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LEARNING OBJECTIVES

By the end of this chapter, you should be able to:

▪ Explain basic radio signal propagation concepts, including frequencies, antennas,

and wireless propagation problems.

▪ Explain the frequency spectrum, service bands, channels, bandwidth, licensed

versus unlicensed service bands, and the type of spread spectrum transmission

used in 802.11 Wi-Fi LANs.

▪ Describe 802.11 Wi-Fi WLAN operation with access points and a switched Ethernet

distribution system to link the access points. Distinguish between BSSs, ESSs,

and SSIDs. Discuss communication between access points.

▪ If you read the box, compare the CSMA/CA+ACK and RTS/CTS media access

control disciplines.

▪ Compare and contrast the 802.11g, 802.22a, 802.11n, and 802.11ac

transmission standards. Discuss emerging trends in 802.11 operation,

including channels with much wider bandwidth, MIMO, beamforming,

and multiuser MIMO.

▪ Briefly discuss the key points of wireless mesh networking.

Wireless LANs I

Chapter 6

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INTRODUCTION

OSI Standards

In Chapter 5, we looked at wired switched Ethernet networks. Technologies for these

networks require both physical and data link layer standards. Consequently, they

use OSI standards. In this chapter and in Chapter 7, we will look at wireless LANs. Like

wired LANs, wireless LANs are also single networks, which require physical and DLL

standards. They too use OSI standards.

Test Your Understanding

1. a) At what layers do wireless LANs operate? b) Do wireless LAN standards come

from OSI or TCP/IP? Explain.

802.11 versus Wi-Fi

Having discussed wireless transmission briefly, we will look at wireless networking’s

widest application, wireless local area networks. Wireless LANs (WLANs) use radio

for physical layer transmission on the customer premises.

Ethernet 802.3 LANs

Require standards at Layer 1 (physical) and Layer 2 (data link)

Therefore, use OSI standards

The 802.3 Working Group of the IEEE 802 Committee creates standards

Wireless LANs

Operate at Layers 1 and 2

Therefore, they are OSI standards

802.11 Wireless LAN Technology

The dominant WLAN technology today

Standardized by the 802.11 Working Group of the IEEE 802 Committee

Wi-Fi Alliance

Industry association of 802.11 equipment manufacturers

Purpose

802.11 standards have many options

Wi-Fi Alliance selects subsets of standards as profiles

Does interoperability testing among vendors on these profiles

Only products that pass can display the Wi-Fi logo on their products

However, sometimes develops new standards

Two have been security nightmares

FIGURE 6-1 802.11 / Wi-Fi Wireless LAN (WLAN) Technology (Study Figure)

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Wireless LANs (WLANs) use radio for physical layer transmission on the customer premises.

In the last chapter, we saw that the 802.3 Working Group of the IEEE’s 802 LAN/

MAN Standards Committee creates Ethernet standards. Other working groups create

other standards. The dominant WLAN standards today are the 802.11 standards, which

are created by the IEEE 802.11 Working Group.

It is common to call the 802.11 standards “Wi-Fi” standards. In fact, the terms have

become almost interchangeable, and we will use them that way in this book. However,

as an IT professional, you should understand the technical difference between

802.11 and Wi-Fi. The term Wi-Fi stems from the Wi-Fi Alliance, which is an industry

consortium of 802.11 product vendors. When the 802.11 Working Group creates

standards, it often creates many options. The Wi-Fi Alliance creates subsets of 802.11

standards with selected options. The Alliance conducts interoperability tests among

products that claim to meet these “profiles.” Only products that pass interoperability

tests may display the Wi-Fi Logo on their products. Products that do not pass are rarely

sold, so when someone picks up a box containing an 802.11 product, they almost always

see the Wi-Fi logo.

Test Your Understanding

2. a) Distinguish between 802.3 standards and 802.11 standards. b) Distinguish

between 802.11 and Wi-Fi.

Wireless LAN Operation

It is possible to have a purely wireless LAN. In organizations today, however, the

normal situation is to have a hybrid switched/wireless single network. Figure 6-2 shows that

corporations already have comprehensive Ethernet switched LANs. These wired LANs

reach almost everywhere on the corporate premises. Wireless clients (wireless devices

Ethernet Wired LAN

Notebook

Client

Radio

Transmission

UTP

Access

Point A

Ethernet

Switch

Server

Needed by

Client

Communication

Router

for Internet

Access

Point B

The Internet

FIGURE 6-2 Hybrid Switched/Wireless 802.11 Network

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are almost always clients) communicate wirelessly (by radio) to an 802.11 wireless

access point, which is typically simply called an access point.

Wi-Fi clients rarely communicate with other 802.11 clients. Instead, they usually

need to reach resources on the main Ethernet LAN. Obviously, clients need to reach

servers, and corporate servers are on the firm’s Ethernet network. In addition, of

course, clients need to reach the Internet, and the firm’s border router that connects

it to the Internet is also on the firm’s Ethernet network. In addition to orchestrating

radio transmissions between itself and the wireless clients it serves, an access point

connects the wireless devices to the firm’s main Ethernet LAN.

In addition to orchestrating radio transmissions between itself and the wireless clients

it serves, an access point connects the wireless devices to the firm’s main Ethernet LAN.

Only small firms can get by with a single access point. Larger firms disperse access

points around their premises so that a wireless client can connect to another access point

when it is moved to a different location.

Test Your Understanding

3. a) Why do wireless clients need access to the firm’s main wired switched Ethernet

network? b) How can firms provide WLAN coverage throughout a large building?

RADIO SIGNAL PROPAGATION

Chapter 5 discussed propagation effects in wired transmission media (UTP and

optical fiber). Propagation effects in wired transmission can be well controlled by

respecting cord distance limits and taking other installation precautions. This is possible

because wired propagation is predictable. If you input a signal, you can estimate

precisely what it will be at the other end of a cord. A wired network is like a faithful,

obedient dog.

Propagation effects in wired transmission can be well controlled by respecting cord

distance limits and taking other installation precautions.

In contrast, radio propagation is very unreliable. Radio signals bounce off

obstacles, fail to pass through walls and filing cabinets, and have other problems we

will look at in this section. Consequently, Wi-Fi networks, which use radio to deliver

signals, are more complex to implement than wired networks. They do not have a

few simple installation guidelines that can reduce propagation effects to nonissues.

Therefore, we will spend more time on wireless propagation effects than we did on

wired propagation effects.

Propagation effects in wireless networks are complex and difficult to implement.

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Test Your Understanding

4. a) In 802.3 Ethernet networks, can simple installation rules usually reduce propagation

effects to nonissues? b) In 802.11 Wi-Fi networks, can simple installation

rules usually reduce propagation effects to nonissues?

Frequencies

Radios for data transmission are called transceivers because they both transmit and

receive. When transceivers send, their wireless signals propagate as waves, as we saw

in Chapter 5. Figure 6-3 again notes that waves have amplitude and wavelength. While

optical fiber waves are described in terms of wavelength, radio waves are described in

terms of another wave characteristic, frequency.

Frequency is used to describe the radio waves used in WLANs.

In waves, frequency is the number of complete cycles per second. One cycle per

second is one hertz (Hz). Metric designations are used to describe frequencies. In the

metric system, frequencies increase by a factor of 1,000 rather than 1,024. The most common

radio frequencies for wireless transceivers range between about 500 megahertz

(MHz) and 10 gigahertz (GHz).

Amplitude

Wavelength

1 Second, 2 Cycles

Wavelength is the physical distance between comparable points on adjacent cycles.

Optical fiber transmission is described in terms of wavelength.

Frequency is the number of cycles per second.

In this case, there are two cycles in 1 second, so the frequency is two hertz (2 Hz).

Radio transmission is measured in terms of frequency.

Amplitude is the power of the wave.

FIGURE 6-3 Electromagnetic Wave

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Test Your Understanding

5. a) What is a transceiver? b) Is wireless radio transmission usually expressed in

terms of wavelength or frequency? c) What is a hertz? d) Convert 3.4 MHz to

a number without a metric prefix. (The use of metric prefixes was discussed in

a box in Chapter 1.) e) At what range of frequencies do most wireless systems

operate?

Antennas

A transceiver must have an antenna to transmit its signal. Figure 6-4 shows that there

are two types of radio antennas: omnidirectional antennas and dish antennas.

􀁲􀀁Omnidirectional antennas transmit signals equally strongly in all directions

and receive incoming signals equally well from all directions. Consequently, the

antenna does not need to point in the direction of the receiver. However, because

the signal spreads in all three dimensions, only a small fraction of the energy

transmitted by an omnidirectional antenna reaches the receiver. Omnidirectional

antennas are best for short distances, such as those found in a wireless LAN or a

cellular telephone network.

􀁲􀀁Dish antennas, in contrast, point in a particular direction, which allows them to

send stronger signals in that direction for the same power and to receive weaker

incoming signals from that direction. (A dish antenna is like the reflector in a

flashlight.) Dish antennas are good for longer distances because of their focusing

ability, although users need to know the direction of the other radio. In addition,

dish antennas are hard to use. (Imagine if you had to carry a dish with you whenever

you carried your cellular phone. You would not even know where to point

the dish!)

Omnidirectional Antenna

Signal spreads in all directions

Rapid signal attenuation

-----

No need to point at receiver

Dish Antenna

Focuses signal in a narrow range

Signals can travel longer distances

-----

Must point at receiver

FIGURE 6-4 Omnidirectional and Dish Antennas

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Test Your Understanding

6. a) Distinguish between omnidirectional and dish antennas in terms of operation.

b) Under what circumstances would you use an omnidirectional antenna?

c) Under what circumstances would you use a dish antenna? d) What type of

antenna normally is used in WLANs? Why?

Wireless Propagation Problems

We have already noted that, although wireless communication gives mobility, wireless

transmission is not very predictable, and there often are serious propagation problems.

Figure 6-5 illustrates five common wireless propagation problems.

Inverse Square Law Attenuation Compared to signals sent through wires

and optical fiber, radio signals attenuate very rapidly. When a signal spreads out

from any kind of antenna, its strength is spread over the area of a sphere. (In omnidirectional

antennas, power is spread equally over the sphere, while in dish antennas,

power is concentrated primarily in one direction on the sphere.)

The area of a sphere is proportional to the square of its radius, so signal strength

in any direction weakens by an inverse square law (1/r2), as Equation 6–1 illustrates.

Here, S1 is the signal strength at distance r1, and S2 is the signal strength at a farther

distance r2.

S2 = S1 * (r1/r2)2 (Equation 6–1)

If you triple the distance (r1/r2 = 1/3), the final signal strength (S2) falls to only

one-ninth (1/32) of its original strength (S1). With radio propagation, you have to be

relatively close to your communication partner unless the signal strength is very high,

a dish antenna is used, or both.

To give a specific example, at 10 meters, the signal strength is 30 milliwatts (mW).

How strong will the signal be at 30 meters?

Transmission Antenna

Laptop

Dead Zone:

Worse at

Higher

Frequencies

Multipath

Interference

Inverse Square Law

Attenuation

1/r^2

Electromagnetic

Interference

(EMI)

Direct Signal

Reflected Signal

Absorptive

Attenuation

Worse at

Higher

Frequencies

FIGURE 6-5 Wireless Propagation Problems

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􀁲􀀁􀀵􀁉􀁆􀀁􀁅􀁊􀁔􀁕􀁂􀁏􀁄􀁆􀀁􀁕􀁓􀁊􀁑􀁍􀁆􀁔􀀁􀀉􀁔􀁐􀀁r1/r2 is 1/3).

􀁲􀀁􀀴􀁐􀀁􀁘􀁆􀀁􀁎􀁖􀁍􀁕􀁊􀁑􀁍􀁚􀀁􀁕􀁉􀁆􀀁􀁔􀁊􀁈􀁏􀁂􀁍􀀁􀁔􀁕􀁓􀁆􀁏􀁈􀁕􀁉􀀁􀁂􀁕􀀁􀀒􀀑􀀁􀁎􀁆􀁕􀁆􀁓􀁔􀀁􀁃􀁚􀀁􀀒􀀐􀀚􀀁􀀉􀀒􀀐􀀔􀀁􀁔􀁒􀁖􀁂􀁓􀁆􀁅􀀊􀀏

􀁲􀀁􀀔􀀑􀀁􀁎􀀸􀀁􀁎􀁖􀁍􀁕􀁊􀁑􀁍􀁊􀁆􀁅􀀁􀁃􀁚􀀁􀀒􀀐􀀚􀀁􀁊􀁔􀀁􀀔􀀏􀀔􀀔􀀁􀁎􀀸􀀏

􀁲􀀁􀀴􀁐􀀁􀁕􀁉􀁆􀀁􀁔􀁕􀁓􀁆􀁏􀁈􀁕􀁉􀀁􀁐􀁇􀀁􀁕􀁉􀁆􀀁􀁔􀁊􀁈􀁏􀁂􀁍􀀁􀁂􀁕􀀁􀀔􀀑􀀁􀁎􀁆􀁕􀁆􀁓􀁔􀀁􀁘􀁊􀁍􀁍􀀁􀁃􀁆􀀁􀀔􀀏􀀔􀀔􀀁􀁎􀀸􀀏

Absorptive Attenuation As a radio signal travels, it is partially absorbed by the

air molecules, plants, and other things it passes through. This absorptive attenuation is

especially bad in moist air and office plants because water is an especially good absorber

of radio signals.

Absorptive attenuation can be confusing because we have already seen inverse

square law attenuation. Yes, wireless propagation suffers from two forms of attenuation.

Inverse square law attenuation is due to the signal spreading out as a sphere and

so becoming weaker at each point on the sphere. Absorptive attenuation is signal loss

through energy absorption.

Wireless transmission suffers from two forms of attenuation—inverse square law attenuation

and absorptive attenuation.

Dead Zones To some extent, radio signals can go through and bend around objects.

However, if there is a dense object (e.g., a thick wall) blocking the direct path between the

sender and the receiver, the receiver may be in a dead zone, also called a shadow zone or

dead spot. In these zones, the receiver cannot get the signal. If you have a mobile phone

and often try to use it within buildings, you may be familiar with this problem.

Multipath Interference In addition, radio waves tend to bounce off walls,

floors, ceilings, and other objects. As Figure 6-7 shows, this may mean that a receiver

will receive two or more signals—a direct signal and one or more reflected signals.

The Situation

Signals spread over the surface of a sphere

As the radius of the sphere increases with distance, the signal weakens

Weakens as the square of the distance

S2 = S1 * (r1/r2)2 (Equation 6–1)

Example

At 10 meters, the signal strength is 30 mW

How strong will it be at 30 m?

The distance triples (so r1/r2 is 1/3).

So we multiply the signal strength at 10 meters by 1/9 (1/3 squared)

30 mW multiplied by 1/9 is 3.33 mW.

So the strength of the signal at 30 meters will be 3.33 mW.

FIGURE 6-6 Inverse Square Law Attenuation (Study Figure)

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The direct and reflected signals will travel different distances and so may be out of

phase when they reach the receiver. For example, one may be at its highest amplitude

while the other is at its lowest, giving an average of zero. If their amplitudes

are the same, they will completely cancel out. In real situation, multiple signals

travelling different paths will interfere, so we call this type of interference multipath

interference.

Multipath interference may cause the signal to range from strong to nonexistent

within a few centimeters. If the difference in time between the direct and reflected signal

is large, some reflected signals may even interfere with the next direct signal. Multipath

interference is the most serious propagation problem at WLAN frequencies.

Multipath interference is the most serious propagation problem at WLAN frequencies.

Electromagnetic Interference (EMI) A final common propagation problem in

wireless communication is electromagnetic interference (EMI). Many devices produce

EMI at frequencies used in wireless data communications. Among these devices are

cordless telephones, microwaves, and nearby access points. Consequently, placing

access points so that they give good coverage without creating excessive mutual

interference is difficult.

Frequency-Dependent Propagation Problems To complicate matters, two

wireless propagation problems get worse as frequency increases.

􀁲􀀁􀀧􀁊􀁓􀁔􀁕􀀍􀀁􀁉􀁊􀁈􀁉􀁆􀁓􀀎􀁇􀁓􀁆􀁒􀁖􀁆􀁏􀁄􀁚􀀁􀁘􀁂􀁗􀁆􀁔􀀁􀁔􀁖􀁇􀁇􀁆􀁓􀀁􀁎􀁐􀁓􀁆􀀁􀁓􀁂􀁑􀁊􀁅􀁍􀁚􀀁􀁇􀁓􀁐􀁎􀀁􀁂􀁃􀁔􀁐􀁓􀁑􀁕􀁊􀁗􀁆􀀁􀁂􀁕􀁕􀁆􀁏􀁖􀁂􀁕􀁊􀁐􀁏􀀁􀁕􀁉􀁂􀁏􀀁

lower-frequency waves because they are absorbed more rapidly by moisture in the

air. Consequently, as we will see in this chapter, WLAN signals around 5 GHz attenuate

more rapidly than signals around 2.4 GHz.

􀁲􀀁􀀴􀁆􀁄􀁐􀁏􀁅􀀍􀀁􀁅􀁆􀁂􀁅􀀁􀁛􀁐􀁏􀁆􀀁􀁑􀁓􀁐􀁃􀁍􀁆􀁎􀁔􀀁􀁈􀁓􀁐􀁘􀀁􀁘􀁐􀁓􀁔􀁆􀀁􀁘􀁊􀁕􀁉􀀁􀁇􀁓􀁆􀁒􀁖􀁆􀁏􀁄􀁚􀀏􀀁􀀢􀁔􀀁􀁇􀁓􀁆􀁒􀁖􀁆􀁏􀁄􀁚􀀁􀁊􀁏􀁄􀁓􀁆􀁂􀁔􀁆􀁔􀀍􀀁

radio waves become less able to go through and bend around objects.

Direct Wave

Low

Amplitude

Reflected Wave

High

Amplitude

Signals Cancel Each Other

Total Amplitude = 0

FIGURE 6-7 Multipath Interference

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Test Your Understanding

7. a) If the signal strength from an omnidirectional radio source is 8 mW at 30 meters,

how strong will it be at 120 meters, ignoring absorptive attenuation? Show your

work. b) Contrast inverse square law attenuation and absorptive attenuation. c) How

are dead zones created? d) Why is multipath interference very sensitive to location?

e) What is the most serious propagation problem in WLANs? f) List some sources of

EMI. g) What two propagation problems become worse as frequency increases?

RADIO BANDS, BANDWIDTH, AND SPREAD

SPECTRUM TRANSMISSION

Service Bands

The Frequency Spectrum The frequency spectrum is the range of all possible

frequencies from zero hertz to infinity, as Figure 6-8 shows.

Service Bands Regulators divide the frequency spectrum into contiguous spectrum

ranges called service bands that are dedicated to specific services. For instance, in

the United States, the AM radio service band lies between 535 kHz and 1,705 kHz. The

FM radio service band, in turn, lies between 88 MHz and 108 MHz. The 2.4 GHz service

band that we will see later in this chapter extends from 2.4 GHz to 2.4835 GHz. There

are also service bands for police and fire departments, amateur radio operators, communication

satellites, and many other purposes.

Channels Service bands are subdivided further into smaller frequency ranges

called channels. A different signal can be sent in each channel because signals in

different channels do not interfere with one another. This is why you can receive different

television channels successfully.

Channel 5

Channel 4

Channel 3

Channel 2

Channel 1

Service

Band

0 Hz

Frequency

Spectrum

(0 Hz to Infinity)

The frequency spectrum is the

range of all possible frequencies

from 0 Hz to infinity.

A service band is a (usually)

contiguous range of the frequency

spectrum dedicated to a specific

purpose, such as FM radio,

emergency response, GPS, etc.

Service bands are divided further

into channels. Signals sent in

different channels do not interfere

with one another.

FIGURE 6-8 The Frequency Spectrum, Service Bands, and Channels

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Test Your Understanding

8. a) Distinguish among the frequency spectrum, service bands, and channels. b) In

radio, how can you send multiple signals without the signals interfering with one

another?

Signal and Channel Bandwidth

􀀧􀁊􀁈􀁖􀁓􀁆􀀁􀀗􀀎􀀔􀀁􀁔􀁉􀁐􀁘􀁆􀁅􀀁􀁂􀀁􀁘􀁂􀁗􀁆􀀁􀁐􀁑􀁆􀁓􀁂􀁕􀁊􀁏􀁈􀀁􀁂􀁕􀀁􀁂􀀁􀁔􀁊􀁏􀁈􀁍􀁆􀀁􀁇􀁓􀁆􀁒􀁖􀁆􀁏􀁄􀁚􀀏􀀁􀀪􀁏􀀁􀁄􀁐􀁏􀁕􀁓􀁂􀁔􀁕􀀍􀀁􀀧􀁊􀁈􀁖􀁓􀁆􀀁􀀗􀀎􀀚􀀁􀁔􀁉􀁐􀁘􀁔􀀁

that real signals do not operate at a single frequency. Rather, real signals spread over a

range of frequencies. This range is called the signal’s bandwidth. Signal bandwidth is

measured by subtracting the lowest frequency from the highest frequency.

A channel also has a bandwidth. For instance, if the lowest frequency of an FM

􀀁􀁄􀁉􀁂􀁏􀁏􀁆􀁍􀀁􀁊􀁔􀀁􀀙􀀚􀀏􀀑􀀁􀀮􀀩􀁛􀀁􀁂􀁏􀁅􀀁􀁕􀁉􀁆􀀁􀁉􀁊􀁈􀁉􀁆􀁔􀁕􀀁􀁇􀁓􀁆􀁒􀁖􀁆􀁏􀁄􀁚􀀁􀁊􀁔􀀁􀀙􀀚􀀏􀀓􀀁􀀮􀀩􀁛􀀍􀀁􀁕􀁉􀁆􀁏􀀁􀁕􀁉􀁆􀀁channel bandwidth

is 0.2 MHz (200 kHz). AM radio channels are 10 kHz wide, FM channels are

200 kHz wide, and television channels are 6 MHz wide. How wide must the channel

bandwidth be? The channel bandwidth must be wide enough for a signal’s bandwidth.

Claude Shannon discovered a remarkable thing about signal transmission.

A signal carrying X bits per second only needs half the bandwidth of a signal carrying

2X bits per second.1 Looked at the other way, if you want to transmit twice as many

bits per second, you need to double your bandwidth. More generally, if you want to be

able to transmit N times as fast, you need N times as much channel bandwidth. High

bandwidth brings high radio transmission speed.

To transmit N times as fast, you need N times as much channel bandwidth.

1 Speaking more precisely, Shannon also found that the signal-to-noise ratio (the ratio of single power to

noise) also affects propagation speed. However, engineers find it far easier to increase speed by increasing

bandwidth than by increasing the signal-to-noise ratio.

Signal

Power

Frequency

Bandwidth

Signal

Lowest

Frequency

Highest

Frequency Frequency is

measured in hertz (Hz)

Signals spread over a range of frequencies.

Faster signals spread over a wider range of frequencies.

This range of frequencies is called the signal’s bandwidth.

Channel bandwidth must be wide enough for the signal’s bandwidth.

FIGURE 6-9 Signal Bandwidth

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Radio channels with large bandwidths are called broadband channels. They

can carry data very quickly. Although the term broadband technically refers only

to the width of a channel, broadband has come to mean “fast,” whether or not radio

is used.

Transmission systems that are very fast are usually called broadband systems even when

they do not use radio channels.

Test Your Understanding

9. a) Does a signal usually travel at a single frequency, or does it spread over a range

of frequencies? b) If the lowest frequency in a channel is 1.22 MHz and the highest

frequency is 1.25 MHz, what is the channel bandwidth? (Use proper metric

notation.) c) If you want to transmit seven times as fast, how much wider must

the channel be? d) Why is large channel bandwidth desirable? e) What do we

call a system whose channels are wide? f) What other types of system do we call

broadband?

The 2.4 GHz and 5 GHz Service Bands

802.11 Wi-Fi WLANs today use two service bands. One is the 2.4 GHz band. The other

is the 5 GHz band.

The 2.4 GHZ Service Band The 2.4 GHz service band is the same in most

countries in the world, stretching from 2.4 GHz to 2.4835 GHz. Radio propagation is

better in the 2.4 GHz service band than it is in the higher-frequency 5 GHz band, where

absorptive attenuation is higher and dead zones are deader. Consequently, propagation

differences are somewhat shorter.

Unfortunately, the 2.4 GHz band only has 83.5 MHz of bandwidth. Traditionally,

each 802.11 channel was 20 MHz wide, although 40 MHz bandwidth channels were

introduced in 802.11n. Furthermore, due to the way channels are allocated, there are

only three possible non-overlapping 20 MHz 802.11 channels, which are centered at

Required Transmission Speed and Required Channel Bandwidth

There is a direct relationship between required transmission speed and required channel bandwidth

Doubling bandwidth doubles the possible transmission speed

Multiplying bandwidth by N makes possible N times the transmission speed

Broadband Channels

Broadband means wide radio channel bandwidth and therefore high speed

Popularly, fast systems are called “broadband” even if they are not radio systems

FIGURE 6-10 Channel Bandwidth and Transmission Speed (Study Figure)

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Channels 1, 6, and 11.2 If nearby access points operate in the same channel, their signals

will interfere with each other unless the access points are far apart. This is called cochannel

interference. If an 802.11n station finds itself in a crowded area, it will drop

back from 40 MHz channels to 20 MHz channels to reduce interference. Of course,

speed roughly drops in half when this happens.

If you have only three access points that can all hear each other, there is no problem

with having only three channels. You simply run each on a different channel, and there

will be no co-channel interference. However, when you have multiple access points that

can all hear each other, Figure 6-12 shows that there is no way to avoid having some

2 Channel numbers were defined for the 2.4 GHz band when channels were narrower. A 20 MHz 802.11

channel overlaps several initially defined channels. Channels 1, 6, and 11 operate in the 2.402 GHz to

2.422 GHz, 2.427 GHz to 2. 447 GHz, and 2.452 GHz to 2.472 GHz frequency ranges, respectively. Note that

there are unused 5 MHz “guard bands” between the channels to prevent inter-channel interference.

The 2.4 GHz Service Band

2.4 GHz to 2.485 GHz

Propagation characteristics are good

For 20 MHz 802.11 channels, only three nonoverlapping channels are possible

Channels 1, 6, and 11

This creates co-channel interference between nearby access points transmitting in the same

channel

Except in very small networks, difficult or impossible to put nearby access points on different channels

(Figure 6-12)

The 5 GHz Service Band

More bandwidth, so between 11 and 24 non-overlapping 20 MHz channels

Makes it easy to have nearby access points operate on non-overlapping channels

Increasing channel bandwidth in newer standards reduces the number of possible channels

FIGURE 6-11 The 2.4 GHz and 5 GHz Service Bands (Study Figure)

Access Point A

Channel 1

Access Point B

Channel 6

Access Point C

Channel 6

Access Point D

Channel 6

Access Point E

Channel 6

Access Point F

Channel 11

Interference

In 802.11g,

nonoverlapping

channels are

1, 6, and 11

FIGURE 6-12 Co-Channel Interference in the 2.4 GHz Service Band

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co-channel interference. You can minimize co-channel interference somewhat by giving

the shared channel to the two access points that are farthest apart, but this will only

reduce interference somewhat.

The 5 GHZ Service Band Wi-Fi can also operate in the 5 GHz service band.

The big advantage of the 5 GHz band is that it is far wider than the 2.4 GHz band. In

contrast to the 2.4 GHz band’s mere three channels, the 5 GHz band provides between

11 and 24 non-overlapping 20 MHz channels today, depending on the frequencies

allocated to this service band in a particular country. In addition, while the 2.4 GHz

band is extremely crowded almost everywhere, it is only recently that companies have

begun to use the 5 GHz band extensively.

The problem with the 5 GHz band has been simple economics. Radio transceivers

in this band are inherently more expensive than they are in the 2.4 GHz band. However,

technological advances have brought 5 GHz radio transceivers down to the price range

that companies and households can now afford. Given the room in the 5 GHz band, this

has led to a gold rush for vendors and users moving into this uncrowded service band.

Adding to the attractiveness of the 5 GHz band, regulators in several countries

have been extending it to add more total bandwidth and therefore more channels. The

United States added more bandwidth in 2003. In 2013, the Federal Communications

Commission announced that it would add 35% more. In contrast, the 2.4 GHz band has

no expansion potential because it is bordered by services that cannot be moved.

In addition, we will see that 802.11n and 802.11ac are using channels much wider

than 20 MHz—up to 160 MHz. Wider channels mean fewer channels in the service

band. Without growth in 5 GHz bandwidth, there would be too little bandwidth in the

service band to permit enough very wide channels.

Test Your Understanding

10. a) In what two service bands does 802.11 operate? b) Which band dominated use

initially? c) How many 20 MHz non-overlapping channels does the 2.4 GHz band

support? d) Why is this a problem? e) Why are companies moving rapidly into

the 5 GHz band? f) How many non-overlapping channels does the 5 GHz band

support? g) Why is it important that governments to add more bandwidth to the

5 GHz band? h) If you triple channel bandwidth, what happens to the number of

channels in the service band?

NORMAL AND SPREAD SPECTRUM TRANSMISSION

Spread Spectrum Transmission

At the frequencies used by WLANs, there are numerous and severe propagation problems.

In these service bands, regulators mandate the use of a form of transmission called

spread spectrum transmission. Spread spectrum transmission is transmission that uses

far wider channels than transmission speed requires.

Spread spectrum transmission is transmission that uses far wider channels than transmission

speed requires.

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Regulators mandate the use of spread spectrum transmission to minimize propagation

problems—especially multipath interference. (If the direct and reflected signals

cancel out at some frequencies within the band, they will be double at other frequencies

and will average out over a wide enough frequency range.)

In commercial spread spectrum transmission, security is not a benefit. The

military uses spread spectrum transmission for security, but it does so by keeping

certain parameters of its spread spectrum transmission secret. Commercial spread

spectrum transmission must make these parameters publicly known to allow parties to

communicate easily.

In wireless LANs, spread spectrum transmission is used to reduce propagation problems,

not to provide security.

Test Your Understanding

11. a) In Wi-Fi service bands, what type of transmission method is required by

regulators? b) What is the benefit of spread spectrum transmission for business

communication? c) Is spread spectrum transmission done for security reasons in

commercial WLANs?

Licensed and Unlicensed Radio Bands

If two nearby transceivers send at the same frequency, their signals will interfere with

each other. To prevent chaos, governments regulate how radio transmission is used. The

International Telecommunications Union, which is a division of the United Nations,

creates worldwide rules that define service bands and specify how individual radio

service bands are to be used. Individual countries enforce these rules but are given

discretion over how to implement controls.

Licensed Radio Bands In licensed radio bands, transceivers must have a government

license to operate. They also need a license change if they move. Commercial

television bands are licensed bands, as are AM and FM radio bands. Government agencies

control who may have licenses in these bands. By doing so, the government limits

interference to an acceptable level. In some licensed bands, the rules allow mobile

hosts to move about while only central transceivers are regulated. This is the case for

mobile telephones.

Unlicensed Radio Bands However, for companies that have wireless access

points and mobile computers, even the requirement to license central antennas (in

this situation, access points) is an impossible burden. Consequently, the International

Telecommunications Union has created a few unlicensed radio bands. In these bands,

a company can add or drop access points any time it chooses. It can also have as many

wireless hosts as it wishes. All 802.11 Wi-Fi networks operate in these unlicensed

radio bands.

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The downside of unlicensed radio bands is that companies must tolerate interference

from others. If your neighbor sets up a wireless LAN next door to yours, you have

no recourse but to negotiate with him or her over such matters as which channels each

of you will use. At the same time, the law prohibits unreasonable interference by using

illegally high transmission power.

Test Your Understanding

12. a) Do WLANs today use licensed or unlicensed bands? b) What is the advantage

of using unlicensed bands? c) What is the downside?

Implementing Spread Spectrum Transmission

Normal versus Spread Spectrum Transmission As noted earlier in our discussion

of the bandwidth and speed, if you need to transmit at a given speed, you must have

a channel whose bandwidth is sufficiently wide.

To allow as many channels as possible, channel bandwidths in normal radio transmission

are limited to the speed requirements of the user’s signal, as Figure 6-14 illustrates.

For a service that operates at 10 kbps, regulators would allocate only enough channel

bandwidth to handle this speed. Adding more channel bandwidth would not increase

speed. It would be pure waste.

In contrast to normal radio transmission, which uses channels just wide enough

for transmission speed requirements, spread spectrum transmission takes the original

signal, called a baseband signal, and spreads the signal energy over a much broader

channel than is required by the transmission speed.

Licensed Radio Bands

If two nearby radio hosts transmit in the same channel, their signals will interfere

Most radio bands are licensed bands, in which hosts need a license to transmit

The government limits licenses to reduce interference

Television bands, AM radio bands, etc. are licensed

In cellular telephone bands, which are licensed, only the central antennas are licensed, not the mobile

phones

Unlicensed Radio Bands

Some bands are set aside as unlicensed bands

Hosts do not need to be licensed to be turned on or moved

802.11 Wi-Fi operates in unlicensed radio bands

This allows access points and hosts to be moved freely

However, there is no legal recourse against interference from other nearby users

Your only recourse is to negotiate

At the same time, you may not cause unreasonable interference by transmitting at illegally high power

FIGURE 6-13 Licensed and Unlicensed Radio Bands (Study Figure)

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Orthogonal Frequency Division Multiplexing There are several spread spectrum

transmission methods. The 802.11 Working Group’s current standards all use

orthogonal frequency division multiplexing (OFDM), which Figure 6-15 illustrates.

In OFDM, each broadband channel is divided into many smaller subchannels

called subcarriers. OFDM transmits part of a frame in each subcarrier. OFDM sends

data redundantly across the subcarriers, so if there is impairment in one or even a few

subcarriers, all of the frame will usually still get through.

Why use subcarriers instead of simply spreading the signal over the entire channel?

The problem is that sending data over a very wide channel reliably is very difficult.

It is much easier to send many slow signals in many small subcarriers.

Test Your Understanding

13. a) In normal radio operation, how does channel bandwidth relate to the bandwidth

required to transmit a data stream of a given speed? b) How does this change in

Note: Height of box indicates bandwidth of channel

Channel bandwidth

required for signal speed

Normal Radio: Transmission:

Bandwidth is

no wider than required

for the signal's speed

Spread Spectrum

Transmission:

Channel bandwidth is

much wider than required

for the signal's speed

Commercial spread spectrum transmission reduces certain propagation effects,

especially multipath interference

Commercial spread spectrum transmission does not provide security as a

military spread spectrum transmission does

FIGURE 6-14 Normal Radio Transmission and Spread Spectrum Transmission

Subcarrier 1 (part of frame)

Subcarrier 2 (another part of frame)

Subcarrier 3 (yet another part of frame)

Bandwidth of

Spread Spectrum Channel

Subcarriers are subchannels

More Subcarriers

FIGURE 6-15 Orthogonal Frequency Division Multiplexing (OFDM)

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spread spectrum transmission? c) What spread spectrum transmission method

dominates today? d) Why does it use subcarriers instead of simply spreading the

data over the entire channel?

802.11 WLAN OPERATION

As Figure 6-16 shows, an 802.11 Wi-Fi LAN typically connects a small number

of mobile devices to a large wired Ethernet LAN because the servers and Internet

access routers that mobile hosts need to use usually are on the wired LAN.3 In 802.11

terminology, the wired Ethernet LAN to which access points connect is a distribution

system (DS).

The wired LAN to which access points connect is a distribution system (DS).

Test Your Understanding

14. In Figure 6-16, what is the distribution system?

Wireless Access Points

When a wireless host wishes to send a frame to a server, it transmits the frame to a wireless

access point.

3 There is a rarely used 802.11 ad hoc mode, in which no wireless access point is used. In ad hoc mode, computers

communicate directly with other computers without using an access point. (In contrast, when an access point

is used, this is called 802.11 infrastructure mode.) In addition, 802.11 can create point-to-point transmission

over longer distances than 802.11 normally supports. This approach, which normally is used to connect nearby

buildings, uses dish antennas.

Distribution System

(Wired Ethernet LAN)

Notebook

Client

Radio

Access Transmission

Point A

Removes packet from

incoming frame,

places it in ongoing frame

Ethernet

Switch

Server

Needed by

Client

Packet

802.11 Frame

Containing Packet

Packet

802.3 Frame

Containing Packet

802.3 Frame

Containing Packet

Packet

Notebook client sends

a packet to the server

on the distribution

system (wired LAN)

FIGURE 6-16 Typical 802.11 Wi-Fi Operation

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As Figure 6-16 shows, when a wireless host transmits to a server on the wired

LAN, it puts the packet in an 802.11 frame.4 An 802.11 frame cannot travel over the 802.3

LAN. Wi-Fi has an entirely different frame organization, and Ethernet switches have no

idea how to handle 802.11 frames. To address this problem, the access point removes

the packet from the 802.11 frame and places the packet in an 802.3 Ethernet frame. The

access point sends this 802.3 frame to Ethernet network, which delivers the frame to

the server. Later, when the server replies, the wireless access point receives the 802.3

frame, removes the packet from the Ethernet frame, and forwards the packet to the

wireless host in a Wi-Fi frame.5

The packet goes all the way from the wireless host to a server. The 802.11 frame travels

only between the wireless host and the wireless access point. The 802.3 frame travels

only between the wireless access point and the server.

Test Your Understanding

15. a) Why must an access point remove an arriving packet from the frame in which

the packet arrives and place the packet in a different frame when it sends the

packet back out?

Basic Service Sets (BSSs)

We need to introduce a bit of jargon at this point. First, a basic service set (BSS) consists

of an access point and the wireless hosts it serves. In Figure 6-17, there are two

BSSs. The basic service set of Access Point A has two wireless hosts, while the BSS of

Access Point B has one. Of course, most BSSs serve many more wireless hosts.

A basic service set (BSS) consists of an access point and the wireless hosts it serves

The access point in a BSS has an identifier called the service set identifier (SSID).

(Note that the term basic is not in the name.) Wireless hosts must know the SSID to associate

with the access point. Fortunately, this information is very easy to learn.

Test Your Understanding

16. a) What is a BSS? (Do not just spell out the acronym.) b) What is an SSID? (Do not

just spell out the acronym.) c) Does the access point have an SSID? d) Why must

wireless devices know the access point’s SSID?

4 802.11 frames are much more complex than 802.3 Ethernet frames. Much of this complexity is needed to counter

wireless propagation problems.

5 This sounds like what a router does. However, a router can connect any two single networks. Access points

are limited to connecting 802.3 and 802.11 networks.

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Extended Service Sets (ESSs), Handoffs, and Roaming

If a mobile host travels too far from a wireless access point, its signal will become too

weak to reach the access point. However, if there is a closer access point, the host can

be handed off to that access point for service. In WLANs, the ability to use handoffs is

also called roaming.6

Roaming requires that both access points belong to the same extended service set. An

extended service set (ESS) is a group of BSSs that 1) are connected to the same distribution

system and 2) in which all access points have the same SSID.

An extended service set (ESS) is a group of BSSs that 1) are connected to the same distribution

system and 2) in which all access points have the same SSID.

We said earlier in this section that one function of access points is to work together

to coordinate service, and we gave roaming as an example of this. In roaming, the two

access points involved have to coordinate the handoff. They do this by communicating

over the distribution system. Specifically, they coordinate via 802.11r messages, which

are nicely named because they deal with roaming.

6 In cellular telephony, which we will see in Chapter 10, the terms handoff and roaming mean different things.

Large Wired LAN

Distribution System (DS)

Access

Point A

(SSID = abc)

Basic

Service

Set

(BSS)

Basic

Service

Set

(BSS)

Extended Service Set (ESS)

Access

Point B

(SSID = abc)

Roaming/

Handoff

A basic service set (BSS) is an

access point and its wireless hosts.

Service set ID (SSID) identifies an

access point

Extended service set (ESS) is a

group of BSSs with the same SSID

that connect via a distribution

system. (In this case, SSID = abc.)

Traveling hosts can be handed off

(roam) to a different BSS in the

same ESS.

FIGURE 6-17 Basic Service Sets, Extended Service Set, Handoff, and Roaming

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Access points also need to contact one another via the distribution system. In roaming,

they coordinate using the 802.11r protocol.

Test Your Understanding

17. a) What is a handoff in 802.11? b) What is the relationship between handoffs and

roaming in Wi-Fi? c) What is an ESS? (Do not just spell out the abbreviation.) d) What

characteristics do all access points in an ESS share? e) How can access points communicate

with each other? f) What is the purpose of the 802.11r standard?

Media Access Control

The access point and all of the wireless hosts it serves transmit and receive in a

single channel. Figure 6-18 shows that if two devices transmit in the same channel

at the same time, their signals will interfere with each other. When a wireless

host or the access point transmits, all other devices must wait. As the number of

hosts served by an access point increases, individual throughput falls because of

this waiting. The box “Media Access Control” discusses how media access control

(MAC) methods govern when hosts and access points may transmit so that collisions

are avoided.7

7 Yes, this is where the term MAC address comes from. Conceptually, Media Access Control is a sublayer of

the data link layer. It applies to Ethernet, Wi-Fi, and other 802.11 standards. Addresses are defined at this layer

so that all 802.11 standards use EUI-48 addresses.

Access

Point

Channel Sharing

The access point and all the hosts it serves transmit in a

single channel. If two devices transmit at the same time, their

signals will collide, becoming unreadable.

Media Access Control (MAC)

MAC methods govern when devices may transmit so that only

one device transmits at a time.

Collision! Laptop

FIGURE 6-18 Hosts and Access Points Transmit on a Single Channel

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Media access control (MAC) methods govern when hosts and access points may transmit

so that collisions can be avoided.

The access point and all of the wireless hosts it serves transmit and receive in a single

channel. When a wireless host or the access point transmits, all other devices must wait.

Test Your Understanding

18. All wireless hosts and the access point that serves them transmit on the same channel.

a) What problem does this cause? b) How does media access control address this

problem? c) Does media access control apply to wireless hosts, access points, or both?

BOX 1

Media Access Control (MAC)

The 802.11 standard has two mechanisms for media access control. The first, CSMA/CA+ACK, is

mandatory. Access points and wireless hosts must support it. The second, RTS/CTS, is optional.8

CSMA/CA+ACK Media Access Control

The mandatory method is Carrier Sense Multiple Access with Collision Avoidance and

Acknowledgement, which is mercifully shortened to CSMA/CA+ACK.

Carrier sense (CS) means to listen to (sense) traffic (the carrier, in radio parlance). Multiple

access (MA) means that this method uses listening to control how multiple hosts can access the

network to transmit. Quite simply, if another device is transmitting, the wireless host or access

point does not transmit.

Collision avoidance (CA) means that the method attempts to avoid two devices transmitting

at the same time. Most obviously, if one device has been sending for some time, two or

more others may be waiting to send. If they both send as soon as the current sender stops, they

will both transmit at the same time. This will cause a collision. Collision avoidance adds a random

delay time to decide which device may transmit first. This works, but it is inefficient because it

adds dead time when no one is transmitting.

ACK means that if the receiver receives a message correctly, it immediately sends an

acknowledgment to the sender, not waiting at all. This is another reason to require stations to

delay before sending when a sender stops transmitting.

If the sender does not receive an ACK, it retransmits the frame. Sending acknowledgments

and retransmissions makes 802.11 Wi-Fi transmission reliable because it provides both

error detection and error correction. CSMA/CA+ACK is the only reliable transmission method we

will see in this book other than TCP. Most early DLL protocols were reliable because transmission

then was unreliable, even in wired networks. Under these circumstances, error correction at the

data link layer made sense. This is no longer true today generally. Wired transmission protocols

such as Ethernet are unreliable. Doing error correction is simply not worth the effort when transmission

errors are rare. We have seen that wireless transmission, however, is encumbered with

propagation problems, and lost or damaged frames are far too common. It makes sense under

these conditions to make 802.11 (and many other wireless protocols) reliable.

8 Actually, if you have even a single host with older 802.11b equipment connected to an access point, RTS/CTS

becomes mandatory. However, 802.11b wireless hosts are almost never encountered anymore.

(continued)

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Thanks to CSMA/CA+ACK, 802.11 is a reliable protocol.

CSMA/CA+ACK works well, but it is inefficient. Waiting before transmission wastes valuable

time. Sending ACKs also is time consuming. Overall, an 802.11 LAN can only deliver throughput

(actual speed) of about half the rated speed of its standard—that is, the speed published in the

standard.

Test Your Understanding

19. a) What does CS mean? (Do not just spell out the abbreviation.) b) How is carrier sensing

used in multiple access? c) Why is CA desirable? d) Does a frame’s receiver transmit an ACK

immediately or after a random delay? e) Is CSMA/CA+ACK reliable or unreliable? f) Why

was 802.11 made reliable? g) Is CSMA/CA+ACK efficient?

Request to Send/Clear to Send (RTS /CTS)

Although CSMA/CA+ACK is mandatory, there is another control mechanism called request to

send/clear to send (RTS/CTS). Figure 6-20 illustrates RTS/CTS. As noted earlier, the RTS/CTS

(continued)

Carrier Sense Multiple Access with Collision Avoidance and Acknowledgement

Mandatory for 802.11 Wi-Fi Operation

Carrier Sensing with Multiple Access

Sender listens for traffic (senses the carrier)

If another device is transmitting, it waits

This controls access by multiple devices that must not transmit simultaneously

Collision Avoidance

When the current sender stops, two or more waiting devices may immediately want to transmit

This will cause a collision

Instead, the devices must wait a randomized amount of time before sending

This usually avoids collision, but it is inefficient

ACK (Acknowledgement) and Reliability

Receiver immediately sends back an acknowledgement

CA random delay for other devices guarantees there will be enough time for an immediate ACK

If sender does not receive the acknowledgement, it retransmits using CSMA/CA+ACK

CSMA/CA plus ACK is a reliable protocol

Reliable transmission protocols are rare for wired networks

However, radio transmission is unreliable enough to warrant it

Inefficiency

There is a lot of waiting with CSMA/CA+ACK

This makes it inefficient

FIGURE 6-19 CSMA/CA+ACK Media Access Control

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protocol is optional. Avoiding RTS/CTS whenever possible is wise because RTS/CTS is much less

efficient, and therefore slower, than CSMA/CA+ACK.

􀁴􀀁 􀀸􀁉􀁆􀁏􀀁􀁂􀀁􀁉􀁐􀁔􀁕􀀁􀁘􀁊􀁔􀁉􀁆􀁔􀀁􀁕􀁐􀀁􀁔􀁆􀁏􀁅􀀍􀀁􀁕􀁉􀁆􀀁􀁉􀁐􀁔􀁕􀀁􀁎􀁂􀁚􀀁􀁔􀁆􀁏􀁅􀀁􀁂􀀁request-to-send (RTS) message to the

wireless access point. This message asks the access point for permission to send messages.

􀁴􀀁 􀀪􀁇􀀁􀁕􀁉􀁆􀀁􀁂􀁄􀁄􀁆􀁔􀁔􀀁􀁑􀁐􀁊􀁏􀁕􀀁􀁓􀁆􀁔􀁑􀁐􀁏􀁅􀁔􀀁􀁃􀁚􀀁􀁃􀁓􀁐􀁂􀁅􀁄􀁂􀁔􀁕􀁊􀁏􀁈􀀁􀁂􀀁clear-to-send (CTS) message, then other

hosts must wait. The host sending the RTS may then transmit, ignoring CSMA/CA.

Although RTS/CTS is widely used, keep in mind that it is only an option, while CSMA/CA

is mandatory. Also, tests have shown that RTS/CTS reduces throughput when it is used even

compared to CSMA/CA.

RTS/CTS makes sense primarily when two wireless clients can both hear the access point

but cannot hear each other. With CSMA/CA+ACK, the two stations may transmit at the same

time. RTS/CTS eliminates this.

Test Your Understanding

20. a) Describe RTS/CTS. b) Is CSMA/CA+ACK required or optional? c) Is RTS/CTS required or

optional? d) Which is more efficient, RTS/CTS or CSMA/CA+ACK? e) When does RTS/CTS

make sense to use?

Host A

RTS

CTS

Host that wishes to

transmit may send a

Request-to-Send (RTS) message

Access Point broadcasts a

Clear-to-Send (CTS) message

4. Other hosts must wait

while Host A transmits

Host A may transmit freely

FIGURE 6-20 Request to Send/Clear to Send Media Access Control

802.11 TRANSMISSION STANDARDS

The 802.11 Working Group has created several WLAN transmission standards since

􀀒􀀚􀀚􀀘􀀏􀀁􀀸􀁆􀀁􀁘􀁊􀁍􀁍􀀁􀁍􀁐􀁐􀁌􀀁􀁂􀁕􀀁􀁕􀁉􀁆􀀁􀁎􀁐􀁔􀁕􀀁􀁊􀁎􀁑􀁐􀁓􀁕􀁂􀁏􀁕􀀁􀁐􀁇􀀁􀁕􀁉􀁆􀁔􀁆􀀁􀁔􀁕􀁂􀁏􀁅􀁂􀁓􀁅􀁔􀀁􀁕􀁐􀁅􀁂􀁚􀀏

Characteristics of 802.11g, 602.11a, 802.11n, and 802.11ac

Figure 6-21 compares the 802.11g, 802.11a, 802.11n, and 802.11ac standards that companies

must support today.

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Characteristic 802.11g 802.11a 802.11n 802.11ac

Status Obsolescent but

still used

Obsolescent but still

used in business

Dominant today

in sales and the

installed base

Growing rapidly.

The wave of the

future

Unlicensed Band(s) 2.4 GHz only 5 GHz only 2.4 GHz and 5 GHz 5 GHz only

Channel

bandwidth

20 MHz 20 MHz 40 MHz, but will

drop back to 20 MHz

if there is interference

on the two

selected channels

80 MHz or

160 MHz

Number of nonoverlapping

channels in the

5 GHz band in

the USA (varies by

country)

NA 20–25 20–25 at 20 MHz

8–12 at 40 MHz

4–6 at 80 MHz

1–2 at 160 MHz

MIMO? No No Yes Yes

Maximum

number of

spatial streams

NA NA 4 8

Multi-User MIMO /

Beamforming?

No No No Yes

Rated Speed 54 Mbps 54 Mbps 100 Mbps to 600

Mbps; 150 to 300

Mbps common.

433 Mbps to

6.93 Gbps; 433

Mbps to 1.3

Gbps common.

FIGURE 6-21 Characteristics of Major 802.11 Wi-Fi Standards

􀁲􀀁􀀵􀁉􀁆􀀁􀀙􀀑􀀓􀀏􀀒􀀒􀁈􀀁􀁔􀁕􀁂􀁏􀁅􀁂􀁓􀁅􀀁􀁊􀁔􀀁􀁐􀁃􀁔􀁐􀁍􀁆􀁕􀁆􀀍􀀁􀁃􀁖􀁕􀀁􀁂􀁄􀁄􀁆􀁔􀁔􀀁􀁑􀁐􀁊􀁏􀁕􀁔􀀁􀁎􀁖􀁔􀁕􀀁􀁔􀁕􀁊􀁍􀁍􀀁􀁅􀁆􀁂􀁍􀀁􀁘􀁊􀁕􀁉􀀁􀁂􀀁􀁄􀁐􀁏-

siderable number of 802.11g devices. It achieved high penetration when it first

brought 54 Mbps speed to the 2.4 GHz band, but 802.11n has largely replaced it.

􀁲􀀁􀀯􀁐􀁕􀀁 􀁎􀁂􀁏􀁚􀀁 􀁑􀁆􀁐􀁑􀁍􀁆􀀁 􀁉􀁂􀁗􀁆􀀁 􀁆􀁗􀁆􀁏􀀁 􀁉􀁆􀁂􀁓􀁅􀀁 􀁐􀁇􀀁 􀀙􀀑􀀓􀀏􀀒􀀒􀁂􀀍􀀁 􀁃􀁖􀁕􀀁 􀁊􀁕􀀁 􀁔􀁂􀁘􀀁 􀁔􀁐􀁎􀁆􀀁 􀁃􀁖􀁔􀁊􀁏􀁆􀁔􀁔􀀁 􀁖􀁔􀁆􀀁

because of its operation in the uncrowded 5 GHz band. Today, it has a small

installed base, but access points operating in the 5 GHz band still must support

802.11a clients.

􀁲􀀁􀀵􀁉􀁆􀀁 􀀙􀀑􀀓􀀏􀀒􀀒􀁏􀀁 􀁔􀁕􀁂􀁏􀁅􀁂􀁓􀁅􀀁 􀁏􀁐􀁘􀀁 􀁅􀁐􀁎􀁊􀁏􀁂􀁕􀁆􀁔􀀁 􀁕􀁉􀁆􀀁 􀁊􀁏􀁔􀁕􀁂􀁍􀁍􀁆􀁅􀀁 􀁃􀁂􀁔􀁆􀀏􀀁 􀀱􀁓􀁐􀁅􀁖􀁄􀁕􀁔􀀁 􀁕􀁉􀁂􀁕􀀁 􀁖􀁔􀁆􀀁 􀁕􀁉􀁊􀁔􀀁

standard have higher speeds than 802.11g and 802.11a products and also have longer

transmission ranges. As prices have fallen, 802.11n has become the low-price

option in Wi-Fi.

􀁲􀀁􀀱􀁓􀁐􀁅􀁖􀁄􀁕􀁔􀀁􀁖􀁔􀁊􀁏􀁈􀀁􀁕􀁉􀁆􀀁􀀙􀀑􀀓􀀏􀀒􀀒􀁂􀁄􀀁􀁔􀁕􀁂􀁏􀁅􀁂􀁓􀁅􀀁􀁓􀁆􀁑􀁓􀁆􀁔􀁆􀁏􀁕􀀁􀁂􀀁􀁔􀁎􀁂􀁍􀁍􀀁􀁃􀁖􀁕􀀁􀁓􀁂􀁑􀁊􀁅􀁍􀁚􀀁􀁈􀁓􀁐􀁘􀁊􀁏􀁈􀀁􀁑􀁂􀁓􀁕􀀁

of the Wi-Fi installed base. Compared to 802.11n, 802.11ac offers very high speeds.

802.11ac is the wave of the future and will probably supplant 802.11n rapidly.

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Test Your Understanding

21. a) Among the four standards listed, which are obsolete? b) Which dominates the

installed base today? c) What is the market position of 802.11ac?

Bands and Channel Bandwidth

Earlier in this chapter, we saw that other things being equal, doubling channel bandwidth

doubles the possible transmission speed. However, service bands have limited

total bandwidth, so doubling channel bandwidth means cutting the number of channels

in half.

802.11g Channel Bandwidth The 802.11g standard operates only in the

crowded 2.4 GHz band. With channel bandwidth of 20 MHz, only three 802.11g

channels are possible.

802.11a Channel Bandwidth The 802.11a standard operates only in the less

crowded 5 GHz band. It also uses 20 MHz channels. This permits many 802.11a channels.

Unfortunately, radios in the 5 GHz band were expensive when 802.11a emerged, so

802.11a never achieved large market share.

802.11n Channel Bandwidth The 802.11n standard can operate in both the

2.4 GHz band and the less-crowded and wider 5 GHz band. It also doubles 802.11g

bandwidth, raising it to40 MHz. This alone roughly doubles speed. However, to

be a good neighbor, when there are stations operating on the three possible 20 MHz

channels, 802.11n products will drop back to a 20 MHz channel bandwidth, losing their

channel bandwidth advantage.

Dual-band 802.11n products also operate in the 5 GHz band. In this higher band,

40 MHz channels are widely available. In other words, 802.11n often reaches its full

expression only in the 5 GHz band. Note, however, in Figure 6-21 that while 802.11n can

use 20 to 25 channels in the 5 GHz band, it can only use 8 to 12 40 MHz channels. Why

fewer than half the channels? The answer is that the 5 GHz band is not a solid band of

frequencies. There are some gaps, and it is easier to fill available ranges with 20 MHz

channels than with 40 MHz channels. We will see that the situation is even worse in

802.11ac.􀀚

802.11ac Channel Bandwidth The 802.11ac standard operates only in the

5 GHz band and has even wider channels than 802.11n. Support for 80 MHz channels

is mandatory, and 160 MHz channels are optional. Doubling and quadrupling channel

bandwidth compared to 802.11n means roughly a doubling and quadrupling of

transmission speeds, other things being equal. Of course, having wider channels

means having fewer channels, and the filling of available ranges with 80 MHz and

􀀚􀀁Why are ranges of channels shown, rather than precise numbers? The answer is that regulatory limits apply

in some situations. For instance, near an airport that uses weather radar, you may not use some parts of the

“available” 5 GHz band.

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160 MHz channels is even harder than with 40 MHz channels. There are only 4–6

channels at 80 MHz and 1–2 channels at 160 MHz.10

Test Your Understanding

22. a) Why is wider channel bandwidth good? b) What is the downside of wider channel

bandwidth? c) What frequency band or bands do 802.11g, 802.11a, 802.11n,

and 802.11ac use? d) For each, compare channel bandwidth and the number of

possible channels.

MIMO

Increasing bandwidth is the easiest way to boost transmission speed, but there is also

a more elegant way to increase speed without increasing bandwidth. Figure 6-22 notes

that standards beyond 802.11g use a technique called multiple input/multiple output

(MIMO) to double, triple, or quadruple transmission speed (or even increase it more)

without increasing channel bandwidth.

The key to higher throughput in MIMO is that the host or access point sends two or

more spatial streams (radio signals) in the same channel between two or more different antennas

on access points and wireless hosts. Earlier, we said that that was impossible. That was

a bit of a lie, actually. It used to be impossible, but newer technology has made this possible.

In the figure, there are two spatial streams. As we saw earlier in this chapter, two

signals in the same channel should interfere with each other. However, the two spatial

streams sent by different antennas will arrive at the two receiving antennas at slightly

different times. Using detection and separation methods based on differences in arrival

times for the two spatial streams, the receiver can separate the two spatial streams in the

same channel and so can read them individually.

Even with only two spatial streams using two antennas each on the sender and

receiver, MIMO can roughly double throughput. Using more antennas and therefore

10 The United States is currently in the process of adding about 35% more capacity to the 5 GHz band. Some of

this will fill spaces between available ranges to give larger available ranges. This will add one or two 160 MHz

channels, again depending on conditions.

Access

Point

Wireless

Host

Antenna

Spatial Stream1

Spatial Stream 2

in same channel

Two spatial streams are sent in the same channel, but from different sending antennas.

The two signals arrive at slightly different times at the two receiving antennas.

This allows the receiver to distinguish between the two signals.

FIGURE 6-22 Multiple Input/Multiple Output (MIMO) Operation

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more spatial streams can increase throughput even more. MIMO is not limited to two

spatial streams.

The 802.11n standard introduced MIMO to Wi-Fi. With two spatial streams,

the rated speed in 802.11n with 40 MHz channels is 300 Mbps. Three spatial streams

raise the rated speed to 450 Mbps, and four raise it to 600 Mbps. The 802.11n standard

requires access points to support four spatial streams, although wireless hosts

are only required to support two spatial streams. Typical speeds in 802.11n products

today have rated speeds of 150 Mbps to 300 Mbps.

The 802.11ac standard, in addition to doubling or quadrupling channel bandwidth

compared to 802.11n, doubles the number of possible spatial streams to eight.

The standard offers 16 possible combinations of bandwidth (80 MHz or 160 MHz)

and number of spatial streams (1 to 8). This creates a large number of possible rated

􀁔􀁑􀁆􀁆􀁅􀁔􀀛􀀁 􀀕􀀔􀀔􀀁 􀀮􀀩􀁛􀀁 􀁕􀁐􀀁 􀀗􀀏􀀚􀀁 􀀨􀀩􀁛􀀏􀀁 􀀱􀁓􀁐􀁅􀁖􀁄􀁕􀁔􀀁 􀁕􀁐􀁅􀁂􀁚􀀁 􀁕􀁚􀁑􀁊􀁄􀁂􀁍􀁍􀁚􀀁 􀁑􀁓􀁐􀁗􀁊􀁅􀁆􀀁 􀁓􀁂􀁕􀁆􀁅􀀁 􀁔􀁑􀁆􀁆􀁅􀁔􀀁 􀁐􀁇􀀁

433 Mbps to 1.3 Gbps.

Another benefit of MIMO, beyond greater transmission speed, is greater transmission

range. Greater propagation distances may permit fewer access points to be

installed, and this will lower equipment and installation cost.11

Test Your Understanding

23. a) How does MIMO use spatial streams to increase transmission speed? b) What

is the main benefit of MIMO? c) What is its other benefit? d) Compare the range of

rated speeds possible with 802.11n and 802.11ac.

Beamforming and Multiuser MIMO

Today, jet fighters use phased array radar systems that are flat dishes with many tiny

antennas spread over the surface. Controlling the relative phases of the signals from these

antennas can focus the radar beam in a particular direction very rapidly. Multiple antennas

on MIMO systems can do the same, focusing the radio power instead of broadcasting

it isotropically (in all directions equally). Figure 6-23 illustrates this beamforming.

Obviously, beamforming means that when the access points transmits to (or

receives from) a wireless device the signal will be stronger. The radio can operate at

lower power or send the signal farther.

Beamforming also allows multiuser MIMO (MU-MIMO), in which the access

point focuses on two wireless devices at the same time. With focused transmissions, it

can communicate with two or more devices simultaneously. This eliminates the time a

device may have to wait before transmitting in order to avoid collisions.

Theoretically, MU-MIMO was possible with 802.11n, but this aspect of the standard

was never developed to a useable degree. With 802.11ac, however, beamforming

and multiuser MIMO are intrinsic parts of the system’s operation and well-defined.

Test Your Understanding

24. a) What is beamforming? b) What benefits can it bring? c) Distinguish between

MIMO and multiuser MIMO.

11 When a station transmits, it modulates the signal for physical layer transmission. (Modulation is covered in

Module B). The 802.11n standard’s best modulation method is 64 QAM. 802.11ac raises this to 256 QAM. This

allows 802.11ac to send a third more bits per clock cycle as 802.1n (8 versus 6 bits per clock cycle).

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Speed, Throughput, and Distance

So far, we have been talking about rated speeds. However, what throughput—actual

speed—can individuals expect to see? The general answer is complicated, but the single

most important word is less. Individual users will always receive less than rated speeds,

often much less.

Rated Speed versus Throughput Rated speed is the number of bits that the

host or access point will transmit per second according to the 802.11 standard it uses.

The aggregate throughput is usually 25% to 50% lower.

Laptop 2

Access Point

with Beamforming

and Multiuser MIMO

Laptop 1

Laptop 3

Beamforming can direct signal energy toward individual devices.

This sends stronger signals to wireless hosts.

Beamforming can also allow simultaneous transmission to and from

multiple devices simultaneously through multiuser MIMO (MU-MIMO).

FIGURE 6-23 Beamforming and Multiuser MIMO

Rated Speed versus Throughput

Total throughput is substantially lower than rated speed—sometimes 50% lower

Aggregate versus Individual Throughput

Access point throughput is aggregate throughput for all devices transmitting and receiving

Its capacity is shared by all stations currently sending or receiving

Individual throughput can be much lower than aggregate throughput

Throughput versus Distance

As distance from the access point increases, signals get weaker

Wireless hosts must use slower but more reliable bit encoding methods

This reduces individual throughput for these devices

FIGURE 6-24 Speed, Distance, and Throughput (Study Figure)

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Aggregate Throughput versus Individual Throughput In addition, the

access point’s aggregate throughput is shared by all users of the access point. Suppose

that the aggregate throughput is 100 Mbps per second and there are 10 users of an access

point. If all 10 transmit or receive simultaneously, then individual throughput would be

about 10 Mbps (actually somewhat less because of time lost in turn-taking). Of course,

it would be rare for all stations to transmit simultaneously. But if even three are sending

and receiving simultaneously, the individual throughput they experience would be

about 33 Mbps.

What percentage of time do hosts transmit or receive? It depends entirely on what

they are doing. Web downloads occur about every 30 seconds and take only a second or

two. In contrast, streaming video creates an almost continuous data stream, consuming

a good deal of the aggregate throughput.

Throughput versus Distance As noted earlier, speed is highest when a

user is near an access point. As the user moves away, speed falls. For each standard,

such as 802.11g, 802.11n, and 802.11ac, there actually is a range of speeds. Each

speed corresponds to a different way of modulating signals.12 The fastest speeds use

aggressive modulation methods that are fast but very sensitive to errors. As a station

moves farther from an access point, the signal weakens and errors increase. The transceiver

must switch to a less aggressive modulation method that is less sensitive to

errors. Unfortunately, this modulation method also transmits more slowly. As a wireless

hosts moves farther from an access point, its transmission speed will fall.

In General Overall, it is impossible to say with any certainty what individual

throughput a user will receive. A rule of thumb that frequently works is that individual

throughput will be a quarter to a third of the rated speed if the access point is not

heavily loaded.

Test Your Understanding

25. a) Distinguish between rated speed, aggregate throughput, and individual

throughput. b) What factors influence individual throughput, given a certain level

of aggregate throughput? c) Why does transmission speed drop as a computer

moves farther from an access point?

Backward Compatibility

When new access points and wireless clients are sold, they must be able to work with

older equipment. For instance, an 802.11ac client must be able to work with an older

802.11n access point. In the same way, an 802.11n access point must be able to work with

an even older 802.11g client. Of course, when an 802.11ac device works with an 802.11n

device, the 802.11ac device must drop back to 802.11n operation. Communication will

take place at 802.11n speeds. This backward compatibility permits products meeting

new standards to be installed gradually over time.

12 Module A discusses common modulation techniques.

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Test Your Understanding

26. a) What is backward compatibility? b) Why is it important? c) When a device that

implements 802.11ac attaches to an 802.11n access point, what standard does it use

to communicate?

Standards and Options

We have looked at many standards in this section. However, just because two products

are compliant with a particular standard, such as dual-band 802.11n, does not mean that

they will have the same performance. This is true because most standards have options.

One, for example, is the number of antennas on an 802.11n wireless access point. 802.11n

calls for up to four antennas on a wireless client. However, most early 802.11n clients had

only two antennas and therefore could only transmit two spatial streams. For a given

piece of equipment, knowing what standards it complies with is not enough. The optional

features of the standard implemented in the device must also be known.

Test Your Understanding

27. a) Why can two products that comply with the same standard perform differently?

b) What implications does this have for making purchases? (The answer is

not in the text.)

WIRELESS MESH NETWORKING

Do Wi-Fi devices in an organization usually connect to a wired switched Ethernet

network? Yes, as we have seen throughout this chapter. Is it possible to avoid this by

building an all-wireless network? The answer is that we are getting close. As Figure 6-26

shows, it is at least theoretically possible for wireless access points and wireless hosts

to organize themselves into a mesh, forwarding frames from one to another until they

reach the wireless destination host. With this arrangement, there is no wired network

involved. The wireless devices provide their own wireless distribution system, with no

need for Ethernet.

The 802.11s standard for mesh networks exists. However, many related standards

need to be developed before mesh networking’s issues can be resolved sufficiently to be

useful in corporations. New standards must address three main issues.

Devices Built for Newer Standards Still Implement Older Standards

This Allows Older and Newer Devices to Communicate

Newer product: 802.11ac in the 5 GHz band

Older product: 802.11n in the 2.4 GHz band

The newer product will drop back to the standard the older product can use

They will communicate using the 802.11n standard in the 2.4 GHz band

They will only get 802.11n speeds

This is Backward Compatibility

FIGURE 6-25 Backward Compatibility (Study Figure)

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􀁲􀀁􀀧􀁊􀁓􀁔􀁕􀀍􀀁􀁎􀁆􀁔􀁉􀁆􀁔􀀁􀁎􀁖􀁔􀁕􀀁􀁃􀁆􀀁􀁔􀁆􀁍􀁇􀀎􀁐􀁓􀁈􀁂􀁏􀁊􀁛􀁊􀁏􀁈􀀏􀀁􀀩􀁐􀁔􀁕􀁔􀀁􀁂􀁏􀁅􀀁􀁂􀁄􀁄􀁆􀁔􀁔􀀁􀁑􀁐􀁊􀁏􀁕􀁔􀀁􀁆􀁏􀁕􀁆􀁓􀀁􀁂􀁏􀁅􀀁􀁍􀁆􀁂􀁗􀁆􀀁􀁕􀁉􀁆􀀁

mesh frequently, and the network must respond immediately to changes.

􀁲􀀁􀀴􀁆􀁄􀁐􀁏􀁅􀀍􀀁􀁊􀁕􀀁􀁘􀁊􀁍􀁍􀀁􀁃􀁆􀀁􀁅􀁊􀁇􀁇􀁊􀁄􀁖􀁍􀁕􀀁􀁕􀁐􀀁􀁂􀁗􀁐􀁊􀁅􀀁􀁐􀁗􀁆􀁓􀁍􀁐􀁂􀁅􀁊􀁏􀁈􀀁􀁂􀁄􀁄􀁆􀁔􀁔􀀁􀁑􀁐􀁊􀁏􀁕􀁔􀀁􀁏􀁆􀁂􀁓􀀁􀁕􀁉􀁆􀀁􀁈􀁆􀁐􀁈􀁓􀁂􀁑􀁉􀁊􀁄􀁂􀁍􀀁

center of the mesh. (Think of sitting in the middle seat at a table during a Christmas

dinner and constantly having to pass food back and forth.) If mesh networking

overloads wireless hosts and access points, it is useless.

􀁲􀀁􀀵􀁉􀁊􀁓􀁅􀀍􀀁􀁂􀁏􀀁􀁆􀁗􀁆􀁏􀀁􀁃􀁊􀁈􀁈􀁆􀁓􀀁􀁊􀁔􀁔􀁖􀁆􀀁􀁊􀁔􀀁􀁔􀁆􀁄􀁖􀁓􀁊􀁕􀁚􀀏􀀁􀀸􀁊􀁕􀁉􀀁􀁏􀁐􀀁􀁄􀁆􀁏􀁕􀁓􀁂􀁍􀀁􀁄􀁐􀁏􀁕_t_i_n_g_u_􀁓􀁐􀁍􀀍􀀁􀁔􀁆􀁄􀁖􀁓􀁊􀁕􀁚􀀁􀁘􀁊􀁍􀁍􀀁􀁉􀁂􀁗􀁆􀀁

to exist between every pair of devices, many of which will have just entered the

mesh and are not well known. This is a recipe for security nightmares.

Test Your Understanding

28. a) What is wireless mesh networking? b) What is the current 802.11 standard for

mesh networking? c) What devices forward frames in a mesh network? d) What

three issues must be overcome to make mesh networking acceptable to corporations?

CONCLUSION

Synopsis

Chapter 5 looked at Ethernet switched local area networks. This chapter and Chapter 7

look at wireless LANs (WLANs). All single networks, whether point-to-point, switched,

or wireless, operate at Layers 1 and 2. OSI standards dominate at those layers, so we can

expect wireless network standards to be OSI standards.

This chapter focuses extensively on physical layer propagation. This detail is

needed because wireless propagation effects are complex. We can predict what will

happen as a signal travels down a copper wire or an optical fiber, but predicting how

strong a radio signal will be at a user’s location is far more difficult. We looked at five

wireless propagation problems: absorptive attenuation, inverse square law attenuation

(yes, there are two types of attenuation), interference, dead zones, and multipath interference.

Multipath interference is the biggest propagation problem in wireless LANs.

Absorptive attenuation and dead zones become worse at higher frequencies.

Wireless LANs use omnidirectional antennas because users would not know

where to point a dish antenna and certainly do not want to carry a dish around. Fixed

Frame

A Forwarding

Host

Wireless

Frame

No wired distribution

system needed

Frames are forwarded by access

points and wireless hosts

Mesh networking is

governed by 802.11s

and related standards

FIGURE 6-26 Wi-Fi Wireless Mesh Networking

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users may use dishes pointing at a distant radio source to have stronger transmission

and reception.

The frequency spectrum consists of all frequencies from 0 Hz to infinity. (Radio

propagation is described by frequency, which is measured in hertz.) Service bands are

(usually) contiguous ranges of the frequency spectrum that are reserved for particular

purposes, such as FM radio, television, or police communication. Service bands

are divided into channels. Signals are sent in a single channel, and signals in different

channels do not interfere with each other. Most commercial wireless services and

corporate WLANs operate between 500 MHz and 10 GHz.

Radio signals do not propagate at a single frequency. They spread over a range of

frequencies, and the spread increases as signal speed increases. Consequently, to carry

fast signals, channels must have wide bandwidths. Doubling bandwidth should double

possible signal speed.

Wireless LANs operate in unlicensed bands, in which you can set up your network the

way you wish. However, you must tolerate interference from nearby WLANs built by others.

Initially, almost all WLAN technology operated in the 2.4 GHz band, in which

radio prices were low. However, there are only three non-overlapping 20 MHz channels

in this band, so nearby access points often interfere with one another. Increasingly,

new WLAN equipment operates in the 5 GHz unlicensed band, in which there are more

channels for a given channel bandwidth. The gap between 2.4 GHz prices and 5 GHz

prices is narrowing. Consequently, the use of the 5 GHz band is growing rapidly.

In the 2.4 GHz and 5 GHz bands, the government requires the use of spread spectrum

transmission, in which the signal is spread far more than it needs to be for its speed.

Current 802.11 Wi-Fi standards use orthogonal frequency division multiplexing (OFDM),

in which the channel is broken into much smaller subchannels called subcarriers. The

frame is transmitted redundantly within the subcarriers. WLAN spread spectrum techniques,

unlike military spread spectrum techniques, provide no security.

In 802.11 WLAN operation, access points normally attach to the firm’s main wired

Ethernet LAN so that wireless clients can access servers and Internet access routers on

the wired LAN. When a wireless host transmits, it sends its packet in an 802.11 frame.

The access point removes the packet from the 802.11 frame, puts it in an 802.3 frame,

and sends the frame to the server or Internet access router. The packet travels all the

way; the 802.11 frame does not. A basic service set (BSS) consists of an access point and

the wireless hosts it serves. The SSID is the name of a radio on an access point. In an

extended service set (ESS), all access points have the same SSID. Among other things,

this permits roaming, which is also called being handed off.

The access point and the stations it serves all transmit in a single channel. Media

access control (MAC) ensures that they take turns transmitting so that their signals do not

interfere. In a box, we looked at 802.11 Wi-Fi’s two MAC protocols. CSMA/CA+ACK is

mandatory. Request to send/clear to send is optional but sometimes useful. Both create

inefficiency by creating dead time in which there is no transmission.

WLAN products on the market follow different 802.11 standards. Figure 6-21

compares four 802.11 transmission standards. One consistent theme for newer versions

is the use of wider channel bandwidths, which bring higher rated speeds. The 802.11g

and 802.11a standards use 20 MHz channels. The 802.11n standard doubles this, except

when there is interference from 802.11g devices in the 2.4 GHz band. The 802.11ac

standard specifies 80 MHz and 160 MHz channels. While 802.11g uses the crowded and

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limited 2.4 GHz band, 802.11a, 802.11n, and 802.11ac can take advantage of the wider

5 GHz band’s far greater total bandwidth.

Another way to boost speed is MIMO, which uses multiple antennas on the

sender and receiver. The signals sent by different antennas are called spatial streams.

The sender can transmit multiple spatial streams in the same channel, and the receiver

will be able to read them. Roughly speaking, transmission speed increases in proportion

to the number of spatial streams. The 802.11g and 802.11a standards do not use

MIMO. The 802.11n standard uses MIMO and can support up to four spatial streams.

The 802.11ac standard can support up to eight.

The 802.11ac standard can also use beamforming, which directs signals to individual

devices instead of broadcasting signals omnidirectionally. Beamforming increases

distance by focusing more of the sender’s power on the receiver without using a dish

antenna. A particularly sophisticated type of beamforming is multiuser MIMO, which

allows multiple stations to communicate simultaneously with a single access point in a

single channel. The access point can use their different spatial streams to separate their

signals. If one station is sending, other stations do not have to wait to send.

It is easy to talk about rated speeds, but throughput is more difficult to discuss.

Throughput is always slower than rated speed, and this is aggregate throughput, which

is shared by all devices actively using an access point. Individual throughput is always

lower than aggregate throughput. In addition, as a station moves farther from the access

point, it must use slower bit encoding processes, further reducing individual throughput.

We looked briefly at two 802.11 mesh networks, which use access points and client

hosts to forward 802.11 frames wirelessly between two wireless hosts. This forwarding

process may require several hops among wireless devices. No Ethernet distribution system

is involved. In Chapter 7, we will continue to look at 802.11 wireless LANs, focusing

on security and management. We will then look at other local wireless technologies,

including Bluetooth.

END-OF-CHAPTER QUESTIONS

Thought Questions

6-1. A building is cube-shaped. It uses 16 access

points, which are on average, 10 meters apart

from one another. The company wishes to

reduced this to 8 meters. About how many

5 GHz access points would the company need

for the building?

6-2. The following matters were not addressed

specifically in the text. However, if you understand

the concepts of Layer 1 and Layer 2

standards, in each case, give your answer and

explain your reasoning. a) Is multipath interference

a Layer 1 or Layer 2 concern? b) Is

media access control a Layer 1 or Layer 2 concern?

c) Is MIMO a Layer 1 or Layer 2 concern?

d) Are wireless propagation problems Layer

1 or Layer 2 concerns? e) Is 802.11ac a Layer 1

or Layer 2 standard? f) Is 802.11r a Layer 1 or

Layer 2 standard?

Perspective Questions

6-3. What was the most surprising thing you learned

in this chapter?

6-4. What was the most difficult part of this chapter

for you?

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LEARNING OBJECTIVES

By the end of this chapter, you should be able to:

▪ Use Xirrus Wi-Fi Inspector with some facility.

▪ Interpret output from Wi-Fi Inspector in specific situations.

▪ Do a site survey.

INTRODUCTION

Wi-Fi analysis programs listen to nearby access points (and sometimes wireless hosts) to

determine such things as how strong their signals are, what types of security they use,

what their SSIDs and BSSIDs are, and sometimes the directions of the individual access

points.

There are many Wi-Fi analysis programs for mobile devices. Many have “stumbler”

in their names in homage to one of the first examples, NetStumbler. This chapter looks

at Wi-Fi Inspector from Xirrus, which runs on Microsoft Windows and which is available

as a free download from Xirrus. A comparable Windows Widget that always remains on

the desktop is also available from Xirrus.

THE FOUR WINDOWS

Figure 6a-1 shows the ribbon menu and four tiled windows that appear when you bring

up Wi-Fi Inspector. This view shows all information in a single window. This is the

default. It is also what you see if you click on Show All in the Layout ribbon.

Chapter 6a

Using Xirrus Wi-Fi Inspector

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The Radar Window (Read the Fine Print)

The most obvious window is the radar window, which shows all access points in the

vicinity. The access points are spread out across the two-dimensional picture.

Relative Direction (Meaningless) It appears that the radar window shows the

relative directions of the access points, much as an air traffic radar display shows the

directions of nearby aircraft. Actually, it does not. The access points are merely spread

out for readability. Direction is meaningless. In this sense, the radar window is misleading.

However, it looks cool.

Distance from the Center (Signal Strength) What does distance from the

center mean? It looks like it means physical distance, as it would on a physical radar

screen. Rather, it means signal strength. Access points that are shown closest to the

center are the strongest, and access points that are the farthest from the center are the

weakest.

FIGURE 6A-1 Four Windows in Wi-Fi Inspector

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Measuring Signal Strength Signal strength gives the RSSI (relative signal

strength indicator) for the access point. Smaller negative numbers are better. For example,

–60 dBm is a very strong signal, while –87 dBm is a very weak signal. In Figure 6a-1,

Nalu24 has a signal strength of –65, which is quite good. Belkin has a signal strength of

–89, which is terrible.

For signal strength, smaller negative numbers are better.

(It’s a double negative.)

Expanding the Radar Window The radar window in its normal small form

can only display four access points. Under the Layout section of the menu, selecting

Radar in the Layout Group will maximize the radar window. This allows up to ten

access point names to be seen. By the way, “network” and “SSID” are synonyms.

Figure 6a-2 shows the expanded radar window. There are only two nearby access

points, so there is no need for a large radar window. However, it certainly is easier to

read the relative indicated signal strength.

FIGURE 6A-2 Expanded Radar Window

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Connection Window

The connection window (in the upper right in Figure 6a-1) shows information about

the access point to which the computer running Wi-Fi Inspector is currently connected

(Nalu24). It shows the SSID (the network name, in this case, Nalu24), the BSSID (the

access point’s MAC address, in this case, Cisco-Linksys:73:22:511), the channel (6), the

signal strength (–65 dBm), and the network mode (802.11n).

In the middle is information about the user’s PC. It shows the user’s MAC address

and configuration information, including the user’s IP address, the IP address of the destination

server, the IP address of the default gateway (router), and the network’s external IP

address given to it by the ISP. (This is a home network.) This information does not tell the

user about nearby access points, but it can be very useful in assessing connection problems.

On the right is a Connect/Disconnect button. Clicking this button shows a list

of potential networks and allows the user’s computer to disconnect from the current

access point and pick another to connect to. The user can also turn off the computer’s

wireless adapter.

The Networks Window

The networks window shows detailed information about each of the nearby access

points. This is what the user goes to when he or she wants detailed information. The row

for the access point to which the user is currently connected is shown in orange. Wi-Fi

Inspector updates the information in the networks window frequently. As Figure 6a-3

shows, the information in this window is detailed.

􀁲􀀁􀀴􀀴􀀪􀀥􀀏􀀁􀀵􀁉􀁆􀀁􀁏􀁆􀁕􀁘􀁐􀁓􀁌􀀁􀁏􀁂􀁎􀁆􀀏

􀁲􀀁􀀴􀁊􀁈􀁏􀁂􀁍􀀁􀀭􀁆􀁗􀁆􀁍􀀁􀁊􀁏􀀁􀁆􀁊􀁕􀁉􀁆􀁓􀀁􀁅􀀣􀁎􀀁􀁐􀁓􀀁􀁑􀁆􀁓􀁄􀁆􀁏􀁕􀁂􀁈􀁆􀀏􀀁􀀳􀁆􀁎􀁆􀁎􀁃􀁆􀁓􀀁􀁕􀁉􀁂􀁕􀀁􀁔􀁎􀁂􀁍􀁍􀁆􀁓􀀁􀁏􀁆􀁈􀁂􀁕􀁊􀁗􀁆􀀁􀁅􀀣􀁎􀀁

numbers indicate higher strength. Next to the number is a colored bar.

􀁲􀀁􀀨􀁓􀁆􀁆􀁏􀀁􀁊􀁔􀀁􀁇􀁐􀁓􀀁􀁔􀁊􀁈􀁏􀁂􀁍􀁔􀀁􀁐􀁇􀀁􀁭􀀘􀀑􀀁􀁅􀀣􀁎􀀁􀁂􀁏􀁅􀀁􀁂􀁃􀁐􀁗􀁆􀀁􀀉􀁭􀀗􀀑􀀁􀁅􀀣􀁎􀀍􀀁􀁆􀁕􀁄􀀏􀀊􀀏

1 The first two octets in a MAC address identify the company making the network adapter in the access point.

Wi-Fi Inspector converts this information into a humanly readable name.

FIGURE 6A-3 Networks Window

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􀁲􀀁􀀺􀁆􀁍􀁍􀁐􀁘􀀁􀁊􀁔􀀁􀁇􀁐􀁓􀀁􀁔􀁊􀁈􀁏􀁂􀁍􀁔􀀁􀁃􀁆􀁕􀁘􀁆􀁆􀁏􀀁􀁭􀀘􀀒􀀁􀁅􀀣􀁎􀀁􀁂􀁏􀁅􀀁􀁭􀀙􀀑􀀁􀁅􀀣􀁎􀀏

􀁲􀀁􀀰􀁓􀁂􀁏􀁈􀁆􀀁􀁊􀁔􀀁􀁇􀁐􀁓􀀁􀁔􀁊􀁈􀁏􀁂􀁍􀁔􀀁􀁃􀁆􀁕􀁘􀁆􀁆􀁏􀀁􀁭􀀙􀀒􀀁􀁅􀀣􀁎􀀁􀁂􀁏􀁅􀀁􀁭􀀚􀀑􀀁􀁅􀀣􀁎􀀏

􀁲􀀁􀀳􀁆􀁅􀀁􀁊􀁔􀀁􀁇􀁐􀁓􀀁􀁭􀀚􀀒􀀁􀁅􀀣􀁎􀀁􀁂􀁏􀁅􀀁􀁃􀁆􀁍􀁐􀁘􀀏

􀁲􀀁􀀯􀁆􀁕􀁘􀁐􀁓􀁌􀀁􀀮􀁐􀁅􀁆􀀏􀀁􀀙􀀑􀀓􀀏􀀒􀀒􀁈􀀍􀀁􀀙􀀑􀀓􀀏􀀒􀀒􀁏􀀍􀀁􀁆􀁕􀁄􀀏

􀁲􀀁􀀥􀁆􀁇􀁂􀁖􀁍􀁕􀀁􀀦􀁏􀁄􀁓􀁚􀁑􀁕􀁊􀁐􀁏􀀏􀀁􀀯􀁐􀁏􀁆􀀍􀀁􀀸􀀦􀀱􀀍􀀁􀀵􀀬􀀪􀀱􀀁􀀉􀁊􀁏􀀁􀀸􀀱􀀢􀀊􀀍􀀁􀁐􀁓􀀁􀀢􀀦􀀴􀀁􀀉􀀙􀀑􀀓􀀏􀀒􀀒􀁊􀀊􀀏

􀁲􀀁􀀥􀁆􀁇􀁂􀁖􀁍􀁕􀀁􀀢􀁖􀁕􀁉􀁆􀁏􀁕􀁊􀁄􀁂􀁕􀁊􀁐􀁏􀀏􀀁􀀰􀁑􀁆􀁏􀀁􀀉􀁏􀁐􀁏􀁆􀀊􀀍􀀁􀀸􀀱􀀢􀀐􀀱􀀴􀀬􀀍􀀁􀀸􀀱􀀢􀀓􀀐􀀱􀀴􀀬􀀍􀀁􀀸􀀱􀀢􀀐􀀙􀀑􀀓􀀏􀀒􀀹􀀍􀀁􀁐􀁓􀀁

WPA2/802.1X.

􀁲􀀁􀀷􀁆􀁏􀁅􀁐􀁓􀀏􀀁􀀵􀁉􀁆􀀁􀁏􀁂􀁎􀁆􀀁􀁐􀁇􀀁􀁕􀁉􀁆􀀁􀁅􀁆􀁗􀁊􀁄􀁆􀀁􀁎􀁂􀁏􀁖􀁇􀁂􀁄􀁕􀁖􀁓􀁆􀁓􀀏

􀁲􀀁􀀣􀀴􀀴􀀪􀀥􀀏􀀁􀀵􀁉􀁆􀀁􀁂􀁄􀁄􀁆􀁔􀁔􀀁􀁑􀁐􀁊􀁏􀁕􀀈􀁔􀀁􀀮􀀢􀀤􀀁􀁂􀁅􀁅􀁓􀁆􀁔􀁔􀀏

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􀁲􀀁􀀧􀁓􀁆􀁒􀁖􀁆􀁏􀁄􀁚􀀏􀀁􀀵􀁉􀁆􀀁􀁄􀁆􀁏􀁕􀁆􀁓􀀁􀁇􀁓􀁆􀁒􀁖􀁆􀁏􀁄􀁚􀀁􀁐􀁇􀀁􀁕􀁉􀁆􀀁􀁄􀁉􀁂􀁏􀁏􀁆􀁍􀀏

􀁲􀀁􀀯􀁆􀁕􀁘􀁐􀁓􀁌􀀁􀀵􀁚􀁑􀁆􀀏􀀁􀀢􀁄􀁄􀁆􀁔􀁔􀀁􀁑􀁐􀁊􀁏􀁕􀀁􀁐􀁓􀀁􀁂􀁅􀀁􀁉􀁐􀁄􀀁􀀉􀁏􀁐􀀁􀁂􀁄􀁄􀁆􀁔􀁔􀀁􀁑􀁐􀁊􀁏􀁕􀀊􀀏

􀁲􀀁􀀨􀁓􀁂􀁑􀁉􀀏􀀁􀀵􀁉􀁊􀁔􀀁􀁊􀁔􀀁􀁂􀀁􀁄􀁉􀁆􀁄􀁌􀁃􀁐􀁙􀀁􀁕􀁉􀁂􀁕􀀁􀁕􀁆􀁍􀁍􀁔􀀁􀀸􀁊􀀎􀀧􀁊􀀁􀀪􀁏􀁔􀁑􀁆􀁄􀁕􀁐􀁓􀀁􀁕􀁐􀀁􀁈􀁓􀁂􀁑􀁉􀀁􀁕􀁉􀁆􀀁􀁔􀁊􀁈􀁏􀁂􀁍􀀁􀁍􀁆􀁗􀁆􀁍􀀁􀁐􀁗􀁆􀁓􀀁

time (checked) or not to do so (unchecked). In the figure, both are checked, so both

will be graphed.

In the figure, the access points are listed in terms of declining signal strength.

However, the networks table can be sorted by any column heading. The user merely

clicks on the column heading.

Figure 6a-4 zooms in on the networks window. In the upper right, there are

instructions to “Right click on SSID name to Locate.” In the section on the radar

window, we saw that the window does not give the physical locations of access points.

The Locate function under networks addresses this lack of physical location in a limited

but interesting way. If you right click on an SSID name such as Nalu24, your computer

begins beeping. If you are far away, it will beep slowly. As you approach it, the beeping

􀁔􀁑􀁆􀁆􀁅􀀁􀁘􀁊􀁍􀁍􀀁􀁃􀁆􀀁􀁊􀁏􀁄􀁓􀁆􀁂􀁔􀁆􀁅􀀏􀀁􀀦􀁔􀁔􀁆􀁏􀁕􀁊􀁂􀁍􀁍􀁚􀀍􀀁􀁚􀁐􀁖􀀁􀁂􀁓􀁆􀀁􀁖􀁔􀁊􀁏􀁈􀀁􀁕􀁉􀁆􀀁􀁏􀁆􀁕􀁘􀁐􀁓􀁌􀀁􀁂􀁏􀁂􀁍􀁚􀁔􀁊􀁔􀀁􀁗􀁆􀁓􀁔􀁊􀁐􀁏􀀁􀁐􀁇􀀁􀁂􀀁

Geiger counter.

Signal History

In the networks window, we saw that the user can check or uncheck whether graphing

should be done. The Signal History window shows these graphs. The graphs in

Figure 6a-4 show that the signal strength for Nalu24 is uniformly excellent, while the

signal strength for Belkin is uniformly poor. Major fluctuations would indicate serious

problems.

FIGURE 6A-4 Locating an Access Point ISBN 1-323-07906-8

􀀁 􀀤􀁉􀁂􀁑􀁕􀁆􀁓􀀁􀀗􀁂􀀁 􀁲􀀁 􀀶􀁔􀁊􀁏􀁈􀀁􀀹􀁊􀁓􀁓􀁖􀁔􀀁􀀸􀁊􀀎􀀧􀁊􀀁􀀪􀁏􀁔􀁑􀁆􀁄􀁕􀁐􀁓 249

Other Groups on the Ribbon

The Layout group on the ribbon is the most-used feature of the Xirrus Wi-Fi Inspector.

Help Group The Help group provides a user’s guide to explain the program’s

detailed functionality. There is also a helpful glossary of terms.

Settings Group The Settings group allows the user to adjust many settings, for

example, expressing RSSI in percentage terms instead of in terms of dBm.

Tests Group The windows in Wi-Fi Inspector provide information visually. The

Tests group allows the user to conduct more detailed tests. These tests are good for

troubleshooting.

TESTS

As just noted, the Tests group actively tests the quality of your service. The tests group

performs three important tests.

Connection Test

The connection test shows how well you are connected to the outside world and to

critical internal devices. Figure 6a-6 shows the results of a connection test. It shows that

Wi-Fi Inspector uses ping to test latency to your DNS server, default gateway (router),

and a host on the Internet (Internet Reachable). It also does a DNS lookup, in this case

for www.google.com.

The test shows that the user has low latency for the default router and an Internet

host. It also shows that the DNS lookup was successful. In color, these are shown in

green, with the word Pass. However, there is relatively high latency to the user’s DNS

server (152 ms). This is indicated by a yellow bar with the text Warning: high latency.

However, the latency is not very high. This connection looks good.

FIGURE 6A-5 Signal History

ISBN 1-323-07906-8

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Speed Test

The speed test takes the user to speedtest.net. Figure 6a-7 shows a test in which there

was a download speed of 14 Mbps and an upload speed of just under 1 Mbps. These are

reasonable numbers.

FIGURE 6A-6 Connection Test

FIGURE 6A-7 Speed Test in Wi-Fi Inspector

ISBN 1-323-07906-8

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Quality Test

Figure 6a-8 shows results from the quality test, which takes you to pingtest.net. The

results give the user’s quality level a B. However, the box on the left notes that the

connection should be fine for anything but gaming.

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is less than 50 miles away. Connecting to a more distant server would increase

latency.

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voice and video, for which jitter can result in jittery voice or video. Again, the

number is fairly good.

􀁲􀀁􀀵􀁉􀁆􀁓􀁆􀀁􀁘􀁂􀁔􀀁􀁛􀁆􀁓􀁐􀀁􀁑􀁂􀁄􀁌􀁆􀁕􀀁􀁍􀁐􀁔􀁔􀀏􀀁􀀵􀁉􀁆􀀁􀁄􀁐􀁏􀁏􀁆􀁄􀁕􀁊􀁐􀁏􀀁􀁂􀁑􀁑􀁆􀁂􀁓􀁔􀀁􀁕􀁐􀀁􀁃􀁆􀀁􀁓􀁆􀁍􀁊􀁂􀁃􀁍􀁆􀀏

􀁲􀀁􀀵􀁉􀁆􀁓􀁆􀀁􀁊􀁔􀀁􀁂􀀁􀀮􀀰􀀴􀀁􀁔􀁄􀁐􀁓􀁆􀀁􀁐􀁇􀀁􀀕􀀏􀀔􀀔􀀏􀀁􀀵􀁉􀁊􀁔􀀁􀁊􀁔􀀁􀁂􀀁􀁕􀁓􀁂􀁅􀁊􀁕􀁊􀁐􀁏􀁂􀁍􀀁􀁔􀁖􀁃􀁋􀁆􀁄􀁕􀁊􀁗􀁆􀀁􀁊􀁏􀁅􀁊􀁄􀁂􀁕􀁐􀁓􀀁􀁐􀁇􀀁􀁗􀁐􀁊􀁄􀁆􀀁

call quality. A MOS score of 5 indicates toll-call quality on the telephone system.

A MOS of 4.33 is quite good.

One caveat is that pingtest.net is a bit “grabby.” It tries to sell you its tools and is

􀁔􀁍􀁊􀁈􀁉􀁕􀁍􀁚􀀁􀁂􀁈􀁈􀁓􀁆􀁔􀁔􀁊􀁗􀁆􀀏􀀁􀀪􀁏􀀁􀁂􀁅􀁅􀁊􀁕􀁊􀁐􀁏􀀍􀀁􀁕􀁉􀁆􀀁􀁔􀁊􀁕􀁆􀀁􀁖􀁔􀁆􀁔􀀁􀀫􀁂􀁗􀁂􀀍􀀁􀁘􀁉􀁊􀁄􀁉􀀁􀁚􀁐􀁖􀀁􀁎􀁂􀁚􀀁􀁉􀁂􀁗􀁆􀀁􀁕􀁐􀀁􀁅􀁐􀁘􀁏􀁍􀁐􀁂􀁅􀀏􀀁

􀀺􀁐􀁖􀀁􀁎􀁂􀁚􀀁􀁂􀁍􀁔􀁐􀀁􀁉􀁂􀁗􀁆􀀁􀁕􀁐􀀁􀁈􀁊􀁗􀁆􀀁􀁂􀀁􀁇􀁊􀁓􀁆􀁘􀁂􀁍􀁍􀀁􀁆􀁙􀁄􀁆􀁑􀁕􀁊􀁐􀁏􀀁􀁕􀁐􀀁􀁕􀁉􀁊􀁔􀀁􀀫􀁂􀁗􀁂􀀁􀁑􀁓􀁐􀁈􀁓􀁂􀁎􀀏

FIGURE 6A-8 Quality Test

ISBN 1-323-07906-8

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ACTIVITIES

Questions

1. Why is the radar window’s image of a radar scope misleading?

2. How would you locate an access point despite the limitations of the radar window?

This will take one to four paragraphs.

3. There is a value of –44 dBm for signal strength. How good is this?

4. How can you sort the networks window?

5. What information does the Connection Test give you?

6. What information does the Speed Test give you?

7. What information does the Quality Test give you?

Activity

Select a building. Go to at least ten locations. At each location, record the information in

the networks window. Also, do a connection and speed test. Write a brief report what

you learned about Wi-Fi service in the building, referring to the data you collected.

ISBN 1-323-07906-8