Wireless and Mobile Protocol

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Fund. Wireless & Mobile Protocol

COSC 4301/5340 Wireless Local Area Networks (WLANs)

Instructor: Dr. Xingya Liu Department of Computer Science

Lamar University

Office: MAES 86 Phone: 409-880-8677

Email: xliu@lamar.edu

Fund. Wireless & Mobile Protocol

The ubiquitous WLAN • Today’s road worriers require access to the Internet everywhere. • WLAN is more than just cable replacement, it provides hassle-

free broadband Internet access everywhere.

• Coverage in ‘hot-spots’ sufficient. • WLAN meets the expectations for easiness, cost and bandwidth.

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Public WLAN

Airport

Railway Station

Campus

Plant

Semi-public WLAN

Office Hospital

Congress hall, Hotel

Corporate WLAN

Office

Home WLAN

Remote Access

Fund. Wireless & Mobile Protocol

Wireless LANs

Infrastructure Network

AP AP

wired network

AP: Access Point

Ad-hoc Network

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Fund. Wireless & Mobile Protocol

Overview of WLANs • Wireless Internet access cheaper and faster

• The number of public Wi-Fi hot spots worldwide will be 5.8 million by 2015, up from virtually nil in 2001. (The Wireless Broadband Alliance projections)

• 350% increase compared to the 1.3 million WiFi hot spots that are live today (Nov. 2011)

• Wireless public area access service revenue grows from $3.9 million in 2001 to $224.7 million in 2005.

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Fund. Wireless & Mobile Protocol

Applications • Home wireless networks

• Enterprise wireless networks

• Public access: Airport, Convention Centers, Cafes, Train Stations

• Hospitals

• Warehouses • Consulting and audit teams

• Dynamic environments, ad agencies, etc.

• Universities

• Historic buildings, older buildings

• Meeting rooms • Retail stores

• Restaurants and car rental agencies

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Fund. Wireless & Mobile Protocol

Basics of Wireless LANs • Advantages:

– Generally works in industrial, scientific, and medical (ISM) band, which is un-licensed and available for public.

– Users can access high speed multimedia applications, with easy implementation, low cost, and wide user acceptance.

• Disadvantages: – Interference problem

• Co-channel interference • Interference between different WLANs in the same frequency band • Interference from non-WLAN devices in the same frequency band

– Weak to provide real-time services, QoS guarantees

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Fund. Wireless & Mobile Protocol

Topologies – Single-Cell Wireless LAN

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Fund. Wireless & Mobile Protocol

Topologies – Single Cell Wireless LAN

• In the figure, there is a backbone wired LAN, such as Ethernet, that supports servers, workstations, and one or more bridges or routers to link with other networks.

• There is a control module (CM) (Access Point (AP)) that acts as an interface to a wireless LAN. (CM = AP)

• The control module includes either bridge or router functionality to link the wireless LAN to the backbone.

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Fund. Wireless & Mobile Protocol

Topologies – Single Cell Wireless LAN

• CM includes some sort of access control logic, such as a polling or token-passing scheme, to regulate the access from the end systems.

• Note that some of the end systems are stand-alone devices such as a workstation or a server.

• In addition, hubs or other user modules (UM) (PORTAL) that control a number of stations off a wired LAN may also be part of the wireless LAN configuration.

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Fund. Wireless & Mobile Protocol

Topologies – Multiple Cell Wireless LAN

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Fund. Wireless & Mobile Protocol

Topologies – Multiple Cell Wireless LAN

• There are multiple control modules interconnected by a wired LAN.

• Each control module supports a number of wireless end systems within its transmission range.

• For example, with an infrared LAN, transmission is limited to a single room; therefore, one cell is needed for each room in an office building that requires wireless support.

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Fund. Wireless & Mobile Protocol

Family of Wireless LAN Standards – IEEE 802.11

• 802.11-1997: released in 1997, clarified in 1999, many amendments – 802.11a – 54Mbps 5GHz- Ratified in 1999 – 802.11b - 11Mbps 2.4GHz- ratified in 1999 – 802.11d – International (country-to-country) roaming – 802.11e - Quality of Service

• Enhance the 802.11 MAC to expand support for applications with Quality of Service requirements

– 802.11F - Inter-Access Point Protocol (IAPP), withdrawn later • Establish an Inter-Access Point Protocol for data exchange via the distribution

system – 802.11g - Higher Data rate (54Mbps) 2.4GHz – 802.11h - Dynamic Frequency Selection and Transmit Power Control

mechanisms for 802.11a – 802.11i - Authentication and security

• Enhance the 802.11 MAC to provide improvements in security – 802.11j – Extensions for Japan

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Fund. Wireless & Mobile Protocol

Family of Wireless LAN Standards – IEEE 802.11

• 802.11-2007: a new release of the standard, merged amendments a , b, d, e, g, h, i, and j with the base standard (July 2007) – 802.11k – Radio resource measurement – 802.11n – Higher data rate using MIMO, both 2.4GHz and 5GHz – 802.11p – WAVE (wireless access for the vehicular environment) – 802.11r – Fast BSS transition – 802.11s – Mesh networking – 802.11u – Third-part authorization of clients, e.g., cellular offload – 802.11v – Wireless network management – 802.11w – Protected management frames – 802.11y – 3.6GHz (3650-3700MHz) operation in the U.S. – 802.11z – Extensions to Direct Link Setup (DLS)

• 802.11-2012: a new release of the standard, merged amendments k , n, p, r, s, u, v, w, y, and z with the 2007 standard (March 2012)

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Fund. Wireless & Mobile Protocol

IEEE 802.11 Reference Architecture

• Station (STA) – terminal with access mechanisms

to the wireless medium and radio contact to the access point

• Basic Service Set (BSS) – group of stations using the same

radio frequency • Access Point

– station integrated into the wireless LAN and the distribution system

• Distribution System – interconnection network to form

one logical network (ESS: Extended Service Set) based on several BSS

• Portal – bridge to other (wired) networks

Distribution System

Portal

Access Point

BSS2

BSS1 Access

Point

STA1

STA2 STA3

ESS

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Fund. Wireless & Mobile Protocol

IEEE 802.11 Reference Architecture

• The smallest building block of a wireless LAN is a basic service set (BSS), which consists of some number of stations executing the same MAC protocol and competing for access to the same shared medium.

• A basic service set may be isolated or it may connect to a backbone distribution system through an access point.

• The MAC protocol may be fully distributed or controlled by a central coordination function housed in the access point.

• The access point functions as a bridge.

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Fund. Wireless & Mobile Protocol

IEEE 802.11 Reference Architecture

• The basic service set generally corresponds to what is referred to as a cell in the literature.

• An extended service set (ESS) consists of two or more basic service sets interconnected by a distribution system.

• Typically, the distribution system is a wired backbone LAN.

• The extended service set appears as a single logical LAN.

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Fund. Wireless & Mobile Protocol

Protocol Architecture

mobile terminal

access point

fixed terminal

application

TCP

PHY

MAC

802.3 MAC

802.3 PHY

application

TCP

802.3 PHY

802.3 MAC

MAC

PHY

LLC

infrastructure network

LLC LLC

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Fund. Wireless & Mobile Protocol

WLAN: IEEE 802.11b • Data rate

– 1, 2, 5.5, 11 Mbit/s, depending on SNR – User data rate max. approx. 6 Mbit/s

• Transmission range – 300m outdoor, 30m indoor Max. data rate ~10m indoor

• Frequency: Free 2.4 GHz ISM-band • Availability: Many products, many vendors

• Quality of Service – Typically Best effort, no guarantees (unless polling is used, limited support

in products)

• Special Advantages/Disadvantages – Advantage: many installed systems, lots of experience, available

worldwide, free ISM-band, many vendors, integrated in laptops

– Disadvantage: heavy interference on ISM-band (Industrial, Scientific, Medical band), no service guarantees, slow relative speed

Limited, WEP (Wired Equivalent Privacy) insecure 18Liu

Fund. Wireless & Mobile Protocol

WLAN: IEEE 802.11a • Data rate

– 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR – User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54) – 6, 12, 24 Mbps mandatory

• Transmission range – 100m outdoor, 10m indoor

• E.g., 54 Mbps up to 5 m, 48Mbps up to 12 m, 36Mbps up to 25 m, 24Mbps up to 30m, 18Mbps up to 40 m, 12Mbps up to 60 m

• Frequency – Free 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz ISM-band

• Availability – Some products, some vendors

• Quality of Service – Typically best effort, no guarantees (same as all 802.11 products)

• Special Advantages/Disadvantages – Advantage: fits into 802.x standards, free ISM-band, uses less crowded 5 GHz band – Disadvantage: data rates may drop fast depending on SNR, propagation condition,

and the distance between sender and receiver, no QoS • Security

– Limited, WEP insecure 19Liu

Fund. Wireless & Mobile Protocol

WLANs – 802.11 Compatibility

• 802.11a and 802.11b share the same MAC layer

• Significant differences at the physical layer. – 802.11b: 2.4 GHz, DSSS – 802.11a: 5 GHz, OFDM – Possible to operate both on the same network concurrently

(using the same access points)

• Interoperability – WECA (Wireless Ethernet Compatibility Alliance):

organization behind Wi-Fi (Wireless Fidelity) that certifies products meeting the 802.11b specification through compatibility testing

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Fund. Wireless & Mobile Protocol

IEEE 802.11 Protocol Architecture

Physical Layer (PHY)

Distributed Coordination Function (DCF)

Point Coordination Function (PCF)

Normal Data Traffic (Asynchronous) Contention Service

Real Time Traffic Contention Free Service

MAC

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Fund. Wireless & Mobile Protocol

MAC Protocol Review • Medium Access Control (MAC)

– Resolve contentions to the shared medium – Especially important for LAN

• Wired LAN – ALOHA

• Send without waiting • Simple, no synchronization

– Slotted ALOHA • Time is divided into equal size slots

– CSMA • Listen before transmit • 1-persistent, non-persistent, p-persistent

– CSMA/CD • Abort colliding transmission

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Fund. Wireless & Mobile Protocol

Wireless LAN MAC • CSMA (Carrier Sense Multiple Access) as Wireless MAC?

– Collision detection is difficult – Hidden and Exposed Terminal Problem makes the use of CSMA an

inefficient technique

• Hidden Terminal Problem – A talks to B – C senses the channel – C does not hear A’s transmission (out of range) – C talks to B – Signals from A and B collide

A B C

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Fund. Wireless & Mobile Protocol

Wireless LAN MAC

• Exposed Terminal Problem – B talks to A – C wants to talk to D – C senses channel and finds it to be busy – C stays quiet (when it could have ideally transmitted)

A B C D Not possible

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Fund. Wireless & Mobile Protocol

Hidden and Exposed Terminal Problems

• Hidden Terminal Problem – More collisions – Waste of resources

• Exposed Terminal Problem – Under-utilization of channel – Lower effective throughput

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Fund. Wireless & Mobile Protocol

Multiple Access with Collision Avoidance (MACA)

• MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance

– RTS (request to send) (20 byte): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet

– CTS (clear to send) (16 byte): the receiver grants the right to send as soon as it is ready to receive

• Signaling packets contain

– sender address – receiver address – packet size

• Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)

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Fund. Wireless & Mobile Protocol

Multiple Access with Collision Avoidance (MACA)

• Hidden Terminal Revisited

– A sends RTS – B sends CTS – C overhears CTS – C inhibits its own transmitter – A successfully sends DATA to B

A B C RTS CTS

DATA CTS

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Fund. Wireless & Mobile Protocol

Multiple Access with Collision Avoidance (MACA)

• Hidden Terminal Revisited

– How does C know how long to wait before it can attempt a transmission?

– A includes length of DATA that it wants to send in the RTS packet

– B includes this information in the CTS packet

– C, when it overhears the CTS packet, retrieves the length information and uses it to set the inhibition time

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Fund. Wireless & Mobile Protocol

Multiple Access with Collision Avoidance (MACA)

• Exposed Terminal Revisited – B sends RTS to A (overheard by C) – A sends CTS to B – C cannot hear A’s CTS – C assumes A is either down or out of range – C does not inhibit its transmissions to D

A B C D RTS RTS

CTS Cannot hear CTS

Tx not inhibited

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Fund. Wireless & Mobile Protocol

Multiple Access with Collision Avoidance (MACA)

• Collision

– Still possible RTS packets can collide! – Binary exponential backoff performed by stations that

experience RTS collisions – RTS collisions not as bad as data collisions in CSMA since

RTS packets are typically much smaller than DATA packets – If DATA packets are of the same size as RTS/CTS packets,

significant overheads

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