Learning Objectives

Tell how IEEE 802.11a networks function, and how they differ from 802.11b networks

List the advantages and disadvantages of an IEEE 802.11g network

Describe the HiperLAN/2 networksCompare low-speed and high-speed WLANsExplain basic and enhanced WLAN security

features

High Speed WLANs

Three standards for high-speed WLANs that transmit at speeds over 15 MbpsIEEE 802.11aIEEE 802.11gHiperLAN/2

All WLANs are concerned with securityHow to prevent unauthorized access

IEEE 802.11a

Approved in 1999, 802.11a transmits at speeds of 5.5 Mbps and 11 Mbps

Great demand for 802.11a WLANS, also called Wi-Fi5, with maximum speed of 54 MbpsDevices use gallium arsenide (GaAs) or silicon

germanium (SiGe) rather than CMOS semiconductors

Increased speed achieved by higher frequency, more transmission channels, multiplexing techniques, and more efficient error-correction

U-NII Frequency Band

802.11b uses unlicensed Industrial, Scientific, and Medical (ISM) band and specifies 14 frequencies

802.11a uses Unlicensed Information Infrastructure (U-NII) bandTable 7-1 compares ISM and U-NIIU-NII is divided into three bands, shown in

Table 7-2U-NII provides more bandwidth, faster

transmission, and increased powerEfforts underway to unify 5 GHz bands globally

ISM vs. U-NII

U-NII Spectrum

Channel Allocation

802.11a WLANs have have 11 channels in USA but requires 25 MHz passbandSee Figure 7.1

Figure 7-2 shows 8 channels in Low and Medium Bands with 20 MHz channel supporting 52 carrier signals, each 200 KHz wideSupports eight networks per AP, as shown in

Figure 7-3IEEE 802.11e Task Group is working on standard

that supports quality of service (QOS)

802.11b Channels

802.11a Channels

Orthogonal Frequency Division Multiplexing

Electromagnetic waves reflect off surfaces and may be delayed in reaching their destinationFigure 7-4 illustrates multipath distortionReceiving device waits until all reflections are

received before it can transmitIncreasing speed of WLAN only causes longer

delays waiting for reflections802.11a uses Orthogonal Frequency

Division Multiplexing (OFDM) to solve this problem

Orthogonal Frequency Division Multiplexing

Dating to 1960s, OFDM’s primary role is to split high-speed digital signal into several slower signals running in parallelSending device breaks transmission into pieces

and sends it over channels in parallelReceiving device combines signals to re-create

the transmissionSee Figure 7-5

Multiple Channels of OFDM

OFDM Breaks 802.11B Ceiling Limit

Slowing down transmissions actually delays reflections, increases total throughput, and results in faster WLANSee Figure 7-6

802.11a specifies eight overlapping channels, each divided into 52 subchannels that are 300 KHz wideOFDM uses 48 subchannels for data and the

remaining four for error correction

OFDM vs. Single Channel

Modulation Techniques Vary Depending on Speed

6 Mbps—phase shift keying (PSK) Encodes 125 Kbps of data on each of 48

subchannels, resulting in 6Mbps data rateSee Figure 7-7

12 Mbps—quadrature phase shift keying (QPSK)Encodes 250Kbps per channel for 12 Mbps data

rateSee Figure 7-8

PSK

QPSK

Modulation Techniques Vary Depending on Speed

24 Mbps—16-level quadrature amplitude modulation (16-QAM)16 different signals can encode 500 Kbps per

subchannelSee Figure 7-9

54 Mbps—64-level quadrature amplitude modulation (64-QAM)Transmits 1,125 Mbps over each of 48

subchannelsSee Figure 7-10

16-QAM

64-QAM

Higher Speeds

Official top speed of 802.11a is 54 MbpsSpecification allows for higher speeds

known as turbo mode or 2X modeEach vendor can develop 2X mode by

combining two frequency channelsProduces 96 subchannels and speeds up to

108 MbpsOther 2X mode techniques include increasing

and reallocating individual carriers and using different coding rate schemes

Error Correction

802.11a transmissions significantly reduce errorsMinimizes radio interference from outside

sources801.11a has enhanced error correction

Forward Error Correction (FEC) transmits secondary copy of information that may be used if data is lost

Uses 48 channels for standard transmissions and 4 for FEC transmissions

802.11a Physical Layer

802.11a changed only physical layerPHY layer is divided into two parts

Physical Medium Dependent (PMD) sublayer defines method for transmitting and receiving data over wireless medium

Physical Layer Convergence Procedure (PLCP) reformats data received from MAC layer into frame that PMD sublayer can transmit

PLCP

Based on OFDM, PLCP frame has three partsPreamble—allows receiving device to prepare

for rest of frameHeader—provides information about frameData—information to be transmitted

See Figure 7-11

802.11a PLCP Frame

Fields in PLCP Frame

SynchronizationRateLengthParity

TailServiceDataPad

802.11a Rate Field Values

Advantages and Disadvantages

AdvantagesGood for area that need higher transmission

speeds

DisadvantagesShorter range of coverageApproximately 225 feet as compared with

375 feet for 802.11b WLAN

IEEE 802.11g

In 2001, IEEE proposed 802.11g draft standard to combine stability of 802.11b with faster data transfer rates of 802.11aOperates in 2.4 GHz ISM frequencyHas two mandatory modes: Complementary Code

Keying (CCK) mode and Orthogonal Frequency Division Multiplexing (OFDM)

Offers two optional modes: Packet Binary Convolutional Coding (PBCC-22) and CCK-ODFM

802.11g products not expected until 2003

HiperLAN/2

Similar to 802.11a, HiperLAN/2 was standardized by European Telecommunications Standards InstituteFigure 7-12 shows protocol stack for HiperLAN/2Has three basic layers: Physical, Data Link, and

ConvergenceProducts based on HiperLAN/2 may appear in

2003

HiperLAN/2 Protocol Stack

Physical Layer

PHY layers of IEEE 802.11a and HiperLAN/2 are almost identicalOperate in 5 GHz bandUse OFDMTransmit up to 54 MbpsConnect seamlessly to wired Ethernet networks

Data Link Layer

HiperLAN/2 centralizes control of RF medium to access point (AP)AP informs clients, known as mobile terminals

(MTs), when they may send dataChannel allocation is based on dynamic

time-division multiple access (TDMA) that divides bandwidth into several time slots

Quality of Service (QOS) refers to dynamically allocated time slots based on needs of MT and condition of network

Radio Link Control (RLC) Sublayer

Three primary functions of RLC sublayerConnection setup procedure and connection

monitoring—authentication and encryptionRadio resource handling, channel monitoring,

and channel selection—automatic transmission frequency allocation (known as Dynamic Frequency Selection (DFS)

Association procedure and reassociation procedure—standardized handoff to nearest AP by roaming MTs

Logical Link Control (LLC) sublayer, also part of Data Link Layer, performs error checking

Convergence Layer

HiperLAN/2 offers seamless high-speed wireless connectivity up to 54 MbpsCan connect to cellular telephone systemsCan connect to Asynchronous Transfer Mode

(ATMs) systems using fiber-optic media and transmitting at 622 Mbps

Can connect to IEEE 1394 (also known as FireWire) high speed external serial bus transmitting at 400 Mbps

Summary: High- and Low-Speed WLANs

May compare different types of WLANsDo not consider them as competing

technologiesRather, they are complementary technologies,

each with its strengths and weaknesses and market niche

HomeRF—combines wireless data, cordless telephony, and streaming media for home networksSupports QoS and transmits from 1/6 Mbps to

10 Mbps

WLAN Summary

IEEE 802.11—provides cable-free access for mobile or fixed location at rate of 1 or 2 Mbps

802.11b (Wi-Fi)—popular choice for business wireless networksTransmits at 11 Mbps on three simultaneous

channels but offers no QoS and uses crowded ISM band

WLAN Summary

802.11a—current leader in business WLANsUses U-NII frequency, allows 8 simultaneous

channels, and transmits at 54 Mbps standard, can be increased to 108 Mbps

802.11g—offers faster data rates while remaining compatible with 802.11b networksUses only three channels and crowded ISM

frequency

WLAN Summary

HiperLAN/2—uses dynamically allocated time slots and dynamic frequency selection for high-speed communicationsPopular in Europe

Table 7-4 compares WLANs

WLAN Comparison

802.11 Security

Greatest strength of WLANs is ability to roam freely

Greatest weakness is risk of unauthorized user receiving RF signalsSome flawed IEEE WLAN security provisions

Basic Security involves two areas:Authenticating usersKeeping transmissions private

Authentication

Verifies user has permission to access network

Each WLAN client can be given Service Set Identifier (SSID) of networkOnly clients that know SSID may connectSSID may be entered manually into wireless

device, but anyone with device has access to network

Access points (APs) may freely advertise SSID to any mobile device within range

Privacy

IEEE standard provides optional Wired Equivalent Privacy (WEP) specification for data encryption Two types of keys used for encryptionPublic key cryptography uses matched public

and private keysIEEE uses shared key cryptography with same

key used for encryption and decryptionThe longer the key, the more secure it isSee Figure 7-13

WEP

WEP Privacy Concerns

In late 2000, researchers revealed “initialization vector” used to encrypt transmissions with WEP were reused about once every five hoursMakes it easy for anyone to collect data to

break WEP encryptionResearches recovered 128-bit WEP key in less

than 2 hoursMany think IEEE WLANs should be

treated as insecure

Enhanced Security

Administrators must use enhanced security measures to prevent WLAN attacks

Four kinds of WLAN attacksHardware theftAccess point impersonationPassive monitoringDenial of service

Additional Security Procedures

IEEE task group working on draft known as IEEE 802.1x to allow centralized authentication of wireless clientsUses Extensible Authentication Protocol (EAP)

—client negotiates authentication protocols with separate authentication server

Uses Remote Authentication Dial-In User Service (RADIUS)—server on wired network sends security keys to wireless client

See Figure 7-14

802.1x Security

Other Security Steps

Use an access control list with MAC addresses of approved clients, as seen in Figure 7-15

Use digital certificates issued by trusted third party for secure, encrypted online communication

Use digital wrapper or gatekeeper that secures data by wrapping around another program or file

Use a Virtual Private Network (VPN), a secure, encrypted connection between two points

Access Control List

Higher Levels of Security

Reduce transmission power used in WLANsDecreases distance radio waves travel, thus

limiting range where hackers can pick up signals

Change default WLAN security settingsKeep WLAN traffic separate from that of

wired networkUse 128-bit WEP keys rather than default

40-bit keys