3.2. Explain the applications of physical RF antenna and antenna system types and identify their basic attributes, purpose, and function.

Several basic concepts about antennas are important to understand. These include knowing what an antenna is, what it does, and understanding the measurements for determining an antenna’s performance.

What Is an Antenna? A conductor is a material that allows an electrical current to flow through it. An alternating current (AC) sent to a conductor generates an electromagnetic field around that conductor. This field will pulse and vary as the AC does. The electromagnetic field will then radiate away from the conductor into space. If another conductor is placed nearby, the electromag- netic field lines that cross this remote conductor will induce on it an electric current that is a copy of the original current.

These conductors can be used as antennas. An antenna is a passive conductor that is used to transmit electromagnetic waves through space. The antenna itself is passive and contains no power or energy to release, but instead relies on the power source to which it is attached. Anten- nas are located at the transition point between the device that creates (or receives) the AC and the air through which the waves are transmitted. A conceptual antenna is shown in Figure 4-1.

Antenna

Source Receiver

Antenna

AC current AC current

Electromagnetic wave

Figure 4-1 Conceptual antenna

Antenna Concepts 121

In telecommunications, transmitting (outgoing) is often abbreviated Tx while receiving (incoming) is Rx.

The simplest antenna is a bare metal wire called a whip antenna. (This type of antenna is found on older automobiles and cellular phones.) Technically, a whip antenna is “a quarter wavelength wire that stands above a ground plane.” The term quarter wavelength indicates that the length of the antenna should be at least one-quarter of the wavelength of the electro- magnetic wave that it is sending or receiving. This means that FM (frequency modulation) radio stations that broadcast at 100 MHz would require an antenna that is 30 inches (75 centimeters) in length. The basic formula for computing the necessary length of a whip antenna in inches is 2952/Frequency (MHz) (or 7500/Frequency (MHz) for centimeters). The term ground plane refers to the area of the electrical ground. All antennas, like any electronic component, have at least two connection points: a connection to a power source and to a ground to form a complete circuit. The electromagnetic field is set up between the whip and the ground plane, with current flowing through the field, completing the circuit. The ground plane should spread out at least a quarter wavelength around the base of the whip. Although the ground plane can be smaller, it will affect the performance of the antenna. A whip antenna is illustrated in Figure 4-2.

After learning of electromagnetic wave experiments through a magazine article, the young Italian engineer Guglielmo Marconi constructed the first transmitter for wireless telegraphy in 1895 using a wire strung between two trees as an antenna. Within two years, he was using this new invention to communicate with ships

at sea. In the early morning of April 15, 1912, a 21-year-old telegrapher in New York City received a wireless message from the wireless station in Newfoundland that had picked up faint SOS distress signals from the steamship Titanic, and the information was promptly spread across the world. This singular event dramatized the importance of this new means of communication.

Quarter wavelength

Ground plane

Whip antenna

Figure 4-2 Whip antenna

122 Chapter 4 Antennas

Antenna Measurements There are several measurements used with antennas. These measurements include Intentional Radiator (IR), Equivalent Isotropically Radiated Power (EIRP), Decibels Isotropic (dBi), and Decibels Dipole (dBd).

Intentional Radiator (IR) The Federal Communications Commission (FCC) labels a system that is used to create and transmit RF signals an intentional radiator (IR). First, it is intentional in that it is specifically designed to send out electromagnetic waves, as compared to other devices that may only create waves as a by-product. (By contrast, an electric motor, which creates RF waves and can even interfere with a wired network, is an unintentional radiator.) Second, it is a radiator designed to radiate out, or send, a signal.

The FCC limits the amount of power that can be generated by an IR. However, when calcu- lating this power, the FCC considers an IR to be all of the components in a system except the antenna.

IR is the power that is directed to the antenna.

Equivalent Isotropically Radiated Power (EIRP) The term isotropic is used to refer to something that has absolute uniformity in all its characteristics in all directions. An isotropic radiator is a source of RF waves that has the exact same magnitude or properties in all directions; that is, because there is no preference in the direction of the radiation, an isotopic radiator sends out its signal uniformly in all directions. A true isotropic radiator is only theoretical and is used as a reference point by which other radiators can be compared.

An isotropic radiator allows for comparisons between different emit- ters regardless of their size or type.

The amount of power that a theoretical isotropic radiator can generate is called the Equiva- lent (also called Effective) Isotropically Radiated Power (EIRP). EIRP is the power radiated out by the wireless system and includes any antenna amplification (gain). As with IR, the FCC limits the amount of EIRP that can radiate from an antenna.

EIRP is the power that is directed from the antenna.

The maximum EIRP for IEEE 802.11b WLANs is 100 mW (20 dBm). The EIRP for 802.11g and 802.11a wireless networks vary by transmission speed and are listed in Table 4-1.

The EIRP for an 802.11n network varies considerably depending upon its configuration.

Antenna Concepts 123

Decibels Isotropic (dBi) Although an isotropic radiator sends its signals out uniformly in all directions, in some instances the RF signal needs to be directed towards a specific receiver, such as in a wireless mesh network. How can this be accomplished? Because an antenna is only a radiator, it is passive and by itself contains no “native” power to release; instead, it must rely upon the power source to which it is attached. If more IR power was sent by the power source to the antenna, the signal would radiate out farther, yet it would radiate out equally in all directions. This means it would take a significant additional amount of power to ultimately reach the receiver and could even interfere with other anten- nas, as shown in Figure 4-3.

The answer to this problem is to send, or focus, the signal in a specific direction towards the receiver. Focusing the signal is known as a passive gain because no additional IR power is added. (If additional IR power was sent to the antenna from the power source this would be an active gain.) If an antenna has a passive gain in one direction, it must have a decrease in the other directions, since no additional energy is being added.

Transmission Speed IEEE 802.11g IEEE 802.11a

24 Mbps and less 50 mW (17 dBm) 40 mW (16 dBm)

36 Mbps 40 mW (16 dBm) 25.1 mW (14 dBm)

48 Mbps 31.6 mW (15 dBm) 20 mW (13 dBm)

54 Mbps 20 mW (13 dBm) 20 mW (13 dBm)

Table 4-1 IEEE 802.11g and the EIRP

Sending antenna (isotropic)

Distance from standard power

Distance from additional power

Receiving antenna

Figure 4-3 Increased power

124 Chapter 4 Antennas

If you were to stand out in a field, look up into the sky, and shout, anyone standing around you could hear your voice. Yet if you were to face one person and cup your hands around your mouth to direct your voice to that person, then others would not be able to hear you clearly if at all. This is the concept behind a passive gain.

This focusing can often be accomplished through the type of antenna. High-gain antennas have longer ranges and higher signal quality, yet must be aimed precisely in a particular direc- tion, whereas low-gain antennas have a shorter range but do not have to be precisely aimed at the receiver. The antenna on a spacecraft that must be directed to a specific point on the earth is an example of a high-gain device; a WLAN antenna is typically a low-gain device.

It is important to use the correct type of antenna in order to meet a network’s needs.

Measuring the passive gain of an antenna is necessary in order to determine if the directed signal can reach its destination or to compare two types of antennas. The passive gain (increase) of power that is “funneled” from an antenna, compared to that of an isotropic radiator sent in all directions, is measured in decibels isotropic (dBi). The dBi is a relative measurement of power, and not an absolute measurement, since it only compares the pas- sive gain with an isotropic radiator. See Figure 4-4.

The dBi is the antenna gain.

Decibels Dipole (dBd) The most basic type of antenna is known as a dipole. A dipole antenna can be made with a single stretched wire that has a connection in the middle and resembles a T shape. The current is fed to the middle of the antenna via the connection and then decreases uniformly from the maximum power at the center out to zero at the ends of the wire. Dipole antennas are the simplest and most practical types of antennas.

Sending antenna (high gain)

Distance from standard power

Receiving antenna

dBi gain

Figure 4-4 dBi gain

Antenna Concepts 125

Dipole antennas were created by Heinrich Hertz in 1886 based on his experimentation on electromagnetic radiation.

Whereas dBi is a relative measurement that compares the passive gain with a theoretical iso- tropic radiator, decibels dipole (dBd) compares the antenna gain against that of a dipole antenna. Because 0 dBd is approximately equivalent to 2.14 dBi (meaning that a standard dipole antenna is 2.14 dBi), to convert from dBd to dBi simply means adding 2.14 to the dBd value. For example, 2.00 dBd ¼ 4.14 dBi.

dBd is the antenna gain compared to a dipole antenna. However, WLAN equipment is rarely measured in dBd; instead, dBi is more commonly used.

Table 4-2 compares the different measurements used with antennas.

Types of Antennas

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1.3.1. Identify RF signal characteristics, the applications of basic RF antenna concepts, and the implementation of solutions that require RF antennas. 1.3.2. Explain the applications of physical RF antenna and antenna system types and identify their basic attributes, purpose, and function.

There are three basic categories of antennas: omnidirectional, semidirectional, and highly directional. Each category then includes multiple types of antennas, each with different charac- teristics for specific applications.

Omnidirectional Antennas The most common type of antenna for a WLAN is an omnidirectional antenna. An omnidi- rectional antenna radiates its signal out horizontally in all directions equally. A user standing 30 feet (9 meters) away on one side of an omnidirectional antenna should receive the same strength signal as a user standing the same distance away on the other side of the antenna.

Name Abbreviation Description Comments

Intentional radiator IR Power directed to the antenna Includes all components except the antenna

Equivalent isotropically radiated power

EIRP Power directed from the antenna

Amount of power that theoretical isotropic radiator can generate

Decibels isotropic dBi Antenna gain Gain compared to theoretical isotropic radiator

Decibels dipole dBd Antenna gain compared to dipole antenna

Rarely used in WLANs

Table 4-2 Antenna measurements

126 Chapter 4 Antennas

Omnidirectional antennas are sometimes used in outdoor WLAN applications, such as in a remote wireless root bridge point-to-multipoint configuration that connects two or more buildings. In this setting, the omnidirectional antenna on the root bridge can transmit simul- taneously to all the other receiving devices located in separate buildings.

Remote wireless bridges are covered in Chapter 2.

Although they can be used in an outdoor configuration, omnidirectional antennas are more commonly used for indoor WLANs. An access point (AP) is usually centrally located in a hallway to cover rooms on each side or in the middle of a warehouse so that the signal is spread equally across the area. Dipole antennas are omnidirectional antennas that are fre- quently used with wireless LANs. A typical dipole antenna for a WLAN is a thin, straight metal rod that is encased in plastic. Figure 4-5 shows a WLAN dipole antenna.

There are three factors to consider with WLAN dipole omnidirectional antennas: horizontal vs. vertical coverage; polarization; and antenna diversity.

Horizontal vs. Vertical Coverage The strength of the signal from an omnidirec- tional antenna flows out 90 degrees from the orientation of the antenna. Thus, even though

Figure 4-5 Dipole antenna

Types of Antennas 127

an omnidirectional antenna radiates its signal out horizontally as well as vertically, it pro- vides less coverage area vertically. If the antenna is in an upright vertical position, most of the signal goes out “sideways” to adjoining rooms. If the antenna is in a horizontal position, “lying flat,” most of the signal goes up to the ceiling and down to the floor.

Suppose a standard omnidirectional antenna is placed in the center of Floor 5 of a multi- story building. Most of its signal will radiate horizontally along Floor 5; only some of the signal will vertically reach Floor 4 and Floor 6 above and below the AP. If a high-gain omnidirectional antenna is placed in the same position, the signal is even “flatter” (that is, more horizontal) and less vertical, meaning that more of the rooms on Floor 5 will receive the signal but less of it will radiate to Floor 4 or Floor 6.

Because WLANs are typically configured to provide the broadest cov- erage areas horizontally to users in multiple rooms on the same floor, the antennas should be positioned vertically (most AP dipoles are hinged so that they can be adjusted). However, if the coverage needs to be increased to accommodate more users on the floors

above and below, the antennas can be adjusted 90 degrees so they are parallel to the floor to provide greater vertical coverage (yet less horizontal coverage).

Polarization The orientation of radio waves as they leave an antenna is known as polar- ization. Waves follow the plane of their electrical fields, and the electric field is parallel to the radiating elements. (The antenna element is the metal part of the antenna that is doing the radiating.) If the antenna is in a vertical position (perpendicular to the ground), then the polarization is said to be vertical; if it is in a horizontal position (parallel to the ground) the polarization is horizontal.

Polarization is typically referred to as being horizontal or vertical, but the actual polarization can be at any angle. Circular polarization is also possible.

Antennas must be polarized alike in order for them to send and receive signals efficiently. An antenna in a horizontal position (that is, a horizontally polarized antenna) will not com- municate well with a vertically polarized antenna, and vice versa.

For an indoor WLAN this is not as significant as for outdoor antennas because as the RF signal bounces off of walls and other indoor objects, the polarization may change. How- ever, for the best reception, a WLAN’s antennas should be in the vertical position, either pointing up to the ceiling or, for a ceiling mounted AP, pointing down to the floor. This allows the polarization of the AP antenna to match polarization of wireless laptops. (Most laptops have antennas built into the sides of the screen so that, when the laptop is opened, the antennas are in the vertical position.) WLAN vertical polarization is shown in Figure 4-6.

As noted above, it is possible to adjust the antennas of the AP 90 degrees so they are parallel to the floor in order to provide greater vertical coverage to floors above and below. However, the reception may be impacted due to polarization.

128 Chapter 4 Antennas

4 Antenna Diversity Many 802.11a/b/g APs have two dipole antennas, as shown in Figure 4-7. Such APs employ antenna diversity to improve the reception. Due to the impact of propagation behaviors (absorption, reflection, scattering, refraction, and diffraction), signals may arrive at the two dipole antennas at different times and with different signal strengths. Instead of being a hindrance, having multiple antennas can offer the AP several “looks” at the same signal, since the signal reaching each antenna will have experienced different interference. After receiving these multiple copies of the same signal, the AP can then select the best signal to use by comparing the signal strengths of the received transmis- sions. Selecting the best signal to use is known as switching.

An AP with multiple external dipole antennas will only use the recep- tion from one antenna.

Antenna diversity can also be used in transmitting as well. Using a technology known as transmit diversity, the AP can transmit on the antenna that most recently received the stron- gest incoming signal.

Semidirectional Antennas Unlike an omnidirectional antenna, which evenly spreads the signal in all directions, a semi- directional antenna focuses the energy in one direction. Generally, a semidirectional antenna will only transmit its signal no more than in a half-circle (180 degrees). For outdoor applica- tions, semidirectional antennas can be used for short- and medium-range remote wireless bridge networks, such as in a point-to-point configuration in which two buildings are con- nected through a wireless network.

A special type of semidirectional antenna used outdoors is known as a sectorized antenna. As its name suggests, a sectorized antenna divides the coverage area into different sectors and

Laptop Laptop

Access point

Figure 4-6 WLAN vertical polarization

Figure 4-7 AP with two dipole antennas

Types of Antennas 129

gives each sector its own radiation pattern, thus improving coverage. For example, a sectorized antenna could divide the entire 360 degrees of coverage area into nine sectors of 40 degrees each. Each sector would then have its own sectorized pattern, as shown in Figure 4-8. A varia- tion of a sectoried antenna is an antenna array, which is comprised of multiple antennas. Yet unlike a sectoried antenna, the power to each antenna in an antenna array does not have to be the same. Instead, different power levels or phase distributions to the individual antennas can be used to customize the transmitted signal.

At an outdoor auto auction of over 300 acres of cars, “walking auctions” allowed the auctioneer and the buyers to move from car to car to place bids and then send that information to the main office via WLAN. Complaints arose that the outdoor WLAN’s omnidirectional antennas were not sufficient. Instead of installing more poles with

additional APs and electrical power throughout the area, sectorized antennas with an increased dBi gain were used to replace the omnidirectional antennas in order to increase the coverage area.

For indoor use, a semidirectional antenna can be used when a group of users are clustered closely together and a broad coverage area is not needed. However, semidirectional antennas generally are not commonly used for indoor WLANs.

Highly-Directional Antennas Highly-directional antennas send a narrowly focused signal beam long distances. These antennas are generally reflective devices that are shaped like a concave dish (that is, like a parabola). The reflector creates a large surface area that the antenna uses to receive and transmit signals, as shown in Figure 4-9. These antennas are used for outdoor long-distance point-to-point wireless links, such as connecting buildings that are up to 25 miles (40 kilo- meters) apart. Highly-directional antennas are not used for indoor WLANs.

Many Web sites contain instructions for modifying a common large wire-mesh Chinese vegetable strainer to connect with a Universal Serial Bus (USB) wireless network interface card adapter to create a homemade parabolic antenna!

Figure 4-8 Sectorized antenna

130 Chapter 4 Antennas

Antenna Coverage Patterns

C W N A

1.3.1. Identify RF signal characteristics, the applications of basic RF antenna concepts, and the implementation of solutions that require RF antennas.

Knowing that an antenna in an upright vertical position transmits most of the signal horizon- tally while only part of the signal radiates above and below the antenna is helpful, yet more precision is needed to determine the best type of antenna and antenna location for the opti- mum coverage area. These antenna coverage patterns involve the Azimuth and elevation, beamwidth, and the Fresnel zone.

Azimuth and Elevation An antenna radiation chart is used to illustrate the radiation pattern of an antenna. See Figure 4-10. They are based on what is called a polar or radar chart that illustrates the RF signal. Antenna radiation charts offer a precise picture of the radiation pattern of an antenna. An explanation of how to interpret these charts is as follows:

● Antenna location. The center of the chart is the location of the antenna. ● Degrees. The degree symbols (from 0 to 360 degrees) along the outer ring of the

chart indicate the coverage pattern around the antenna in degrees. For example, an omnidirectional antenna would cover a larger portion of the chart than a highly- directional antenna.

● Outer circle. The outer circle represents a 100-percent gain.

Figure 4-9 Highly-directional antenna

Antenna Coverage Patterns 131

● Inner circles. The inner circles represent a gain that is less than 100 percent. For example, a pattern that does not extend fully to the outer circle yet only extends to the –3 dB mark indicates only a 50-percent gain, while a pattern that extends to the –10 dB mark indicates only a 10-percent gain.

The coverage patterns in an antenna radiation chart do not include antenna gain. In addition, these charts do not represent power levels or distance in feet (or meters) of coverage.

A radiation pattern has both a horizontal as well as a vertical coverage area, and these are different. To account for these differences, separate antenna radar charts are used to illus- trate each. The Azimuth chart represents the horizontal coverage area, while the elevation chart is used to show the vertical coverage area. That is, the Azimuth chart shows the signal pattern from looking down from the ceiling at a vertically polarized antenna, while the ele- vation chart illustrates the signal pattern looking at the same antenna from a side view. Figure 4-11 illustrates the radiation pattern for an omnidirectional dipole antenna in 3D and radiation charts in Azimuth (RF projecting out from the antenna) and elevation (RF projecting up and down from the antenna). Note that the Azimuth horizontal coverage area is uniform (because it is an omnidirectional antenna) whereas the area of the elevation coverage is in two lobes that extend out from the antenna. By contrast Figure 4-12 shows the same three illustrations with a semidirectional antenna, where the Azimuth pattern is not uniform in all directions but instead is more focused in one direction, as is the elevation.

–3

–10

30330

300

270

240

210

180

150

120

–30

–40

–50

–20

Figure 4-10 Antenna radiation chart

132 Chapter 4 Antennas

Table 4-3 lists the maximum degrees of coverage of Azimuth and elevation charts for the three types of antennas.

Beamwidth An antenna’s beamwidth indicates how narrow or wide the transmission is. Beamwidth is considered a measure of the antenna’s “half-power” range. The beamwidth is determined by first locating the peak radiation intensity, and then locating the points on either side of the peak that represent half the power (–3 dB) of the peak intensity. The distance between the half-power points is the beamwidth. Both horizontal and vertical beamwidths can be mea- sured, as illustrated in Figure 4-13.

60120

150

180

210

240

270

Dipole Azimuth Plane Pattern

Phi 5 90

Phi 5 270 300

330

–20 –10

(dB)

60120

150

180

150

120

Dipole Elevation Plane Pattern

–20 –10

(dB)

Dipole 3D Radiation Pattern

0 0

Figure 4-11 Radiation patterns for omnidirectional antenna

Antenna Coverage Patterns 133

Because half the power expressed in decibels is –3 dB, the beam- width is sometimes referred to as the 3 dB beamwidth.

60120

150

180

150

120

Semidirectional Antenna Azimuth Plane Pattern

Phi 5 0

Phi 5 180

(dB)

Semidirectional Antenna Elevation Plane Pattern

Phi 5 90

Phi 5 270

60120

150

180

150

120

–20 –10 0

(dB)

Semidirectional Antenna 3D Radiation Pattern

z y

–20 –10

Figure 4-12 Radiation patterns for semidirectional antenna

Antenna Type Maximum Azimuth

(degrees) Maximum Elevation

(degrees)

Omnidirectional 360 90

Semidirectional 135 65

Highlydirectional 16 21

Table 4-3 Maximum Azimuth and elevation

134 Chapter 4 Antennas

Fresnel Zone Suppose you were to stand outside and look off into the distance to see your friend. This would require that there were no large objects in the path of your vision—such as trees, large buildings, hills and mountains, or even the curvature of the earth—to obscure your friend. In other words, you would need a visual line of sight from where you stood to see your friend.

The earth curves approximately 8 inches every mile, so an object 100 feet tall would not be visible 15 miles away.

For outdoor RF transmissions, a clear path is likewise necessary between the antennas. How- ever, this RF line of sight is more demanding than a visual line of sight. With visual line of sight, it might be possible to look through a line of trees or other objects to see your friend in the distance. However, with RF line of sight, an object that is even close to the path of the RF transmission—although not entirely obscuring the RF line of sight—can still impact the signal. A “hard” object like a building protruding into the signal path can deflect part of the signal and cause it to reach the receiving antenna out of phase, thus reducing the power or cancelling out the signal. Leaves or other “soft” objects protruding into the path can reduce the strength of the signal.

Even though you can see the receiving antenna, you do not necessar- ily have a good quality RF link to that location.

The Fresnel zone (fre-NEL) is an elliptical area immediately surrounding the visual line of sight for RF transmissions and is shown in Figure 4-14. Each Fresnel zone (theoretically there are an infinite number of zones between two antennas) is an ellipsoidal or sausage-like shape. The zone varies in thickness, depending on the length of the signal path and signal frequency. The signal strength is strongest in Zone 1, which is a straight line from sender to receiver. Sig- nal strength decreases in each successive zone. Obstacles in the first Fresnel zone will create signals that will be 0 to 90 degrees out of phase, in the second zone they will be 90 to 270 degrees out of phase, in third zone, they will be 270 to 450 degrees out of phase, etc.

–3 dB

–3 dB–3 dB

Figure 4-13 Beamwidth

Antenna Coverage Patterns 135

Odd-numbered Fresnel zones are considered constructive while even- numbered zones are destructive.

Determining where the zones are located, the necessary clearance for Zone 1, and the extent of the impact of objects can be done through calculations. For example, when sending a signal 5 miles (8 kilometers) with an obstruction at 2 miles (3.2 kilometers) at 2.4 GHz will result in Zone 1 being 104 feet (31 meters) at its widest point. The general rule of thumb is that 60 percent of the first Fresnel zone must be clear of obstacles. In this example the 60-percent radius of the no-obstacles area would be 31 feet (9.6 meters).

Although the Fresnel zone could be calculated by hand, several Web sites have different calculators you can use to determine different RF behaviors such as the Fresnel zone. In Hands-On Projects 4-1 and 4-2 you will use these online calculators.

The steps for mitigating obstructions in the Fresnel zone include: ● Raising the antenna mounting point on the existing structure ● Increasing the height of an existing tower ● Building a new structure that is tall enough to mount the antenna ● Locating a different mounting point on another building or tower for the antenna ● Cutting down trees

Multiple-Input Multiple-Output (MIMO)

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1.3.1. Identify RF signal characteristics, the applications of basic RF antenna concepts, and the implementation of solutions that require RF antennas.

2.1.2. Comprehend the differences between, and explain the different types of, spread-spectrum technologies and how they relate to the IEEE 802.11-2007 standard’s (as amended and including 802.11n-draft2.0) PHY clauses.

4.2.1. Identify and explain how to solve the following WLAN implementation challenges using features available in enterprise class WLAN equipment.

Transmitting antenna

Receiving antenna

Zone 1

Zone 2

Zone 3

Figure 4-14 Fresnel zone

136 Chapter 4 Antennas

A technology known as multiple-input multiple-output (MIMO) has revolutionized wireless LAN as well as wireless WAN communications over the past decade. It is important to under- stand this new technology and its signal processing techniques.

What Is MIMO? It has long been known that a wireless device with multiple receive (Rx) antennas can dra- matically improve wireless transmissions by either selecting the stronger incoming signal or combining the individual signals at the receiver. However, IEEE 802.11a/b/g devices can only transmit or receive on a single antenna at a given time, no matter how many antennas they may have. A wireless system that uses a single antenna is called a single-input single- output (SISO) system. A SISO system is characterized by having only one radio chain, or a radio with supporting infrastructure such as devices to amplify the signal or convert an ana- log signal into a digital signal. A SISO radio chain is shown in Figure 4-15.

In the mid-1990s, research predicted that a significant improvement in RF performance could be achieved by not only having multiple antennas but actually using them simultaneously to transmit and receive. This research, which soon led to new wireless systems that utilized mul- tiple simultaneous antennas, was called Multiple-Input Multiple-Output (MIMO). MIMO is characterized by having a radio chain for each antenna, as illustrated in Figure 4-16.

Using multiple antennas at the receiver and transmitter has revolu- tionized today’s wireless communications. Most high-rate wide-area wireless systems, such as 4G mobile phone technologies like Long Term Evolution (LTE) and WiMAX, use MIMO technologies.

Because of its advantages, the IEEE made MIMO the heart of 802.11n. The speed of an 802.11n wireless network can be as much as 600 Mbps using MIMO and other enhance- ments. (By some estimates MIMO alone contributes 40 percent to the increase in speed.)

Tx Rx

Processor Radio

Processor Radio ProcessorRadio

ProcessorRadio

Figure 4-16 MIMO radio chain

Tx Antennas Rx Antennas

Input

Radio chain

Processor Radio Output

Radio Processor

Figure 4-15 SISO radio chain

Multiple-Input Multiple-Output (MIMO) 137

For this reason the 802.11n standard is sometimes called HT (MIMO) for High Throughput Multiple-Input Multiple-Output.

Typically, 802.11n APs and internal Mini-PCI-e wireless client network interface card adapters have a maximum of three radio chains, while external adapters have at most two radio chains. Mobile devices, like smartphones, usually have only one radio chain.

MIMO Signal Processing Techniques MIMO can take advantage of the following signaling processing techniques to create high throughput: spatial diversity, spatial multiplexing, maximal ratio combining, and transmit beam forming.

Spatial Diversity An RF signal does not take a direct path straight from the transmitter to the receiver. Instead, multiple copies of the signal are transmitted, and these various copies may bounce off objects in the area before reaching the receiver at slightly different times. The result is multiple copies of the signal arriving at the receiver at different times, having traveled along different paths (multipath). Although the difference between the signals, known as delay spread, is measured in nanoseconds, it can negatively impact the reception because these cop- ies are “added” to the primary signal. This can result in downfade, corruption, or nulling.

Multipath and delay spread are covered in Chapter 3.

Due to the advantages of spatial diversity, what if an intentional delay was inserted into a copy of the transmission? This is the idea behind Cyclic Shift Diversity (CSD). CSD sends a “normal” version of the signal transmitted on one Tx antenna while a “shifted” version of the same signal is sent by one or more Tx antennas. Each Tx antenna also adds a prefix after shifting the symbol so that the receiving device is aware of the shift. Now what if, instead of a single transmission being sent from one antenna to another antenna, the same transmission was sent simultaneously from multiple antennas and was received on multiple antennas? Each transmission would experience a different multipath, yet there would be a high probability that, while some signals would undergo deep “fades,” others would not. Having diverse signals sent from multiple antennas to multiple antennas could improve the overall reliability of the transmission. That is why multiple antennas are used to reduce the effect of multipath and delay spread.

This technique for combating multipath by using multiple Tx and Rx antennas is called spatial diversity (sometimes called MIMO diversity). Spatial diversity can increase the reliability of an RF signal by sending the same transmission out from different antennas. Each transmission will take different paths (called spatial paths) between Tx and Rx antennas. (The signals take diverse paths because the antennas are spaced apart.) Using these different paths improves reliability because it is unlikely that all of the paths will be degraded in the same way. Spatial diversity, which sends redundant streams in parallel, can also improve the range because it allows a larger amount of signal to gather at the Rx antenna.

138 Chapter 4 Antennas

Spatial diversity is like sending three disaster relief trucks, all filled with the same cargo, to victims of a flood. If all three trucks left the warehouse at the same time, yet took different routes to the destina- tion through the disaster area, the odds are high that at least one of the trucks will be able to make it through.

Spatial Multiplexing Although spatial diversity can improve reliability and range, it does not increase the speed of the wireless transmission. Instead of sending the same trans- mission out through multiple Tx antennas, what if the data was first split up and then sent out over multiple Tx antennas, with the receiver merging it back together? That is, instead of sending redundant streams in parallel, what if the antenna sent independent streams in parallel? This would significantly increase the speed: three Tx antennas sending three streams to three Rx antennas could result in a three-fold increase.

This technique is called spatial multiplexing. Spatial multiplexing techniques can increase performance by sending independent streams of information at the same time over the same fre- quencies. The advantage of spatial multiplexing is a significant increase in speed without the need for any additional power or bandwidth. Spatial multiplexing is illustrated in Figure 4-17.

Due to space and power constraints, wireless network interface card adapters have fewer antennas whereas APs have more antennas to ensure higher performance. In contrast, mobile devices such as cell phones and tablets usually have only one WLAN radio chain with one antenna.

MIMO and spatial multiplexing are used in 802.11n networks. A specific numbering system is used to indicate the number of Tx antennas, Rx antennas, and spatial multiplexed streams. For example, an 802.11n wireless device designated as 2x3:2 is interpreted as follows:

● 2—maximum number of transmit (Tx) antennas that can be used by the radio ● 3—maximum number of receive (Rx) antennas that can be used by the radio ● 2—maximum number of data spatial streams the radio can use

This means that the radio in this wireless device can transmit on two antennas and receive on three antennas but can only send or receive two data streams.

Common configurations of 802.11n devices are 2x2:2, 2x3:2, 3x3:2, and 3x3:3. Although the IEEE standard allows up to 4x4:4, improve- ments beyond 3x3 are small. Generally 3x3:3 APs are the configura- tion for high-end 802.11n networks while most clients are 2x3:2.

Data to be sent

Data received

Tx Rx

b1 b2 b3 b4 b5 b6

b1 b3 b5

b2 b4 b6

b1 b3 b5

b2 b4 b6

Split

Radio

Processor

Radio

Processor

Merge

Figure 4-17 Spatial multiplexing

Multiple-Input Multiple-Output (MIMO) 139

And if more antennas than spatial streams are available, such as 3x3:2, Space Time Block Coding (STBC) can be used. STBC sends a redundant copy of part or all of the transmited signal on the unused antenna(s). This can increase the reliability of the signal at the receiver and also reduces the Signal-to-Noise ratio (SNR) error rate. STBC is an optional feature in the 802.11n standard.

Maximal Ratio Combining (MRC) When a client device with only one antenna (such as an older 802.11a/b/g device) transmits to an 802.11n AP with multiple MIMO antennas, the AP will receive multiple copies of the multipath signal on each of its Rx antennas, with each sig- nal at a different phase. The AP then processes the multiple received signals into one “reinforced” signal by adjusting their phases and amplitudes to form the best possible signal. The algorithm that the 802.11n AP uses to do this is called the maximal ratio combining (MRC) algorithm. MRC multiplies each received signal by a weight factor that is proportional to the signal ampli- tude, so that a strong signal is further amplified while weaker signals are not used (although in some cases, information from weaker signals may be combined with the stronger signal).

MRC helps the 802.11n MIMO AP to hear the client better, yet it does not improve the reception of transmissions sent to the non- MIMO client from the AP.

Transmit Beam Forming (TxBF) An option specified by the IEEE for 802.11n net- works to improve the reliability of transmissions by reducing outside signal interference (noise) is transmit beam forming (TxBF). TxBF can use different directions and beamwidths, which can be generated either by the microprocessor chip attached to the radio chains (called chip- based beamforming) or by the antenna. Antenna beamforming can be static (a fixed radiation pattern is used) or dynamic (the radiation pattern can change for each transmitted frame).

TxBF utilizes a process in which the sending device codes or assigns weights to the signals before sending them. These weights depend on the transmitter’s best estimate of the environ- ment. The information required to make this estimate can be obtained through implicit or explicit feedback. With explicit feedback, the receiver makes a series of computations and sends them to the transmitter, which then uses them to configure how to make the best transmissions. With implicit feedback, the transmitter assumes that the environment is the same in both directions and creates the information by tracking incoming information.

Although TxBF is part of the IEEE 802.11n standard, it is highly com- plex and not widely implemented in 802.11n devices.

Antenna Installation

C W N A

1.1.1. Define and explain the basic concepts of RF behavior.

1.2.1. Understand and apply the basic components of RF mathematics.

1.3.3. Describe the proper locations and methods for installing RF antennas.

1.4.1. Identify the use of the following WLAN accessories and explain how to select and install them for optimal performance and regulatory domain compliance.

140 Chapter 4 Antennas

Although most 802.11a/b/g APs have dual external dipole antennas, like those pictured earlier in Figure 4-7, most new 802.11n APs only have internal antennas. However, the use of MIMO spatial multiplexing can still extend the range of these devices beyond that of an 802.11a/b/g device with antennas.

The antenna spacing for internal antennas needs to be greater than half the wavelength, which means the internal antennas are spaced at 4.9 inches (12.5 centimeters) for 2.4 GHz or 2.3 inches (6 centi- meters) for 5 GHz.

Some enterprise-grade APs have connections for attaching an external antenna, and external antennas are used for wireless MANs and WANs. Installing an external antenna involves positioning it at the best location, using the correct installation accessories, and measuring antenna performance.

Location Because WLAN systems use omnidirectional antennas to provide the broadest area of cover- age, APs should be located near the middle of the coverage area. Generally, the AP can be secured to the ceiling or high on a wall. It is recommended that APs should be mounted as high as possible for two reasons: to avoid obstructions for the RF signal and to deter thieves from stealing the device.

In buildings with a false ceiling (also called a drop or suspended ceiling), there is a temptation to simply remove a ceiling tile, place the AP in the space above the ceiling, and then replace the tile. However, this should not be done unless a special enclosure surrounds the AP and its antennas. The air-handling space above drop ceilings (and sometimes even between the walls and under structural floors) is used to circulate and otherwise handle air in a building. These spaces are called plenums. Placing an access point in a plenum can be a hazard, because if an electrical short in the AP were to cause a fire, it could generate smoke in the plenum that would be quickly circulated throughout the building. If it is necessary to place an AP in a plenum, it is important to place it within a special plenum-rated enclosure to meet fire safety code requirements.

Outdoor antennas are usually affixed to a pole or mast, which serves the dual purpose of improving reception and discouraging thieves. Antenna-mounting systems are available that allow you to secure the antenna, with the option of unlocking it quickly to lower the antenna for repair or maintenance.

Safety should always be considered when installing an outdoor antenna, particularly when the antenna is to be mounted high on a pole or a rooftop. These outdoor installations gener- ally require long ladders, trips to high rooftops, or climbing towers and can be dangerous due to the heights and the need to work with electrical and RF equipment. It is recommended that outdoor antenna installation be left to professionals.

The FCC has ruled that local municipalities, cities, or neighborhood groups cannot impose restrictions on installations of 802.11 WLAN products on property controlled by a user, except where public safety is a concern.

Antenna Accessories A number of devices can “accessorize” an antenna by providing additional functionality and safety. Often a transmission problem can be resolved by adding one of these accessories to

Antenna Installation 141

the antenna system. These accessories include amplifiers, attenuators, RF cable and connec- tors, and lightning arrestors.

Amplifier An amplifier is a device that amplifies or increases the amplitude of an RF sig- nal. Often it is necessary to “boost” the strength of a signal to compensate for its loss of power. This loss may be the result of the distance between the AP and the wireless device, or it could be due to a loss from the cable connecting a wireless device to its antenna (as when an external antenna is attached to an AP).

Amplifiers can be of two types. A unidirectional amplifier increases the RF signal level before it is injected into the transmitting antenna. A bidirectional amplifier boosts the RF signal before it is injected into a device that contains or is directly connected to the antenna, such as an AP. Most amplifiers for WLANs are bidirectional.

In the past, amplifiers used in 802.11 WLAN bands had to be certi- fied by the Federal Communications Commission (FCC) and mar- keted, sold, and installed as a package containing the amplifier, the transmitter, any specific cable, and the specific antenna. All the pro- ducts had to be sourced from a single vendor and in the configura-

tion in which they were certified; any changes to the package required a new certification. However, that is no longer the case. It is now permissible to purchase amplifiers and devices from separate sources as single entities. However, the amplifier must be certified with the spe- cific transmitter and antenna it will be installed with and it must also follow the same EIRP limits as the transmitter itself.

Attenuators Whereas an RF amplifier increases the signal prior to transmission, an RF attenuator decreases the RF signal. An RF attenuator may be used when the gain of an antenna did not match the power output of an AP (in order to fully comply with FCC regu- lations regarding power output of a WLAN) or for testing purposes.

RF attenuators can be either fixed-loss attenuators or variable-loss attenuators. Fixed-loss attenuators limit the RF power by a set amount, whereas variable-loss attenuators allow the user to set the amount of loss. Fixed-loss attenuators are the only type permitted by the FCC for WLAN systems.

RF Cables and Connectors Connecting antennas, amplifiers, and attenuators to an AP or wireless device requires the correct cables and connectors. Some basic rules for select- ing cables and connectors include:

● The connector should match the electrical capacity of the cable and device to which it is connected, along with the type and gender of connector.

● Only high-quality connectors and cables from well-known suppliers should be used. ● Cable lengths should be as short as possible. ● Cables should match the electrical capacity of the connectors. ● Whenever possible, you should purchase premanufactured cables rather than cutting

and splicing them together. ● Use RF signal splitters sparingly. An RF signal splitter is a small device with one input

and two or more outputs, meaning that a splitter divides the power in the input signal

142 Chapter 4 Antennas

to multiple outputs. Whenever an RF signal is split, its original strength decreases by one-half.

Because 802.11n uses MIMO, you cannot use an RF signal splitter on 802.11n equipment.

Lightning Arrestor Just as an antenna is designed to pick up RF signals, it also can inadvertently pick up high electrical discharges from a nearby lightning strike (or contact with a high-voltage electrical source). A lightning arrestor limits the amplitude and disturb- ing interference voltages by channeling them to the ground.

A lightning arrestor will not protect equipment from a direct light- ning strike.

A lightning arrestor, illustrated in Figure 4-18, is designed to be installed between the antenna cable and the wireless device. One end of the lightning arrestor is connected to the antenna while the other end connects to the wireless device. The ground lug is connected to a cable that is grounded. If the arrestor is installed outdoors, the cable should be connected to a ground rod, which is a metal rod inserted in the earth. Indoors, the cable should be connected to the structural steel of the building or to a grounded electrical panel.

Measuring Antenna Performance Several measurements relate to the performance of RF transmissions from antennas. These include link budget, system operating margin (SOM), and voltage standing wave ratio (VSWR).

Link Budget Figure 4-19 illustrates the gains ( þ ) and losses (–) in power from the trans- mitter to the receiver known as the link budget. The link budget is a rough calculation of all known elements of the link between the various wireless components to determine if the sig- nal will have the proper strength when it reaches its destination. To make an accurate link budget calculation, you need the following information:

● Antenna gain ● Free space path loss

Connects to grounded cable

Connects to antenna

Connects to

wireless device

Figure 4-18 Lightning arrestor

Antenna Installation 143

● Frequency of the link ● Loss of each connector at the specified frequency ● Number of connectors used ● Path length ● Power of the transmitter ● Total length of transmission cable and loss per unit length at the specified frequency

System Operating Margin (SOM) The system operating margin (SOM) is the dif- ference (measured in decibels) between the received signal level and the signal level that is required by that radio to assure that the transmission can be decoded without errors. (The SOM is also referred to as the fade margin.) The SOM is the difference between the signal received and the radio’s specified receiver sensitivity (the signal needed for a good reception). The formula is SOM ¼ Rx signal (dBm) � Rx sensitivity (dBm).

System operating margin (SOM) is the difference between “what you get” and “what you need.”

Voltage Standing Wave Ratio (VSWR) The term impedance is applied to any electrical entity that impedes or hinders the flow of current. Often impedance is a result of a mismatch between two devices. The Voltage Standing Wave Ratio or VSWR (pronounced viswar) is a measure of how well an electrical load is impedance-matched to its source. A perfect impedance match is the maximum power transferred from the source, yet in reality that will not occur.

The value of VSWR is expressed as a ratio, with 1 as the denominator, such as 2:1, 3:1, and 10:1. A perfect impedance match of the maximum power transferred from the source corre- sponds to a VSWR of 1:1. When the VSWR is indicated in dB it is called the return loss. Table 4-4 lists a comparison of VSWR and return loss values.

Tx antenna gain (1) Rx antenna gain (1)

Transmitter Receiver

Tx cable (2)

Connector (2) Connector (2)

Rx cable (2)

Free space path loss (–)

Figure 4-19 Link budget

144 Chapter 4 Antennas

For WLANs, if one part of the equipment has different impedance than another part, the RF signal may be reflected back within the device itself. Not only does this cause a loss of signal strength, the reflected power can actually burn out the device’s electronics.

Chapter Summary ■ An antenna is used to send electromagnetic waves to a receiving device. The antenna

has no energy or power; instead, it is supplied power from the power source to which it is connected. Four measurements are used with antennas. An intentional radiator (IR) is the power directed to the antenna. The Equivalent Isotropically Radiated Power (EIRP) is the power directed from the antenna. Antenna energy can be focused through the use of high-gain antennas. By contrast, low-gain antennas have a shorter range but do not have to be precisely aimed at the receiver. The passive gain (increase) of power that is “funneled” from an antenna compared to that of an isotropic radiator sent in all directions is measured in decibels isotropic (dBi). The dBi is a relative mea- surement and is not an absolute measurement of power, since it is only comparing the passive gain with an isotropic radiator. Decibels dipole (dBd) compares the antenna gain against that of a dipole antenna.

■ There are three categories of antennas. An omnidirectional antenna radiates its signal out horizontally in all directions equally and is used extensively in indoor WLANs. This type of antenna also radiates its signal vertically but provides less coverage area vertically than horizontally. That is because the strength of the signal flows out 90 degrees from the orientation of the antenna. The orientation of the radio waves as they leave the antenna is known as polarization. If the antenna is in a vertical position (perpendicular to the ground) the polarization is vertical, whereas if it is in a horizon- tal position parallel to the ground the polarization is horizontal. Many have two dipole antennas and employ antenna diversity to improve the reception. A semidirec- tional antenna focuses the energy in one direction, while a highly-directional antenna sends a narrowly focused signal beam long distances.

■ A chart used to illustrate the radiation pattern of an antenna is called an antenna radiation chart. These charts are used to create Azimuth charts that represent the hor- izontal coverage area and elevation charts to show the vertical coverage area. An antenna’s beamwidth indicates a transmission’s width. The Fresnel zone is an elliptical area immediately surrounding the visual line of sight for RF transmissions. Each

VSWR Return Loss (dB)

1.0202 40

1.0220 39

1.0245 38

1.0285 37

Table 4-4 VSWR and return loss values

Chapter Summary 145

Fresnel zone is an ellipsoidal shape, with the zones varying in thickness depending on the length of the signal path and signal frequency.

■ A technology known as multiple-input multiple-output (MIMO) has revolutionized wireless LAN as well as wireless WAN communications over the past decade. MIMO systems have separate radio chains connected to each antenna. MIMO can take advantage of different signaling processing techniques in order to create high through- put. Spatial diversity can increase the reliability of an RF signal by sending the same transmission out from different antennas that will take different paths between Tx and Rx antennas. Using these different paths improves reliability because of the unlikel- hood that all of the paths will be degraded the same. Spatial multiplexing techniques can increase performance by sending independent streams of information at the same time over the same frequencies, resulting in a significant increase in speed without any additional power or additional bandwidth. When a client device that has only one antenna such as an older 802.11a/b/g device) transmits to an 802.11n AP with multi- ple MIMO antennas, the AP receives multiple copies of the multipath signal on each of its Rx antennas, with each signal at a different phase. The AP then processes those copies using the maximal ratio combining (MRC) algorithm. An option for 802.11n networks to improve the reliability of transmissions by reducing outside signal inter- ference (noise) is transmit beam forming (TxBF), which uses complex antenna systems to allow for different directions and beamwidths to be used.

■ Because WLAN systems use omnidirectional antennas to provide the broadest area of coverage, APs should be located near the middle of the coverage area, usually secured to the ceiling or high on a wall. Outdoor antennas are often affixed to poles or masts. It is recommended that outdoor antenna installation be left to profes- sionals. An amplifier is a device that amplifies or increases the amplitude of an RF signal, often necessary to “boost” the strength of a signal to compensate for its loss of power. An RF attenuator decreases the RF signal. An RF signal splitter is a small device with one input and two or more outputs that divide the power in the input signal to multiple outputs. Just as an antenna is designed to pick up RF signals, it also can inadvertently pick up high electrical discharges from a nearby lightning strike. A lightning arrestor limits the amplitude and disturbing interference voltages by channeling them to the ground.

■ There are several measurements that relate to the performance of RF transmissions from antennas. The link budget is a rough calculation of all known elements of the link to determine if the signal will have the proper strength when it reaches its desti- nation. The system operating margin (SOM), or fade margin, is the difference, mea- sured in decibels, between the received signal level and the signal level that is required by that radio to assure that the transmission can be decoded without errors. The Volt- age Standing Wave Ratio or VSWR is a measure of how well an electrical load is impedance-matched to its source.

Key Terms active gain The gain in which additional power was sent to the antenna from the power source. amplifier A device that amplifies or increases the amplitude of an RF signal.

146 Chapter 4 Antennas

antenna array Multiple antennas that can be customized to send an optimal signal. antenna diversity The ability of an access point to examine multiple copies of a received transmission and then select the best signal. antenna radiation chart A chart used to illustrate an antenna’s radiation pattern. antennas A passive conductor used to transmit electromagnetic waves through space. Azimuth chart A chart that represents the horizontal coverage area of an antenna. beamwidth A measurement of a transmission’s width. bidirectional amplifier A device that increases the RF signal before it is injected into the device that contains or is directly connected to the antenna. conductor A material that allows an electrical current to flow through it. Cyclic Shift Diversity (CSD) A technique that sends a normal version of the signal along with a shifted version of the same signal. decibels dipole (dBd) A measurement that compares the antenna gain against that of a dipole antenna. decibels isotropic (dBi) The passive gain of power that is funneled from an antenna compared to that of an isotropic radiator sent in all directions. dipole An antenna consisting of a single stretched wire with connection in the middle. elevation chart A chart that represents the vertical coverage area of an antenna. Equivalent (also called Effective) Isotropically Radiated Power (EIRP) The amount of power that a theoretical isotropic radiator can generate. explicit feedback A TxBF technique in which the receiver makes a series of computations and sends them to the transmitter, which then uses them to configure how to make the best transmissions. fade margin The difference between the received signal level and the signal level that is required by that radio to assure that the transmission can be decoded without errors. fixed-loss attenuator An RF attenuator that limits the RF power by a set amount. Fresnel zone An elliptical area immediately surrounding the visual line of sight of an RF transmission. ground rod A metal rod inserted in the earth to ground an antenna. high-gain antennas Antennas that have longer ranges and higher signal quality than low-gain antennas yet must be aimed precisely in a particular direction. highly-directional antenna An antenna that sends a narrowly focused signal beam long distances. HT (MIMO) High Throughput Multiple-Input Multiple-Output for IEEE 802.11n WLANs that utilize a radio chain for each antenna. implicit feedback Information that is computed by the receiver and sent back to the transmitter for use in antenna configuration. intentional radiator (IR) A system used to create and transmit RF signals as defined by the Federal Communications Commission (FCC). isotropic radiator A source of RF waves that have the exact same magnitude or properties in all directions. lightning arrestor A device that limits the amplitude and disturbing interference voltages by channeling them to the ground.

Key Terms 147

link budget A rough calculation of all known elements of a link to determine if a signal will have the proper strength when it reaches its destination. low-gain antennas Antennas with a shorter range than high-gain antennas that do not have to be precisely aimed at the receiver. maximal ratio combining (MRC) The algorithm a MIMO AP uses when it receives multiple copies of a signal from a non-MIMO device. MIMO diversity A MIMO technique of sending the same transmission out on different paths from different antennas. Multiple-Input Multiple-Output (MIMO) A system that uses one radio chain for each antenna so that each antenna can simultaneously transmit and receive signals. omnidirectional antenna An antenna that radiates its signal out horizontally in all directions equally. passive gain The gain in which no additional power is added. plenum The air-handling space above drop ceilings that is used to circulate and handle air in a building. polarization The orientation of radio waves as they leave an antenna. radio chain A radio with supporting infrastructure such as devices to amplify the signal or convert an analog signal into a digital signal. return loss The VSWR as measured in dB. RF attenuator A device that decreases the RF signal and is used when the gain of an antenna did not match the power output of an AP. RF line of sight A theoretical straight line between a transmitter and the receiver. RF signal splitter A device that divides the power in the input signal to multiple outputs. sectorized antenna An antenna that divides the coverage area into different sectors and gives each sector its own antenna. semidirectional antenna An antenna that focuses energy in one direction. sensitivity The signal strength needed for a good reception. single-input single-output (SISO) A wireless communication system that uses one radio chain. Space Time Block Coding (STBC) An 802.11n option that sends a redundant copy of part or all of the transmited signal on an unused antenna. spatial diversity A MIMO technique of sending the same transmission out from different antennas that will take different paths. spatial multiplexing A MIMO technique of sending independent streams of information at the same time over the same frequencies. switching The process of choosing which antenna reception to accept. system operating margin (SOM) The difference between the received signal level and the signal level that is required by that radio to assure that the transmission can be decoded without errors. transmit beam forming (TxBF) An option for reducing outside signal interference by using complex antenna systems to allow for different directions and beamwidths. transmit diversity The ability of an access point to transmit on the antenna that most recently received the strongest incoming signal.

148 Chapter 4 Antennas

unidirectional amplifier A device that increases the RF signal level before it is injected into the transmitting antenna. variable-loss attenuator An RF attenuator that allows the user to set the amount of loss. visual line of sight An unobstructed straight line between objects. Voltage Standing Wave Ratio (VSWR) A measure of how well an electrical load is impedance-matched to its source.

Review Questions 1. An intentional radiator (IR) is .

a. power directed from an antenna

b. power directed to an antenna

c. one-eighth of a wavelength

d. another name for a mini-DPI antenna

2. is the power radiated out by the wireless system and includes any antenna gain.

a. Antenna Power Source (APS)

b. Radiation Power (RP)

c. Equivalent Isotropically Radiated Power (EIRP)

d. Dipole Equivalent Isotropic Radiator (DEIR)

3. The passive gain increase of power that is funneled from an antenna compared to that of an isotropic radiator is called .

a. dBi

b. dBp

c. KHz

d. IRP

4. compares the antenna gain against that of a dipole antenna.

a. IERP-D

b. dBB

c. ERP

d. dBd

5. The orientation of radio waves as they leave an antenna is called .

a. polarization

b. phase

c. orientation

d. radiation form

Review Questions 149

6. APs with multiple antennas employ to improve reception.

a. transmit diversity

b. polar waves

c. switch master configuration (SMC)

d. antenna diversity

7. Generally, a semidirectional antenna will only transmit its signal no more than degrees.

a. 22.5

b. 45

c. 90

d. 180

8. An antenna that divides the entire 360 degrees of coverage area into eight sectors of 45 degrees each is called a .

a. dipole antenna

b. sectorized antenna

c. T-antenna

d. Phased Dipole Array Configuration (DPAC)

9. The outer circle of an antenna radiation chart represents .

a. a gain that is less than 100 percent

b. a gain that is 100 percent

c. the location of the antenna

d. the power to the antenna

10. A(n) chart shows the vertical coverage area of an antenna.

a. Azimuth

b. elevation

c. Azure

d. meter

11. Beamwidth is considered a measure of the antenna’s range.

a. half-power

b. quarter-power

c. full-power

d. sector-power

150 Chapter 4 Antennas

12. The general rule is that percent of the first Fresnel zone must be clear of obstacles.

a. 60

b. 75

c. 90

d. 100

13. A SISO system has how many radio chains?

a. 1

b. 2

c. 3

d. 4

14. sends redundant streams in parallel.

a. Co-Op Beamwaving

b. Muxplexing

c. SISO diversity

d. Spatial diversity

15. A wireless device designated as 2x3:1 has how many Rx antennas?

a. 1

b. 2

c. 3

d. 6 (3x2)

16. The maximal ratio combining (MRC) algorithm is used when .

a. a non-MIMO device sends to a MIMO device

b. a MIMO device sends to a non-MIMO device

c. a SISO device sends to a SISO device

d. a MIMO device sends to a MIMO device

17. Each of the following is a good location for installing an AP except:

a. on a ceiling.

b. high on a wall.

c. in a plenum without a special enclosure.

d. on a pole.

18. Amplifiers for wireless LANs are .

a. unidirectional

b. bidirectional

c. beamform directional

d. attenuated

Review Questions 151

19. Another name for system operating margin (SOM) is .

a. additive margin

b. buffer space

c. reserve margin

d. fade margin

20. When the VSWR is indicated in dB it is called the .

a. downfade

b. dBVSWR

c. return loss

d. power ratio

Hands-On Projects

Project 4-1: Using Online Calculators to Compute RF Behavior—Part I Several Web sites have calculators you can use to determine different RF behaviors. In this project, you will use the Swiss Wireless site to compute

power, cable loss, antenna gain, free space path loss, link budget, Fresnel zone, and diffraction.

1. Use your Web browser to go to www.swisswireless.org/wlan_calc_en.html.

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions then open a search engine and search for “Swiss Wireless”.

2. Review the information in the Power section. Recall that the reference point that relates the logarithmic relative decibel (dB) scale to the linear milliwatt scale is known as the dBm, and this reference point specifies that 1 mW ¼ 0 dBm and is a measurement of absolute power. This calculator will convert from watts to dBm.

3. Click in the Watts box and type 1, which is the maximum power level for an IEEE WLAN.

4. Click dBm ,-W to convert from watts to dBm. What is the dBm of 1 W ? Record your answer.

5. Scroll down to display the Loss in coaxial cable at 2.45 GHz section.

6. Next to Choose type of cable, select Aircom.

7. Click in the Length (meter) box and type 1.75. Click m-.dB. What is the loss at this length? Record your answer.

8. Select other types of cables and note the losses.

152 Chapter 4 Antennas

9. Scroll down to display the Antenna section. Remember that dBi refers to the gain of an antenna. Next to Frequency Band, select 2.41–2.48 GHz (Wifi 802.11b, 802.11g, Bluetooth) if it is not already selected.

10. Next to Antenna diameter in meters, type .1 (for 3.9 inches), which is the size of an optional antenna that could be added to an AP.

11. Click D-. dB. What is the maximum theoretical gain? Record your answer.

12. Scroll down to display the Free space loss section. Remember that as the RF signal propagates from the antenna, it spreads out and weakens.

13. Next to Frequency Band, select 2.41–2.48 GHz (WiFi 802.11b; 802.11g; Bluetooth).

14. The maximum distance of an 802.11b WLAN is 375 feet (114 meters). Next to kilo- meters type .1143.

15. Click dB,- km. What is the free space path loss? Record your answer.

16. Change the frequency band to 5.15–5.85 GHz (802.11a, Hiperlan 2).

17. Click dB,- km. How does the free space path loss for 802.11a compare with 802.11g? Record your answer.

18. Scroll down to display the Link budget section. Enter these values for a sample WLAN:

● Transmit � Transmit output power: 115 dBm � Cable loss (negative value!): –6 dB � Antenna gain: 12 dBi

● Propagation � Free space loss (negative value!): –81.561 dB

● Reception � Antenna gain: 12 dBi � Cable loss (negative value!): –4 dB � Receiver sensitivity (generally negative value): –82 dBm

19. Click Compute. Record this value.

20. Scroll down to display the Propagation: Fresnel ellipsoid section.

21. Next to Distance “D” between transmitter and receiver [meters] type 114. This is the maximum distance of an 802.11b WLAN.

22. Next to Distance “d” between transmitter and obstacle [meters], type 65. This assumes an obstacle at about the halfway point.

23. Click Compute radius. Record this value.

24. Scroll down to display the Propagation: Diffraction section.

25. Suppose you have two APs, affixed to ceilings, that are communicating with each other, and that you want to determine the power loss resulting from an obstacle between them that is located 1.6 feet (.5 meters) above the two APs? Next to Height “h” between antenna top and obstacle top [meters] type .5.

Hands-On Projects 153

26. Next to Distance “D1” between transmitter and obstacle [meters] type 70.

27. Next to Distance “D2” between receiver and obstacle [meters] type 35.

28. Click Power loss. Record this value.

Project 4-2: Using Online Calculators to Compute RF Behavior—Part II In this project you will use Web sites to compute EIRP and the loss associated with different cable lengths.

1. The amount of power that a theoretical isotropic radiator can generate is called the Equivalent (also called Effective) Isotropically Radiated Power (EIRP). EIRP is the power radiated out by the wireless system and includes any antenna amplification (gain). Use your Web browser to go to www.distributed-wireless.com/calculators/EIRP.html.

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions then open a search engine and search for “DWG EIRP calculator”.

2. To calculate the EIRP of a typical WLAN enter these values:

● Transmitter power: 10 ● Power unit: dBm ● Your loss in dB: 2 ● Antenna gain: 19

3. Click Get EIRP. Record this value.

4. Cables, connectors, and even lightning arrestors can affect the link budget. You will now calculate the loss caused by these antenna accessories. Use your Web browser to go to www.afar.net/rf-link-budget-calculator/.

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions then open a search engine and search for “Afar RF Link Budget Calculator”.

5. Accept the default values under Input.

6. Enter these values under Cable Loss Calculator:

● Cable type: other ... ● Loss per 100 ft: 6.0 ● Cable length: 4 ● Number of connectors: 2

154 Chapter 4 Antennas

7. Click Tx Cable. Record these values.

8. Now change the cable length to 2.

9. Click Tx Cable. What is the difference between this value and the one calculated previously?

10. Close all windows.

Project 4-3: Downloading and Installing a Wireless Monitor Gadget Gadgets are small, mini-applications that run on the desktop and provide easy access to commonly used information and tools. In this project you will down-

load and install the Xirrus Wi-Fi Monitor gadget, which provides information in a format similar to an antenna radiation chart.

1. Use your Web browser to go to www.xirrus.com/library/wifitools.php.

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions then open a search engine and search for “Xirrus Wi-Fi Monitor gadget”.

2. Scroll down to the section Download Gadget.

3. Under Gadget for Windows 7 or Vista click Download Vista Gadget v1.2.

4. Click Save and specify the location for the download.

5. When the download has finished click Open.

6. Double-click the application file. Click Allow if asked.

7. Click the Install button.

8. Minimize all open windows to expose the gadget on the desktop.

9. Move the mouse pointer over the gadget to display the gadget’s configuration options.

10. Click the gadget’s Larger size button to increase the gadget’s size.

11. Click the gadget’s wrench to open the gadget’s option screen.

12. Next to Sweep Type select Radar.

13. Under Display Units select dBm. Click OK.

14. On the gadget click Show networks ...

15. Click the name of a wireless network under the SSID column. The signal strength of the network you clicked should appear on the radiation chart.

16. Click the name of another WLAN and note its strength.

17. Close all windows.

Hands-On Projects 155

Case Projects

Case Project 4-1: Antenna Measurements Use the Internet to identify two different models of antennas and record their IR, EIRP, dBi, and dBd. What can you say about these two antennas when comparing them? Which antenna is better? Which would you recommend for a WLAN? Why? Record your research and write a one-paragraph explanation about their differences.

Case Project 4-2: Types of Antennas Use the Internet to find Azimuth and elevation charts for an omnidirectional antenna, a semidirectional antenna, and a highly directional antenna. Print each of these charts and compare them. What can you say about their coverage patterns? Write a one-paragraph explanation of the charts.

Case Project 4-3: MIMO Research the history of MIMO. How is it being used in cellular telephony, in wireless MANs, and in wireless WANs? What are its disadvantages? What are its advantages? What can be expected in the future regarding MIMO? Write a one-page paper on your findings.

Case Project 4-4: AP Security Enclosures In some areas of high traffic it may be necessary to secure an AP so that it is not stolen. Use the Internet to research AP security enclosures. Find three enclosures and create a table com- paring their features. Be sure to include pricing information. Which of these would you select? Why? Write a one-paragraph explanation of your choice.

Case Project 4-5: Nautilus IT Consulting Nautilus IT Consulting (NITC), the computer technology business that helps organizations with IT solutions, has asked for your help.

2 & Up! is a children’s consignment clothier with several locations in the area. 2 & Up! is considering migrating to new IEEE 802.11n equipment, yet the IT staff is divided on the value of this upgrade. You have been asked to give a presentation at a working lunch meet- ing about the advantages of 802.11N and MIMO.

1. Create a PowerPoint presentation of eight or more slides that covers MIMO. Your slides should explain what MIMO is, how it works, its advantages, and what it could do for 2 & Up!. Because you will be addressing the IT staff of 2 & Up!, this presenta- tion should contain technical information.

2. In response to your presentation, 2 & Up! sent Nautilus IT Consulting a list of ques- tions, one of which regards compatibility of 802.11n devices with 802.11a/b/g devices. Create a one-page explanation of how devices from these two standards can or cannot coexist, and give your opinion as to which direction 2 & Up! should take.

156 Chapter 4 Antennas

chapter5

Physical Layer Standards

After completing this chapter you should be able to:

• List and describe the different wireless modulation technologies • Explain the features in the 802.11b Physical Layer Standards • Describe the technologies found in the 802.11a PHY standards • Explain how the 802.11g Physical Layer Standards are different from the other standards • List the features in the 802.11n PHY standards

157 Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

The sport of football is known for its bone-jarring collisions. When a 180-pound gunner (a player who sprints down the field on a kickoff) runs head-on into a 275-pound line- man, these violent collisions cause the crowd to jump to its feet. Yet it’s not uncommon for the gunner to have trouble getting back on his feet. That’s because the force exerted in such a collision can exceed 100 times the force of gravity, or the same that passengers experience in a car crash. Among professional, college, and youth football players, almost 300,000 head concussions from these collisions occur annually.

Players who suffer a brain injury during a football game may experience headache, nausea, and short-term memory loss (it seldom involves a loss of consciousness), yet they are unlikely to ask to be taken out of the game for fear of losing their spot in the lineup. This makes it difficult for coaches and trainers to assess the extent of the injury and to know when to take them out or allow them back in the game. There is a growing concern about when—and even if—a player who suffers a violent collision should return to the field of play. Should a player who is groggy after a big hit be allowed back in the game? Although the National Football League (NFL) in 2009 cre- ated new rules that mandate that a player exhibiting any signs of head injury must leave the field for the remainder of the day, no such rules exist in college or at youth levels. And in the long term, what can be done to minimize football head traumas? The answer to these questions may rely on new devices that use wireless technology.

The Head Impact Telemetry (HIT) System was developed by Simbex. It combines six strategically positioned accelerometers (a device that measures acceleration, much like those found in a car’s airbag system), a temperature sensor, nonvolatile onboard memory, a battery pack, and a wireless transceiver into a single package that fits inside a football player’s helmet—the package adds just 6 ounces (170 grams) to the weight of a typical helmet. This allows the marshmallow-shaped spring-mounted accelerometers to rest against a player’s head so that the movement of the skull can be measured instead of the movement of the helmet itself. An even more compact system developed by X2Impact has tiny accelerometers and gyroscopes embedded into the player’s mouth guard. The HIT system automatically captures data when any single sensor detects an acceleration that exceeds 10 times the force of gravity. Data is captured regarding the magnitude, duration, and location of the impact from 12 milliseconds before the system is triggered to 28 milliseconds following the impact.

This data is then immediately transmitted wirelessly from the helmet to the side- lines. The helmet’s transceiver uses a frequency-hopping spread-spectrum protocol that transmits in the 902–928-MHz frequency range (if both teams are using HIT hel- mets the radio signals can be differentiated so that one team will not pick up another team’s signals). A receiver located on the sidelines receives the “hit data”

Real World Wireless

158 Chapter 5 Physical Layer Standards

In 1978 the International Organization for Standardization (ISO) released a set of specifica- tions that was intended to describe how dissimilar computers could be connected through a network. Called the Open Systems Interconnection (OSI) reference model, this set of specifi- cations demonstrated networking concepts by portraying networking as a series of related steps that occur in networked communication. The model breaks network communication into seven different layers. Within each layer, different networking tasks are performed by hardware and/or software. Each layer interacts with the layer above it (by providing services) and the layer below it (by receiving services) and also logically links to the same layer on the corresponding sending or receiving device. After a revision in 1983, the OSI reference model is still used extensively today. The layers and their functions are outlined in Table 5-1.

The value of using layers in the OSI reference model is that it divides networking into a series of tasks and then illustrates how those tasks relate to each other.

and is connected to a laptop computer, which then displays that information along with a three-dimensional graphic of the human head and indicators to show where the player was hit. Trainers and coaches can then make an immediate medical deter- mination of what action needs to be taken regarding the player. At the end of the game all of the data is combined with the data from all other players.

An early version of the HIT system, introduced in 2003, outfitted four football players from two colleges who were monitored through 35 practices and 10 games. Researchers recorded roughly 3,300 head hits and found that, on average, players endured 50 impacts strong enough to trigger the system during the course of a single game. The average acceleration caused by the hits was 40 times the force of gravity per blow (the same level of impact delivered by the gloved fist of a professional boxer), and at least twice per game the players took hits to the head of 100 times the force of gravity. This data was compared against the Gadd Severity Index (GSI), which is a method developed by car crash researchers for describing just how jarring a blow someone has received. While a human head can withstand GSI values up to 1000 with- out serious injury, the blows endured by these players ranged as high as 1599. Today hundreds of college teams and many professional teams use the HIT system.

The data accumulated has revealed some interesting information. For example, dif- ferent positions on football teams sustain different types of blows to the head. Line- men sustain frontal blows that are usually low impact blows, yet there are several blows per game. In contrast, wide receivers receive fewer, but harder blows. And linebackers sustain higher accelerations than linemen. This may lead to specific hel- mets being designed for different positions. In addition, coaches can use this data to determine if tackling is being taught correctly. The HIT system is also used in hockey at the college and youth level, as well as in studies in equestrian sports, snowboard- ing, soccer, and even in the military.

Chapter 5 Physical Layer Standards 159

In this chapter you learn about IEEE 802.11 wireless LAN functions at the lowest layer of the OSI reference model, the Physical layer. Because the Physical layer primarily deals with turn- ing frames into electrical impulses for transmission, you will begin by exploring the different wireless modulation schemes. Then you examine each of the IEEE WLAN standards and see how they are implemented at the Physical layer.

Wireless Modulation Technologies

C W N A

2.1.2. Comprehend the differences between, and explain the different types of spread-spectrum technologies and how they relate to the IEEE 802.11-2007 standard’s (as amended and including 802.11n-draft2.0) PHY clauses.

2.1.3. Identify the underlying concepts of how spread-spectrum technology works.

The two primary radio frequency (RF) modulation techniques are narrowband transmission and spread spectrum.

Narrowband Transmission RF signals that are transmitted on only one frequency are known as narrowband transmis- sions. A broadcast radio station using narrowband transmission would tell listeners to “tune to 88.5” because this is the one frequency on which the radio signal is transmitted. Narrowband transmissions, illustrated in Figure 5-1, require more power for the signal to be transmitted because the signal must exceed the noise level, or the total amount of out- side interference (noise), by a substantial margin.

The primary advantage of a narrowband transmission is the efficiency that comes from using only a small portion of the spectrum for its transmission. However, there are several disadvantages of narrowband transmissions for WLANs. Narrowband transmissions

Layer Name Function

7 Application Interacts with software applications to provide the interface for network services.

6 Presentation Handles how the data is represented and formatted for the user.

5 Session Permits the devices on the network to hold ongoing communications across the network. Handles session setup, data or message exchanges, and tear-down when the session ends.

4 Transport Ensures that error-free data is given to the user. It handles the setup and tear-down of connections.

3 Network Picks the route packets take and handles addressing of packets for delivery.

2 Data Link Provides the means to transfer data between network entities and detect and correct errors.

1 Physical Sends signals to the network or receives signals from the network.

Table 5-1 OSI layers and functions

160 Chapter 5 Physical Layer Standards

require more power and yet are vulnerable to interference from another radio signal being transmitted at or near the same frequency. Much like an accident on a one-lane road can stop all traffic, a single interfering signal at or near the broadcast frequency can render a narrowband transmission ineffective. The IEEE 802.11 standards do not permit the use of narrowband transmissions for WLANs.

Spread-Spectrum Transmissions An alternative to narrowband transmission is spread-spectrum transmission. Whereas a nar- rowband transmission sends a strong signal using a small portion of the spectrum, spread spectrum transmits a weaker signal across a broader portion of the radio frequency band, as seen in Figure 5-2.

Spread spectrum has several advantages over narrowband transmission. These include:

● Resistance to narrowband interference. Spread-spectrum transmission is more resistant to outside interference. This is because any interference affects only a small portion of the signal being transmitted instead of impacting the entire signal as with narrowband.

2.41 2.42 2.43 2.44 2.45 2.46 2.47

Noise level

Power (Amplitude)

2.40

Frequency

Figure 5-1 Narrowband transmission

2.41 2.42 2.43 2.44 2.45 2.46 2.47

Noise level

Spread spectrum

Narrowband Power

(Amplitude)

2.40

Frequency

Figure 5-2 Narrowband vs. spread spectrum transmission

Wireless Modulation Technologies 161

In this way spread-spectrum transmissions help keep traffic moving, much like multiple lanes of a highway: although an accident in one lane of an four-lane highway is inconvenient, cars can use the three other lanes to move around it and keep going.

● Resistance to spread-spectrum interference. Because each spread-spectrum transmitter uses a different set of procedures for transmitting its signal, spread-spectrum transmis- sions typically do not interfere with other spread-spectrum signals.

● Lower power requirements. Since spread spectrum requires less power to transmit the signal, the radio sending unit does not require as much energy as a narrowband unit.

● Less interference on other systems. The spread-spectrum signal is transmitted beneath the noise level. This means that other radio receivers that pick up the signal would consider it to be “standard” noise and ignore it. The result is that spread-spectrum transmissions do not generally interfere with other radio transmissions.

● More information transmitted. Spread-spectrum transmission can send more bits at one time than a similar narrowband transmission.

● Increased security. The fact that other radio receivers see spread-spectrum transmissions as noise and ignore it provides an additional degree of security for spread-spectrum transmissions in that other radio receivers cannot easily eavesdrop on the transmission.

● Resistance to multipath distortion. Although not entirely eliminated with spread- spectrum transmission, the amount of multipath distortion is reduced. This is due to the low power level at which spread spectrum is transmitted.

Spread-spectrum transmission uses three methods to spread the signal over a wider area. The methods are frequency-hopping spread spectrum, direct sequence spread spectrum, and orthogonal frequency division multiplexing.

Frequency-Hopping Spread Spectrum (FHSS) The historical background of frequency-hopping spread spectrum helps to illustrate this technology. In the early days of World War II Nazi German warships developed a method for jamming the radios that guided U.S. torpedoes. Film actress Hedy Lamarr, whose first husband manufactured mili- tary aircraft control systems, and music composer George Antheil were Hollywood neigh- bors who happened to strike up a conversation regarding how to prevent torpedoes from being jammed. Antheil proposed that rapid changes in radio frequencies could be coordi- nated in much the same way he had coordinated sixteen synchronized player pianos in one of his music pieces. Within a short time they fleshed out their idea and applied in 1941 for a patent for a “Secret Communication System.” Their proposed device used slotted paper rolls similar to player-piano rolls in order to synchronize the frequency changes in a radio trans- mitter and receiver using 88 frequencies (the same number of keys on a piano). They received a U.S. patent for their idea the following year.

Although Lamarr and Antheil’s idea was never implemented during the war, once electronics were cheaply available in the late 1950s the military used the concept as a basic tool for securing military communications. However, by this time the patent had expired and the duo never received any royalties.

Instead of transmitting on a single frequency, frequency-hopping spread spectrum (FHSS) uses a range of frequencies, called the bandwidth (the difference between the upper and

162 Chapter 5 Physical Layer Standards

lower frequencies), that change during the transmission. With FHSS, a short burst is trans- mitted at one frequency, followed by a short burst transmitted at another frequency, and so on, until the entire transmission is completed.

Figure 5-3 illustrates how FHSS functions. The transmission starts by sending a burst of data at the 2.44-GHz frequency for 100 milliseconds (ms or thousandths of a second). The amount of time that a transmission occurs on a specific frequency is called the dwell time. The transmission frequency then quickly changes. The time to change is called the hop time and is measured in microseconds (μs) or millionths of a second. In this example the frequency is changed to 2.41 GHz and transmits at that frequency for the next 100 milliseconds. After another hop time (usually about 200 μs) at the third 100 milliseconds the transmission takes place at the 2.42-GHz frequency. This continual switching of frequencies takes place until the entire transmission is complete. The sequence of changing frequencies is called the hopping code. In Figure 5-3, the hopping code is 2.44-2.41-2.42-2.40-2.43.

The receiving station must also know the hopping code in order to correctly collect the incoming transmissions in the right sequence.

2.40

2.41

2.42

2.43

2.44

2.45

F re

q u e n c y (

G H

z )

100 200 300 400 500

Time (milliseconds)

2nd

1st

3rd

4th

5th

Figure 5-3 FHSS transmission

Wireless Modulation Technologies 163

If an FHSS transmission encounters interference on a particular frequency, then that part of the signal is retransmitted on the next frequency of the hopping code. Figure 5-4 shows that the sec- ond transmission was interfered with, so it was retransmitted on the frequency that would nor- mally carry the third transmission. All subsequent transmissions are then moved to the next fre- quency of the hopping code. Because FHHS transmits short bursts over a wide range of frequencies, the extent of any interference will be very small and can easily be corrected by error checking. In addition, FHSS signals will have a minimal interference on other signals.

Frequencies are often listed to the thousandths position, but it can become tedious as well as error-prone to designate a FHSS hopping code as 2.457-2.422-2.442-2.432. As an alterna- tive, a numeric value called the channel is assigned to a frequency range and that value is used instead. (When expressed in this form, the hopping code 2.457-2.422-2.442-2.432 becomes channels 10-3-7-5.) A channel’s frequency range typically has a center frequency, along with lower and upper limits. For example, Channel 6 may be designated as 2.437 GHz (center), although the actual range for Channel 6 may be from 2.426 GHz (lower) to 2.448 GHz (upper).

Consider a typical television receiving digital transmissions. Instead of tuning it to video frequency 199.25 MHz and audio frequency 203.75 MHz, it’s much easier instead to just turn to Channel 11, which has been preset to those frequencies.

2.40

2.41

2.42

2.43

2.44

2.45

F re

q u e n c y (

G H

z )

100 200 300 400 500

Time (milliseconds)

2nd Scrambled

1st

2nd Resent

3rd

4th

Figure 5-4 FHSS error correction

164 Chapter 5 Physical Layer Standards

The FCC has established restrictions on FHSS to minimize interference between systems. All FHSS systems in the 900-MHz band must change frequencies or “hop” through 50 channels and cannot spend more than 400 ms on one frequency over a span of 20 seconds. For trans- missions in the 2.4-GHz band, FCC restrictions are divided between those devices that use a minimum of 75 hop channels (called full-channel FHSS) and those that use fewer channels (reduced-channel FHSS). The specifications for full-channel FHSS include:

● Peak transmit (TX) power cannot exceed 1 W ( þ30 dBm) ● Hop channel separation is not less than 25 KHz ● Hop channels are selected randomly ● Average dwell time equal on each frequency ● Maximum dwell time 400 ms (in any 30-second time period)

Due to speed limitations FHSS is not widely implemented in WLAN systems.

Bluetooth technology uses FHSS by dividing the 2.4-GHz frequency into 79 different frequencies spaced 1 GHz apart. In one second of Bluetooth transmission the frequency will change 1,600 times, or once about every 625 μs.

Direct Sequence Spread Spectrum (DSSS) The second type of spread-spectrum technology is direct sequence spread spectrum (DSSS). DSSS uses an expanded redundant code to transmit each data bit; that is, each bit is converted to a series of bits before being transmitted. Figure 5-5 illustrates a simplified model of DSSS. In the first frame of the figure there are three original data bits to be transmitted: 1, 0, and 1. Instead of transmitting these three bits, a different sequence of bits is inserted, as shown in the middle frame of the figure. This bit pattern is called the chipping code (a single radio bit is sometimes referred to as a chip). In this example, the chipping code is 1001: for every original 1 bit the chipping code

Original data bits to be transmitted: 101

Chipping code: 1001 for 1 bit 0110 for 0 bit

0 0 0 0 0

111111

DSSS signal transmitted: 0110-0110-0110

00 0 0

11 1111

0 0

Figure 5-5 DSSS

Wireless Modulation Technologies 165

of a 1 (1001) is substituted instead. The inverse of that code, 0110, is substituted for the original 0 bit. (The chipping code in this figure is illustrated using only 4 bits for the sake of simplicity whereas most DSSS chipping codes are either 11 or 22 bits in length). The final step is to add the original data bit to the chipping code, as seen in the bottom pane of the figure, which results in the signal that is transmitted. The addition of the original bit to the chipping code is accomplished by the Boolean operation of exclusive or (XOR).

When the bits to be transmitted are consecutive 1 bits, an extra 0 is placed between them in the chipping code so that it becomes [1001][0][1001], which would result in a transmitted code of [0110] [1][0110].

There are several advantages to using DSSS:

● Error correction. With FHSS a single corrupted bit requires that specific bit to be retransmitted, as illustrated in Figure 5-4 above. DSSS, on the other hand, can recover the original data using advanced statistical techniques without the need for any retransmission. DSSS can actually recover from not just one corrupted bit but from multiple corrupted bits in a transmission.

● Less interference on other systems. If the DSSS signal is picked up by an unintended device, the signal will appear as low-powered noise and will be ignored.

● Shared frequency bandwidth. Using DSSS makes it possible to share the frequency band with similar devices. Known as colocation, this is achieved by assigning each device a unique chipping code in order that all the transmissions can use the same frequency yet remain separate. The transmission of one network would only appear as noise to another network and would be filtered out.

● Security. If an eavesdropper picked up the signal of the original data bit, it would be a simple task to read the message. However, a DSSS transmission that has been intercepted is far harder to decipher.

The FCC restrictions on DSSS include a maximum output power level of 1 W and a minimum chipping code of 10 bits.

Orthogonal Frequency Division Multiplexing (OFDM) The third spread- spectrum technology is orthogonal frequency division multiplexing (OFDM). Strictly speak- ing, OFDM is not a spread-spectrum technology. However, because its properties are similar to that of spread spectrum, OFDM is usually classified as this type of technology. Although OFDM dates back to the mid-1960s, it has been adopted as the modulation method of choice for virtually all new wireless technologies used today.

Two basic concepts are involved with OFDM. The first is breaking down the transmission into separate parts and sending each part in parallel simultaneously. That is, instead of send- ing one long stream of data as with FHSS or DSSS, OFDM sends the transmission in paral- lel across several channels. Sending multiple signals simultaneously is called multiplexing. Consider a single-lane toll road with one toll booth that handles automobile traffic. Cars would bottleneck every morning and afternoon during rush hour trying to go through the

166 Chapter 5 Physical Layer Standards

one toll booth. The solution is not to raise the cars’ speed limit going through the toll booth, but to add more toll booths on parallel lanes. This in effect is what OFDM does: it uses multiple transmission paths in parallel for traffic.

The second concept is that these parallel transmissions are sent at a slower rate. Although it may seem counter-intuitive, OFDM actually increases the overall speed by sending the data more slowly. Consider again the drivers on the single-lane toll road, when a sudden thun- derstorm dumps a large amount of water. A driver, finding that her windshield is constantly being splashed by the driver ahead of her, may decide to slow down (“back off”) enough to keep a longer distance between her and the car in front to avoid the splashes.

This is the idea behind the slower ODFM transmissions. Because of multipath distortion, the RF signals (called symbols) bounce off objects such as walls and furniture, delaying them from reaching the receiver (called delay spread). When they do arrive they interfere with each other by being “added” to the primary signal, resulting in downfade, corruption, or nulling. This add- ing is known as intersymbol interference (ISI). Even though a delay, called the guard interval (GI), is built into the receiver to allow for these late-arriving symbols, those that exceed the guard interval can still cause ISI. The slower speed used in OFDM allows a symbol to “back off” from the earliest symbol, thus reducing the risk for signals to arrive “on top” of each other. In other words, sending a slower signal creates a longer period of time in which the later multi- path signals can arrive without impacting the earlier signal. OFDM is illustrated in Figure 5-6.

Orthogonal means at a right angle to. The signals in OFDM are created so they are at right angles to each other, letting them be closely spaced together yet not causing ISI.

Symbol 1First symbol

Last symbol

First symbol

OFDM

FHSS and DSSS

Guard interval

Intersymbol interference

Time

Last symbol

Symbol 1 Symbol 2

Symbol 2 Symbol 3

Symbol 1

Symbol 1 Symbol 2

Symbol 2 Symbol 3

Figure 5-6 OFDM

Wireless Modulation Technologies 167

Because OFDM transmissions are sent in parallel, more data can be sent in a given period of time. And even though the transmissions are sent slower to prevent ISI, overall the total throughput is increased.

Comparison of Modulation Technologies Each modulation technology—narrowband, FHSS, DSSS, and OFDM—has its own strengths and weaknesses.

Narrowband transmissions require more power and are vulnerable to interference from another radio signal being transmitted at or near the same frequency. Therefore, narrowband transmissions are not appropriate for WLANs using small APs and even smaller wireless net- work interface card adapters.

The communication resilience of FHSS transmissions means they are less prone to interfer- ence from outside signals than DSSS. WLAN systems that use FHSS have the potential for a higher number of colocation units than DSSS. (As many as 12 to 15 colocated FHSS can share frequency in a given area as opposed to only three DSSS systems.) Yet DSSS has the potential for greater transmission speeds over FHSS. FHSS transmits a short burst on one frequency that is typically only 1 MHz, while DSSS transmits on a frequency that is 22 MHz wide. The amount of data that a DSSS channel can send and receive is much greater than that of FHSS. DSSS has a potential bandwidth of up to 11 Mbps whereas FHSS can only transmit at a maximum of 3 Mbps. Because of its higher throughput, DSSS systems are preferred over FHSS for 802.11b WLANs.

However, the dramatically increased throughput that can be achieved by OFDM has made it the leader today among the modulation schemes. OFDM is used in IEEE 802.11a/g/n net- works (not in 802.11 or 802.11b). Because it can support speeds of up to 600 Mbps for IEEE 802.11n and 54 Mbps for 802.11a/g networks, OFDM is the preferred modulation technique for faster WLANs.

IEEE 802.11 Physical Layer Standards

C W N A

2.1.4. Identify and apply the concepts which make up the functionality of spread-spectrum technology.

2.2.1. Identify, explain, and apply the frame types and frame exchange sequences covered by the IEEE 802.11-2007 standard.

2.2.2. Identify and apply regulatory domain requirements.

2.2.3. OSI model layers affected by the 802.11-2007 standard and amendments.

2.2.4. Use of ISM and UNII bands in Wi-Fi networks.

3.2.1. Describe and apply concepts surrounding WLAN frames.

3.2.3. Define, describe, and apply IEEE 802.11 coordination functions and channel access methods and features available for optimizing data flow across the RF medium.

168 Chapter 5 Physical Layer Standards

IEEE wireless standards follow the OSI model with some modifications. The IEEE has divided the Data Link layer into two sublayers: the Logical Link Control (LLC) sublayer, which provides a common interface, reliability, and flow control, and the Media Access Control (MAC) sublayer, which appends physical addresses to the frame. These are illustrated in Figure 5-7. The reason for this alteration was to allow higher-level protocols, such as those operating in the Network layer, to interact with Data Link layer protocols without regard for Physical layer specifications.

The IEEE has also subdivided the Physical layer (PHY) for WLANs into two sublayers, as seen in Figure 5-8. The Physical Medium Dependent (PMD) sublayer makes up the standards for both the characteristics of the wireless medium and defines the method for transmitting and receiving data through that medium. The second sublayer of the PHY layer is the Physical Layer Convergence Procedure (PLCP) sublayer. The PLCP sublayer performs two basic func- tions: it reformats the data received from the MAC layer (when transmitting) into a frame that

OSI Model

Network layer

Data Link layer

Physical layer

IEEE Model

Network layer

Logical Link Control

Media Access Control

Physical Layer Convergence Procedure (PLCP)

Physical Medium Dependent (PMD)

Figure 5-8 PHY sublayers

OSI Model

Network layer

Data Link layer

Physical layer

IEEE Model

Network layer

Logical Link Control

Media Access Control

Physical layer

Figure 5-7 Data Link sublayers

IEEE 802.11 Physical Layer Standards 169

the PMD sublayer can transmit, as shown in Figure 5-9, and it “listens” to the medium to determine when the data can be sent.

The IEEE standards specify that the features of a WLAN must be transparent to the upper layers of the IEEE model. That is, the functions of the PHY and MAC layers should provide full implementation of all of the WLAN features so that no modifications are needed at any other layers. This makes the PHY and MAC layers of the IEEE 802.11 wireless LANs function like those of other IEEE network standards such as Ethernet in sharing the same LLC layer. Because all of the WLAN features are isolated in the PHY and MAC layers, any network operating system or LAN application will run on an IEEE WLAN without modifica- tion. However, in order to accomplish this, the standard requires that some networking features that are usually associated with higher level layers be performed at the MAC layer for WLANs.

The specifics of the PMD and PLCP sublayers for IEEE WLANs can be divided into those that apply to the different IEEE standards: 802.11b, 802.11a, 802.11g, and 802.11n WLANs.

The original IEEE 802.11 standard of 1997 defined a local area net- work that provides cable-free data access for clients in a mobile or fixed location at a rate of up to 2 Mbps using either FHSS or DSSS RF transmissions or infrared transmissions. Yet due to its slow speed it never enjoyed widespread favor and was soon replaced by 802.11a/b.

IEEE 802.11b Physical Layer Standards The basic purpose of the 802.11 PHY layer is to send the signal to the network and receive the signal from the network. To perform this function the PHY layer is divided into two parts: the PMD and the PLCP sublayers.

Physical Layer Convergence Procedure Standards The PLCP standards for 802.11b are based on DSSS. The PLCP must reformat the data received from the MAC layer (when transmitting) into a frame that the PMD sublayer can transmit. A PLCP frame is illustrated in Figure 5-10.

PHY layer

MAC layerMAC frame

PLCP Converts into PMD frame

PMD frame

PMD Prepares for transmitting

PMD frame

Figure 5-9 PCLP sublayer function

170 Chapter 5 Physical Layer Standards

There is often confusion regarding the terms used to describe data units. A packet refers to a message sent by protocols operating at the Network layer of the OSI Reference Model. A datagram is basically the same as a packet, although the term is also used to refer to a message sent at a higher level of the OSI. The term frame refers to messages

that travel at low levels of the OSI, while the terms Protocol Data Unit (PDU) and Service Data Unit (SDU) refer to protocol messages.

The frame is made up of three parts: the preamble, the header, and the data. The preamble prepares the receiving device for the rest of the frame, whereas the header provides informa- tion about the frame itself. The data portion of the PLCP frame is the information that is actually being transmitted. The size of the data or payload can be from 1 to 16,384 bits. A description of the fields is as follows:

● Synchronization. The Synchronization field consists of alternating 0’s and 1’s. It alerts the receiving device that a message may be on its way so that the receiving device will then synchronize with the incoming signal.

● Start Frame Delimiter. The Start Frame Delimiter is always the same bit pattern (1111001110100000) and it defines the beginning of a frame.

● Signal Data Rate. The speed of the signal is designated by the Signal Data Rate field. ● Length. The value of the length of the frame is contained in the Length field. ● Header Error Check. The Header Error Check field contains a value that the receiving

device can use to determine if the data was received correctly. ● Data. The data or payload can be from 1 to 16,384 bits, and that value is contained in

this Data field.

The PLCP frame preamble and header are always transmitted at 1 Mbps. This was designed to allow a slower sending device (like an 802.11) to talk to a faster receiving device (like an 802.11b) by using the slowest speed. The disadvantage of using the lowest common denominator speed is that two faster devices must still fall back to the 1-Mbps transmission rate for the pream- ble and header. However, the data can be sent at the faster rate once the connection is established.

80 16 8 8 8 1 to 16,384

Transmission speed (Mbps)

Parts Preamble Header Data

1 Mbps

Synchronization Start frame

delimiter

LengthSignal data rate

Header error check

DataService

1, 2, 5.5, or 11 Mbps

Size (bits)

Figure 5-10 802.11b PLCP frame

IEEE 802.11 Physical Layer Standards 171

Another advantage of the slower PCLP preamble and header trans- mission speed is that the slower signal can cover a larger area than a faster signal can.

Physical Medium Dependent Standards Once the PCLP has created the frame, it then passes it on to the PMD sublayer of the PHY layer. The job of the PMD is to translate the binary 1’s and 0’s of the frame into radio signals that can be used for transmission. The PMD can transmit the data at 11, 5.5, 2, or 1 Mbps.

The 802.11b standard uses the Industrial, Scientific and Medical (ISM) band for its transmis- sions. The 802.11b standard specifies 14 frequencies that can be used, beginning at 2.412 GHz and incrementing by .005 GHz (except for channel 14). These are listed in Table 5-2.

The United States and Canada use channels 1-11; channels 12-14 are used in Europe, France, and Japan.

IEEE 802.11b transmissions in the 2.4-GHz frequency are designed to be þ /�11 MHz from the channel center frequency. However, some of the transmission may still encroach onto other frequencies up to 30 MHz from the channel center, so that they actually consume five overlapping channels. For example, transmitting on channel 6 may cause inter- ference on channels 5 and 7 as well as limited interference on channels 4 and 8. This leaves

Channel Number Lower Frequency Center Frequency Upper Frequency

1 2.401 2.412 2.423

2 2.406 2.417 2.428

3 2.411 2.422 2.433

4 2.416 2.427 2.438

5 2.421 2.432 2.443

6 2.426 2.437 2.448

7 2.431 2.442 2.453

8 2.436 2.447 2.458

9 2.441 2.452 2.463

10 2.446 2.457 2.468

11 2.451 2.462 2.473

12 2.456 2.467 2.478

13 2.461 2.472 2.483

14 2.473 2.484 2.495

Table 5-2 802.11b ISM channels

172 Chapter 5 Physical Layer Standards

5only three nonoverlapping (simultaneously usable) 20-MHz channels—1, 6, and 11—as seen in Figure 5-11.

The 802.11b standard specifies two different types of modulation to be used. For transmis- sions at 1 Mbps, a two-level phase shift key (PSK) known as differential binary phase shift keying (DBPSK) is specified. The phase change for PSK bit 0 is 0 degrees, whereas the phase change for bit 1 is 180 degrees. For transmissions at 2, 5.5, and 11 Mbps, a four- level phase change called differential quadrature phase shift keying (DQPSK) is used. Instead of having only two variations in phases for 0 and 1, the four-level phase change has four variations in phases for the bit combinations 00, 01, 10, and 11.

The 802.11b standard also outlines the type of DSSS coding to be used. DSSS uses the expanded redundant code (usually called the Barker code in this context) to transmit each data bit. The Barker code is used when 802.11b is transmitting at 1 Mbps or 2 Mbps. However, to transmit at rates above 2 Mbps (known as High-Rate DSSS or HR-DSSS) the complementary code keying (CCK) technology is used instead. CCK uses a shorter chipping code (8 bits instead of 11) yet uses different chipping codes for different sequences of bits. This coding technique consists of a set of 64 8-bit code words. As a set, these code words have unique mathematical properties that allow them to be correctly distinguished from one another by a receiver. The 5.5-Mbps rate uses CCK to encode 4 bits per carrier, while the 11-Mbps rate encodes 8 bits per carrier. Table 5-3 summarizes the IEEE 802.11b Physical layer standards.

IEEE 802.11a Physical Layer Standards The 802.11a standards are significantly different from the 802.11b standards. The differences have to do with increasing the speed from 11 Mbps to 54 Mbps, the PLCP frame contents, and modulation techniques.

Channel 3 Channel 7 Channel 11

Channel 2 Channel 10Channel 6

Channel 1 Channel 5 Channel 9

Channel 4 Channel 8

2400 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462 2483

Figure 5-11 2.4 GHz nonoverlapping channels

Transmission Speed (Mbps) Modulation DSSS Coding Technique

1 Differential binary phase shift keying (DBPSK) Barker code

2 Differential quadrature phase shift keying (DQPSK) Barker code

5.5 Differential quadrature phase shift keying (DQPSK) Complementary code keying (CCK)

11 Differential quadrature phase shift keying (DQPSK) Complementary code keying (CCK)

Table 5-3 IEEE 802.11b Physical layer standards

IEEE 802.11 Physical Layer Standards 173

Higher Speed Enhancements The 802.11a standards outline a WLAN that operates at speeds faster than that of 802.11b: up to 54 Mbps (with an optional proprietary speed of 108 Mbps) compared to only 11 Mbps with 802.11b. The differences that allow the IEEE 802.11a standards to achieve these higher speeds are all in the Physical layer. These enhancements include use of OFDM, using a higher frequency, and utilizing a more efficient error-correction scheme.

OFDM OFDM offers significant improvements in speed over FHSS or DSSS. OFDM sepa- rates the transmission into multiple parts and sending each part in parallel simultaneously. Even though these parallel transmissions are sent more slowly it reduces ISI. A large portion of the increase in speed in 802.11a is due to the use of OFDM

UNII Frequency Band While the 802.11b standard uses the unlicensed Industrial, Scien- tific and Medical (ISM) band for its transmissions, the 802.11a standard uses another unli- censed band, the Unlicensed National Information Infrastructure (UNII). The UNII band is intended for devices that will provide short-range, high-speed wireless digital communica- tions. The Federal Communications Commission (FCC) has segmented the 300 MHz of UNII spectrum into four bands, each with a maximum power limit. These bands and their maximum power outputs are seen in Table 5-4.

The original IEEE 802.11a standard did not include the UNII-2 Extended band. In 2003 the FCC added an additional 255 MHz of spectrum in the 5.470–5.725 band, which increased the amount of spectrum for 802.11a devices by almost 80 percent. This was an attempt to better harmonize the U.S. spectrum with that of other nations.

The 802.11a standard enables faster rates because of the higher frequencies and increased power at which it operates. The total bandwidth available for IEEE 802.11a WLANs using UNII is almost four times that available for 802.11b networks using the ISM band. The ISM band offers only 83 MHz of spectrum in the 2.4-GHz range while the UNII band offers 300 MHz. However, the range of coverage with 802.11a is less than that of 802.11b.

One of the attractive features of 802.11a is that it uses the 5-GHz UNII band, which is less cluttered with consumer and business elec- tronic devices (such as cordless phones, microwave ovens, and Blue- tooth devices) all transmitting on the same frequency that can cause interference with 802.11b WLANs. However, a growing number of devices are using the UNII band that can cause interference.

UNII Band Frequency (GHz) Maximum Power Output (mW)

UNII-1 (Low Band) 5.15–5.25 50

UNII-2 (Middle Band) 5.25–5.35 250

UNII-2 (Extended) 5.47–5.725 250

UNII-3 (High Band) 5.725–5.825 1000

Table 5-4 UNII characteristics

174 Chapter 5 Physical Layer Standards

Not all countries permit transmissions in all of the UNII bands. In addition, the maximum power output can also vary between countries. Although this is not a problem for WLANs contained within a single country, multinational companies and individuals who travel inter- nationally may be required to maintain different networks in different countries.

In order to reduce interference, 802.11a WLANs using UNII also incorporate a technology known as transmit power control (TPC). An 802.11a AP that supports TPC sends this infor- mation to the wireless devices and also indicates the maximum transmit power allowed in the WLAN and the transmit power the AP is currently using. The device then responds with its own transmit power capability. The AP uses this data to determine the maximum power for this WLAN network segment. The radio power can be adjusted dynamically to reduce inter- ference with other devices while still maintaining sufficient power for the WLAN.

Error Correction IEEE 802.11a also handles errors differently than 802.11b, thus achiev- ing in an increase in speed. First, the number of errors is significantly reduced by the nature of 802.11a transmissions. Because transmissions are sent over parallel subchannels using OFDM, radio interference from an outside source is minimized. Instead of the interference impacting the entire data stream, it will generally only affect one subchannel.

Error correction in 802.11a is also enhanced. Forward Error Correction (FEC) transmits a secondary copy along with the primary information. Of the 52 subchannels, 48 are used for standard transmissions and 4 are used for FEC transmissions. If part of the primary transmis- sion is lost, the secondary copy can be used to recover (through sophisticated algorithms) the lost data. This eliminates the need to retransmit if an error occurs, which in turn saves time. Because of its high speed, 802.11a can accommodate the FEC overhead with a negligible impact on performance.

PLCP Frames The PLCP for 802.11a is based on OFDM instead of DSSS. The PLCP must reformat the data received from the MAC layer (when transmitting) into a frame that the PMD sublayer can transmit. An example of an 802.11a PLCP frame is illustrated in Figure 5-12.

Like an 802.11b frame, the 802.11a frame is made up of three parts: the preamble, the header, and the data. The preamble allows the receiving device to prepare for the rest of the frame, the header provides information about the frame itself and the data portion of

X 4 1 12 16

Transmission speed (Mbps)

Parts Preamble Header Data

6, 9, 12, 18, 24, 36, 48, or 54 Mbps6 Mbps

Synchronization Rate LengthRe- served

Size (bits)

Data

Parity

Tail Service

Tail

Pad

Figure 5-12 802.11a PLCP frame

IEEE 802.11 Physical Layer Standards 175

the PLCP frame is the information that is to actually be transmitted. A description of the fields is as follows:

● Synchronization. The Synchronization field consists of 10 repetitions of a short train- ing sequence signal and two repetitions of a long training sequence signal. The purpose of these signals is to establish timing and frequency with the receiver. The Synchroni- zation field is transmitted in 16 microseconds.

● Rate. The Rate field, which is 4 bits in length, specifies the transmission rate of the data field. The rate field values are shown in Table 5-5.

● Length. This field contains the value that indicates the length of the Data field, from 1 to 4095.

● Parity. Parity is used for error checking. ● Tail. This field indicates the end of the Header. All 6 bits are all set to zero. ● Service. The Service field is used to synchronize with the receiver. The first 7 bits are

set to zero while the remaining 9 bits are reserved for future use and are also set to zero. Although the Service field is part of the header, it is transmitted at the same rate as the Data field.

● Data. The actual data to be transmitted is contained in this field. The length of the data field is from 1 to 4095 bits.

● Pad. The IEEE standard specifies that the number of bits in the data field must be a multiple of 48, 96, 192, or 288. If necessary the length of the data field may need to be “padded” with extra bits, which are found in this field.

Modulation Techniques The modulation techniques used to encode the 802.11a data vary depending upon the speed:

● 6 Mbps. At this speed, phase shift keying (PSK) is used. The change in the starting point of the cycle varies depending on if a 0 or a 1 bit is being transmitted, as shown in Figure 5-13. PSK can encode 125 Kbps of data per each of the 48 subchannels, resulting in a 6,000 Kbps (125 Kbps � 48) or 6 Mbps data rate.

Data Rate (Mbps) Rate Field Contents

6 1101

9 1111

12 0101

18 0111

24 1001

36 1011

48 0001

54 0011

Table 5-5 802.11a rate field values

176 Chapter 5 Physical Layer Standards

● 12 Mbps. Whereas PSK only has a change in starting point, the type of modulation used at this speed also has a change in amplitude. Known as quadrature phase shift keying (QPSK), it can double the amount of data encoded over PSK to 250 Kbps per channel, which produces a 12-Mbps (250 Kbps x 48) data rate. QPSK is illustrated in Figure 5-14.

● 24 Mbps. Transmitting at 24 Mbps requires a 16-level quadrature amplitude modula- tion (16-QAM) technique. 16-QAM has 16 different signals that can be sent, as shown in Figure 5-15. Whereas QPSK requires two signals to send 4 bits, 16-QAM can transmit the same in only one signal. For example, to transmit the bits 1110, QPSK would send 11 and then 10 by modifying the phase and amplitude. 16-QAM would only send one signal (1110). 16-QAM can encode 500 Kbps per subchannel.

● 54 Mbps. Data rates of 54 Mbps are achieved by using 64-level quadrature amplitude modulation (64-QAM). 64-QAM, illustrated in Figure 5-16, can transmit 1.125 Mbps over each of the 48 subchannels.

Although 54 Mbps is the “official” top speed of 802.11a, the IEEE specification also allows for higher speeds as well. These higher speeds are known as turbo mode or 2X mode. 2X mode can be developed by each vendor and is not specified in the IEEE standard.

-3 -2 -1 +1 +2 +3

0 bit

Change in amplitude

1 bit Change in

starting point

Figure 5-13 Phase shift keying (PSK)

-3 -2 -1 +1 +2 +3

01 bit

Change in amplitude

11 bit

Change in starting point

00 bit 10 bit

-1

Figure 5-14 Quadrature phase shift keying (QPSK)

IEEE 802.11 Physical Layer Standards 177

Developers cannot further increase the complexity of the modulation on the subcarriers beyond the maximum 54-Mbps rate because of the amount of noise allowed. Instead, ven- dors can use other techniques such as combining frequency channels, increasing and reallo- cating the individual carriers, and using different coding rate schemes.

-3 -1 +1 +3

Change in amplitude

Change in starting point

-1

-3

01 0100 01

01 0000 00

10 0111 01

10 0011 00

01 1000 10

01 1100 11

10 1011 10

10 1111 11

Figure 5-15 16-level quadrature amplitude modulation (16-QAM)

-7 -5 -3 -1 +1 +3 +5 +7

Change in amplitude

Change in starting point

-1

-3

010 010011 010

010 011011 011

111 010110 010

111 011110 011

010 111011 111

010 110011 110

111 111110 111

111 110110 110

-5

-7

010 100011 100

010 101011 101

001 111000 111

001 110000 110

001 100000 100

001 101000 101

010 001011 001

010 000011 000

001 010000 010

001 011000 011

001 001000 001

001 000000 000

111 001110 001

111 000110 000

100 010101 010

100 011101 011

100 001101 001

100 000101 000

111 100110 100

111 110110 101

100 111101 111

100 110101 110

100 100101 100

100 101101 101

Figure 5-16 64-level quadrature amplitude modulation (64-QAM)

178 Chapter 5 Physical Layer Standards

Table 5-6 summarizes the IEEE 802.11a Physical layer standards.

Channel Allocation With 802.11b, the available frequency spectrum (2.412–2.484 GHz) is divided into 11 useable channels, only three of which are nonoverlapping channels available for simultaneous operation. This is illustrated in Figure 5-17.

GHz

2.402 2.412 2.422 2.432 2.442 2.452 2.462 2.472

2.407 2.417 2.427 2.437 2.447 2.457 2.467

Channel 1

Channel 2

Channel 3

Figure 5-17 802.11b channels

Transmission Speed (Mbps) Modulation

6 Phase shift keying (PSK)

12 Quadrature phase shift keying (QPSK)

24 16-level quadrature amplitude modulation (16-QAM)

54 64-level quadrature amplitude modulation (16-QAM)

Table 5-6 802.11a characteristics

Channel numbers

5.16 5.18 5.20 5.22 5.24 5.26 5.28 5.30 5.32 5.34

36 40 44 48 52 56 60 64

300 KHz

1 2 3 4 5 51 52

20 MHz

Channel 48

Carriers

Figure 5-18 802.11a channels

IEEE 802.11 Physical Layer Standards 179

Within each 802.11a frequency channel, there is a channel 20 MHz wide that supports 52 carrier signals, with each signal 300 KHz wide, as shown in Figure 5-18. The center points for the eight channels in this figure are 5.18, 5.20, 5.22, 5.24, 5.26, 5.28, 5.30, and 5.32 GHz. Table 5-7 shows a list of 802.11a channel frequencies used in the United States.

The original 802.11a standard specified 12 nonoverlapping channels, with 4 channels each spread across three areas of usage: indoor, indoor/outdoor, and outdoor usage. However, these areas were virtu- ally ignored. A later revision added 11 additional channels.

IEEE 802.11a networks have 555 MHz spread across 23 nonoverlapping channels. Because there are an increased number of available channels, more users can access more bandwidth when the WLAN is managed correctly. There are additional advantages of having more channels. When multiple APs are used, more users can be supported by assigning specific

Channel Frequency (GHz)

36 5.180

40 5.200

44 5.220

48 5.240

52 5.260

56 5.280

60 5.300

64 5.320

100 5.500

104 5.520

108 5.540

112 5.560

116 5.580

136 5.680

140 5.700

149 5.745

153 5.765

157 5.785

161 5.805

165 5.825

Table 5-7 802.11a characteristics

180 Chapter 5 Physical Layer Standards

channels to users associated with specific APs. In addition, if your neighbor is also using his own 802.11a WLAN within range of your network it could cause interference and band- width contention. However, the fact that more channels are available means this is not a problem, because it is possible to set the AP to use a different channel to reduce or eliminate the interference.

IEEE 802.11g Physical Layer Standards The 802.11g standard uses the 2.4-GHz ISM frequency instead of the UNII band used by 802.11a. The standard outlines two mandatory modes along with one optional mode. The first mandatory mode is the same CCK mode used by 802.11b at 11 and 5.5 Mbps. The second mandatory mode for the faster 54 Mbps, known as extended rate PHYs (ERP), uses OFDM like the 802.11a standard but still in the 2.4-GHz frequency. The optional mode can transmit at 22 Mbps and is known as PBCC-22 (Packet Binary Convolutional Coding), which has speeds between 6 and 54 Mbps. The 802.11g standards are summa- rized in Table 5-8.

The 802.11g standard provides greater throughput than 802.11b networks and covers a broader area than 802.11a networks while retaining backward compatibility with 802.11b devices. However, 802.11g does have some disadvantages. First, it offers only three nonover- lapping channels, compared to 23 channels for 802.11a. Second, in an environment where both 802.11b and 802.11g devices are transmitting, all 802.11g devices will drop to only 11 Mbps speeds.

Transmission Speed (Mbps) Modulation DSSS Coding Technique

1 Differential binary phase shift keying (DBPSK)

Barker code

2 Differential quadrature phase shift keying (DQPSK)

Barker code

5.5 Differential quadrature phase shift keying (DQPSK)

Complementary code keying (CCK)

6 (mandatory speed) OFDM n/a

11 Differential quadrature phase shift keying (DQPSK)

Complementary code keying (CCK)

12 (mandatory speed) OFDM n/a

18 (optional speed) OFDM n/a

22 (optional speed) PBCC-22 n/a

24 (mandatory speed) OFDM n/a

36 (optional speed) OFDM n/a

48 (optional speed) OFDM n/a

54 (optional speed) OFDM n/a

Table 5-8 IEEE 802.11g Physical Layer standards

IEEE 802.11 Physical Layer Standards 181

Some vendors are now implementing proprietary techniques that will allow each device in a “mixed mode” setting of 802.11b and 802.11g devices to transmit at its highest available speed.

IEEE 802.11n Physical Layer Standards Enhancements to the IEEE 802.11n PHY contribute to the increased speed and range of 802.11n networks. These enhancements include 40-MHz channels and variable guard interval. The results can be categorized in the modulation and coding schemes tables.

40-MHz Channels The bandwidth of the channel determines the speed of the transmis- sion and is a measure of the efficiency of the radio. This efficiency, known as spectral efficiency, is measured in the number of bits per Hertz. The IEEE 802.11 and 802.11b using DSSS have a channel spacing that is 22 MHz wide while 802.11a/g using OFDM have 20-MHz-wide channels. The spectral efficiency of 802.11b is 0.5 bits per Hertz (or 11 Mbps in 22 MHz), while 802.11a and 802.11g have a higher spectral efficiency, as much as 2.7 bits per Hertz at 54 Mbps.

IEEE 802.11n WLANs can use channels that are either 20 MHz or 40 MHz wide. The 40-MHz channels are actually two adjacent 20-MHz channels that are “bonded” together. The two channels are known as the primary channel and the secondary channel. Yet instead of simply doubling the data rate, using a 40-MHz channel can actually increase the rate even more. Each 20-MHz channel has a small amount of space reserved at the top and the bottom of the frequency to reduce interference from other channels. Because channel bond- ing does not need the space between the two bonded channels, it can instead be used to increase the channel size even more.

Due to regulatory issues, not all countries allow channel bonding 40- MHz systems.

However, because channel bonding consumes two adjacent channels, it could result in an increased probability of interference with other WLANs. Although it is unlikely to be a problem with 802.11n operating in the 5-GHz frequency (it has 23 nonoverlapping chan- nels), this is not the case when 802.11n is using the 2.4-GHz frequency where only three of the eleven channels are nonoverlapping. For an 802.11n to use channel bonding it would consume nine of the eleven channels (the center frequency plus four channels on the left and four on the right), result in interference with virtually any other 802.11b/g/n 2.4-GHz network operating in the area.

Two safeguards can protect against interference caused by channel bonding. First, 802.11n WLANs using 40-MHz channels are required to listen for other wireless devices.

182 Chapter 5 Physical Layer Standards

The 802.11n AP can automatically move to another channel or switch to 20-MHz oper- ation if another AP starts operating in either half of the designated 40-MHz channel. This is known as Dynamic Frequency Selection (DFS). In another option, called Phased Coexistence Operation (PCO), an 802.11n AP alternates between using 20MHz and 40- MHz channels.

Variable Guard Interval The guard interval (GI) is a “quiet” period between OFDM symbols that allows for the arrival of late symbols over long multipaths without impacting the earlier symbols. IEEE 802.11a/b/n WLANs use 800 nanoseconds as the GI. However, if the environment does not result in significant multipath then 802.11n can also use a reduced guard interval of 400 nanoseconds, known as a variable guard interval (VGI). This reduces the symbol time from 4 microseconds to 3.6 microseconds and can increase the rate of transmission.

Modulation and Coding Scheme (MCS) The original IEEE 802.11 DSSS trans- mitted a symbol of 11 chips lasting one microsecond using a phase shift key (PSK) tech- nique. At 1 Mbps, a single symbol was sent every microsecond. The 2-Mbps rate was achieved by sending two symbols each microsecond using a four-phase PSK. IEEE 802.11b coded more bits into each symbol using DQPSK in order to achieve data rates of 11 Mbps. For IEEE 802.11a/g, a symbol lasts four microseconds, including an 800 nano- second GI. For the highest data rate, 54 Mbps, each symbol carries 216 data bits spread out over 48 subcarriers using 64-QAM.

However, for IEEE 802.11n networks, there are a wide number of options for transmit- ting. Although it continues to use OFDM and a four-microsecond symbol, similar to 802.11a/g, 802.11n increases the number of subcarriers in each 20-MHz channel from 48 to 52. (This marginally increases the data rate to a maximum of 65 Mbps for a single- transmit radio in IEEE 802.11n.) The IEEE 802.11n standard allows a selection of eight data rates for a transmitter to use and also increases the number of transmitters allowable to four. When using 40-MHz channels, 802.11n increases the number of subcarriers avail- able to 108. Similarly, eight data rates are provided for each transmitter for the 40-MHz channel.

In total there are 77 possible combinations of these factors—modulation, convolutional coding rate (a type of error-correcting code), guard interval, channel width, and number of spatial streams—for 802.11n. The IEEE standard defines a Modulation and Coding Scheme (MCS) that outlines the different combinations and assigns an index number to each scheme. MCS 0–31 define the same modulation and coding that is used on all streams, while MCS 32–76 describe mixed combinations that can be used to modulate two to four streams (for example, MCS 33 defines using 16-QAM on spatial stream #1 and QPSK on stream #2 while MCS 77 refers to using 64-QAM on streams #1–3 with 16- QAM on stream #4). All 802.11n APs are required to support at least MCS 0–15, while 802.11n wireless devices must support MCS 0–7. All other MCS values, including those associated with 40-MHz channels and VGI, are optional. A partial MCS table is shown in Table 5-9.

IEEE 802.11 Physical Layer Standards 183

Chapter Summary ■ Radio frequency signals that are transmitted on only one frequency are called narrow-

band transmissions. Several disadvantages of narrowband transmissions for WLANs make it unusable. An alternative to narrowband transmission is spread-spectrum transmission. Whereas a narrowband transmission sends a strong signal using a small portion of the spectrum, spread spectrum transmits a weaker signal across a broader portion of the radio frequency band. Spread-spectrum transmission uses three methods to spread the signal over a wider area. The methods are frequency-hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), and orthogonal frequency division multiplexing (OFDM). Instead of transmitting on a single frequency, FHSS uses a range of frequencies. DSSS uses an expanded redundant code to transmit each

Data Rate (Mbps) 20-MHz Channel

Data Rate (Mbps) 40-MHz Channel

MCS Index Spatial Streams

Modulation Type 800 ns GI 400 ns GI 800 ns GI 400 ns GI

0 1 BPSK 6.5 7.20 13.50 15.00

1 1 QPSK 13.00 14.40 27.00 30.00

2 1 QPSK 19.50 21.70 40.50 45.00

3 1 16-QAM 26.00 28.90 54.00 60.00

4 1 16-QAM 39.00 43.30 81.00 90.00

5 1 64-QAM 52.00 57.80 108.00 120.00

6 1 64-QAM 58.50 65.00 121.50 135.00

7 1 64-QAM 65.00 72.20 135.00 150.00

8 2 BPSK 13.00 14.40 27.00 30.00

9 2 QPSK 26.00 28.90 54.00 60.00

10 2 QPSK 39.00 43.30 81.00 90.00

11 2 16-QAM 52.00 57.80 108.00 120.00

12 2 16-QAM 78.00 86.70 162.00 180.00

13 2 64-QAM 104.00 115.60 216.00 240.00

14 2 64-QAM 117.00 130.00 243.00 270.00

15 2 64-QAM 130.00 144.40 270.00 300.00

16 3 BPSK 19.5 21.70 40.50 45.00

… … … … … … …

31 4 64-QAM 260.00 288.90 540.00 600.00

Table 5-9 Modulation and Coding Scheme (MCS)

184 Chapter 5 Physical Layer Standards

data bit. OFDM breaks the transmission into separate parts to send each part in par- allel simultaneously. These parallel transmissions are sent more slowly to minimize the impact of delay spread.

■ IEEE wireless standards follow the OSI model with some modifications. The IEEE has divided the Data Link layer into two sublayers: the Logical Link Control (LLC) sublayer (which provides a common interface, reliability, and flow control) and the Media Access Control (MAC) sublayer (which appends physical addresses to the frame). The IEEE standards specify that the features of a WLAN must be transparent to the upper layers of the IEEE model so that the PHY and MAC layers of the IEEE 802.11 wireless LANs function like those of other IEEE network standards (such as Ethernet) in sharing the same LLC layer. Because all of the WLAN features are isolated in the PHY and MAC layers, any network operating system or LAN application will run on an IEEE WLAN without modification.

■ For IEEE 802.11b the PLCP standards are based on DSSS. The 802.11b standard uses the Industrial, Scientific and Medical (ISM) band for its transmissions and specifies 14 frequencies that can be used, beginning at 2.412 GHz and incrementing by .005 GHz. These transmissions in the 2.4-GHz frequency are designed to be þ /–11 MHz from the channel center frequency. This results in only three nonoverlap- ping (simultaneously usable) 20-MHz channels: 1, 6, and 11.

■ The 802.11a standards are significantly different from the 802.11b standards. While the 802.11b standard uses the unlicensed Industrial, Scientific and Medical (ISM) band for its transmissions, the 802.11a standard uses another unlicensed band, the Unli- censed National Information Infrastructure (UNII). The 802.11a standard enables faster rates because of the higher frequencies and increased power at which it operates. The total bandwidth available for IEEE 802.11a WLANs using UNII is almost four times that available for 802.11b networks using the ISM band. IEEE 802.11a also handles errors differently than 802.11b, which results in an increase in speed. Within each 802.11a frequency channel there is a channel 20 MHz wide that supports 52 carrier signals, each 300 KHz wide. IEEE 802.11a networks have 555 MHz spread across 23 nonoverlapping channels. Because of the increased number of available channels, more users can access more bandwidth when the WLAN is managed correctly.

■ The 802.11g standard uses the 2.4-GHz ISM frequency instead of the UNII band used by 802.11a. The 802.11g standard provides greater throughput than 802.11b net- works and covers a broader area than 802.11a networks while retaining backward compatibility with 802.11b devices. However, 802.11g does have some disadvantages. First, the number of nonoverlapping channels available is still only three with 802.11g compared with eight channels for 802.11a. Second, in an environment where both 802.11b and 802.11g devices are transmitting, all 802.11g devices will drop to only 11 Mbps speeds.

■ Enhancements to the IEEE 802.11n PHY contribute to the increased speed and range of 802.11n networks. IEEE 802.11n WLANs can use channels that are either 20 MHz or 40 MHz wide. The 40-MHz channels are actually two adjacent 20-MHz channels that are “bonded” together. There are two safeguards to protect against the interfer- ence as a result of channel bonding. While IEEE 802.11a/b/n WLANs use 800

Chapter Summary 185

nanoseconds as the guard interval, if the environment does not result in significant multipath then 802.11n can also use a reduced guard interval of 400 nanoseconds, known as a variable guard interval (VGI). In total there are 77 possible combinations of factors for 802.11n. The IEEE standard defines a Modulation and Coding Scheme (MCS) that outlines the different combinations and assigns an index number to each scheme.

Key Terms 16-level quadrature amplitude modulation (16-QAM) A modulation technique that sends 16 different signals simultaneously. 2X mode A proprietary transmission scheme that doubles the effective rate of an 802.11a network. 64-level quadrature amplitude modulation (64-QAM) A modulation technique that can transmit 1.125 Mbps over each of 48 subchannels. bandwidth The difference between the upper and lower frequencies. Barker code The bit pattern used in direct sequence spread spectrum (DSSS) modulation. channel A numeric value assigned to a frequency range. chipping code The bit pattern used in direct sequence spread spectrum (DSSS) modulation. colocation Sharing a frequency band between similar devices. communication resilience Term used to describe transmissions that are less prone to interference. complementary code keying (CCK) A coding technique used in 802.11b networks that consists of a set of 64 8-bit code words. convolutional coding rate A type of error-correcting code. differential binary phase shift keying (DBPSK) A two-level phase shift key used in 802.11b networks. differential quadrature phase shift keying (DQPSK) A four-level phase change used in 802.11b networks. direct sequence spread spectrum (DSSS) A wireless modulation technique that uses an expanded redundant code to transmit each data bit. dwell time The amount of time that a transmission remains on a specific frequency in FHSS. Dynamic Frequency Selection (DFS) A technology in which IEEE 802.11n WLANs using 40-MHz channels can automatically move to another channel or switch to 20-MHz operation to minimize interference. extended rate PHYs (ERP) A mandatory mode for the faster 54 Mbps in IEEE 802.11g. Forward Error Correction (FEC) An IEEE 802.11a error correction technique that transmits a secondary copy along with the primary information. frequency-hopping spread spectrum (FHSS) A modulation technique that uses a range of frequencies that change during the transmission.

186 Chapter 5 Physical Layer Standards

full-channel FHSS A FHSS technology in which the devices use a minimum of 75 hop channels. guard interval (GI) A delay built-in into the receiver to allow for late-arriving symbols. High-Rate DSSS (HR-DSSS) Transmission rates above 2 Mbps in IEEE 802.11b. hopping code The sequence of changing frequencies in FHSS. hop time The time it takes to change a frequency in FHSS. intersymbol interference (ISI) Signal interference as a result of multipath transmission. microseconds (μs) One millionth of a second. milliseconds (ms) One thousandth of a second. Modulation and Coding Scheme (MCS) A system that assigns a numeric value to each of the 77 possible transmission combinations IEEE 802.11n. multiplexing The process of sending multiple signals simultaneously. narrowband transmission Radio signals that are sent on only one radio frequency or a very narrow portion of the frequencies. Open Systems Interconnection (OSI) reference model A seven-layer model that conceptually illustrates the steps of networking. orthogonal frequency division multiplexing (OFDM) A modulation technique that splits a single high-speed digital signal into several slower signals running in parallel. PBCC-22 (Packet Binary Convolutional Coding) An optional 802.11g technique for transmitting at 22 Mbps. Phased Coexistence Operation (PCO) An optional IEEE 802.11n technology that alternates between using 20-MHz and 40-MHz channels. Physical Layer Convergence Procedure (PLCP) A Physical layer sublayer that reformats the data received from the MAC layer (when transmitting) into a frame that the PMD sublayer can transmit “listens” to the medium to determine when the data can be sent. Physical Medium Dependent (PMD) A Physical layer sublayer that defines the standards for both the characteristics of the wireless medium and the method for transmitting and receiving data through that medium. primary channel The first channel of two bonded IEEE 802.11n channels. quadrature phase shift keying (QPSK) An IEEE 802.11a modulation technique that increases the amount of data encoded to 250 Kbps per channel. reduced-channel FHSS A FHSS technology in which the devices use fewer than 75 hop channels. secondary channel The second channel of two bonded IEEE 802.11n channels. spectral efficiency The efficiency of the radio measured in the number of bits per hertz. spread-spectrum transmission A technique that takes a narrow, weaker signal and spreads it over a broader portion of the radio frequency band. symbols Radio frequency signals. transmit power control (TPC) An IEEE 802.11a technology to reduce interference. turbo mode A proprietary transmission scheme used to double the effective rate of an 802.11a network. variable guard interval (VGI) An IEEE 802.11n technology that uses a reduced guard interval of 400 nanoseconds.

Key Terms 187

Review Questions 1. Which of the following is an advantage of narrowband transmissions?

a. the number of chips used

b. interference

c. the amount of power used

d. the amount of spectrum used

2. Each of the following is an advantage of spread-spectrum transmissions except:

a. resistance to narrowband interference.

b. higher power requirements.

c. more information transmitted.

d. resistance to multipath distortion.

3. Each of the following is a type of spread-spectrum technology except:

a. FHSS.

b. DSSS.

c. UNII.

d. OFDM.

4. is the difference between the upper and lower frequencies.

a. Bandwidth

b. Spread spectrum

c. IRP

d. Guard interval

5. In FHSS the amount of time used to change from one frequency to the next is called the .

a. dwell time

b. phase

c. hop time

d. modulation event (ME)

6. Which of the following technologies use a chipping code?

a. FHSS

b. DSSS

c. OFDM

d. ISM

188 Chapter 5 Physical Layer Standards

7. When using each device is assigned a unique chipping code so that all the transmissions can use the same frequency yet remain separate.

a. Chip Allocation Standard (CAS)

b. shared bandwidth

c. PMDS

d. colocation

8. With OFDM, the is intended to reduce intersymbol interference (ISI).

a. presymbol response

b. vertical symbol delay (VSD)

c. dwell time

d. guard interval

9. The sublayer reformats the data received from the higher layer into a frame that can be transmitted.

a. Physical Medium Dependent (PMD)

b. Physical Layer Convergence Procedure (PLCP)

c. Media Access Control (MAC)

d. Logical Link Control (LLC)

10. DSSS is used by .

a. 802.11b

b. 802.11a

c. 802.11n

d. 802.11h

11. The PLCP frame preamble and header in 802.11b are always transmitted at Mbps.

a. 1

b. 2

c. 5.5

d. 11

12. Each of the following channels is a nonoverlapping frequency in 802.11b except:

a. 1.

b. 6.

c. 8.

d. 11.

Review Questions 189

13. Which of the following is not a characteristic of IEEE 802.11a?

a. It uses ISM instead of UNII.

b. The bandwidth is 300 MHz.

c. The range of coverage is less than 802.11b.

d. The fastest speed is 54 Mbps with an optional proprietary speed of 108 Mbps.

14. transmits a secondary copy along with the primary information to account for errors.

a. Forward Error Correction (FEC)

b. Multiplexing

c. Bandwidth diversity

d. DSSS framing

15. How many nonoverlapping channels are available in IEEE 802.11a?

a. 3

b. 12

c. 23

d. 56

16. Two safeguards to protect 802.11n channel bonding from interfering with other WLANs are Dynamic Frequency Selection (DFS) and .

a. Phased Coexistence Operation (PCO)

b. MIMO Substitution

c. Channel Allocation

d. MIMO Branching (MIBRA)

17. The ability to adjust the “quiet” period between OFDM symbols to allow for the arrival of late symbols in 802.11n is called .

a. ISI delay

b. space allocation

c. modulation adjustment

d. variable guard interval (VGI)

18. All of the possible combinations of different factors for 802.11n transmissions are contained in the .

a. UNII Procedures Manual

b. Wi-Fi Table

c. IEEE 802.11n Resource Guide

d. Modulation and Coding Scheme (MCS)

190 Chapter 5 Physical Layer Standards

19. The abbreviation μs stands for .

a. microsecond

b. millisecond

c. nanosecond

d. kilosecond

20. A numeric value that is assigned to a frequency range is called the .

a. downfade

b. bandwidth

c. modulation

d. channel

Hands-On Projects

Project 5-1: Comparing WLAN Utilization Statistics Using a Wireless Network Simulator—Part 1 Wireless network simulators allow WLAN technicians to simulate a network as an aid to designing a network. In this project you will install and use the WLAN simulator Pamvotis.

1. Use your Web browser to go to pamvotis.org/.

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions then open a search engine and search for “Pamvotis WLAN simulator”.

2. Scroll down to the Download and Installation Instructions section.

3. Next to Setup (Windows only) click Download.

4. Follow the instructions to install Pamvotis on your computer.

5. Launch the Pamvotis application.

6. First configure an IEEE 802.11 network at 1 Mbps running for 150 seconds (2.5 min- utes). Under Events Configuration change Simulation Time (sec): to 150.

7. Under Statistics Results Configuration, click Choose Statistics.

8. Under Available Statistic Results, check Throughput (Kb/s) or kilobits per second, Throughput (pkts/s) or packets per second, and Utilization. The Mean Values checkbox is selected by default.

9. Click OK to return to the Pamvotis 1.1—WLAN Simulator screen.

10. Under Nodes Configuration, click Configure.

11. In the Physical Layer section, click IEEE 802.11.

12. In the Nodes section, change Number of nodes to 25.

Hands-On Projects 191

13. In the Data Rate section, click 1Mb/s.

14. Click OK to return to the Pamvotis 1.1—WLAN Simulator screen.

15. Click Run Simulation. When the simulation finishes click Close.

16. Click the View Results button.

17. Click Throughput_bits.txt and then click Open to view the raw data results of the simula- tion. This is the number of bits that a node successfully transmitted in a specific time interval. Scroll through this data to see the results. Close this file when you are finished.

18. Click the View Results button.

19. Click Througput_packets.txt and then click Open. This is the number of packets that a node successfully transmitted in a specific time interval. Scroll through this file and close it when you are finished.

20. Click the View Results button.

21. Click Utilization.txt and then click Open. The utilization is the percentage of the channel capacity the node occupied and is calculated as the node’s throughput in bits per second divided by the node’s data rate. Scroll through this file and close it when you are finished.

22. Click the View Results button.

23. Click Mean_Values.txt and then click Open. This contains the mean (average) of the accumulated values for the simulation.

24. Click File and Save As. Save this file as 802.11 1Mbps Mean_Values.txt.

25. Next, you will create a second simulation of an IEEE 802.11 network at 2 Mbps running for 150 seconds (2.5 minutes). Under Nodes Configuration click Configure.

26. In the Data Rate section click 2Mb/s.

27. Click OK to return to the Pamvotis 1.1—WLAN Simulator screen.

28. Click Run Simulation. When the simulation finishes click Close.

29. Click the View Results button.

30. Click Mean_Values.txt to view the results and then click Open.

31. Click File and Save As. Save this file as 802.11 2Mbps Mean_Values.txt.

32. Leave Pamvotis open for the next project.

Project 5-2: Comparing WLAN Utilization Statistics Using a Wireless Network Simulator—Part 2 Wireless network simulators allow WLAN technicians to simulate a network as an aid to designing a network. In this project you will continue to create WLAN simulations and then compare them.

1. If Pamvotis is not open, then launch the application.

2. Now you will create a third simulation of an IEEE 802.11g network at 54Mbps running for 150 seconds (2.5 minutes). Under Nodes Configuration, click Configure.

192 Chapter 5 Physical Layer Standards

3. In the Physical Layer section, click IEEE 802.11g.

4. In the Data Rate section, click 54Mb/s.

5. Click OK to return to the Pamvotis 1.1—WLAN Simulator screen.

6. Click Run Simulation. When the simulation finishes click Close.

7. Click the View Results button.

8. Click Mean_Values.txt to view the results and then click Open.

9. Click File and Save As. Save this file as 802.11g 54Mbps Mean_Values.txt.

10. Now you will create a fourth simulation of an IEEE 802.11a network at 54 Mbps running for 150 seconds (2.5 minutes). Under Nodes Configuration, click Configure.

11. In the Physical Layer section, click IEEE 802.11a.

12. Click OK to return to the Pamvotis 1.1—WLAN Simulator screen.

13. Click Run Simulation. When the simulation finishes click Close.

14. Click the View Results button.

15. Click Mean_Values.txt to view the results and then click Open.

16. Click File and Save As. Save this file as 802.11a 54Mbps Mean_Values.txt.

17. Close Pamvotis.

18. Next, you will import data from these four simulations into Microsoft Excel 2010. Launch Excel on your computer.

19. Click the Data tab on the Ribbon.

20. Click From Text in the Get External Data group.

21. Navigate to the first file, 802.11 1Mbps Mean_Values.txt in the C:\Program Files\ Pamvotis 1.1 folder, and click Import.

22. Be sure that the file type is Delimited and click Next.

23. Be sure that Tab is selected under Delimiters.

24. Click Treat consecutive delimiters as one. Click Next.

25. Click Finish.

26. When the Where do you want to put the data? question appears, select cell A1 if it is not already selected. Click OK. The information is now imported into Excel.

27. Move the cell selector two rows below the last row of the imported data.

28. Follow the same steps to import the other remaining files, 802.11 2Mbps Mean_Values.txt, 802.11g 54Mbps Mean_Values.txt, and 802.11a 54Mbps Mean_Values.txt.

29. Pay attention to the Utilization (msec) column. What can you say about the utilization for these different standards? Write a one-paragraph summary of why they are different.

30. Save the Excel file as Project 5-2 and then close Excel.

31. Close all windows.

Hands-On Projects 193

Project 5-3: Downloading and Installing Xirrus Wi-Fi Inspector Wireless Monitor In this project you will download and install the Xirrus Wi-Fi Inspector, an enhanced version of the Xirrus Monitor gadget.

1. Use your Web browser to go to www.xirrus.com/library/wifitools.php.

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions then open a search engine and search for “Xirrus Wi-Fi Inspector”.

2. If necessary, scroll down to the section Xirrus Wi-Fi Inspector.

3. Under Download Xirrus Wi-Fi Inspector, click Wi-Fi Inspector v1.2.0 (or the current version).

4. Click Save and specify the location for the download.

5. When the download has finished, navigate to the location of the downloaded file.

6. Double-click the application file.

7. Click the Next button and accept the default settings to install Xirrus Wi-Fi Inspector.

8. If necessary, launch the application.

9. In the Layout box, click Show All.

10. Under Radar, note the dBm of the different wireless networks.

11. Under Networks, select one of the networks and note its Signal (dBm), Network Mode, Chan- nel and Frequency. What have you learned in this chapter about each of those elements?

12. Under Networks, check Graphs for each of the wireless networks that you can detect. The signal strength will be graphed under Signal History. If necessary, hold down the ALT key and click the mouse to change the scale of the graph.

13. If Wi-Fi Inspector is installed on a portable device carry it to another location and note any changes. What types of objectives impact the signal strength the most? Which impact it the least? Why? What happens to the dBm as you move?

14. How helpful is this information?

15. Close all windows.

Case Projects

Case Project 5-1: FHSS Uses Although FHSS is not used for WLANs, it is found in other technologies and applications. Use the Internet to research how FHSS is used. Why was it cho- sen for these uses? What advantages make it the best choice? Write a one-page paper on your findings.

194 Chapter 5 Physical Layer Standards

Case Project 5-2: Multiplexing One of the key elements of OFDM is its use of multiplexing. Multiplexing is commonly found in different telecommunications technologies. Using the Internet, research the topic of multiplexing. Where is it used? Why is it chosen? What can multiplexing save in transmis- sions? Write a one-page paper about your research.

Case Project 5-3: Combating Interference What steps should you take when radio frequency interference impacts an IEEE 802.11 net- work? Using the Internet and other sources, research preventive measures for reducing inter- ference from such transmitters as cordless telephones, microwave ovens, and WLAN signals from a neighbor’s system. Create a paper that lists the different potential sources and inter- ference and what can be done to mitigate them.

Case Project 5-4: Channel Bonding While 802.11n can make adjustments if channel bonding impacts another WLAN, 802.11a/g does not have these capabilities. If your upstairs apartment neighbor is using channel bonding and your downstairs neighbor is using the other channel, you could have no available frequency for your AP. How would you handle this situation? Should you be able to claim that their equipment is encroaching upon your airspace? Using the Internet and other sources, research this dilemma. What is your opinion on this controversy? How would you handle a situation like this?

Case Project 5-5: Nautilus IT Consulting Nautilus IT Consulting (NITC), the computer technology business that helps organizations with IT solutions, has asked for your help. Craig’s Discount Furniture is considering migrating to IEEE 802.11n, but one of the com- pany’s IT staff members states that the benefits of 802.11n do not outweigh the time and expense it would take to upgrade from 802.11g. Although some of the IT staff say that 802.11n has advantages, they are unsure exactly what these advantages are and if they would be useful to the organization. You have been asked to create a presentation compar- ing 802.11n with 802.11g.

1. Create a PowerPoint presentation of eight or more slides that compares 802.11n and 802.11g. Be sure to include the technical reasons behind the 802.11n enhancements. Because you will be addressing the IT staff, this presentation should contain technical information.

2. After your presentation, Craig’s Discount Furniture is leaning towards implementing 802.11n. However, they are unsure which of the variations of 802.11n they should use for their warehouse, which covers 30,000 square feet yet is essentially open with no interior walls. Using the MCS, decide on an 802.11n technology that would be most appropriate for this setting. Create a one-page memo that summarizes your choice and explains why Craig’s Discount Furniture should explore this option.

Case Projects 195

chapter6

Media Access Control Layer Standards

After completing this chapter you should be able to:

• Describe the three WLAN service sets • Explain the features of MAC frames and MAC frame types • Describe the MAC functions of discovering, joining, and transmitting on a WLAN

197 Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

Since the ratification of the IEEE 802.11n standard in September 2009, the number of 802.11n devices sold has increased much faster than first anticipated. Less than one year after its ratification, almost 1,700 “N” devices were certified by the Wi-Fi Alliance, including not only access points (APs) but also telephone handsets, digital cameras, and even flat-panel TVs. IEEE 802.11a/b/g devices were outsold by 802.11n devices by almost 40,000 units in a three-month time span in mid-2010. In fact, during the first half of 2010, over 420,000 802.11n APs were sold in North America, compared to 520,000 units in all of 2009.

And who is leading the way in the purchase and installation of 802.11n devices? The answer is colleges and universities. The primary reason is because of the expecta- tions of today’s students. Many students have never connected their laptop to a wired network with an Ethernet patch cable, yet they expect to have universal net- work access to play games, watch online videos, and even do the occasional home- work assignment. In addition, most students have multiple wireless devices—game consoles, smartphones, tablets, printers, and Internet alarm clocks—that all require constant wireless access.

This is creating a dramatic change in the design and installation of campus net- works. Today’s WLANs at colleges and universities are mission-critical production net- works that are optimized for high capacity as well as high performance. These campus WLANs are designed to be the primary data network for students and faculty, re- placing wired Ethernet. This has resulted in a significant decrease in the use of wired Ethernet ports. At Carnegie Mellon University in Pennsylvania the school has deacti- vated all wired ports in each of its dorm rooms (students can still request a wired port). Brandeis University in Massachusetts renovated four student dormitories and only provides 802.11n wireless connectivity for their students. The estimated cost for rewiring the four dorms was $200,000, while the actual cost of installing the wireless network was less than $80,000 due to savings in capital, licensing, maintenance, and operations. Many schools are focusing on capacity over coverage, designing the net- work for a maximum of 15 clients per AP in order to provide adequate data rates.

However, WLANs have grown overburdened as more students bring wireless gear onto campus and use it for high-capacity downloads or watching movies. Some schools are looking at transmit beam forming as an option to improve performance, while others are utilizing techniques to exploit 802.11n’s multiple antennas for improved signal reliability to give users a more consistent signal without fading or dropping. New IEEE 802.11 amendments that address power management to improve battery life, improve support for location data, and give wireless devices more “intelligence” to work with APs to improve signal quality may also help.

Real World Wireless

198 Chapter 6 Media Access Control Layer Standards

As you learned in the last chapter, the IEEE 802.11 standard specifies that all WLAN features are implemented in the PHY and MAC layers. Isolating all of the WLAN functions in the bot- tom two layers of the IEEE 802.11 standard does not mean that new features cannot be added to the existing upper layers; a new feature can be added as long as it does not modify the two lower layers. Because no modifications are needed at any of the other layers, existing software designed to meet other IEEE 802 standards will correctly operate on 802.11 WLANs. Any network operating system or LAN application that functions on a standard LAN will also run on a WLAN without modification. One of the original reasons for this design was to enable support for a wide range of protocols, including Novell IPX/SPX, Micro- soft NetBEUI, and AppleTalk, but since TCP/IP has become the standard network protocol this is no longer the “selling point” that it once was.

In this chapter you will learn about the three types of WLAN configurations. You will also look in detail at the IEEE 802.11 MAC layer standard that implements WLAN features.

WLAN Service Sets

C W N A

3.1.2. Define, describe, and apply the following concepts associated with WLAN service sets.

3.2.2. Identify methods described in the IEEE 802.11-2007 standard for locating, joining, and maintaining connectivity with an IEEE 802.11 WLAN.

A service set is all of the devices that are associated with an 802.11 WLAN. There are three different wireless LAN service set configurations: the basic service set, the extended service set, and the independent basic service set.

Basic Service Set A Basic Service Set (BSS) is defined as one or more wireless client devices (called stations or STAs) that are served by a single AP. These devices send all transmissions to the AP as well as receive transmissions from it. Although by definition it is not required that the AP be connected to another network, practically speaking the BSS would have limited functionality if the AP were not connected: the stations would only be able to communicate between each other but not to any other devices or networks outside the BSS. A BSS with one AP connected to a wired network is shown in Figure 6-1.

The BSS must be assigned a unique identifier to differentiate it from other WLANs. This “logical network name” is known as the Service Set Identifier (SSID) and is created by the administrator of the WLAN. The Basic Service Set Identifier (BSSID) is a separate identifier that is the media access control (MAC) address of the AP. The BSSID is included in the header of frames that are transmitted by the AP and stations for a variety of identification purposes.

The SSID can be an alphanumeric string from 2– 32 characters long.

WLAN Service Sets 199

The physical area of radio frequency (RF) coverage provided by the AP of a BSS is called a Basic Service Area (BSA). The advertised BSA of an 802.11b network states that a station can be up to 375 feet (114 meters) away from the AP. IEEE 802.11g networks have a similar BSA to 802.11b networks, 802.11a networks have a smaller BSA, and 802.11n networks have a larger BSA.

However, whereas the BSA is the geographical limit of the RF signal, it is not always the same as the practical coverage area of a BSS. This is because there are several factors that can impact the practical distance a station can be away from the AP and still have an accept- able throughput. These factors include:

● Obstructions. Walls, doors, ceilings, elevators, and other signal obstructions can limit the distance the RF signal may travel.

● Number of users. Because all devices in the BSS share the same medium (similar to an Ethernet network using a hub instead of a switch), having too many users in a single BSS can result in poor throughput rates.

● Applications. A wireless user reading text-based e-mail consumes less bandwidth than a user watching a high-definition movie on a WLAN. Due to the fact that the medium is shared, this can have an impact on throughput.

● Distance from AP. As a mobile device moves farther away from the AP the transmis- sion speed decreases. Known as dynamic rate switching, this allows a station to remain connected albeit at a slower speed. This means that a wireless mobile user on the edge of the BSA will have a slower connection speed than a user closer to the AP.

A general rule is 15–25 users per BSS.

File Server

LaptopLaptop

Figure 6-1 Basic Service Set (BSS)

200 Chapter 6 Media Access Control Layer Standards

Extended Service Set The two limitations of a BSS—a relatively small number of users being confined to a small geographical area—can be overcome with an Extended Service Set (ESS). An ESS is com- prised of two or more BSS networks that are interconnected. By using multiple APs an ESS can accommodate additional users over a wider area, as seen in Figure 6-2. In an ESS a group of APs are configured with the same SSID (logical network name) to create a single distributed WLAN.

Because these APs form a single logical network they must exist in one Layer 2 broadcast domain so that when any station sends a message it goes to all other stations in the WLAN.

The APs in an ESS should be positioned so that the cells overlap to facilitate the movement between cells known as roaming. When a user carrying a wireless device enters into the range of more than one AP the station will choose an AP based on signal strength or packet error rates. Once that station is accepted by the AP it “tunes” to the radio channel at which the AP is transmitting. The station continues to survey the appropriate radio frequencies at regular inter- vals to determine if a different AP can provide better service. If it finds one (perhaps because the user has moved closer to it) then the station associates with the new AP (this process is called a handoff), tuning to the different radio frequency of the new AP. To the user it is seamless because there has not been an interruption of service. This is known as Layer 2 roaming.

One of the weaknesses of the IEEE 802.11 standard is that it does not specify how a handoff should take place. Because roaming between APs of different vendors can sometimes be a problem, some industry experts recommend that all APs in an ESS be from the same vendor.

File Server

LaptopLaptop

Figure 6-2 Extended Service Set (ESS)

WLAN Service Sets 201

However, if a router separates the APs and each AP resides in a separate subnet, as illus- trated in Figure 6-3, this may cause problems. When a user roams from one AP coverage area to another then a new IP address must be assigned. Connectivity can be temporarily lost and running applications may have to be restarted. This is called Layer 3 roaming.

One way to roam between APs that are separated by a router is to implement Mobile IP. Mobile IP provides a mechanism within the TCP/IP protocol to better support mobile com- puting. With Mobile IP, computers are given a home address, which is a static IP number on their home network. The computer also has a home agent, which is a forwarding mecha- nism that keeps track of where the mobile computer is located. When the computer roams to another network (called the foreign network) a foreign agent provides routing services to the mobile computer. The foreign agent assigns the mobile computer a new—yet temporary—IP number. This new IP number is known as the care-of address. The computer then registers the care-of address with its home agent, as shown in Figure 6-4.

When a frame is sent to the computer’s home address, the home agent intercepts the frame. It then encapsulates (or tunnels) that frame into a new frame with the care-of address as the des- tination address. It then redirects it to the foreign agent, which sends it on to the computer now

BSSID = 198.146.118.10

198.146.118.1

subnet

210.113.9.1

subnet

BSSID = 210.113.9.10

Switch Switch

Router

LaptopLaptop

Figure 6-3 Layer 3 roaming

Home address 216.22.8.4

Home agent 198.146.118.20 Foreign agent

Moved to new network

Care-of address

Internet

Figure 6-4 Computer relocated in Mobile IP

202 Chapter 6 Media Access Control Layer Standards

6 located on the foreign network. This process is shown in Figure 6-5. Mobile IP enables a host to be identified by a single IP number even as it moves from one network to another. This movement is seamlessly achieved without the intervention or the knowledge of either the mobile user or the sending computer.

Mobile IP was the first protocol to offer transparent mobility.

The means by which multiple BSS networks in an ESS share information is through the distribution system (DS). The DS is how an AP determines what communication needs to take place with other APs in the ESS or with the wired network. The DS decides if it is neces- sary to exchange frames for stations in their own BSSs, to exchange frames with a wired network (typically through the Ethernet switch to which each AP is connected), or to forward frames to another BSS (to follow stations as they roam from one BSS to another). The distribution system media can be a wired network to which the APs are connected, a wireless radio within the APs, or even a special purpose device that interconnects the APs and provides the required distribution services. A wireless configuration that is used to connect the APs is called a wireless distribution system (WDS).

A DS is not a network; rather, it is a thin layer in each AP that determines the destination for traffic received from a BSS. The DS decides if traffic should be relayed back to a station in the same BSS, forwarded through the distribution system media to another AP, or sent to the wired network to a destination not in the ESS.

Independent Basic Service Set Both BSS and ESS operate in what is called infrastructure mode since stations communicate through an AP. An Independent Basic Service Set (IBSS) is a wireless network that does not use an AP and thus cannot connect to another network. In this type of network, which is also known as peer-to-peer or ad hoc mode, wireless devices communicate directly between themselves, as seen in Figure 6-6.

Frame to home address

216.22.8.4

198.146.118.20

Encapsulated frame

Home agent

Care-of address is 198.146.118.20

Foreign agent

216.22.8.4

Internet

Figure 6-5 Encapsulated Mobile IP frame

WLAN Service Sets 203

The different service sets have their own applications for different uses. An IBSS is useful for quickly and easily setting up a wireless network anywhere that users need to share data between themselves but do not need a connection to the Internet or an external network. For example, an IBSS is useful when a wireless user needs to quickly send a last-minute document to an associate in a hotel meeting room. A BSS has the flexibility to connect users to wired networks. An ESS provides the greatest range of functions: it allows more users to cover a broader wireless area as well as connect to external networks.

802.11 Media Access Control Layer Frame Formats and Types

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3.1.1. Summarize the processes involved in authentication and association.

3.2.1. Describe and apply the concepts surrounding WLAN frames.

3.2.3. Define, describe, and apply IEEE 802.11 coordination functions and channel access methods and features available for optimizing data flow across the RF medium.

IEEE wireless standards follow the Open Systems Interconnection (OSI) model with some modifications. The IEEE has divided the Data Link layer into two sublayers: the Logical Link Control (LLC) sublayer, which provides a common interface, reliability, and flow control, and the Medium Access Control (MAC) sublayer, which appends physical addresses to the frame. The functions that are performed at the MAC sublayer involve different frame formats and types.

MAC Frame Formats Information regarding the MAC frame formats generally focuses on the data units and issues that involve interoperability.

Data Units The OSI model uses the term data unit to describe the sets of data that move through the OSI layers. A Service Data Unit (SDU) describes a specific unit of data that has been passed down from a higher OSI layer to a lower layer but has not yet been encapsu- lated by that lower layer. A Protocol Data Unit (PDU) specifies the data that will be sent to

Laptop

Laptop Laptop

Figure 6-6 Independent Basic Service Set (BSS)

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the peer protocol layer at the receiving device instead of that being sent to a lower layer level (an SDU). Thus the PDU at a given layer is the SDU of the layer below. The process of changing an SDU to a PDU involves an encapsulation process in which the lower layer adds headers, footers, or both to the SDU to transform it into a PDU.

The SDU can be thought of as the payload of a given PDU.

Figure 6-7 illustrates the process in an 802.11 network using SDUs and PDUs. The steps are:

1. The Network Layer (Layer 3) sends data to the LLC sublayer of the Data Link Layer (Layer 2). This data unit is called the MAC Service Data Unit (MSDU) and contains data from Layers 3-7 along with LLC data.

2. The LLC then sends that data unit to the MAC sublayer where the MAC header information is then added. This data unit becomes the MAC Protocol Data Unit (MPDU), sometimes known as simply an IEEE 802.11 frame.

3. When the MPDU is sent to the Physical Layer Convergence Procedure (PLCP) sublayer in the Physical Layer (Layer 1), it is then called the PLCP Service Data Unit (PSDU).

4. The PSDU is then passed to the Physical Medium Dependent (PMD) sublayer that creates the PLCP Protocol Data Unit (PPDU) by adding a header and other data to it.

5. The PPDU is then transmitted as a series of bits to the receiving device.

When discussing data units, remember that a binary digit or bit is a 1 or 0 while a byte is usually eight bits. However, since a byte is not eight bits in all computer systems, octet is sometimes used instead to refer to a true sequence of eight bits.

Network Layer (Layer 3)

Step 1

Step 2

MSDU

Step 3 PSDU

Sending Device Step 5

010000010101101

Receiving Device

Step 4 PPDU

PSDU

PPDU

MPDU

LLC

PLCP

PMD

PLCP

PMD

MAC

MSDU

MPDU

LLC

MAC Data Link Layer

(Layer 2)

Physical Layer (Layer 1)

Data Link Layer (Layer 2)

Network Layer (Layer 3)

Figure 6-7 SDUs and PDUs

802.11 Media Access Control Layer Frame Formats and Types 205

The IEEE 802.11n standard adds an additional element to data units to increase throughput. Figure 6-8 shows a series of frames that are being transmitted from an Ethernet wired network to an AP, and the destination for each of these frames is the same wireless device. Although the data payload will be unique for each frame, other parts of the frame (PHY Preamble, PLCP Header, MAC Header, and Trailer) are “overhead” and contain informa- tion that may not change for each frame. However, because the overhead is transmitted with each frame this takes additional time.

The 802.11n adds a feature to reduce the amount of overhead transmitted and thus increase overall throughput. Aggregate MAC Service Data Unit (A-MSDU) allows multiple MSDUs to be combined (aggregated) together. All MSDUs within the single A-MSDU must be addressed to the same receiver. Aggregate MAC Protocol Data Unit (A-MPDU) allows multi- ple MPDUs to be aggregated together. Like A-MSDU, the MPDUs within the single A-MPDU must be addressed to the same receiver. Figure 6-9 illustrates A-MSDU and A-MPDU.

PHY Preamble

Overhead Payload Overhead

Frame 1

Frame 2

Frame 3

Frame 4

PLCP Header MAC Header Data Trailer

PHY Preamble PLCP Header MAC Header Data Trailer

Figure 6-8 802.11 Overhead and payload

MSDU - 1 MSDU - 2

A-MSDU

A-MPDU

MSDU - 1

MPDU - 1

MSDU - 2

MPDU - 1 MPDU - 2

MSDU - 3 MSDU - 4

MSDU - 3

MPDU - 2

MSDU - 4

Figure 6-9 A-MSDU and A-MPDU

206 Chapter 6 Media Access Control Layer Standards

Interoperability It is important for different systems to be able to understand each other, that is, to have interoperability. The primary areas of interoperability for WLANs are in two particular areas: 802.11 interoperating 802.3 frame types, and high-throughput (HT) devices interoperating with non-HT slower speed wireless networks.

IEEE 802.11 and 802.3 Frames Because most WLANs operate in infrastructure mode (either BSS or ESS), it is important that the 802.11 frames of a wireless network interoperate with an 802.3 Ethernet network. Because these wireless and wired networks share a common IEEE 802 foundation, the frames share a similar format.

Besides interoperating with 802.3 Ethernet networks, an 802.11 frame can also function on an 802.5 Token Ring network.

One area of difference between 802.11 and 802.3 is the frame size, known as maximum transmission unit (MTU). The MTU of an Ethernet 802.3 frame is 1,500 bytes while the MTU of an 802.11 frame is 2,304 bytes. The A-MSDU increases the MTU to 7,935 while A-MPDU allows up to 64k bytes.

MTU is not the same as the TCP value maximum segment size (MSS). The MSS name is actually misleading because it refers to the maximum amount of data that a segment can hold and does not include the TCP headers. Whereas MTU is the size of the entire packet, MSS is the size of the payload. In some consumer broad-

band routers and game consoles the parameter that is called MTU is in fact MSS.

The differences in frame size can be easily addressed to provide interoperability between the networks. The three major options are:

● Fragmentation. If an IP layer receives a message to be sent across the network it looks at the size of the message and then computes how large the IP frame would be after adding its additional header information. If the total length is longer than what can be accommodated, the frame is fragmented into multiple frames.

● Jumbo frames. A network interface adapter can be configured to accept frames that are larger than 1,500 bytes yet less than 9,000 bytes. This is known as jumbo frame support. However, in order to minimize fragmentation all devices on the network would need to be set to the same size, including any switches and networking equipment.

● Lowest common denominator. Most wireless devices automatically set the default MTU at the lowest common denominator of 1,500 bytes.

Another difference between 802.11 and 802.3 frames is the number of address fields. An 802.3 frame has only a Source Address (SA) and a Destination Address (DA), while an 802.11 frame has additional fields. However, this causes no interoperability issues.

802.11 Media Access Control Layer Frame Formats and Types 207

IEEE 802.11n HT and 802.11a/b/g Because of the significant differences between high- throughput (HT) 802.11n and non-HT 802.11a/b/g, an 802.11n AP can tell 802.11n wireless devices to change “on the fly” to one of four HT Operation Modes in order to interoperate with slower devices. These modes are:

● HT Greenfield Mode (Mode 0). In Greenfield Mode all of the stations in the BSS or ESS are 802.11n devices operating at the same HT speed with the same parameters. If an 802.11a/b/g device should roam into this BSA it would not be able to access the WLAN.

● HT Nonmember Protection Mode (Mode 1). The HT Nonmember Protection Mode is the “legacy” mode of transmitting. All stations—including 802.11n—use the 802.11a/b/g format to ensure backwards compatibility. None of the HT enhancements are utilized.

● HT 20 MHz Protection Mode (Mode 2). All of the stations in the HT 20 MHz Protection Mode are 802.11n HT devices. If a 20-MHz-only HT device associates to a 20/40-MHz AP, then protection must be used to prevent the 20-MHz-only station from transmitting at the same time.

● HT Mixed Mode (Mode 3). Both 802.11n and 802.11a/b/g devices can inter- operate in the same BSA in HT Mixed Mode. Although this enables backwards compatibility, it does so at a price. The HT 802.11n devices must transmit a legacy format preamble followed by an HT format preamble. The legacy preamble tells the 802.11a/b/g devices to avoid transmitting over the HT frames that are sent to and from the 802.11n devices. Although sending two preambles adds additional over- head, it still allows the HT stations to take advantage of HT features.

MAC Frame Types Figure 6-10 illustrates a MAC frame within the Physical Layer Convergence Procedure (PLCP) frame. The purpose of the PLCP frame is essentially to establish synchronization between the receiving device and the incoming frame, inform the device about the number

Synchronization LengthStart frame

delimiter

Signal data rate

Header

error check

Parts DataPreamble Header

Service Data

16 16 16 48 0–18,43248 48 48 3216

Frame

control

Address 3 Address 4 QoS

control

Duration/

Address 1 Sequence

control

Frame

body

Size

(bits)

Address 2 Frame

sequence check

DataMAC HeaderParts

Figure 6-10 MAC frame within PLCP frame

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of bytes in the frame, and tell what speeds the sending device supports. Within the Data sec- tion of the PLCP frame is the MAC frame. The first three fields (Frame Control, Duration/ ID, and Address 1) and the last field (FSS) are required in all MAC frames; the other fields (Address 2, Address 3, Sequence Control, Address 4, QoS Control, and Frame Body) are present only in certain frame types and subtypes.

There are three main types of MAC frames. These are management frames, control frames, and data frames.

Management Frames Management frames are used to set up the initial communica- tions between a device and the AP (for infrastructure mode) or between stations (for ad hoc mode), and then maintain the connection. This is necessary because of the dynamic nature of a WLAN in which a station needs to locate an AP in its territory and then access the WLAN as well as eventually disconnect from the network, all of which requires multiple exchanges of frames.

On a wired network the type of management for a WLAN is not necessary.

The format of a management frame is illustrated in Figure 6-11. The Frame control field contains information such as the current version number of the standard and if encryption is being used. The Duration field contains the number of microseconds needed to transmit; this value will vary depending upon which mode of wireless trans- mission is being used. The Address 1 (Destination address) and Address 2 (Source address) fields contain the addresses of the receiving and sending devices, respectively. The Sequence control field contains the sequence number for the packet and packet fragment number.

Some of the types of management frames and their functions include:

● Authentication frame. An authentication frame is used by the access point in deter- mining whether to accept or reject a wireless device from entering the network.

● Association request frame. An association request frame allows an AP to allocate resources for a wireless device.

● Association response frame. When an AP accepts or rejects a wireless device it sends an association response frame.

16 16 16 1 to 18,49648 48 48 48

Frame control

BSSIDDuration Address 1 (Destination

address)

Sequence control

Frame body

DataHeader

Address 2 (Source

address)

Frame check

sequence

Parts

Size (bits)

Figure 6-11 Management frame

802.11 Media Access Control Layer Frame Formats and Types 209

● Beacon frame. An AP regularly transmits a beacon frame to announce its presence and send wireless information to all devices that are within range.

● Deauthentication frame. A device sends a deauthentication frame to another device if it wants to end a communication.

● Disassociation frame. A wireless device sends a disassociation frame to another device if it wishes to end the connection.

● Probe request frame. A device sends a probe request frame when it needs to obtain information from another device.

● Probe response frame. After it receives a probe request frame, a device will respond with a probe response frame containing capability information, supported data rates, etc.

● Reassociation request frame. If a wireless device roams into a different cell it sends a reassociation request frame to the new AP.

● Reassociation response frame. After it receives a reassociation request frame, an access point sends a reassociation response frame containing an acceptance or rejection notice to the wireless device requesting reassociation.

Control Frames Control frames are the second type of MAC frame. After the connec- tion among the stations and AP is established, control frames provide assistance in deliver- ing frames that contain the data by controlling access to the medium. One type of control frame is illustrated in Figure 6-12.

Data Frames The third type of MAC frame is the data frame, which carries the informa- tion to be transmitted to the destination device. The format of a data frame is illustrated in Figure 6-13. The fields Address 1, Address 2, Address 3 and Address 4 contain the address of the BSSID, the destination address, the source address, the transmitter address or the receiver address. Their contents vary, depending upon the mode of transmission.

Frame control

Duration Receiver address

Transmitter address

Frame check

sequence

16 16 48 48 48 Size (bits)

Figure 6-12 Control frame

Frame

control

Duration

Sequence

control

1 to 18,496

Data

Frame

check sequence

Size (bits)

Address 2

Address

Address 1

Figure 6-13 Data frame

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Because data frames contain the actual information that is being transmitted, such as an e-mail message, the content of the data frame is forwarded to the upper layers on the receiving device. The management and control frame contents are not forwarded.

MAC Operations

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2.1.4. Identify and apply the concepts which make up the functional- ity of spread-spectrum technology.

3.1.1. Summarize the processes involved in authentication and association.

3.1.3. Explain and apply the power management features of WLANs.

3.2.1. Describe and apply the concepts surrounding WLAN frames.

3.2.2. Identify methods described in the IEEE 802.11-2007 standard for locating, joining, and maintaining connectivity with an IEEE 802.11 WLAN.

3.2.3. Define, describe, and apply IEEE 802.11 coordination functions and channel access methods and features available for optimizing data flow across the RF medium.

The MAC layer plays a role in several functions in a WLAN. These functions can be broken into three classifications: discovering a WLAN, joining the WLAN, and transmitting on a WLAN.

Discovering the WLAN The first major function of the MAC layer involves defining procedures for a station to dis- cover a WLAN. When a station is powered on or roams into an area of wireless coverage, it must find the AP. This means that the AP must transmit the appropriate information and the station must be looking for that information. This discovery process can be done by passive scanning or active scanning.

Because ESS WLANs are by far the most common types of networks, the study of the MAC operations will focus on these types of wire- less networks.

Passive Scanning Passive scanning depends upon the AP “advertising” itself. At regular intervals, the AP sends a beacon frame to both announce its presence and to provide the necessary information for wireless stations wanting to join the network. This process is known as beaconing. The beacon frame provides the “pulse” of the wire- less network and is an orderly means for stations to establish and also maintain communications.

A beacon frame is a type of management frame and its format follows the standard struc- ture of a management frame. The Destination address is always set to all ones (or 255.255.255.255 in dotted decimal notation), which is the standard IP broadcast address

MAC Operations 211

for a network or a segment (for example, if the segment IP network address is 199.93.29.0 then the broadcast address would be 199.93.29.255). This forces all stations to receive and process each beacon frame.

The body of the beacon frame contains the following fields:

● Beacon interval. The beacon interval tells the amount of time between beacon transmissions.

● Timestamp. The timestamp value forces all wireless stations to update their local clock and thus synchronize with the access point.

● SSID. The SSID identifies a specific wireless LAN. ● Supported rates. The beacon frame carries information that lists the transmission rates

that a particular wireless LAN supports. ● Parameter sets. Information about which wireless modulation scheme is being used

(such as frequency-hopping spread spectrum or direct sequence spread spectrum) along with information about that scheme (such as the hopping pattern and dwell time for a FHSS network).

● Capability information. Capability information provides the requirements of the wireless stations if they want to connect to this wireless network.

In an ad hoc network with no APs, each wireless station assumes the responsibility for sending a beacon. After receiving a beacon frame, each station first waits the amount of time specified in the beacon interval. Then each station waits an additional random delay time, and the station that completes its random delay time

first will then send the next beacon frame. This random delay time rotates the responsibility for sending beacons to all wireless stations.

The beacon interval is normally sent once every 100 milliseconds (ms), although it can be modified. Increasing the beacon interval (and thus decreasing the number of beacon frames transmitted) can decrease wireless network traffic but may result in problems when roaming because timely information about availability of the network can be missed. Decreasing the beacon interval (and increasing the rate of beacons) makes the roaming process faster but increases network traffic and thus decreases overall throughput.

If a beacon frame is delayed because of other network traffic, the actual time between beacons may be longer than the beacon inter- val. However, wireless stations can compensate by resynchronizing with the timestamp in the next beacon.

As the AP transmits it beacon frame, the station is looking for those frames. This process is known as passive scanning. With passive scanning, a station changes to the different chan- nels that it supports and listens for a beacon frame for a set period of time. Once a station receives a beacon frame with the SSID, it can then attempt to join the network. On ESS wireless networks with multiple access points, the station might actually receive beacon

212 Chapter 6 Media Access Control Layer Standards

frames from several APs. In this case, the wireless device would generally attempt to join the network with the strongest signal strength.

While a station is performing a channel scan it cannot transmit or receive data traffic. To minimize this impact, when not actively trans- mitting data many stations will continually scan for incoming bea- cons. Or, they might scan only a single alternate channel one at a time. This continual scanning allows the station to create a real-time

inventory of all available APs so that the information is immediately available if it roams into the range of another AP or if it becomes disconnected it can quickly reconnect to the WLAN.

Active Scanning The second type of scanning is active scanning. In active scanning, the station first sends out a management probe request frame on an available channel. This probe request frame can be a directed probe that contains a specific SSID that the device is searching for (only APs with a matching SSID will reply with a probe response) or it can be broadcast probe with a null value as the SSID (all APs will respond). Active scanning is illus- trated in Figure 6-14. Like the beacon frame, the probe response frame has the information the station needs to connect to the wireless LAN.

The difference between passive scanning and active scanning essen- tially comes down to which device initiates the discovery. In passive scanning, the AP starts the process by sending out a frame that says “Here I am,” while in active scanning the station sends out a frame that says, “Is anybody out there?”

Joining the WLAN Once a wireless device discovers the WLAN, it next requests to join the network. This is a two-fold process known as authentication and association.

Laptop

2. AP responds with

probe response frame

1. Laptop sends

probe request frame Probe response frame

Probe request frame

Figure 6-14 Active scanning

MAC Operations 213

Sometimes a behavior model or mathematical abstraction can be used to illustrate and trace the flow of the authentication and associ- ation processes. This behavior model is called finite-state machine (FSM) or just a state machine.

Authentication In a standard wired LAN, a user sits down at a desktop computer con- nected to the network by a patch cable and then logs into the network with a username and password. The user is authenticated after the network device is already connected to the net- work. This is because physical access to the wired network can be restricted by walls and locked doors and only authorized devices are assumed to be connected to the network. However, because wireless LANs cannot limit access to the RF signal by walls or doors, wireless authentication requires the wireless device (and not the individual user) to be authenticated prior to being connected to the network. IEEE 802.11 authentication is a process in which the AP accepts a station.

Two types of authentication are supported by the 802.11 standard. Open system authentication is the basic (and the default) method. After discovering the network through passive scanning or active scanning and receiving the necessary information, the wireless device sends an association request frame to the AP, which contains information such as the WLANs SSID and the data rates that the device can support. After receiving the association request, the AP then responds with an association response frame, which contains either an acceptance or rejection notice. Open system authentication is virtually a “handshake” between the AP and station in which the station establishes its identity (but the AP does not) and is illustrated in Figure 6-15.

A second type of authentication method (which is optional) is shared key authentication. With shared key authentication, both the AP and the station are given the same key value in advance. (In other words, they “share” the key value.) The station first sends an authentication frame to the AP, and the AP responds with an authentication frame

Laptop

2. AP responds with

association response frame

1. Laptop sends association

request frame You are authenticated

I need to connect to

SSID “Bill”

Figure 6-15 Open system authentication

214 Chapter 6 Media Access Control Layer Standards

that contains a block of text known as the challenge text. The station must encrypt the text with its key value and return it to the AP in an authentication frame. The AP will then decrypt what was returned with its own key to see if it matches the original chal- lenge text. If it does, the AP sends an authentication frame signifying the result of the authentication. Shared key authentication is based upon the fact that only preapproved wireless devices have been given the shared key. Shared key authentication is illustrated in Figure 6-16.

Both types of authentication are weak. With open system authenti- cation, the wireless device only has to know the SSID in order to be authenticated. The SSID can easily be retrieved from another authen- ticated device or even from the AP itself if it uses passive scanning. In addition, the SSID is initially transmitted in an unencrypted form,

so an attacker could capture those wireless packets and view the SSID. The weakness of shared key authentication is that the key must be installed manually on each wireless device, and managing a secret key for all wireless devices can leave the door open for the key to be uncovered.

Association Once a wireless device is authenticated, the final step is to be accepted into the wireless network. This is known as association. Once the AP verifies that the SSID of the station matches that of the wireless network (open system authentication) or that the

Laptop

4. AP compares encrypted text with its own

AP encrypted text Laptop encrypted text

5. AP sends authentication

frame back to device

3. Laptop encrypts

challenge text with

shared key and returns

2. AP sends

challenge text

Encrypted textchallenge text

1. Laptop sends

authentication frame

Key

Now is the time for all good men to come

to the aid of their country.

Now is the time for all good men to come

to the aid of their country.

5(kdjfswo*3l; 3(*FLsd0_)F*

JiqLFUIDp457 9i2&4536HF7e6

5(kdjfswo*3l;

3(*FLsd0_)F*

JiqLFUIDp457 9i2&4536HF7e6

5(kdjfswo*3l;

3(*FLsd0_)F*

JiqLFUIDp457 9i2&4536HF7e6

I want to authenticate

24005slk

Figure 6-16 Shared key authentication

MAC Operations 215

challenge texts match (shared key authentication), then the wireless device is authenticated. Association is the process by which the station “registers” with the AP so that the AP reserves memory space in the AP and establishes an association ID for it. The AP sends an association response frame that includes information such as the association ID and supported data rates.

Transmitting on the WLAN The IEEE 802.11 standard specifies three procedures for transmitting on the WLAN: the dis- tributed coordination function, the point coordination function, and the hybrid coordination function.

Distributed Coordination Function (DCF) The distributed coordination function defines two procedures: the Carrier Sense Multiple Access with Collision Detection and Request to Send/Clear to Send.

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) Because the wireless medium is shared, rules for cooperation among the wireless stations are necessary. These different ways of sharing are called channel access methods. One type of channel access method is known as contention. The “philosophy” of contention is that devices con- tend, or compete, with each other to use the network medium. With contention, any device can attempt to transmit a message at any time. However, if two devices send frames at the same time, a collision results and the frames become unintelligible. One way to prevent net- work collisions is to employ the same principles that people use in polite conversation: first, listen to make sure no one else is talking. If someone is talking, then wait; if no one else is talking, then go ahead and speak.

The IEEE 802.3 Ethernet standard specifies contention with this type of “politeness” as its channel access method. Known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD), it specifies that before a networked device starts to send a frame it should first listen on the wire (called carrier sensing) to see if any other device is cur- rently transmitting. If it senses traffic, it waits until that traffic is finished. If it hears no traffic, then the device can send its frame. However, what if two devices simultaneously listen, hear nothing on the cable, and then both start to send at exactly the same time? A collision would still result. CSMA/CD specifies that each device must always continue to listen while sending its frame. If it hears a collision, each device stops sending data and instead broadcasts a “jam” signal over the network. This tells all other devices to wait before sending any frames. The two sending computers then pause for a random amount of time (called a backoff interval) before attempting to resend. CSMA/CD is illus- trated in Figure 6-17.

However, CSMA/CD cannot be used for wireless networks for two reasons:

● Difficult to detect collisions. Collision detection is very difficult with wireless transmis- sions. With CSMA/CD, the stations must be able to transmit and listen at the same time. However, in radio systems the signal from a transmitting station is so strong that it will overpower that same station’s ability to simultaneously receive a transmission. That means, while it is transmitting, a station “drowns out” its own ability to detect a collision.

216 Chapter 6 Media Access Control Layer Standards

● Hidden node problem. A second factor that makes collision detection difficult with wireless transmission is that all stations would have to be able to detect transmissions from all other stations at all times. In a wireless environment, a station might not be in range of all other stations. In Figure 6-18 wireless Devices 1 and 2 are within range of

Listen

while

sending

Listen

while

sending

Collision

Send

jam

signal

Send

jam

signal

Wait 20.64

microseconds

Wait 97.85

microseconds

Wait 97.85

microseconds

Hears no

traffic so

sends

Jam

Data

A B

C D

PC 1

PC 2

PC 3

PC 1

PC 2

PC 3

PC 1

PC 2

PC 3

PC 1

PC 2

PC 3

Figure 6-17 CSMA/CD

MAC Operations 217

the AP but not within range of each other. If Device 2 “listens” and hears no traffic, it might assume there are no transmissions taking place, while actually Device 1 is already transmitting. This is known as the hidden node problem.

Instead of using CSMA/CD, the 802.11 standard uses an access method known as the Distributed Coordination Function (DCF). The DCF specifies that a modified procedure known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) be used. Whereas CSMA/CD is designed to handle collisions when they occur, CSMA/CA attempts to avoid collisions altogether.

Consider for a moment a family with several children on vacation at a remote cabin, with only one cell phone that can receive service. When will there be the most conflict over who gets to use the phone? Usually it will be immediately after someone talking on the phone has hung up. This is because everyone else has been forced to wait until that telephone conversa- tion was over and now they all want to use the phone at the same time. The same is true with a contention channel access method like Ethernet CSMA/CD: the time at which the most collisions occur is immediately after a station completes its transmission. This is because all other stations wanting to transmit have been waiting for the medium to clear so they can send their frames. Once the medium is clear they all try to transmit at the same time, which results in more collisions and delays. CSMA/CD handles the collisions by having the two stations responsible for the collision wait a random amount of time (the backoff interval) before attempting to resend.

Think about the situation with one cell phone again. Suppose the parents at the cabin created a reward system based on household duties to perform (cleaning a room, mowing the lawn, etc.) for the week. Each week every child receives a “Telephone Wait Time” number. The child who has completed all of his duties gets a Telephone Wait Time of one minute, while a child who has not completed his duties gets a Telephone Wait Time of five minutes. Whenever the telephone became available, each child has to wait the amount of his or her Telephone Wait Time before trying to use the phone. If both Braden and Mia wanted to use the phone, but Mia had to wait only one minute while Braden had to wait five minutes, Mia would obviously get to go first and Braden would be forced to wait until Mia’s call was completed. Because everyone’s wait time is different,

Device 2Device 1

Figure 6-18 Hidden node problem

218 Chapter 6 Media Access Control Layer Standards

there would be no contention for using the phone once it was available. This analogy is similar to wireless CSMA/CA. Instead of making just the two stations responsible for the collision wait a random amount of time before attempting to resend after the collision, CMSA/CA has all stations wait a random amount of time after the medium is clear. This significantly reduces the number of collisions.

With wireless CSMA/CA, the amount of time that a station must wait after the medium is clear is called the slot time. Each station must wait a random amount of slot times as its backoff interval. For example, the slot time for an 802.11b WLAN is 20 microseconds (μs). If a wireless device’s backoff interval is 3 slot times, then it must wait 60 μs (20 μs � 3 slot times) before attempting to transmit. Because CMSA/CA has all stations wait a random amount of time after the medium is clear, the number of collisions is significantly reduced.

A second way in which CSMA/CA reduces collisions is by using explicit frame acknowledgment. An acknowledgment frame (abbreviated ACK) is sent by the receiving device back to the sending device to confirm that the data frame arrived intact. If the ACK frame is not returned a problem is assumed to have occurred and the data frame is transmitted again. This explicit ACK mechanism handles interference and other radio-related problems. CSMA/CA and ACK are illustrated in Figure 6-19.

IEEE 802.11n adds a feature known as block acknowledgment, which is necessary for A-MPDU aggregation. The block acknowledgment mechanism in 802.11n supports multiple MPDUs in an A-MPDU. When an A-MPDU from one station is received and errors are found in some of aggregated MPDUs, the receiving node sends a block ACK only acknowledging the correct MPDUs. The sender then will only retransmit nonacknowl- edged MPDUs.

Client D Client A

Client C

Sends data 1

Wait 20 microseconds3

R ec

ei ve

s A C K

Se nd

s da

Wait 40 microseconds3

Wait 60 microseconds3

Client B

Wait 80 microseconds3

Figure 6-19 CSMA/CA and ACK

MAC Operations 219

Block acknowledgment only applies to A-MPDU but not A-MSDU. This is because when an MSDU is incorrect the entire A-MSDU needs to be transmitted.

Request to Send/Clear to Send (RTS/CTS) Although CSMA/CA reduces the potential for collisions, it does not eliminate them altogether. The hidden node problem, which is the result of stations that are out of range of each other and do not know that the other exists, cannot be solved by CSMA/CA. The 802.11 standard provides an option that can be used when collisions occur due to a hidden node. This option is known as virtual carrier sensing or the Request to Send/Clear to Send (RTS/CTS) protocol. When this option is used it solves the hidden node problem and provides additional protection against collisions.

CSMA/CA is technically physical carrier sensing.

Consider again the situation of the vacationing family with one cell phone. Suppose that someone has just finished using the phone, and now Braden and Mia want to make a call. However, Mom announces that she is expecting a call from her business partner and that nobody can use the phone for the next hour until she receives the call. (Because the service is spotty at the cabin, she does not want to risk missing call waiting.) She has essentially reserved the phone for period of time for a special call.

This analogy is similar to RTS/CTS. A Request to Send (RTS) frame is transmitted by a station to the access point. This frame contains a duration field that indicates the length of time needed for both the transmission and the returning ACK frame. The AP as well as all stations that can receive the RTS frame are alerted that the station needs to reserve the medium for a specific period of time. Each receiving station stores that information in its net allocation vector (NAV). No station can transmit if the NAV contains a value other than zero. The access point then responds to the station with a Clear to Send (CTS) frame that alerts all devices that medium is now being reserved and they should suspend any transmissions. Once the station receives the CTS frame it can then proceed with transmit- ting its frame.

RTS and CTS frames are control frames and not management frames.

The RTS/CTS protocol imposes significant overhead upon the WLAN with the transmission of its RTS and CTS frames. The RTS/CTS protocol is especially taxing when short data packets are being transmitted. For this reason the 802.11 standard allows that when the RTS/CTS option is invoked, short data packets may still be transmitted without RTS/CTS. This is known as the RTS threshold. Only packets that are longer than the RTS threshold are transmitted using RTS/CTS.

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Because the RTS/CTS protocol imposes additional overhead, it gener- ally is not used unless there is no other way to improve network performance.

Another means of reducing collisions is fragmentation, or dividing the data to be trans- mitted from one large frame into several smaller ones. Sending many smaller frames instead of one large frame reduces the amount of time that the wireless medium is being used and likewise reduces the probability of collisions. If the length of a data frame to be transmitted exceeds a specific value, the MAC layer will divide or fragment that frame into several smaller frames. Each fragmented frame is given a fragment num- ber (the first fragmented frame is 0, the next frame is 1, and so on). The frames are then transmitted to the receiving station. The receiving station receives the frame, sends back an ACK, and then is ready to receive the next fragment (a station can receive fragmen- ted frames from up to three different senders). Upon receiving all of the fragments, they are reassembled based on their fragment numbers, back into the one original frame. Fragmentation can reduce the probability of collisions and may be considered as an alternative to RTS/CTS. However, fragmentation does have additional overhead associ- ated with it. (It requires a separate ACK from the receiving station for each fragmented frame)

Fragmentation does not always have to be used separately from RTS/ CTS. The 802.11 standard permits them to be used simultaneously.

Variations of RTS/CTS are also used as protection mechanisms. Because the technolo- gies behind 802.11a/b/g/n are significantly different, having a mix of devices based on these different standards may result in transmission problems. The CTS-to-self is used when 802.11g devices are mixed with 802.11b devices together. The 802.11g device first sends a CTS message using an 802.11b rate and technology to reserve the medium that all devices will hear. It then immediately sends its data frame and ACK at the higher 802.11g rate. The HT Dual-CTS Protection is used with 802.11n devices in a mixed environment with 802.11a/b/g devices. The 802.11n device sends a RTS to the AP, which responds with two CTS frames: one in the 802.11n format and one in the non-802.11n format. This ensures that all devices—802.11a/b/g/n—receive the message.

Besides HT Dual-CTS Protection, another protection mechanism is the HT L-SIG Protection. In a PPDU header, a subfield within the Signal field known as the L-Length subfield for 802.11n frames indicates the length of the PSDU (from 1 to 4,095 octets) that is used to determine the number of octet being transmitted between the MAC and the PHY. When transmitting in HT-Greenfield Mode or HT-Mixed Mode the L-Length subfield is used in conjunction with a Rate subfield (which is set to 6 Mbps) to control the length of time that an 802.11a/b/g station will refrain from transmitting.

The 802.11 standard also defines different interframe spaces (IFS) or “time gaps.” These are standard spacing intervals between the transmissions of the data frames. Instead of

MAC Operations 221

being just “dead space,” these time gaps are used for special types of transmissions. The IFS are:

● Short IFS. Short IFS (SIFS) is used for immediate response actions such as ACK and has the highest level of priority.

● Point Coordination Function IFS. Point Coordination Function IFS (PIFS) is the time used by a device to access the medium after it has been asked and then given approval to transmit.

● Distributed Coordination Function IFS. Distributed Coordination Function IFS (DIFS) is the standard interval between the transmission of data frames.

● Extended IFS. Extended IFS space (EIFS) is used when frames must be retransmitted. ● Arbitration IFS. Arbitration IFS (AIFS) is used when setting priorities to different types

of transmissions. ● Reduced IFS. Used by 802.11n devices, Reduced IFS (RIFS) reduces the amount of

“dead space” required between OFDM transmissions, yet is restricted to Greenfield deployments.

Figure 6-20 illustrates a transmission by a station using direct sequence spread spectrum (DSSS) that has been assigned three slot times. (Remember that the amount of time that a station must wait after the medium is clear is given as the number slot times, and the total amount of waiting time, known as the backoff interval, is calculated by multiplying the number of slot times by the length of each slot time, which is 20 μs). The station starts listening (carrier sensing) before transmitting. If there is no traffic at the completion of the DIFS (50 μs), the station starts transmitting. When the transmission is over, the receiving device sends back an acknowledgment (ACK) in the SIFS gap, acknowledging that the transmission was successful. Once received, the process starts all over again with the station carrier sensing at the next DIFS. This time, if no traffic is detected, the station starts its backoff interval of 60 microseconds (20 μs � 3 slot times). At the end of each slot time interval (20 μs) the wireless device again listens for traffic. If at the end of its backoff interval the station still detects no traffic, then it transmits its second frame. Once the ACK packet is received, the process resumes again.

DIFS

S I F S

A C K

Time in microseconds

DIFS

0 20 40 • • • 500 520 540 560 580 600 620 • • • 1000 1020 1040 1060 1080 1100 1120 1140

S I F S

A C K

DIFS A

Frame #1

A Frame

A Backoff interval

Figure 6-20 CSMA/CA with one station transmitting

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When two stations need to transmit, it becomes more complicated, as shown in Figure 6-21. Wireless station A is using DSSS with three slot times while wireless station B has only two slot times. Station A begins carrier sensing and then transmitting its first frame. Station B then begins carrier sensing while Station A’s first frame is being sent. Because it detects traf- fic, Station B waits. Once Station A has received its ACK, both stations begin carrier sensing during the second DIFS. At the end of the second DIFS Stations, A and B begin their backoff interval. Because the number of Station B’s slot times is only two (20 μs � 2 slot times), it will finish its backoff interval before Station A (20 μs � 3 slot times). Station B then begins transmitting its first packet.

Because each station continues to listen for traffic at the end of each of its time slots, Station A detects that Station B is now transmitting. Station A “remembers” that it has already counted off two of its slot times. Station A must now wait until B’s transmission and acknowledgment is complete and the next DIF begins. After the DIF gap time Station A and B both begin their backoff interval. However, this time Station A only has to wait one slot time. This is because Station A already waited for two of its slot times previously. If a station is “bumped” by another station from transmitting, it only has to wait the remaining number of time slots and not start all over again. This increases the probability that those stations that are waiting will transmit sooner than a new station will.

Point Coordination Function (PCF) The contention channel access method, in which any computer can attempt to transmit a message at any time, is the basis for CSMA/CA. Another type of channel access method is polling. With this method, each device is polled,

Station A starts carrier sensing

DIFS Station

A Frame

S I F S

A C K

Time in microseconds

DIFS

0 20 40 60 • • • 500 520 540 560 580 600 • • • 1000 1020 1040 1060 1080 1100

A backoff interval

(1 and 2)

Station A

waits

A backoff interval

(3)

A Frame

Station B starts carrier sensing

Station B

S I F S

DIFS

S I F S

DIFS DIFS

DIFS

Station B

waits

B backoff interval

(1)

B Frame

B backoff interval

(1 and 2)

Station B

waits

Figure 6-21 CSMA/CA with two stations transmitting

MAC Operations 223

or asked, in sequence if it wants to transmit: if the answer is yes, then it is given permission to transmit while all other devices must wait, whereas if the answer is no, then the next device in sequence is polled. Polling effectively prevents collisions because every device must wait until it receives permission before it can transmit. However, polling can have an impact on performance.

The 802.11 standard provides for an optional polling function known as Point Coordination Function (PCF). With PCF the access point serves as the polling device or “point coordinator.” It queries each device in an orderly fashion to determine if the device needs to transmit. The point coordinator begins by sensing the medium, just as all other devices do, after a SIFS time gap during which an ACK was transmitted. However, whereas the other devices must wait through the duration of the DIFS time gap, the point coordinator has to wait only through the PIFS time gap. Because the PIFS is shorter than the DIFS time gap, the point coordinator will gain control of the medium before any other devices, as seen in Figure 6-22.

If the point coordinator hears no traffic at the end of the PIFS time gap, then it sends out a beacon frame to all stations. One field of this frame contains a value that indicates the length of time that PCF (polling) will be used instead of DCF (contention), and stations store it in their NAV field. After the stations receive this beacon frame they must stop any transmission for that length of time. The point coordinator then sends out another frame to a specific station, granting it permission to transmit one frame to any destina- tion. If it has nothing to send, then that station returns a null data frame to the point coordinator.

Because each station can be told the length of time that PCF will be used instead of DCF, the 802.11 standard allows a WLAN to alternate between PCF (polling) and DCF (contention). The point coordinator waits during the first PIFS and then sends out a beacon frame, “reserving” the network for a specific amount of

time. During this time, a station is polled and a PCF frame is transmitted. At the conclu- sion of that transmission, the time expires and the WLAN returns to the default DCF method, allowing another station to transmit based on contention. When that frame is completed, the point coordinator can again take control of the medium by sending out another beacon frame.

Frame transmitted

DIFS

0 20 40 • • • 500 520 540

S I F S

PIFS Time in microseconds

Figure 6-22 DIFS versus PIFS time gap

224 Chapter 6 Media Access Control Layer Standards

Hybrid Coordination Function (HCF) A third channel access method, which is also optional like PCF, is the hybrid coordination function (HCF). The HCF allows for different types of wireless traffic to be given different levels of priority. The foundation of HCF is that it schedules access to the channel by allocating transmission opportunities (TXOP) to the stations. Each TXOP has a starting time and a maximum duration, which is the time interval the station has the medium all to itself. This means that a station with TXOP can transmit multiple data frames without entering the backoff procedure. This reduction in overhead means that more frames can be sent in a given period of time.

There are two access mechanisms specified by the HCF. The first is known as Enhanced Distributed Channel Access (EDCA). EDCA divides transmissions into four different access category (AC) classes:

● Background (AC_BK) ● Best Effort (AC_BE) ● Video (AC_VI) ● Voice (AC_VO)

Traffic that has been “tagged” with a higher priority AC (such as AC_VO) is given priority over traffic with a lower priority (like AC_BK).

The second HCF mechanism is HCF Controlled Channel Access (HCCA). Like PCF, HCCA uses polling along with centralized scheduling that is controlled by the AP (called the hybrid coordinator or HC). Each station that requires priority of its frames sends that information to the HC, which then assigns a TXOP to the station. Each station is polled and allowed to transmit its packets until its TXOP duration elapses. Yet unlike PCF, the HC can start a polling period at different times.

There is concern that the original HCCA mechanism may not be ade- quate to support real-time voice communication. This is because of the high overhead in polling stations that do not have messages to be transmitted.

Chapter Summary ■ A service set is all of the devices that are associated with an 802.11 WLAN. There

are three WLAN service set configurations. A Basic Service Set (BSS) is as one or more stations that are served by a single AP. The physical area of RF coverage is called a Basic Service Area (BSA). An Extended Service Set (ESS) is comprised of two or more BSS networks that are interconnected to accommodate additional users over a wider area who can roam between AP coverage areas. When all APs are on the same subnet it is called Layer 2 roaming; when the APs are on separate subnets it is known as Layer 3 roaming. Mobile IP provides a mechanism within the TCP/IP protocol to bet- ter support mobile computing. An Independent Basic Service Set (IBSS), also known as peer-to-peer or ad hoc mode, is a wireless network that does not use an AP and cannot connect to another network; instead, the wireless devices communicate directly between themselves.

Chapter Summary 225

■ The OSI model uses the term data unit to describe the sets of data that move through the OSI layers. A Service Data Unit (SDU) describes a specific unit of data that has been passed down from a higher OSI layer to a lower layer but has not yet been encapsulated by that lower layer. A Protocol Data Unit (PDU) specifies the data that will be sent to the peer protocol layer at the receiving device instead of that being sent to a lower layer level (an SDU). The 802.11n adds a feature to reduce the amount of overhead transmitted and thus increase overall throughput by combining or aggregat- ing specific SDUs and PDUs.

■ Because most WLANs operate in infrastructure mode, it is important that the 802.11 frames of a wireless network interoperate with an 802.3 Ethernet network. Because these wireless and wired networks share a common IEEE 802 foundation, the frames share a similar format. Any differences in frame size can be addressed to provide interoperability between the networks. Because of the significant differences between 802.11n HT and non-HT 802.11a/b/g, an 802.11n AP can tell 802.11n wireless devices to change “on the fly” to one of four HT Operation Modes in order to inter- operate with slower devices.

■ There are three main types of MAC frames. Management frames are used to set up the initial communications between a device and the access point (for infrastructure mode) or between stations (for ad hoc mode), and then maintain the connection. Control frames provide assistance in delivering frames that contain the data by controlling access to the medium. Data frames carry the information to be transmitted to the des- tination device.

■ The first major function of the MAC layer involves defining procedures for a station to discover a WLAN. This discovery process can be done by passive scanning or active scanning. Passive scanning depends upon the AP “advertising” itself. At regular inter- vals, the AP sends a beacon frame to both announce its presence and to provide the necessary information for wireless stations wanting to join the network. In active scanning, the station first sends out a management probe request frame on an avail- able channel. Wireless authentication requires the wireless device and not the individ- ual user to be authenticated prior to being connected to the network. IEEE 802.11 authentication is a process in which the AP accepts a station. There are two types of authentication supported by the 802.11 standard. Open system authentication is the basic and the default method. With shared key authentication both the AP and the station are given the same key value in advance. The station first sends an authentica- tion frame to the AP, and the AP responds with an authentication frame that contains a block of text known as the challenge text. The station must encrypt the text with its key value and return it to the AP in an authentication frame. The AP will then decrypt what was returned with its own key to see if it matches the original challenge text. Once a wireless device is authenticated the final step is to be accepted into the wireless network. This is known as association.

■ The IEEE 802.11 standard specifies three procedures for transmitting on the WLAN. Distributed Coordination Function (DCF) specifies that a modified procedure known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) be used. Whereas CSMA/CD is designed to handle collisions when they occur, CSMA/CA attempts to avoid collisions altogether. CSMA/CA can also address collisions through frame acknowledgment and block acknowledgment. The 802.11 standard provides an

226 Chapter 6 Media Access Control Layer Standards

option that can be used when collisions occur due to a hidden node. This option is known as virtual carrier sensing or the Request to Send/Clear to Send (RTS/CTS) protocol. When this option is used, it solves the hidden node problem and provides additional protection against collisions. Variations of RTS/CTS are also used as protection mechanisms.

■ The 802.11 standard provides for an optional polling function known as Point Coordination Function (PCF). With PCF, the access point serves as the polling device or “point coordinator” and queries each device in an orderly fashion to determine if the device needs to transmit. A third channel access method, which is also optional like PCF, is the hybrid coordination function (HCF). The HCF allows for different types of wireless traffic to be given different levels of priority. The foundation of HCF is that it schedules access to the channel by allocating transmission opportunities (TXOP) to the stations. Each TXOP has a starting time and a maximum duration, which is the time interval the station has the medium all to itself.

Key Terms access category (AC) The classes of an EDCA. active scanning A process in which a station first sends out a management probe request frame on an available channel. ad hoc mode A wireless network that does not use an AP. Aggregate MAC Protocol Data Unit (A-MPDU) A data unit that allows multiple MPDUs to be aggregated together. Aggregate MAC Service Data Unit (A-MSDU) A data unit that allows multiple MSDUs to be combined together. Arbitration IFS (AIFS) An interframe space that is used when setting priorities to different types of transmissions. association The final step in the process of a station being accepted into the wireless network. authentication The process of a station being accepted by the AP into the WLAN. backoff interval A random amount of time that two sending devices pause after a collision. Basic Service Area (BSA) The physical area of RF coverage provided by the AP of a BSS. Basic Service Set (BSS) One or more stations that are served by a AP. Basic Service Set Identifier (BSSID) The media access control (MAC) address of the AP. beaconing The process by which the AP sends a beacon frame to both announce its presence and to provide the necessary information for wireless stations wanting to join the network. block acknowledgment An IEEE 802.11n feature that supports multiple MPDUs in an A-MPDU. broadcast probe A probe request frame sent by a station with a null value as the SSID so that all APs will respond. care-of address A new and temporary IP number assigned in Mobile IP.

Key Terms 227

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) The IEEE 802.11 standard that is designed to handle collisions when they occur. Carrier Sense Multiple Access with Collision Detection (CSMA/CD) A channel access method used by Ethernet. carrier sensing The process in which a network device first listens on the wire to see if any other device is currently transmitting. challenge text The text that is encrypted in shared key authentication. channel access methods The different ways of sharing the network medium. contention A channel access method in which devices contend or compete with each other to use the network medium. control frames Frames that provide assistance in delivering frames that contain the data by controlling access to the medium. CTS-to-self A process used when 802.11g devices are mixed with 802.11b devices together. data frame Frame that carries the information to be transmitted to the destination device. directed probe A probe request frame sent by a station that contains a specific SSID that the device is searching for. Distributed Coordination Function (DCF) The standard IEEE 802.11 contention method. Distributed Coordination Function IFS (DIFS) An interframe space that is the standard interval between the transmission of data frames. distribution system (DS) A system that is used by an AP to determine what communication needs to take place with other APs in the ESS or with the wired network. distribution system media The media, either wired network, a wireless radio, or special purpose device, that interconnects APs. dynamic rate switching A technology that allows a station farther away from an AP to still remain connected to the network but at a slower speed. Enhanced Distributed Channel Access (EDCA) An HCF that divides transmissions into four different classes. Extended IFS space (EIFS) An interframe space that is used when frames must be retransmitted. Extended Service Set (ESS) Two or more BSS networks that are interconnected. foreign agent A device that provides routing services to the mobile computer in Mobile IP. foreign network A different network in Mobile IP. frame acknowledgment An acknowledgment frame sent by the receiving device back to the sending device to confirm that the data frame arrived intact. Greenfield Mode A mode in which all of the stations in the BSS or ESS are 802.11n devices operating at the same HT speed with the same parameters. handoff The process by which a station associates with a new AP. HCF Controlled Channel Access (HCCA) The process of using polling along with centralized scheduling that is controlled by the AP. hidden node problem A station that is within range of an AP but not another station. home agent A forwarding mechanism in Mobile IP that keeps track of where the mobile computer is located.

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HT 20 MHz Protection Mode An HT Operation Mode in which a 20-MHz-only HT device associated with a 20/40-MHz AP cannot transmit simultaneously with a 40-MHz device. HT Dual-CTS Protection A protection mechanism used with 802.11n devices in a mixed environment with 802.11a/b/g devices. HT L-SIG Protection A protection mechanism used with 802.11n devices in a mixed environment with 802.11a/b/g devices. HT Mixed Mode A mode in which both 802.11n and 802.11a/b/g devices can interoperate in the same BSA. HT Nonmember Protection Mode A mode in which all stations use the non-HT 802.11a/b/g format to ensure backwards compatibility. HT Operation Modes Different modes that allow faster HT devices to interoperate with slower devices. hybrid coordination function (HCF) The IEEE 802.11 optional function that allows for different types of wireless traffic to be given different levels of priority. Independent Basic Service Set (IBSS) A wireless network that does not use an AP. infrastructure mode A wireless network that uses an AP. interframe spaces (IFS) The standard spacing intervals between the transmissions of the data frames. jumbo frame support The ability of network devices to accept frames that are between 1,500 and 9,000 bytes. Layer 2 roaming A roaming process that occurs between APs on the same subnet. Layer 3 roaming A roaming process that occurs between APs on a different subnet. MAC Protocol Data Unit (MPDU) A data unit that is simply an IEEE 802.11 frame. MAC Service Data Unit (MSDU) A data unit that contains data from Layers 3-7 along with LLC data. management frames Frames that are used to set up the initial communications between a device and the access point (for infrastructure mode) or between stations (for ad hoc mode), and then maintain the connection. maximum transmission unit (MTU) The frame size in an IEEE 802.11 network. Mobile IP A mechanism within the TCP/IP protocol to better support mobile computing. net allocation vector (NAV) The field in which the time reserved for the medium in RTS/ CTS is stored. open system authentication The process of a station sending an association request frame to an AP to be accepted into the WLAN. passive scanning A process in which a station changes to the different channels that it supports and listens for a beacon frame for a set period of time. peer-to-peer A wireless network that does not use an AP. PLCP Protocol Data Unit (PPDU) A data unit that is created by adding a header and other information to it. PLCP Service Data Unit (PSDU) The result of the PMDU sent to the PLCP sublayer. Point Coordination Function (PCF) The IEEE 802.11 optional polling function. Point Coordination Function IFS (PIFS) An interframe space used by a device to access the medium after it has been asked and then given approval to transmit.

Key Terms 229

polling A channel access method in which each device is polled or asked in sequence if it wants to transmit. Protocol Data Unit (PDU) A unit of data that will be sent to the peer protocol layer at the receiving device instead of that being sent to a lower layer level. Reduced IFS (RIFS) An interframe space that is used by 802.11n devices to reduce the amount of “dead space” required between OFDM transmissions. Request to Send/Clear to Send (RTS/CTS) An optional IEEE 802.11 channel access method that reserves the medium for a period of time. roaming The movement between cells. RTS threshold Transmitted short data packets without RTS/CTS. Service Data Unit (SDU) A specific unit of data that has been passed down from a higher OSI layer to a lower layer but has not yet been encapsulated by that lower layer. Service Set Identifier (SSID) A logical network name that is a unique identifier to differentiate WLANs. shared key authentication The process of a station encrypting text in order to be accepted into the WLAN. Short IFS (SIFS) An interframe space used for immediate response actions such as ACK and has the highest level of priority. slot time The amount of time that a station must wait after the medium is clear. transmission opportunities (TXOP) The process of scheduling access to the channel by allocating to the stations. virtual carrier sensing An optional polling IEEE 802.11 channel access method. wireless distribution system (WDS) A distribution system that provides services through a wireless infrastructure.

Review Questions 1. How many different service sets are available in IEEE 802.11?

a. three

b. four

c. five

d. six

2. A(n) consists of one or more stations that are served by a single access point.

a. Basic Service Set (BSS)

b. Hybrid Service Set (HSS)

c. Extended Service Set (ESS)

d. Independent Extended Service Set (IESS)

230 Chapter 6 Media Access Control Layer Standards

3. Each of the following is a name for a service that that does not use an AP except:

a. ad hoc.

b. peer-to-peer.

c. Basic Service Set.

d. Independent Basic Service Set.

4. A(n) describes a specific unit of data that has been passed down from a higher OSI layer to a lower layer but has not yet been encapsulated by that lower layer.

a. Extended MAC Protocol Data Unit (X-MPDU)

b. Aggregate Data Unit (ADU)

c. Protocol Data Unit (PDU)

d. Service Data Unit (SDU)

5. What is a restriction that applies to A-MPDUs?

a. They cannot use OFDM.

b. All of the A-MPDUs within the single A-MPDU must be addressed to the same receiver.

c. They must all share the same hop time.

d. There are no restrictions.

6. Each of the following is an option for addressing differences in frame size except:

a. lowest common denominator.

b. Jumbo frames.

c. fragmentation.

d. maximum transmission unit splicing (MTUS).

7. In mode an IEEE 802.11a/b/g device will not be able to access the AP.

a. HT Nonmember Protection

b. Greenfield

c. HT Mixed

d. non-HT

8. frames are used to set up the initial communications between a device and the access point.

a. Control

b. Management

c. Initialization

d. Data

Review Questions 231

9. is when the AP transmits its beacon frame and a station is looking for those frames.

a. Passive scanning

b. Beacon reconnaissance (BR)

c. SSID allocation

d. WLAN resource allocation

10. What happens if a probe request frame contains a null value?

a. All APs will respond.

b. No APs will respond.

c. Only APs with a null SSID will respond.

d. An error message is generated and sent to the AP.

11. is the process in which an AP accepts a station into the WLAN.

a. Authentication

b. Authorization

c. Acceptance

d. Reception

12. Which of the following uses a challenge text?

a. Open system authentication

b. Shared key authentication

c. Challenge authorization

d. Encryption management

13. Which of the following is the default access method for IEEE 802.11 WLANs?

a. Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

b. Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

c. Carrier Sense Multiple Access with Collision Mitigation (CSMA/CM)

d. Carrier Sense Multiple Access with Collision Routing (CSMA/CR)

14. Which of the following is false regarding block acknowledgment?

a. It only applies to IEEE 802.11n.

b. It supports multiple MPDUs in an A-MPDU.

c. The receiving node sends a block ACK only acknowledging the correct MPDUs.

d. The sender then will retransmit all MPDUs.

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15. reserves the medium for a period of time and handles the hidden node problem.

a. Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

b. NVA

c. Request to Send/Clear to Send (RTS/CTS)

d. DFC

16. Which is used when 802.11g devices are mixed with 802.11b devices together?

a. CTS-to-self

b. HT Dual-CTS Protection

c. HT L-SIG Protection

d. RTS/GHS

17. Which of the following interframe spaces is used for immediate response actions such as ACK and has the highest level of priority?

a. Distributed Coordination Function IFS (DIFS)

b. Arbitration IFS (AIFS)

c. Short IFS (SIFS)

d. Point Coordination Function IFS (PIFS)

18. is an optional polling mechanism.

a. Point Coordination Function (PCF)

b. Polling Contact

c. RTS/CTS

d. Synchronization Access Method

19. The hybrid coordination function (HCF) allows for different types of wireless traffic to be given different levels of .

a. speed

b. priority

c. frame size

d. backoff procedures

20. Each of the following is an Enhanced Distributed Channel Access (EDCA) access category (AC) except:

a. Background (AC_BK).

b. Foreground (AC_FG).

c. Best Effort (AC_BE).

d. Video (AC_VI).

Review Questions 233

Hands-On Projects

Project 6-1: Creating a Bootable Linux USB Flash Drive with Network Protocol Analyzer Software Capturing data, control, and management frames from a wireless network using a Windows-based network protocol analyzer software application

can be difficult. This is because wireless network interface card adapters can operate in one of six different modes: master (when the card acts as an AP), managed (when the station acts as a normal client), repeater, mesh, ad-hoc, or monitor mode (also called Radio Frequency Monitor or RFMON). When in monitor mode, a card can capture frames without first being associated with an AP. Prior to Microsoft Windows Vista, the Microsoft Windows Network Driver Interface Specification (NDIS) did not support monitor mode, and only data frames could be displayed. In later versions of Windows (Vista and 7), some support for monitor mode was added, yet this is dependent upon specific types of cards. Unlike Windows, Linux supports monitor mode so that most cards and their drivers can easily display all three types of frames. Using Linux does not require that the operating system and protocol analyzer software be installed on a hard drive; instead, a “live” bootable CD or USB flash drive containing Linux and selected applications can turn any computer into a Linux-based protocol analyzer without using the hard drive. In this project you will create a USB flash drive that contains Linux and the Backtrack package that contains the Wireshark network protocol analyzer software.

In order to complete this project you will need a computer with a fast Internet connection and a USB flash drive that is at least 2GB. As an option, you can create a live DVD instead of using a USB flash drive.

1. First you will download Unetbootin that allows you to create a live bootable USB flash drive. Use your Web browser to go to unetbootin.sourceforge.net.

2. Click the Download (for Windows) button.

3. Follow the instructions to download the Unetbootin executable file onto your computer.

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions then open a search engine and search for “Unetbootin”.

4. Next you will download Backtrack, which contains the Linux operating system and the Linux-based Wireshark network protocol analyzer software. Use your Web browser to go to www.backtrack-linux.org.

It is not unusual for Web sites to change the location of where files are stored. If the URL above no longer functions then open a search engine and search for “Backtrack”.

234 Chapter 6 Media Access Control Layer Standards

5. Click Downloads.

6. Enter your contact information if requested. Select these settings for the correct Back- track version:

● Release: Select the latest release of Backtrack. ● WM Flavor: KDE. ● Arch: 32 bit. ● Image: ISO ● Download: Direct

Note that BackTrack is an extremely large application and may take up to three hours to download with a fast Internet connection.

7. Click the Click to Download button. After the BackTrack download is completed, insert the USB flash drive into the computer and note the letter drive assigned to the flash drive.

8. Launch Windows Explorer and navigate to the location of the Unetbootin file.

9. Launch the Unetbootin application by double-clicking the filename.

10. Click the radio button Diskimage.

11. Click the browse button (labeled “…”) and select the downloaded Backtrack file and click Open.

12. Verify that the Type setting is USB Drive.

The Type setting should be USB Drive and not Hard Drive. If Hard Drive is selected it will erase the entire contents of the hard drive.

13. Verify that the Drive is set to the letter assigned to the USB flash drive you inserted in Step 7.

14. Click OK. Depending on the computer it could take anywhere from 5 to 20 minutes to complete the process.

15. Click Exit after the installation is complete. Close all windows.

Project 6-2: Launch the Linux Wireshark Network Protocol Analyzer The Wireshark wireless network protocol analyzer can capture and display control, management, and network frames. In this project you will capture a set of WLAN frames.

Hands-On Projects 235

You should only capture frames from your own WLAN or one that is approved for you to use. You should not capture frames from a foreign WLAN without the owner’s permission.

1. Insert the USB flash drive into a computer that contains a wireless network interface card adapter.

2. Reboot the computer.

3. If the computer is not configured to launch from a USB flash drive press the appro- priate key to change the boot sequence so that the USB drive is the first drive from which the computer launches. If that is not available, press the appropriate key to enter the ROM BIOS and change the boot order settings so that the USB drive is first.

4. Press Enter to select Default.

5. When the root@root:~# prompt appears, type iwconfig and press Enter. Note the interface that is associated with IEEE 802.11.

6. When the root@root:~# prompt appears, type iwlist and press Enter. Note the channel number of the WLAN.

7. When the root@root:~# prompt appears, type iwconfig interface channel number. For example, if the interface is wlan0 on channel 11 type iwconfig wlan0 channel 11. When you are finished, press Enter.

8. When the root@bt:~# prompt appears, type airmon-ng start interface. For example, if the interface is wlan0 type airmon-ng start wlan0. When you are finished, press Enter.

9. When the root@bt:~# prompt appears type startx and press Enter.

10. Click the K Menu icon (the first icon in the lower left corner).

11. Click Backtrack and Information Gathering and Network Analysis and Network Traffic Analysis and then Wireshark.

12. When the Wireshark application starts, click Capture.

13. Click Interfaces.

14. Click Start next to the device mon0.

15. Allow Wireshark to collect 1–2 minutes of frames. Within this time, be sure that someone on the same WLAN is performing a network activity such as surfing the Web.

16. Click Capture and then Stop to stop collecting packets.

17. Click File and Save As to save your capture to a data file. In the File name text box type Lastname-Project 6-2.pcap, where Lastname is your last name.

18. In the Save in folder box navigate to a location to save the file.

236 Chapter 6 Media Access Control Layer Standards

You will not be able to save your file to the same USB flash drive from which you booted the computer.

19. Leave Wireshark open for the next project.

Project 6-3: Analyze WLAN Statistics The Wireshark wireless network protocol analyzer can display statistics regarding the captured frames. In this project you look at statistics about those frames.

1. In Wireshark click Statistics and then Summary. View the summary data about your packet capture.

2. Note the total time of packet captures in the Between first and last packet line. In the Packets line the total number of packets capture in this time is displayed, while the Avg. packets/sec line displays the average number of packets transmitted each second. Does this value surprise you? Why is it so high? How would it compare to a wired network?

3. Note the Avg. MBit/sec for this capture. Is it what you would have expected given the type of IEEE WLAN that is being used? Click the Close button.

4. Click Statistics and then Protocol Hierarchy. Expand this screen to full size. What per- centage of frames were management frames? What percentage were data frames? Why the difference? Click the Close button.

5. Click Statistics and then click IO Graphs to display a graph of the rate at which packets are sent and received. Be sure the Tick interval: is set to 1 sec and the Pixels per tick: is at 5. What can you say about the graph? Is there a surge in packets? Why?

6. Now change the Tick interval: to .1 sec. Scroll to the beginning of the graph and then back to the end. Click the Close button.

7. Next view the exchanges (“conversations”) between endpoints. Click Statistics and then click Conversation List and finally WLAN. Is there anything significant about these con- versations? Click the Close button.

8. Click Statistics and then click Endpoint List and finally WLAN to display the number of packets sent and received (Tx and Rx) by the Aps. (The different APs are identified by their vendor name, such as Netgear and Cisco.) Click the Close button.

9. Now view a graphical representation of the different frames. Click Statistics and then click Flow Graph. When the Wireshark: Flow Graph dialog box appears, click OK. Note the relationship between the beacon frames and data frames. Click the Close button and then the Cancel button.

10. Finally, click Statistics and then click WLAN Traffic. Which type of frames were trans- mitted most frequently? Why? Click the Close button.

11. Leave Wireshark open for the next project.

Hands-On Projects 237

Project 6-4: Analyze WLAN Frames The Wireshark wireless network protocol analyzer allows you to display the contents of frames. In this project you will analyze those frames.

1. In the top pane of Wireshark, click a frame that says Beacon frame in the Info column.

2. That frame is displayed in the lower pane. Expand all of the lines to show their full detail and then scroll back up to the top.

3. Answer these questions about the beacon frame:

a. What is the length of the beacon frame?

b. Is Frequency-Hopping Spread Spectrum (FHSS) being used?

c. What is the data rate of this beacon frame?

d. What are the supported rates advertised by this beacon frame from the AP? (Note that you will find this under Supported Rates and Extended Supported Rates if the AP supports them.)

4. Now scroll down to a data frame and click it. Answer these questions about the data frame:

a. What is the length of the data frame? How does it compare with the beacon frame?

b. What is the SSI signal strength?

c. Is this frame one that is being retransmitted?

5. Locate an acknowledgment frame, which is a type of control frame. Scroll through the details of this frame.

6. Close all windows.

Studying the contents of the different types of frames is an excellent means to learn how WLANs actually function.

Case Projects

Case Project 6-1: Mobile IP Mobile IP is the underlying technology for supporting a variety of mobile data and wireless networking applications besides WLANs. For example, specific cell phone technologies rely on Mobile IP to enable the relay of

messages. Use the Internet to research Mobile IP and how it is being used today. What are its advantages and disadvantages? What are some of its applications? What are the predictions for Mobile IP use in the future? Write a one-page paper on your findings.

238 Chapter 6 Media Access Control Layer Standards

Case Project 6-2: Wireless Distribution Systems One option for a distribution system media can be a wireless radio within the APs. This wireless configuration is called a wireless distribution system (WDS). Use the Internet to research WDS. How do they function? What are their strengths as well as limitations? In what setting is a WDS preferable? Write a one-page paper about your research.

Case Project 6-3: Jumbo Frame Support Jumbo frames provide an easy way to address the differences in frame size to provide inter- operability between wired and wireless networks. Research Jumbo frames on the Internet. Are they widely used? Why or why not? What are their strengths? What are their weak- nesses? In what applications should they be used? Create a one-page paper on your research.

Case Project 6-4: HT Operation Modes Table Because of the significant differences between 802.11n HT and non-HT 802.11a/b/g, an 802.11n AP can tell 802.11n wireless devices to change to one of four HT Operation Modes in order to interoperate with slower devices. Create a table of the four HT Operation Modes. Include the name of the mode, its mode number, the setting in which it is used, its advantages, and its disadvantages.

Case Project 6-5: Nautilus IT Consulting Nautilus IT Consulting (NITC) is a computer technology business that helps organizations with IT solutions. NITC has asked for your help.

Castleview Medical Associates operates several physician offices, clinics, and free-standing surgical centers in a large region. Castleview wants to upgrade to IEEE 802.11n technology, but is unsure if DCF, PCF, or HCF would be best for its operations

1. Create a PowerPoint presentation of eight or more slides that covers a comparison of these three types of functions. Include strengths and weaknesses of each. Because you will be addressing Castleview’s IT staff this presentation should contain technical information.

2. After your presentation Castleview’s IT staff is still struggling with the best approach. Create a one-page memo that contains your choice and why they should explore this option.

Case Projects 239

chapter7

WLAN Management and Architectures

After completing this chapter you should be able to:

• Describe the features of an autonomous access point architecture • Explain the characteristics and features of a controller-based architecture • Describe the differences between multiple- and single-channel architecture models • Explain what a wireless network management system is and how it functions • Describe the characteristics of basic and enhanced power management technologies

241 Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

Early IEEE 802.11 wireless LANs (WLANs) typically utilized a Basic Service Set (BSS) or an Extended Service Set (ESS) with one or more access points (APs). Although these service set configurations still remain the preferred choice for settings with a limited number of users, such as for homes or SOHOs, for enterprise settings with hundreds of users—and a corre- sponding high number of APs—the task of individually managing all of the APs proved to be burdensome. A technician would have to roam into the range of each of the APs in order to

An automobile insurer is expanding a program that tracks how and when drivers use their cars. This results in lower premiums for safe drivers and higher premiums for risky drivers. And wireless technology is at the heart of it.

Progressive Insurance now offers its Snapshot® discount program, first started in June 2008, in over 20 different states. Customers who sign up for the program receive a device about the size of the palm of your hand. This device is plugged into the car’s on-board diagnostic port (ODB), a connection that has been required in all cars since 1996. Through the ODB, the Snapshot device records such information as the number of miles driven, the time of day that the driving takes place, and the number of sudden stops. (It does not record the vehicle’s location by GPS or whether the driver is speeding.) This information is then automatically transmitted wirelessly to Progressive. (Earlier models required drivers to remove the device and connect it to their home computers either through a cable or wireless connection to upload the data.) Drivers can go online at any time to view their information and rates.

Drivers with good driving habits—gentle braking, driving fewer miles than the average driver for that state, and limited driving during peak hours or between mid- night and 4:00 AM—can receive a discount on their car insurance. These discounts can be as much as 60 percent less than normal rates. However, drivers with bad driv- ing habits could see their insurance increase by up to 9 percent. Drivers can drop out of the Snapshot program at virtually any time and not have any accumulated infor- mation used against them. And Progressive states that they will not use the Snapshot data to settle a claim unless the owner of the vehicle first grants permission.

All indications are that Progressive will continue to expand this program using wireless technology. They already own several patents on the technology, including one that is for a “method of determining a cost of automobile insurance for a selected period based upon monitoring, recording and communicating data represen- tative of operator and vehicle driving characteristics during said period, whereby the cost is adjustable by relating the driving characteristics to predetermined safety standards.” Insurance industry analysts predict that other insurance companies will soon have their own Snapshot-like devices using wireless technology.

Real World Wireless

242 Chapter 7 WLAN Management and Architectures

make the changes, taking several days of work just to make a single configuration change. A centralized management structure was simply not available.

However, as WLANs have matured, new architectures based on new wireless equipment have been developed. Many of these new architectures have the goal of improving the management of the wireless network.

In this chapter the different types of WLAN management and architectures will be discussed. First you will learn about the three general categories of WLAN architectures: autonomous access point architectures, controller-based architectures, and other architectures. Next, the differences between single- and multiple-channel architecture network models will be explored. Finally, you will learn how to manage these architectures through a wireless network manage- ment system and you will learn how WLANs handle power management.

Autonomous Access Point Architectures

C W N A

3.2.3. Define, describe, and apply IEEE 802.11 coordination functions and channel access methods and features available for optimizing data flow across the RF medium.

4.4.1. Define, describe, and implement autonomous APs.

4.4.3. Define, describe, and implement distributed WLAN architectures.

The most common WLAN architecture is one with a “stand-alone” AP. These APs are known as independent or autonomous access points because they are separate from other autono- mous access points. All of the “intelligence” for wireless management, authentication, and encryption is contained within the AP itself to provide service to wireless stations. Because everything is self-contained in these devices they are also called fat access points.

Autonomous APs are covered in Chapter 2.

Autonomous access point architectures include network connectivity along with a lengthy list of features. There are both advantages as well as limitations to this type of architecture.

Network Connectivity The network connectivity of WLANs using an autonomous access point depends on its type of service set. A service set is all of the devices that are associated with an 802.11 WLAN. There are three different wireless LAN service set configurations: the BSS, the ESS, and the independent basic service set (IBSS). The IBSS is a wireless network that does not use an AP and thus cannot connect to another network. Although by definition a BSS is not required that the AP be connected to another network, practically speaking the BSS would have limited functionality if the AP were not connected: the stations would only be able to communicate between each other but not to any other devices or networks outside the BSS. An ESS is composed of two or more BSS networks that are interconnected. By using multiple APs, an ESS can accommodate additional users over a wider area. This type of configuration is called a distributed WLAN architecture, in which

Autonomous Access Point Architectures 243

multiple APs form a non-centralized (distributed) network through a wireless connection (instead of a centralized network with a single AP at the center).

Service sets are covered in Chapter 6.

Feature Sets The features found in autonomous access points vary by vendor as well as by the setting for which the AP is designed (home, small office home office, or enterprise). Many of these fea- tures are designed to enhance the configuration, installation, and management of the AP. Some of the typical features sets include:

● Ability to add 5-GHz IEEE 802.11a support to a 2.5-GHz IEEE 802.11g AP ● Built-in security and manageability features ● External antenna connection ● Support for Wireless Distribution Systems (WDS) ● Support for large numbers of wireless stations ● Auto network connect and dynamic rate shifting ● Power over Ethernet (PoE) options ● Diversity radio antenna ● Security to limit unauthorized stations access to network resources ● Adjustable transmit power ● Ability to configure parameters, run diagnostics, and monitor performance from

anywhere on the network using a Web browser ● Support for standards-based management protocols

In addition to these feature sets, two additional features add enhanced capabilities to autono- mous access points: quality of service and wireless virtual LANs (VLANs).

Quality of Service (QoS) The Distributed Coordination Function (DCF) contention method was designed to be an “equal and fair” approach to wireless transmissions: each wireless station has the same opportunity as all other stations for accessing the medium (sometimes called “airtime fairness”). Although DCF generally works well for data transmis- sions, the same is not true for real-time traffic that is time dependent. These types of trans- missions—like voice and video—depend heavily upon each frame arriving in sequence, whereas general data transmissions are not as sensitive to time. Delays in voice and video transmission can result in a video that freezes on the screen or a conversation that has gaps of dead space. DCF cannot distinguish between voice, video, and data frames to assure that time-sensitive frames have a priority over data-only frames. The capability to prioritize dif- ferent types of frames is known as Quality of Service (QoS).

Distributed Coordination Functions are covered in Chapter 6.

244 Chapter 7 WLAN Management and Architectures

QoS on WLANs has become increasingly important due to the widespread adoption of Voice over IP (VoIP) telephony, which uses Internet Protocol (IP-based) data packet switching networks to transmit voice communications. However, implementing QoS over WLANs poses several chal- lenges. Although it is possible to use Point Coordination Function (PCF) or a combination of DCF and PCF for QoS, most wireless manufacturers have chosen not to provide the optional PCF ser- vice in their equipment. This has resulted in some vendors offering proprietary wireless QoS, which forces customers to adopt one brand of equipment for all wireless hardware.

In 2004, the Wi-Fi Alliance released its QoS specification known as Wi-Fi Multimedia (WMM). WMM is modeled after a wired network QoS prioritization scheme. WMM out- lines four levels of prioritization for WLAN QoS. These are summarized in Table 7-1.

In 2005 the IEEE released its own set of QoS standards for WLANs known as IEEE 802.11e-2005. This standard is based on the optional hybrid coordination function (HCF). There are two access mechanisms specified by the HCF. The first is known as Enhanced Distributed Channel Access (EDCA). EDCA divides transmissions into four different access categories or ACs (WMM is based on EDCA and is considered a subset of it). The second HCF mechanism is HCF Controlled Channel Access (HCCA), which uses polling along with centralized scheduling controlled by the AP.

Currently there are four recognized solutions to QoS: proprietary; WMM (some APs support both proprietary and WMM); EDCA; or split dual-band (using a dual-band AP and dedicating the 5 GHz to VoIP and the 2.4 GHz to data). HCCA is a very powerful, complex coordina- tion function, and it is unclear if it will gain widespread acceptance.

Wireless Virtual LANs (VLANs) A wired network can be segmented by constructing a virtual local area network (VLAN). A VLAN is a logical grouping of network devices within a larger physical network. VLANs do not require that all of the devices (or their users) be physically located together; they can be dispersed anywhere throughout the network. For example, a VLAN group may consist of all the accounting department employees, even if they are scattered across different floors of an office building or even in different buildings.

The key to VLANs is the ability of the network switch to correctly direct packets. Switches allo- cate an access port for each network device and then keep a record of that device’s media access control (MAC) address with the port number. When a VLAN packet arrives at the switch, it can determine which devices are part of that VLAN and only send the packet out those ports. This means that network switches are able to send VLAN packets to the correct ports because of the way in which the VLAN packets are identified. One standard for marking VLAN packets

WMM Access Category Description

WMM Voice Priority The highest priority; facilitates multiple high-quality voice calls

WMM Video Priority Prioritizes video traffic higher than regular data traffic but not as high as voice traffic

WMM Best Effort Priority Includes traffic from applications that are not time sensitive

WMM Background Priority Includes low-priority traffic, such as file transfers or print jobs

Table 7-1 Wi-Fi Multimedia (WMM)

Autonomous Access Point Architectures 245

is the IEEE 802.1q standard that supports trunking (trunking means that a single cable is used to support multiple virtual LANs). The 802.1q standard is called an internal tagging mechanism because it inserts a 4-byte “tag” header within the existing Ethernet packet.

Just as with wired networks, wireless VLANs can also be used to segment traffic, yet without adding additional network hardware. For example, an organization may set up two wireless VLANs: the first is for employee access, in which employees can see the company’s files and databases through the network, while a second VLAN is for guest access, limited only to Internet access or files available for any user. Employees can configure their wireless network interface card client adapters to use the Service Set Identifiers (SSID) Employee while guests would use the SSID Guest. When the devices associate to the same AP, they automatically become part of their respective wireless VLAN. And because wired devices attached through the switch can also belong to the same VLAN, wireless VLAN and wired VLAN devices can share subnets or can belong to completely different subnets.

Another benefit of using multiple SSIDs and VLANs is that different security features can be configured for each VLAN group.

Wireless VLANs can be configured in one of two ways. The difference depends upon which device separates the packets and directs them to different networks. In Figure 7-1, separating

Packets separated hereSwitch

Laptop Laptop

Accounting VLAN

Laptop Laptop

Marketing VLAN

AP AP

File server

Accounting VLAN Accounting VLAN Marketing VLAN

File serverPC

Figure 7-1 Packets separated at switch

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packets in this wireless VLAN is done by the switch. Each AP is connected to a separate port on the switch and represents a different VLAN. As packets destined for the wireless LAN arrive at the switch, the switch separates the packets and sends them to the appropriate AP (VLAN).

Yet this configuration has limitations. For example, if a wireless user in the accounting depart- ment is part of the Accounting VLAN, what happens when that user roams to the Marketing VLAN supported by another AP? The user may no longer have access to the Accounting VLAN and then is unable to use the network resources. Reconfiguring the network to make each VLAN accessible from every AP across the enterprise may not always be possible.

A more flexible approach is illustrated in Figure 7-2, where the AP is responsible for separating the packets. Under this configuration a user can roam into different areas of cov- erage and still be connected to the correct VLAN. The key to this configuration is that dif- ferent VLANs are transmitted by the AP on different SSIDs. This enables only the clients associated with a specific VLAN to receive those packets. Access points that support wireless VLANs may support 16 or more multiple SSIDs (and thus multiple VLANs).

If you were to look at an AP configured for multiple VLANs it would appear as 16 different wireless networks.

Packets separated here Packets separated here

Switch

Laptop

Accounting VLAN

Laptop

Marketing VLAN Accounting VLAN Marketing VLAN

AP AP

File server

Accounting VLAN Accounting VLAN Marketing VLAN

File serverPC

Figure 7-2 Packets separated at AP

Autonomous Access Point Architectures 247

Wireless VLANs allow a single AP to service different types of users. Wireless VLANs are not only found on enterprise-grade APs, they are also supported by many SOHO and con- sumer APs. Configuring wireless VLANs can also prevent a user who connects to a wireless network from accessing sensitive computers or files.

Advantages and Limitations Autonomous access point architectures have several advantages. As the foundation of WLANs for several years they have matured to include a strong array of features, including QoS, wireless VLANs, and other options. In addition, the growth (scalability) of a distributed WLAN architecture using autonomous APs is straightforward and virtually unlimited: to add more capacity to the WLAN all that is needed is to add more APs. This allows for a WLAN to start small and grow in manageable increments. In addition, because all of the intelligence is in the AP there are no other devices that must be added.

Yet because each autonomous AP is essentially independent, each AP must be separately con- figured, managed, and maintained. Each AP operates as a separate node that is configured with its own unique settings (such as channel number and power settings), leading to coordi- nation problems. For example, when an autonomous AP hears another AP on the same channel, it cannot determine if that AP is part of the same network or if it belongs to another neighboring network. Adding a new AP to a larger existing WLAN may require significant configurations to multiple APs. Table 7-2 lists some limitations of the autonomous access point architecture.

Controller-Based Architectures

C W N A

2.3.1. Define the roles of the organizations in providing direction, cohesion, and accountability within the WLAN industry.

4.4.2. Define, describe, and implement WLAN controllers that use centralized and/or distributed forwarding.

The limitations of the autonomous access point architecture can be addressed by a second type of WLAN architecture that does not have at its focal point an autonomous AP. Sometime called

Procedure Description Limitation

Layer 2 Roaming Seamless station roaming across APs Must install and configure new AP and may need to reconfigure some existing APs

Layer 3 Roaming Station roaming across separate subnets Not possible with autonomous APs; may need to implement Mobile IP

Management Adjust configuration settings as necessary Must visit each AP to change settings; no centralized management possible

Load Balancing Equalize number of stations across multiple APs so that all APs have manageable loads

Must be done on a manual basis; cannot automatically spread load across multiple APs

Table 7-2 Autonomous access point architecture limitations

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controller-based architectures, these configurations rely upon a wireless LAN controller (WLC) at the heart of the network. With a single WLC centrally configured, the settings can then be automatically distributed to all APs.

WLCs are covered in Chapter 2.

The APs used in controller-based architectures are different from the APs used in an autono- mous access point WLAN and in WLCs.

Access Points In an autonomous access point architecture, the AP consists of three major parts: an antenna and a radio transmitter/receiver to send and receive wireless signals, special bridging software to interface wireless devices to other devices, and a wired network interface that allows it to connect by cable to a standard wired network in a BSS or ESS. Its basic functions are to act as the “base station” for the wireless network to receive and forward all wireless transmis- sions as well as to act as a bridge between the wireless and wired networks.

Access points in a controller-based architecture are significantly different. There are three main types of APs that are found in this architecture: lightweight, mesh, and captive portal APs.

Lightweight APs In a controller-based architecture, autonomous APs are replaced with lightweight access points, also called thin access points. A lightweight access point does not contain the management and configuration functions that are found in autonomous access points; instead, these features are contained in the centralized WLC. Lightweight access points only have simplified radios for wireless communication between devices and a media converter for accessing the wired network.

Lightweight APs only handle the real-time MAC layer functionality within themselves; all other (non-real-time) MAC functionality is processed by the WLC. This type of division is referred to as a split MAC architecture. Because the wireless controller manages the AP configurations individually, the configuration of each lightweight AP is not necessary.

Lightweight APs cannot function independently of a wireless LAN controller.

One of the most significant benefits to lightweight access points is a decrease in the total cost of ownership (TCO), which is the total cost of owning a product that includes acquisi- tion, setup, support, ongoing maintenance, service, and all operating expenses. Autonomous access points require not only initial deployment but also individual management. For exam- ple, in an autonomous access point architecture with 250 APs, spending only 30 minutes per year on each AP would result in 125 person hours a year, or over three months of maintenance in a five-year period (exclusive of any troubleshooting or errors). However, all lightweight AP can be centrally managed from a single WLC, resulting in significant reduction in TCO.

Controller-Based Architectures 249

Figure 7-3 illustrates the projected labor costs of an autonomous access point compared with a lightweight access point.

Some WLAN controllers can support up to 250 lightweight access points.

Lightweight Mesh APs A mesh access point does not have to be individually con- nected by a cable to the existing wired network. Instead, each mesh access point communi- cates wirelessly with the next closest mesh access point. Dozens—or even hundreds—of mesh access points can communicate between themselves to create a wireless mesh network (WMN). Only one mesh access point must be physically connected to the wired network, and all other mesh access points transparently “hop” through each other to reach the mesh access point with the wired connection.

Just as an autonomous AP can be replaced with a lightweight AP, a standard mesh AP can be replaced with a lightweight mesh AP. Lightweight mesh APs can also be centrally config- ured and managed through a WLC.

Captive Portal APs A home user typically installs a WLAN in order to allow multiple desktop and mobile stations to share access to the Internet. After the network connection is established, simply launching a Web browser will give the user immediate and unlimited access to the Internet.

However, in a public area that is served by a WLAN, opening a Web browser will rarely give immediate Internet access. This is because the owner of the WLAN usually wants to advertise themselves as providing this service, or because they want the user to read and accept an Accept- able Use Policy (AUP) before using the WLAN. And sometimes a “general” authentication, such

10 50 100 250

Number of Access Points

L a b o r

C o s ts

500 1000

Projected labor costs with autonomous access points

Projected labor costs with lightweight access points

Figure 7-3 Autonomous AP vs. lightweight AP labor costs

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as a password given to all current hotel guests, must be entered before being given access to the network. This type of information, approval or authentication can be supported through a captive portal AP. A captive portal AP uses a standard Web browser to provide information, give the wireless user the opportunity to agree to a policy, or present valid login credentials.

Using a Web browser instead of a custom program for captive portal APs ensures that the captive portal APs work with virtually all computers and operating systems.

When a user on a wireless station connects to a public WLAN with a captive portal AP, he will typically launch a Web browser to access the Internet. Instead of displaying the requested Web page, the browser first displays a special screen that provides information and then requires a user response (such as click “I Agree” or enter a valid password). Once the user enters this information, he is verified by the captive portal AP; all other network access is blocked until these credentials are approved. A captive portal AP is illustrated in Figure 7-4.

Although a captive portal AP is not a required part of a controller-based architecture, this is typically the architecture in which it is most often found. A dedicated captive portal AP is not even required for displaying information and authenticating the wireless user. Some cap- tive portal alternatives have two parts: a client portion and a separate authentication server. The client portion uses firewall rules to control traffic going through the router. When a new user tries to access a Web site, the client will transparently redirect the user to the authentication server. The client also talks to the authentication server every few minutes to update it on vital statistics including uptime, load, traffic count per client, and to let it know that it is still connected.

Web server

Internet

Access to Internet blocked

until AUP accepted

Captive portal access point

Laptop

Acceptable use policy

I have read and accept

the terms of the AUP

I decline the use of the

Internet

Figure 7-4 Captive portal AP

Controller-Based Architectures 251

Several software-only open-source captive portals are available.

Wireless LAN Controllers (WLCs) In some ways a WLC is akin to its wired network counterpart, the switch. A switch joins multi- ple computers together within one local area network (LAN). There are two types of switches. An unmanaged switch provides no management capabilities in the operation of the switch and only allows the network devices to communicate. A series of light emitting diode (LED) lights give limited information about the link status of the devices and their activity, but nothing more. A managed switch supports both control and monitoring of the network. This control of the network allows the network manager to adjust the communication parameters. For example, in a setting with a large amount of electric noise the data rate can be dropped and cer- tain automatic negotiation features disabled to ensure good transmissions. This can even be done on a port-by-port basis. Monitoring the network is accomplished by using SNMP (Simple Network Management Protocol), which provides information such as the number of bytes transmitted and received, the number of frames transmitted and received, the number of errors, and port status. All of this information can be viewed on a port-by-port basis.

Although a wired managed switch is similar to a WLC in that it can control and monitor the network (a WLAN controller is sometimes called a wireless switch), a WLC has several enhanced features. One of these features is network placement. There are three recognized functional areas of a network as illustrated in Figure 7-5. These include:

● Core layer. The core layer is considered the backbone of the network and includes all high-end switches and high-speed fiber cables. At this layer of the network frames are not routed through the WLAN; rather, this layer is concerned with speed and ensures the reliable delivery of frames.

● Distribution layer. The distribution layer, also called the workgroup layer, includes LAN-based routers and Layer 3 switches. These devices ensure that frames are prop- erly routed between subnets and VLANs.

● Access layer. At the access layer devices such as hubs and switches are located. This is because at the access layer (also called the desktop layer) the function is to connect client nodes to the network and ensure that frames are delivered to end user computers.

Depending on different factors (for example, how the WLC will be integrated with the exist- ing wired network) a WLC can be placed at either the core, distribution, or access layer. Generally most WLC are positioned at the core layer.

Most WLAN controller products encapsulate the IEEE 802.11 frames into an IP tunnel that then creates a point-to-point link between the WLAN controller and the lightweight AP.

Another advantage of a WLC is network connectivity. As wireless stations move through a WLAN, a handoff procedure must occur between APs as authentication information is trans- ferred from one AP to another. The length of time necessary for this procedure can have an adverse effect in WLAN systems using voice over wireless VoIP. However, this handoff

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procedure is eliminated with WLC and lightweight APs because all authentications are performed in the wireless controller. Also, most WLCs offer automated tools that can simulate the environment and help predict the best locations for APs. Even after deployment, many wireless controllers can establish the best channel and power settings for the wireless network.

There are also additional WLC features. These include:

● WLAN profiles. A WLAN profile is a set of specific configurations that can be applied to different wireless stations. This allows for different virtual WLANs to be created, each with its own SSID and managed through a WLC. Each WLAN profile can be different. For example, one profile may support QoS while another does not.

● Multiple BSSIDs. The Basic Service Set Identifier (BSSID) is the MAC address of the AP and is included in the header of frames that are transmitted by the AP and stations for a variety of identification purposes. A WLC can have multiple virtual BSSIDs, enabling a lightweight AP to support multiple virtual WLANs. This means that several BSSs can be created to function within the area supported by a single lightweight AP.

● Scalability. Using a WLC allows for rapid scalability or wireless network growth: mul- tiple lightweight APs can more easily be added to the network and centrally configured and managed instead of installing autonomous APs.

Router

Core layer

Access layer

Distribution layer

Router

Wide area network router

LaptopLaptop

Access point

Figure 7-5 Network functional layers

Controller-Based Architectures 253

● Client roaming. A wireless station that roams from one lightweight AP to another lightweight AP supported by the same WLC, supported by different WLCs, and even across WLCs on different subnets is fully supported.

Yet there are disadvantages to the controller-based architecture. Currently all devices are pro- prietary, so that the WLCs and APs must be from the same vendor. The Internet Engineering Task Force (IETF) Control and Provisioning of Wireless Access Points (CAPWAP) Working Group is developing a protocol that will allow any vendor’s WLC to communicate with any lightweight AP, but there is no timetable yet for the new standard and none of the companies involved have yet demonstrated cross-vendor interoperability. Another limitation is that many WLC products still do not provide true convergence of the wired and wireless networks but only ease some of the management burdens of WLANs. Many wireless experts claim that what is needed is a comprehensive solution that takes full advantage of existing tools, knowledge, resources, and the wired infrastructure to address security, deployment, and control issues.

Other Architectures

C W N A

4.4.7. Define and describe alternative WLAN architectures.

Besides autonomous access point architectures and control-based architectures, there are other types of WLANs. These include WLAN arrays, cooperative control, and mesh networks.

WLAN Arrays A WLAN array is a proprietary product marketed and sold by Xirrus. This device, which resembles a round consumer-grade smoke detector, replaces a standard WLC installed in a rack in a server closet. The WLAN array contains a WLC that can be directly connected to as many as 16 integrated APs or IAPs. Within the WLAN array housing, the IAPs are arranged in a circular configuration around the WLC. Each IAP provides a separate radio frequency (RF) channel of highly directional coverage. Because the channels are physically adjacent, they can form a multichannel region of coverage around the WLAN array.

The WLAN array also includes a switch and a firewall.

Cooperative Control Cooperative control is a technology marketed and sold by Aerohive. The cooperative control wireless LAN architecture enables APs to communicate and coordinate with each other with- out the need for a WLC; in short, each AP contains the capabilities of a WLC. A special cooperative control autonomous AP known as the HiveAP is central to the network. Multi- ple HiveAPs can be organized into groups (hives) that share control information between other HiveAPs to enable functions like roaming, security, and load balancing.

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Cloud Management Managing wireless networks that are separated geographically, such as a remote campus or branch office on the opposite side of the city from the main facility, usually requires either two separate support teams at each location or one team that travels between sites. This can result in increased costs (maintaining two support teams) or increased time to resolve problems (wait- ing for the support team to travel to the remote site). As an alternative a WLAN architecture based on using the ubiquitious Internet can be implemented. Known as cloud management, this system allows APs and other network equipment to be connected through the Internet and then viewed and managed from one central location. When each device connects to an Internet- based (“cloud”) centralized management system all the devices can be remotely controlled and configured. In addition, diagnostic tests can be run and even updates can be applied.

Wireless Mesh Networks An alternative to an autonomous access point and a lightweight access point is a mesh access point. A mesh access point does not have to be individually connected by a cable to the exist- ing wired network. Instead, each mesh access point communicates wirelessly with the next closest mesh access point. Dozens—or even hundreds—of mesh access points can communi- cate between themselves to create a wireless mesh network (WMN). Only one mesh access point must be physically connected to the wired network, and all other mesh access points transparently “hop” through each other to reach the mesh access point with the wired con- nection. In addition, there are multiple interconnected paths through which the signal can reach the wired mesh access point.

WMNs are covered in Chapter 2.

Multiple-Channel Architecture vs. Single-Channel Architecture Models

C W N A

4.4.5. Define, describe, and implement a multiple-channel architec- ture network model.

4.4.6. Define, describe, and implement a single-channel architecture network model.

There are two architecture models for WLANs: multiple-channel architecture and single- channel architecture. Proponents of each of these models claim that that their approach is superior, whereas in reality both models may have their own respective applications.

Multiple-Channel Architecture (MCA) The 802.11b standard uses the Industrial, Scientific and Medical (ISM) band for its transmis- sions. The 802.11b standard specifies 14 frequencies that can be used, beginning at 2.412 GHz and incrementing by .005 GHz (except for Channel 14). IEEE 802.11b transmissions in the 2.4-GHz frequency are designed to be þ/�11 MHz from the channel center frequency. However, some of the transmission may still encroach onto other frequencies up to 30 MHz

Multiple-Channel Architecture vs. Single-Channel Architecture Models 255

from the channel center, so that they actually consume five overlapping channels (for example, transmitting on channel 6 may cause interference on channels 5 and 7 as well as limited on channels 4 and 8). This leaves only three nonoverlapping (simultaneously usable) 20-MHz channels: 1, 6, and 11. IEEE 802.11a networks have 555 MHz spread across 23 nonover- lapping channels.

Channel allocation is covered in Chapter 5.

Managing the radio frequency spectrum of 802.11b/g—and to a lesser degree 802.11a—wireless networks can be challenging. Setting all of the APs to the same channel number would result in reduced throughput because each station must wait a longer period of time for its turn to transmit (called cochannel interference). To eliminate this interference it is necessary to arrange the coverage areas of the APs so that one channel does not interfere with an adjacent channel. In Figure 7-6, only channels 1, 6 and 11 are used as nonoverlap- ping channel numbers. Each cell is separated from other cells so that no two adjacent cells have the same channel number in order to reduce interference (known as adjacent channel interference). This type of WLAN is called a multiple-channel architecture or MCA because more than one channel is in the wireless network.

One of the keys to an MCA is to have the correct cell size in order to minimize adjacent channel interference. This is especially true when “scaling” or adding additional capacity to the WLAN. The most common approach, called the micro-cell architecture, creates small areas of coverage. Typically, in order to add wireless network capacity, more APs are added while the transmission power of all APs is reduced to minimize potential interference. This can usually provide acceptable network throughput if the site has been properly surveyed to identify the best locations for the APs. In addition, the configuration of BSSIDs and ESSIDs can be made easier in a micro-cell architecture.

Single-Channel Architecture (SCA) The fundamental reason why multiple APs in a MCA are necessary is because the interference range of wireless devices exceeds their useful communication range. That is, devices that are too far apart to communicate can still be close enough to interfere with each other. An alter- native to MCA that addresses this weakness is the single-channel architecture (SCA). Instead of having each cell use a different channel as in WCA, WLANs using SCA have all of the APs use the same channel. Each AP has overlapping coverage that forms a continuous region on a single channel, thereby reducing interference. The SCA architecture is accomplished through

6 1 11 116 1

11 6 1 11 6 1

6 1 11 116 1

11 6 1 11 6 1

Figure 7-6 Nonoverlapping cells

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