Data and Computer Data and Computer CommunicationsCommunications

Eighth EditionEighth Editionby William Stallingsby William Stallings

Lecture slides by Lawrie BrownLecture slides by Lawrie Brown

Chapter 17 – Chapter 17 – Wireless Wireless LANLANss

High Speed High Speed LANLANssInvestigators have published numerous reports of birds taking turns vocalizing; the bird spoken to gave its full attention to the speaker and never vocalized at the same time, as if the two were holding a conversationResearchers and scholars who have studied the data on avian communication carefully write the (a) the communication code of birds such has crows has not been broken by any means; (b) probably all birds have wider vocabularies than anyone realizes; and (c) greater complexity and depth are recognized in avian communication as research progresses.

—The Human Nature of Birds, Theodore Barber

Overview of Wireless LANsOverview of Wireless LANs use wireless transmission mediumuse wireless transmission medium issues ofissues of high prices, low data rates, high prices, low data rates,

occupational safety concerns, & licensing occupational safety concerns, & licensing requirements now addressedrequirements now addressed

key application areas:key application areas: LAN extensionLAN extension cross-building interconnectcross-building interconnect nomadic accessnomadic access ad hoc networkingad hoc networking

Single Cell Single Cell LAN ExtensionLAN Extension

Multi Cell Multi Cell LAN ExtensionLAN Extension

Cross-Building InterconnectCross-Building Interconnect connectconnect LANs in nearby buildings LANs in nearby buildings pointpoint-to-point wireless link-to-point wireless link

NotNot a LAN per se a LAN per se connectconnect bridges or routers bridges or routers

Nomadic AccessNomadic Access linklink LAN hub & mobile data terminal LAN hub & mobile data terminal

laptoplaptop or notepad computer or notepad computer enable enable employee to transfer data from employee to transfer data from

portable computer to serverportable computer to server also also useful in extended environment such useful in extended environment such

as campus or cluster of buildingsas campus or cluster of buildings usersusers move around with portable computers move around with portable computers maymay wish access to servers on wired LAN wish access to servers on wired LAN

Infrastructure Wireless LANInfrastructure Wireless LAN

Ad Hoc NetworkingAd Hoc Networking temporary temporary peerpeer-to-peer network-to-peer network

Wireless LAN RequirementsWireless LAN Requirements throughput - efficient use wireless mediumthroughput - efficient use wireless medium no of nodes - no of nodes - hundreds hundreds of nodes across multiple cellsof nodes across multiple cells connection to backbone LAN -connection to backbone LAN - using using control modules control modules service area - 100 to 300 mservice area - 100 to 300 m low low power consumption - for long battery life power consumption - for long battery life on mobileson mobiles transmission robustness and securitytransmission robustness and security collocated network operationcollocated network operation license-free operationlicense-free operation handoff/roaminghandoff/roaming dynamic configuration - adynamic configuration - additionddition, deletion, and relocation , deletion, and relocation

of end systems without disruption to usersof end systems without disruption to users

TechnologyTechnology infrared (IR) LANsinfrared (IR) LANs

individual individual cell of IR LAN limited to single roomcell of IR LAN limited to single room IR IR light does not penetrate opaque wallslight does not penetrate opaque walls

spread spectrum LANsspread spectrum LANs mostlymostly operate in ISM (industrial, scientific, and operate in ISM (industrial, scientific, and

medical) bandsmedical) bands nono Federal Communications Commission (FCC) Federal Communications Commission (FCC)

licensing is required in licensing is required in USAUSA narrowband microwavenarrowband microwave

microwavemicrowave frequencies but not use spread spectrum frequencies but not use spread spectrum some require FCC licensingsome require FCC licensing

Infrared LANsInfrared LANs constructed using infrared portion of spectrumconstructed using infrared portion of spectrum strengthsstrengths

spectrumspectrum virtually unlimited hence high rates possible virtually unlimited hence high rates possible unregulated spectrumunregulated spectrum infrared infrared shares some properties of visible lightshares some properties of visible light

• reflection covers room, walls isolate networksreflection covers room, walls isolate networks inexpensiveinexpensive and simple and simple

weaknessesweaknesses background background radiation, e.g. sradiation, e.g. sunlight,unlight, indoor lighting indoor lighting powerpower limited by concerns for eye safety and power limited by concerns for eye safety and power

consumptionconsumption

Infrared LANsInfrared LANsTransmission TechniquesTransmission Techniques

directed-beam IRdirected-beam IR pointpoint-to-point links-to-point links rangerange depends on power and focusing depends on power and focusing for indoorfor indoor use can set up token ring LAN use can set up token ring LAN IR transceivers positioned so data circulates in ringIR transceivers positioned so data circulates in ring

omnidirectionalomnidirectional singlesingle base station with line of sight to other stations base station with line of sight to other stations actsacts as a multiport repeater as a multiport repeater otherother stations stations useuse directional beam to it directional beam to it

diffuseddiffused configuration configuration stationsstations focused / aimed focused / aimed atat diffusely reflecting ceiling diffusely reflecting ceiling

Spread Spectrum LANSpread Spectrum LANConfigurationConfiguration

usually usually use multiple-cell arrangementuse multiple-cell arrangement adjacent cells use different center frequenciesadjacent cells use different center frequencies configurations:configurations:

hub hub • connected connected toto wired LANwired LAN• connectconnect to stations on wired LAN and in other cells to stations on wired LAN and in other cells• may domay do automatic handoff automatic handoff

peer-to-peerpeer-to-peer• nono hub hub• MAC algorithm such as CSMA used to control accessMAC algorithm such as CSMA used to control access• for adfor ad hoc LANs hoc LANs

Spread Spectrum LANsSpread Spectrum LANsTransmission Issues Transmission Issues

licensing licensing regulations differ between countriesregulations differ between countries UUSASA FCC allows in ISM band: FCC allows in ISM band:

spread spread spectrumspectrum ( (1W), 1W), veryvery low power low power ((0.5W)0.5W)• 902 - 928 MHz (915-MHz band)902 - 928 MHz (915-MHz band)• 2.4 - 2.4835 GHz (2.4-GHz band)2.4 - 2.4835 GHz (2.4-GHz band)• 5.725 - 5.825 GHz (5.8-GHz band)5.725 - 5.825 GHz (5.8-GHz band)

2.4 GHz also in Europe and Japan2.4 GHz also in Europe and Japan interferenceinterference

many devicesmany devices around 900 MHz: cordless telephones, around 900 MHz: cordless telephones, wireless microphones, and amateur radiowireless microphones, and amateur radio

fewerfewer devices at 2.4 GHz; microwave oven devices at 2.4 GHz; microwave oven littlelittle competition at 5.8 GHz competition at 5.8 GHz

IEEE 802 StandardsIEEE 802 StandardsStandard Scope

Medium access control (MAC): One common MAC for WLANapplications

Physical layer: Infrared at 1 and 2 Mbps

Physical layer: 2.4-GHz FHSS at 1 and 2 MbpsIEEE 802.11

Physical layer: 2.4-GHz DSSS at 1 and 2 Mbps

IEEE 802.11a Physical layer: 5-GHz OFDM at rates from 6 to 54 Mbps

IEEE 802.11b Physical layer: 2.4-GHz DSSS at 5.5 and 11 Mbps

IEEE 802.11c Bridge operation at 802.11 MAC layer

IEEE 802.11d Physical layer: Extend operation of 802.11 WLANs to newregulatory domains (countries)

IEEE 802.11e MAC: Enhance to improve quality of service and enhancesecurity mechanisms

IEEE 802.11f Recommended practices for multivendor access pointinteroperability

IEEE 802.11g Physical layer: Extend 802.11b to data rates >20 Mbps

IEEE 802.11hPhysical/MAC: Enhance IEEE 802.11a to add indoor andoutdoor channel selection and to improve spectrum andtransmit power management

IEEE 802.11i MAC: Enhance security and authentication mechanisms

IEEE 802.11j Physical: Enhance IEEE 802.11a to conform to Japaneserequirements

IEEE 802.11kRadio resource measurement enhancements to provideinterface to higher layers for radio and networkmeasurements

IEEE 802.11m Maintenance of IEEE 802.11-1999 standard with technicaland editorial corrections

IEEE 802.11n Physical/MAC: Enhancements to enable higher throughput

IEEE 802.11p Physical/MAC: Wireless access in vehicular environments

IEEE 802.11r Physical/MAC: Fast roaming (fast BSS transition)

IEEE 802.11s Physical/MAC: ESS mesh networking

IEEE802.11,2

Recommended practice for the Evaluation of 802.11 wirelessperformance

IEEE 802.11u Physical/MAC: Interworking with external networks

IEEE 802 TerminologyIEEE 802 TerminologyAccess point (AP) Any entity that has station functionality and provides

access to the distribution system via the wirelessmedium for associated stations

Basic service set(BSS)

A set of stations controlled by a single coordinationfunction

Coordination function The logical function that determines when a stationoperating within a BSS is permitted to transmit andmay be able to receive PDUs

Distribution system(DS)

A system used to interconnect a set of BSSs andintegrated LANs to create an ESS

Extended service set(ESS)

A set of one or more interconnected BSSs andintegrated LANs that appear as a single BSS to the LLClayer at any station associated with one of these BSSs

MAC protocol dataunit (MPDU)

The unit of data exchanged between two peer MACentites using the services of the physical layer

MAC service data unit(MSDU)

Information that is delivered as a unit between MACusers

Station Any device that contains an IEEE 802.11 conformant MACand physical layer

IEEE 802.11 ArchitectureIEEE 802.11 Architecture

IEEE 802.11 - BSSIEEE 802.11 - BSS basic service set (BSS) building blockbasic service set (BSS) building block maymay be isolated be isolated may connect to backbone distribution may connect to backbone distribution

system (DS) through access point (AP)system (DS) through access point (AP) BSS generally corresponds to cell BSS generally corresponds to cell DS can be switch, wired network, or DS can be switch, wired network, or

wireless networkwireless network have independent BSS (IBSS) with no APhave independent BSS (IBSS) with no AP

Extended Service Set (ESS)Extended Service Set (ESS) possible configurations:possible configurations:

simplest is simplest is each station belongs to single BSSeach station belongs to single BSS can have two can have two BSSs overlapBSSs overlap a stationa station can participate in more than one BSS can participate in more than one BSS associationassociation between station and BSS dynamic between station and BSS dynamic

ESS is twoESS is two or more or more BSSBSS interconnected by interconnected by DSDS appearsappears as single logical LAN to LLC as single logical LAN to LLC

IEEE 802 ServicesIEEE 802 ServicesService Provider Used to support

Association Distributionsystem

MSDU delivery

Authentication Station LAN access andsecurity

Deauthentication Station LAN access andsecurity

Dissassociation Distributionsystem

MSDU delivery

Distribution Distributionsystem

MSDU delivery

Integration Distributionsystem

MSDU delivery

MSDU delivery Station MSDU delivery

Privacy Station LAN access andsecurity

Reassocation Distributionsystem

MSDU delivery

Services - Message Services - Message DistributionDistribution

distribution servicedistribution service primary service used by stations to exchange primary service used by stations to exchange

MAC frames when frame must traverse DSMAC frames when frame must traverse DS if stations in same BSS, distribution service if stations in same BSS, distribution service

logically goes through single AP of that BSSlogically goes through single AP of that BSS integrationintegration service service

enables transfer of data between 802.11 LAN enables transfer of data between 802.11 LAN station and station and oneone on an integrated 802.x LAN on an integrated 802.x LAN

Association Related ServicesAssociation Related Services DSDS requires info about stations within ESS requires info about stations within ESS providedprovided by association-related services by association-related services stationstation must associate must associate before before

communicatingcommunicating 33 mobility transition types: mobility transition types:

no transition -no transition - stationary stationary or in single BSS or in single BSS BSS transition - BSS transition - between between BSS in same ESS BSS in same ESS ESS transition: ESS transition: betweenbetween BSS in different ESS BSS in different ESS

Association Related ServicesAssociation Related Services DS needs identity of destination statioDS needs identity of destination statio

stationsstations must maintain association with AP must maintain association with AP within current BSSwithin current BSS

3 services relate to this requirement: 3 services relate to this requirement:  Association - establishes initial association Association - establishes initial association

between station and APbetween station and AP Reassociation - to tReassociation - to transfer an ransfer an association toassociation to

another another APAP Disassociation - byDisassociation - by station or APstation or AP

Medium Access ControlMedium Access Control MAC layer covers three functional areasMAC layer covers three functional areas

reliablereliable data delivery data delivery access access controlcontrol securitysecurity

Reliable Data DeliveryReliable Data Delivery 802.11 physical / MAC layers unreliable802.11 physical / MAC layers unreliable

noise, interference, and other propagation effects noise, interference, and other propagation effects result in loss of framesresult in loss of frames

even with error-correction codes, frames may not even with error-correction codes, frames may not successfully be receivedsuccessfully be received

cancan be dealt with at a higher layer, e.g. TCP be dealt with at a higher layer, e.g. TCP moremore efficient to deal with errors at MAC level efficient to deal with errors at MAC level 802.11 includes frame exchange protocol802.11 includes frame exchange protocol

station receivingstation receiving frame returns acknowledgment frame returns acknowledgment (ACK) frame(ACK) frame

exchangeexchange treated as atomic unit treated as atomic unit if if no no ACK within short period of timeACK within short period of time, , retransmitretransmit

Four Frame ExchangeFour Frame Exchange can use four-frame exchange for better reliability can use four-frame exchange for better reliability

sourcesource issues a Request to Send (RTS) frame to dest issues a Request to Send (RTS) frame to dest destinationdestination responds with Clear to Send (CTS) responds with Clear to Send (CTS) after receiving CTS, source transmits data after receiving CTS, source transmits data destinationdestination responds with ACK responds with ACK

RTS alerts all stations within range RTS alerts all stations within range of source of source that that exchange is under wayexchange is under way

CTS alerts all stations within range of destination CTS alerts all stations within range of destination other stationsother stations don’t transmit to avoid collision don’t transmit to avoid collision RTS/CTS exchange is required function of MAC RTS/CTS exchange is required function of MAC

but may be disabledbut may be disabled

Media Access ControlMedia Access Control

Distributed Coordination Distributed Coordination FunctionFunction

DCF sublayer DCF sublayer usesuses CSMA CSMA if station has frame to send it listens to mediumif station has frame to send it listens to medium if medium idle, station may transmitif medium idle, station may transmit else else waits until current transmission complete waits until current transmission complete

nono collision detection since on wireless collision detection since on wireless networknetwork

DCF includes delays that act as a priority DCF includes delays that act as a priority schemescheme

IEEE IEEE 802.11 802.11 Medium Medium

Access Access Control Control

LogicLogic

Priority IFS ValuesPriority IFS Values SIFS (short IFS) SIFS (short IFS)

forfor all immediate response actions all immediate response actions (see later) (see later) PIFS (point coordination function IFS)PIFS (point coordination function IFS)

usedused by the centralized controller in PCF by the centralized controller in PCF scheme when issuing pollsscheme when issuing polls

DIFS (distributed coordination function IFS)DIFS (distributed coordination function IFS) used used as minimum delay for asynchronous as minimum delay for asynchronous

frames contending for accessframes contending for access

SIFS UseSIFS Use SIFS givesSIFS gives highest priorityhighest priority

overover stations waiting PIFS or DIFS time stations waiting PIFS or DIFS time SIFS used in following circumstances:SIFS used in following circumstances:

Acknowledgment (ACK)Acknowledgment (ACK)• station station responds with ACK after waiting SIFS gapresponds with ACK after waiting SIFS gap• for efficient collision detect & multi-frame transmissionfor efficient collision detect & multi-frame transmission

Clear to Send (CTS)Clear to Send (CTS)• station station ensures data frame gets through by issuing RTSensures data frame gets through by issuing RTS• and waits for CTS response from destinationand waits for CTS response from destination

Poll responsePoll response• see Point coordination Function (PCF) discussion nextsee Point coordination Function (PCF) discussion next

PIFS and DIFS UsePIFS and DIFS Use PIFS used by centralized controllerPIFS used by centralized controller

for issuing for issuing pollspolls hashas precedence over normal contention traffic precedence over normal contention traffic but not SIFSbut not SIFS

DIFS used for all ordinary asynchronous DIFS used for all ordinary asynchronous traffictraffic

IEEE 802.11 MAC TimingIEEE 802.11 MAC TimingBasic Access MethodBasic Access Method

Point Coordination Function Point Coordination Function (PCF)(PCF)

alternativealternative access method implemented on top of DCF access method implemented on top of DCF pollingpolling by centralized polling master (point coordinator) by centralized polling master (point coordinator) uses uses PIFS when issuing pollsPIFS when issuing polls pointpoint coordinator polls in round-robin to stations configured coordinator polls in round-robin to stations configured

for pollingfor polling when poll issued, polled station may respond using SIFSwhen poll issued, polled station may respond using SIFS if point coordinator receives response, it issues another poll if point coordinator receives response, it issues another poll

using PIFSusing PIFS if no response during expected turnaround time, coordinator if no response during expected turnaround time, coordinator

issues pollissues poll coordinator could lock out async traffic by issuing pollscoordinator could lock out async traffic by issuing polls have a superframe intervalhave a superframe interval defined defined

PCF Superframe TimingPCF Superframe Timing

IEEE 802.11 MAC Frame IEEE 802.11 MAC Frame FormatFormat

Control FramesControl Frames Power Save-Poll (PS-Poll) Power Save-Poll (PS-Poll)

requestrequest AP transmit buffered frame when in power-saving mode AP transmit buffered frame when in power-saving mode Request to Send (RTS)Request to Send (RTS)

firstfirst frame in four-way frame exchange frame in four-way frame exchange Clear to Send (CTS)Clear to Send (CTS)

second second frame in four-way exchangeframe in four-way exchange AcknowledgmentAcknowledgment (ACK) (ACK) Contention-Free (CF)-endContention-Free (CF)-end

announces end of contention-free period part of announces end of contention-free period part of PCFPCF CF-End + CF-Ack: CF-End + CF-Ack:

acknowledges CF-end to end contention-free period and release acknowledges CF-end to end contention-free period and release stations from stations from associatedassociated restrictions restrictions

Data Frames – Data CarryingData Frames – Data Carrying eight eight data frame subtypes, data frame subtypes, inin two groups two groups first first four carry upper-level data four carry upper-level data DataData

simplestsimplest data frame, contention data frame, contention oror contention-free use contention-free use Data + CF-Ack Data + CF-Ack

carriescarries data data and and acknowledges previously received data acknowledges previously received data during contention-free periodduring contention-free period

Data + CF-PollData + CF-Poll used by point coordinator to deliver data & req sendused by point coordinator to deliver data & req send

Data + CF-Ack + CF-PollData + CF-Ack + CF-Poll combines Data + CF-Ack and Data + CF-Pollcombines Data + CF-Ack and Data + CF-Poll

Data Frames – Data Frames – Not Data CarryingNot Data Carrying

otherother four data frames do not carry user data four data frames do not carry user data Null FunctionNull Function

carriescarries no data, polls, or acknowledgments no data, polls, or acknowledgments carriescarries power mgmt bit in frame control field to AP power mgmt bit in frame control field to AP indicatesindicates station is changing to low-power state station is changing to low-power state

otherother three frames (CF-Ack, CF-Poll, CF-Ack + three frames (CF-Ack, CF-Poll, CF-Ack + CF-Poll) same as corresponding frame in CF-Poll) same as corresponding frame in preceding list but without datapreceding list but without data

Management FramesManagement Frames used to manage used to manage communications between communications between

stations and Apsstations and Aps such assuch as management of associations management of associations

requestsrequests, response, reassociation, , response, reassociation, dissociation, and authenticationdissociation, and authentication

802.11 Physical Layer802.11 Physical Layer802.11 802.11a 802.11b 802.11g

Availablebandwidth 83.5 MHz 300 MHz 83.5 MHz 83.5 MHz

Unlicensedfrequency ofoperation

2.4 - 2.4835 GHzDSSS, FHSS

5.15 - 5.35 GHzOFDM

5.725 - 5.825GHz OFDM

2.4 - 2.4835 GHzDSSS

2.4 - 2.4835 GHzDSSS, OFDM

Number of non-overlappingchannels

3(indoor/outdoor)

4 indoor4

(indoor/outdoor)4 outdoor

3(indoor/outdoor)

3(indoor/outdoor)

Data rate perchannel 1, 2 Mbps

6, 9, 12, 18,24, 36, 48, 54

Mbps

1, 2, 5.5, 11Mbps

1, 2, 5.5, 6, 9,11, 12, 18, 24,36, 48, 54 Mbps

Compatibility 802.11 Wi-Fi5 Wi-Fi Wi-Fi at 11 Mbpsand below

Original 802.11 Physical Layer Original 802.11 Physical Layer - DSSS- DSSS

Direct-sequence spread spectrum (DSSS)Direct-sequence spread spectrum (DSSS) 2.4 GHz ISM band at 1 Mbps and 2 Mbps2.4 GHz ISM band at 1 Mbps and 2 Mbps up to seven channels, each 1 Mbps or 2 Mbps, can up to seven channels, each 1 Mbps or 2 Mbps, can

be usedbe used dependsdepends on bandwidth allocated by various national on bandwidth allocated by various national

regulationsregulations 13 in most European countries13 in most European countries oneone in Japan in Japan

each channel bandwidth 5 MHzeach channel bandwidth 5 MHz encodingencoding scheme DBPSK for 1-Mbps and DQPSK scheme DBPSK for 1-Mbps and DQPSK

for 2-Mbps using an 11-chip Barker seqfor 2-Mbps using an 11-chip Barker seq

Original 802.11 Physical Layer Original 802.11 Physical Layer - FHSS- FHSS

Frequency-hopping spread spectrum Frequency-hopping spread spectrum 2.4 GHz ISM band2.4 GHz ISM band at 1 Mbps and 2 Mbpsat 1 Mbps and 2 Mbps 2323 channels channels in Japan in Japan 70 70 channelschannels in in USA USA signalsignal hopping between multiple channels based on a hopping between multiple channels based on a

pseudonoise sequencepseudonoise sequence 1-MHz channels are used1-MHz channels are used

hoppinghopping scheme adjustable scheme adjustable twotwo-level Gaussian FSK modulation for 1 Mbps-level Gaussian FSK modulation for 1 Mbps four-level GFSK modulation used for 2 Mbpsfour-level GFSK modulation used for 2 Mbps

Original 802.11 Physical Layer Original 802.11 Physical Layer – – InfraredInfrared

omnidirectionalomnidirectional range uprange up to 20 m to 20 m 1 Mbps uses 1 Mbps uses 16-PPM (pulse position modulation)16-PPM (pulse position modulation)

4 data bit group mapped to one of 16-PPM symbols4 data bit group mapped to one of 16-PPM symbols eacheach symbol a string of 16 bits symbol a string of 16 bits each 16-bit string has fifteen 0s and one binary 1each 16-bit string has fifteen 0s and one binary 1

2-Mbps has each group of 2 data bits is mapped 2-Mbps has each group of 2 data bits is mapped into one of four 4-bit sequencesinto one of four 4-bit sequences each sequence consists of three 0s and one binary 1each sequence consists of three 0s and one binary 1

intensityintensity modulation is used for transmission modulation is used for transmission

802.11a802.11a uses 5-GHz band (different to other variants)uses 5-GHz band (different to other variants)

supports higher data rates, is less clutteredsupports higher data rates, is less cluttered orthogonal frequency division multiplexing (OFDM)orthogonal frequency division multiplexing (OFDM)

multiplemultiple carrier signals at different frequencies carrier signals at different frequencies somesome bits on each channel bits on each channel

upup to 48 subcarriers modulated using BPSK, to 48 subcarriers modulated using BPSK, QPSK, 16-QAM, or 64-QAMQPSK, 16-QAM, or 64-QAM subcarrier frequency spacing 0.3125 MHzsubcarrier frequency spacing 0.3125 MHz convolutionalconvolutional code at rate of 1/2, 2/3, or 3/4 provides code at rate of 1/2, 2/3, or 3/4 provides

forward error correctionforward error correction combination of modulation technique and coding rate combination of modulation technique and coding rate

determines data ratedetermines data rate

802.11a Physical Frame802.11a Physical Frame

802.11b802.11b extension extension of 802.11 DS-SS schemeof 802.11 DS-SS scheme

with data rates of 5.5 and 11 Mbpswith data rates of 5.5 and 11 Mbps chipping chipping rate 11 MHzrate 11 MHz

same assame as original DS-SS scheme original DS-SS scheme Complementary Complementary Code Keying (CCK) Code Keying (CCK)

modulation givesmodulation gives higher data rate with same higher data rate with same bandwidth & chipping ratebandwidth & chipping rate

also Packet Binary Convolutional Coding also Packet Binary Convolutional Coding (PBCC) for future higher rate use(PBCC) for future higher rate use

11-Mbps CCK Modulation 11-Mbps CCK Modulation SchemeScheme

802.11b Physical Frame802.11b Physical Frame

802.11g802.11g higherhigher-speed extension to 802.11b-speed extension to 802.11b operates in 2.4GHz bandoperates in 2.4GHz band compatible with 802.11b devicescompatible with 802.11b devices combinescombines physical layer encoding physical layer encoding

techniques used in 802.11 and 802.11b to techniques used in 802.11 and 802.11b to provide service at a variety of data ratesprovide service at a variety of data rates ERP-OFDM for 6, 9, 12, 18, 24, 36, 48, ERP-OFDM for 6, 9, 12, 18, 24, 36, 48,

54Mbps rates54Mbps rates ERP-PBCC for 22 & 33Mbps ratesERP-PBCC for 22 & 33Mbps rates

Data Rate vs Distance (m)Data Rate vs Distance (m)Data Rate (Mbps) 802.11b 802.11a 802.11g

1 90+ — 90+

2 75 — 75

5.5(b)/6(a/g) 60 60+ 65

9 — 50 55

11(b)/12(a/g) 50 45 50

18 — 40 50

24 — 30 45

36 — 25 35

48 — 15 25

54 — 10 20

Access and Privacy ServicesAccess and Privacy Services - Authentication- Authentication

authenticationauthentication used to establish used to establish stationstation identity identity wiredwired LAN LANss assume physical connection gives assume physical connection gives

authority to useauthority to use LANLAN notnot a valid assumption for wireless LAN a valid assumption for wireless LANss 802.11 supports several authentication schemes802.11 supports several authentication schemes does does not mandate any particular schemenot mandate any particular scheme from relatively insecure handshaking to public-key from relatively insecure handshaking to public-key

encryptionencryption 802.11 requires mutually acceptable, successful 802.11 requires mutually acceptable, successful

authentication before associationauthentication before association

Access and Privacy ServicesAccess and Privacy Services Deauthentication & PrivacyDeauthentication & Privacy

DeauthenticationDeauthentication invoked invoked whenever an existing authentication is to be whenever an existing authentication is to be

terminatedterminated PrivacyPrivacy

used to prevent messages being read by otherused to prevent messages being read by otherss 802.11 802.11 allows optional use of encryptionallows optional use of encryption

original WEP security features were weakoriginal WEP security features were weak subsequently 802.11i and WPA alternatives subsequently 802.11i and WPA alternatives

evolved giving better securityevolved giving better security

SummarySummary wireless LAN alternativeswireless LAN alternatives IEEE 802.11 architecture and servicesIEEE 802.11 architecture and services 802.11 Media Access Control802.11 Media Access Control 802.11 Physical Layers802.11 Physical Layers

802.11, 802.11a, 802.11b, 802.11g802.11, 802.11a, 802.11b, 802.11g Security considerationsSecurity considerations