Data Communications,Data Networks,computer communications,multiplexing,spread spectrum,protocol architecture,data link protocols,signal encoding techniques,transmission media,asynchronous transfer mode,routing,cellular networks,lan,wan,man,high speed lans

1. Data and ComputerData and Computer
CommunicationsCommunications
Eighth EditionEighth Edition
by William Stallingsby William Stallings
Lecture slides by Lawrie BrownLecture slides by Lawrie Brown
Chapter 17 –Chapter 17 – WirelessWireless LANLANss

2. High SpeedHigh Speed LANLANss
Investigators 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 conversation
Researchers 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

3. 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, & licensingoccupational 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

4. Single CellSingle Cell LAN ExtensionLAN Extension

5. Multi CellMulti Cell LAN ExtensionLAN Extension

6. Cross-Building InterconnectCross-Building Interconnect
 connectconnect LANs in nearby buildingsLANs in nearby buildings
 pointpoint-to-point wireless link-to-point wireless link

NotNot a LAN per sea LAN per se
 connectconnect bridges or routersbridges or routers

7. Nomadic AccessNomadic Access
 linklink LAN hub & mobile data terminalLAN hub & mobile data terminal

laptoplaptop or notepad computeror notepad computer

enableenable employee to transfer data fromemployee to transfer data from
portable computer to serverportable computer to server
 alsoalso useful in extended environment suchuseful in extended environment such
as campus or cluster of buildingsas campus or cluster of buildings

usersusers move around with portable computersmove around with portable computers

maymay wish access to servers on wired LANwish access to servers on wired LAN

8. Infrastructure Wireless LANInfrastructure Wireless LAN

9. Ad Hoc NetworkingAd Hoc Networking
 temporarytemporary peerpeer-to-peer network-to-peer network

10. Wireless LAN RequirementsWireless LAN Requirements
 throughput - efficient use wireless mediumthroughput - efficient use wireless medium
 no of nodes -no of nodes - hundredshundreds of nodes across multiple cellsof nodes across multiple cells
 connection to backbone LAN -connection to backbone LAN - usingusing control modulescontrol modules
 service area - 100 to 300 mservice area - 100 to 300 m
 lowlow power consumption - for long battery lifepower 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

11. TechnologyTechnology
 infrared (IR) LANsinfrared (IR) LANs

individualindividual cell of IR LAN limited to single roomcell of IR LAN limited to single room

IRIR light does not penetrate opaque wallslight does not penetrate opaque walls
 spread spectrum LANsspread spectrum LANs

mostlymostly operate in ISM (industrial, scientific, andoperate in ISM (industrial, scientific, and
medical) bandsmedical) bands

nono Federal Communications Commission (FCC)Federal Communications Commission (FCC)
licensing is required inlicensing is required in USAUSA
 narrowband microwavenarrowband microwave

microwavemicrowave frequencies but not use spread spectrumfrequencies but not use spread spectrum

some require FCC licensingsome require FCC licensing

12. Infrared LANsInfrared LANs
 constructed using infrared portion of spectrumconstructed using infrared portion of spectrum
 strengthsstrengths

spectrumspectrum virtually unlimited hence high rates possiblevirtually unlimited hence high rates possible

unregulated spectrumunregulated spectrum

infraredinfrared shares some properties of visible lightshares some properties of visible light
• reflection covers room, walls isolate networksreflection covers room, walls isolate networks

inexpensiveinexpensive and simpleand simple
 weaknessesweaknesses

backgroundbackground radiation, e.g. sradiation, e.g. sunlight,unlight, indoor lightingindoor lighting

powerpower limited by concerns for eye safety and powerlimited by concerns for eye safety and power
consumptionconsumption

13. Infrared LANsInfrared LANs
Transmission TechniquesTransmission Techniques
 directed-beam IRdirected-beam IR

pointpoint-to-point links-to-point links

rangerange depends on power and focusingdepends on power and focusing

for indoorfor indoor use can set up token ring LANuse 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 stationsbase station with line of sight to other stations

actsacts as a multiport repeateras a multiport repeater

otherother stationsstations useuse directional beam to itdirectional beam to it
 diffuseddiffused configurationconfiguration

stationsstations focused / aimedfocused / aimed atat diffusely reflecting ceilingdiffusely reflecting ceiling

14. Spread Spectrum LANSpread Spectrum LAN
ConfigurationConfiguration
 usuallyusually use multiple-cell arrangementuse multiple-cell arrangement
 adjacent cells use different center frequenciesadjacent cells use different center frequencies
 configurations:configurations:

hubhub
• connectedconnected toto wired LANwired LAN
• connectconnect to stations on wired LAN and in other cellsto stations on wired LAN and in other cells
• may domay do automatic handoffautomatic handoff

peer-to-peerpeer-to-peer
• nono hubhub
• MAC algorithm such as CSMA used to control accessMAC algorithm such as CSMA used to control access
• for adfor ad hoc LANshoc LANs

15. Spread Spectrum LANsSpread Spectrum LANs
Transmission IssuesTransmission Issues
 licensinglicensing regulations differ between countriesregulations differ between countries
 UUSASA FCC allows in ISM band:FCC allows in ISM band:

spreadspread spectrumspectrum ((1W),1W), veryvery low powerlow 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 ovendevices at 2.4 GHz; microwave oven

littlelittle competition at 5.8 GHzcompetition at 5.8 GHz

16. IEEE 802 StandardsIEEE 802 Standards
Standard Scope
Medium access control (MAC): One common MAC for WLAN
applications
Physical layer: Infrared at 1 and 2 Mbps
Physical layer: 2.4-GHz FHSS at 1 and 2 Mbps
IEEE 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 new
regulatory domains (countries)
IEEE 802.11e
MAC: Enhance to improve quality of service and enhance
security mechanisms
IEEE 802.11f
Recommended practices for multivendor access point
interoperability
IEEE 802.11g Physical layer: Extend 802.11b to data rates >20 Mbps
IEEE 802.11h
Physical/MAC: Enhance IEEE 802.11a to add indoor and
outdoor channel selection and to improve spectrum and
transmit power management
IEEE 802.11i MAC: Enhance security and authentication mechanisms
IEEE 802.11j
Physical: Enhance IEEE 802.11a to conform to Japanese
requirements
IEEE 802.11k
Radio resource measurement enhancements to provide
interface to higher layers for radio and network
measurements
IEEE 802.11m
Maintenance of IEEE 802.11-1999 standard with technical
and 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
IEEE
802.11,2
Recommended practice for the Evaluation of 802.11 wireless
performance
IEEE 802.11u Physical/MAC: Interworking with external networks

17. IEEE 802 TerminologyIEEE 802 Terminology
Access point (AP) Any entity that has station functionality and provides
access to the distribution system via the wireless
medium for associated stations
Basic service set
(BSS)
A set of stations controlled by a single coordination
function
Coordination function The logical function that determines when a station
operating within a BSS is permitted to transmit and
may be able to receive PDUs
Distribution system
(DS)
A system used to interconnect a set of BSSs and
integrated LANs to create an ESS
Extended service set
(ESS)
A set of one or more interconnected BSSs and
integrated LANs that appear as a single BSS to the LLC
layer at any station associated with one of these BSSs
MAC protocol data
unit (MPDU)
The unit of data exchanged between two peer MAC
entites using the services of the physical layer
MAC service data unit
(MSDU)
Information that is delivered as a unit between MAC
users
Station Any device that contains an IEEE 802.11 conformant MAC
and physical layer

18. IEEE 802.11 ArchitectureIEEE 802.11 Architecture

19. IEEE 802.11 - BSSIEEE 802.11 - BSS
 basic service set (BSS) building blockbasic service set (BSS) building block
 maymay be isolatedbe isolated
 may connect to backbone distributionmay connect to backbone distribution
system (DS) through access point (AP)system (DS) through access point (AP)
 BSS generally corresponds to cellBSS generally corresponds to cell
 DS can be switch, wired network, orDS can be switch, wired network, or
wireless networkwireless network
 have independent BSS (IBSS) with no APhave independent BSS (IBSS) with no AP

20. Extended Service Set (ESS)Extended Service Set (ESS)
 possible configurations:possible configurations:

simplest issimplest is each station belongs to single BSSeach station belongs to single BSS

can have twocan have two BSSs overlapBSSs overlap

a stationa station can participate in more than one BSScan participate in more than one BSS

associationassociation between station and BSS dynamicbetween station and BSS dynamic
 ESS is twoESS is two or moreor more BSSBSS interconnected byinterconnected by DSDS
 appearsappears as single logical LAN to LLCas single logical LAN to LLC

21. IEEE 802 ServicesIEEE 802 Services
Service Provider Used to support
Association Distribution
system
MSDU delivery
Authentication Station LAN access and
security
Deauthentication Station LAN access and
security
Dissassociation Distribution
system
MSDU delivery
Distribution Distribution
system
MSDU delivery
Integration Distribution
system
MSDU delivery
MSDU delivery Station MSDU delivery
Privacy Station LAN access and
security
Reassocation Distribution
system
MSDU delivery

22. Services - MessageServices - Message
DistributionDistribution
 distribution servicedistribution service

primary service used by stations to exchangeprimary 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 serviceif stations in same BSS, distribution service
logically goes through single AP of that BSSlogically goes through single AP of that BSS
 integrationintegration serviceservice

enables transfer of data between 802.11 LANenables transfer of data between 802.11 LAN
station andstation and oneone on an integrated 802.x LANon an integrated 802.x LAN

23. Association Related ServicesAssociation Related Services
 DSDS requires info about stations within ESSrequires info about stations within ESS
 providedprovided by association-related servicesby association-related services
 stationstation must associatemust associate beforebefore
communicatingcommunicating
 33 mobility transition types:mobility transition types:

no transition -no transition - stationarystationary or in single BSSor in single BSS

BSS transition -BSS transition - betweenbetween BSS in same ESSBSS in same ESS

ESS transition:ESS transition: betweenbetween BSS in different ESSBSS in different ESS

24. Association Related ServicesAssociation Related Services
 DS needs identity of destination statioDS needs identity of destination statio

stationsstations must maintain association with APmust maintain association with AP
within current BSSwithin current BSS
 3 services relate to this requirement:3 services relate to this requirement:

Association - establishes initial associationAssociation - establishes initial association
between station and APbetween station and AP

Reassociation - to tReassociation - to transfer anransfer an association toassociation to
anotheranother APAP

Disassociation - byDisassociation - by station or APstation or AP

25. Medium Access ControlMedium Access Control
 MAC layer covers three functional areasMAC layer covers three functional areas

reliablereliable data deliverydata delivery

accessaccess controlcontrol

securitysecurity

26. Reliable Data DeliveryReliable Data Delivery
 802.11 physical / MAC layers unreliable802.11 physical / MAC layers unreliable

noise, interference, and other propagation effectsnoise, interference, and other propagation effects
result in loss of framesresult in loss of frames

even with error-correction codes, frames may noteven with error-correction codes, frames may not
successfully be receivedsuccessfully be received
 cancan be dealt with at a higher layer, e.g. TCPbe dealt with at a higher layer, e.g. TCP
 moremore efficient to deal with errors at MAC levelefficient to deal with errors at MAC level
 802.11 includes frame exchange protocol802.11 includes frame exchange protocol

station receivingstation receiving frame returns acknowledgmentframe returns acknowledgment
(ACK) frame(ACK) frame

exchangeexchange treated as atomic unittreated as atomic unit

ifif nono ACK within short period of timeACK within short period of time,, retransmitretransmit

27. Four Frame ExchangeFour Frame Exchange
 can use four-frame exchange for better reliabilitycan use four-frame exchange for better reliability

sourcesource issues a Request to Send (RTS) frame to destissues 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 dataafter receiving CTS, source transmits data

destinationdestination responds with ACKresponds with ACK
 RTS alerts all stations within rangeRTS alerts all stations within range of sourceof source thatthat
exchange is under wayexchange is under way
 CTS alerts all stations within range of destinationCTS alerts all stations within range of destination
 other stationsother stations don’t transmit to avoid collisiondon’t transmit to avoid collision
 RTS/CTS exchange is required function of MACRTS/CTS exchange is required function of MAC
but may be disabledbut may be disabled

28. Media Access ControlMedia Access Control

29. Distributed CoordinationDistributed Coordination
FunctionFunction
 DCF sublayerDCF sublayer usesuses CSMACSMA

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

elseelse waits until current transmission completewaits until current transmission complete
 nono collision detection since on wirelesscollision detection since on wireless
networknetwork
 DCF includes delays that act as a priorityDCF includes delays that act as a priority
schemescheme

30. IEEEIEEE
802.11802.11
MediumMedium
AccessAccess
ControlControl
LogicLogic

31. Priority IFS ValuesPriority IFS Values
 SIFS (short IFS)SIFS (short IFS)

forfor all immediate response actionsall immediate response actions (see later)(see later)
 PIFS (point coordination function IFS)PIFS (point coordination function IFS)

usedused by the centralized controller in PCFby the centralized controller in PCF
scheme when issuing pollsscheme when issuing polls
 DIFS (distributed coordination function IFS)DIFS (distributed coordination function IFS)

usedused as minimum delay for asynchronousas minimum delay for asynchronous
frames contending for accessframes contending for access

32. SIFS UseSIFS Use
 SIFS givesSIFS gives highest priorityhighest priority

overover stations waiting PIFS or DIFS timestations waiting PIFS or DIFS time
 SIFS used in following circumstances:SIFS used in following circumstances:

Acknowledgment (ACK)Acknowledgment (ACK)
• stationstation 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)
• stationstation 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

33. PIFS and DIFS UsePIFS and DIFS Use
 PIFS used by centralized controllerPIFS used by centralized controller

for issuingfor issuing pollspolls

hashas precedence over normal contention trafficprecedence over normal contention traffic

but not SIFSbut not SIFS
 DIFS used for all ordinary asynchronousDIFS used for all ordinary asynchronous
traffictraffic

34. IEEE 802.11 MAC TimingIEEE 802.11 MAC Timing
Basic Access MethodBasic Access Method

35. Point Coordination FunctionPoint Coordination Function
(PCF)(PCF)
 alternativealternative access method implemented on top of DCFaccess method implemented on top of DCF
 pollingpolling by centralized polling master (point coordinator)by centralized polling master (point coordinator)
 usesuses PIFS when issuing pollsPIFS when issuing polls
 pointpoint coordinator polls in round-robin to stationscoordinator polls in round-robin to stations
configured for pollingconfigured for 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 anotherif point coordinator receives response, it issues another
poll using PIFSpoll using PIFS
 if no response during expected turnaround time,if no response during expected turnaround time,
coordinator issues pollcoordinator issues 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 defineddefined

36. PCF Superframe TimingPCF Superframe Timing

37. IEEE 802.11 MAC FrameIEEE 802.11 MAC Frame
FormatFormat

38. Control FramesControl Frames
 Power Save-Poll (PS-Poll)Power Save-Poll (PS-Poll)

requestrequest AP transmit buffered frame when in power-saving modeAP transmit buffered frame when in power-saving mode
 Request to Send (RTS)Request to Send (RTS)

firstfirst frame in four-way frame exchangeframe in four-way frame exchange
 Clear to Send (CTS)Clear to Send (CTS)

secondsecond 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 ofannounces 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 releaseacknowledges CF-end to end contention-free period and release
stations fromstations from associatedassociated restrictionsrestrictions

39. Data Frames – Data CarryingData Frames – Data Carrying
 eighteight data frame subtypes,data frame subtypes, inin two groupstwo groups
 firstfirst four carry upper-level datafour carry upper-level data
 DataData

simplestsimplest data frame, contentiondata frame, contention oror contention-free usecontention-free use
 Data + CF-AckData + CF-Ack

carriescarries datadata andand acknowledges previously receivedacknowledges previously received
data during contention-free perioddata during 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

40. Data Frames –Data Frames –
Not Data CarryingNot Data Carrying
 otherother four data frames do not carry user datafour data frames do not carry user data
 Null FunctionNull Function

carriescarries no data, polls, or acknowledgmentsno data, polls, or acknowledgments

carriescarries power mgmt bit in frame control field to APpower mgmt bit in frame control field to AP

indicatesindicates station is changing to low-power statestation 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 inCF-Poll) same as corresponding frame in
preceding list but without datapreceding list but without data

41. Management FramesManagement Frames
 used to manageused to manage communications betweencommunications between
stations and Apsstations and Aps
 such assuch as management of associationsmanagement of associations

requestsrequests, response, reassociation,, response, reassociation,
dissociation, and authenticationdissociation, and authentication

42. 802.11 Physical Layer802.11 Physical Layer
802.11 802.11a 802.11b 802.11g
Available
bandwidth
83.5 MHz 300 MHz 83.5 MHz 83.5 MHz
Unlicensed
frequency of
operation
2.4 - 2.4835 GHz
DSSS, FHSS
5.15 - 5.35 GHz
OFDM
5.725 - 5.825
GHz OFDM
2.4 - 2.4835 GHz
DSSS
2.4 - 2.4835 GHz
DSSS, OFDM
Number of non-
overlapping
channels
3
(indoor/outdoor)
4 indoor
4
(indoor/outdoor)
4 outdoor
3
(indoor/outdoor)
3
(indoor/outdoor)
Data rate per
channel
1, 2 Mbps
6, 9, 12, 18,
24, 36, 48, 54
Mbps
1, 2, 5.5, 11
Mbps
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 Mbps
and below

43. Original 802.11 Physical LayerOriginal 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,up to seven channels, each 1 Mbps or 2 Mbps,
can be usedcan be used
 dependsdepends on bandwidth allocated by variouson bandwidth allocated by various
nationalnational regulationsregulations

13 in most European countries13 in most European countries

oneone in Japanin Japan
 each channel bandwidth 5 MHzeach channel bandwidth 5 MHz
 encodingencoding scheme DBPSK for 1-Mbps andscheme DBPSK for 1-Mbps and
DQPSK for 2-Mbps using an 11-chip Barker seqDQPSK for 2-Mbps using an 11-chip Barker seq

44. Original 802.11 Physical LayerOriginal 802.11 Physical Layer
- FHSS- FHSS
 Frequency-hopping spread spectrumFrequency-hopping spread spectrum

2.4 GHz ISM band2.4 GHz ISM band at 1 Mbps and 2 Mbpsat 1 Mbps and 2 Mbps

2323 channelschannels in Japanin Japan

7070 channelschannels inin USAUSA

signalsignal hopping between multiple channels based on ahopping between multiple channels based on a
pseudonoise sequencepseudonoise sequence

1-MHz channels are used1-MHz channels are used
 hoppinghopping scheme adjustablescheme 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

45. Original 802.11 Physical LayerOriginal 802.11 Physical Layer
–– InfraredInfrared
 omnidirectionalomnidirectional
 range uprange up to 20 mto 20 m
 1 Mbps uses1 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 bitssymbol 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 mapped2-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 transmissionmodulation is used for transmission

46. 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 frequenciescarrier signals at different frequencies

somesome bits on each channelbits 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 providescode at rate of 1/2, 2/3, or 3/4 provides
forward error correctionforward error correction

combination of modulation technique and coding ratecombination of modulation technique and coding rate
determines data ratedetermines data rate

47. 802.11a Physical Frame802.11a Physical Frame

48. 802.11b802.11b
 extensionextension 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
 chippingchipping rate 11 MHzrate 11 MHz

same assame as original DS-SS schemeoriginal DS-SS scheme

ComplementaryComplementary Code Keying (CCK)Code Keying (CCK)
modulation givesmodulation gives higher data rate with samehigher data rate with same
bandwidth & chipping ratebandwidth & chipping rate

also Packet Binary Convolutional Codingalso Packet Binary Convolutional Coding
(PBCC) for future higher rate use(PBCC) for future higher rate use

49. 11-Mbps CCK Modulation11-Mbps CCK Modulation
SchemeScheme

50. 802.11b Physical Frame802.11b Physical Frame

51. 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 encodingphysical layer encoding
techniques used in 802.11 and 802.11b totechniques 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

52. 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

53. Access and Privacy ServicesAccess and Privacy Services
- Authentication- Authentication
 authenticationauthentication used to establishused to establish stationstation identityidentity
 wiredwired LANLANss assume physical connection givesassume physical connection gives
authority to useauthority to use LANLAN
 notnot a valid assumption for wireless LANa valid assumption for wireless LANss
 802.11 supports several authentication schemes802.11 supports several authentication schemes
 doesdoes not mandate any particular schemenot mandate any particular scheme
 from relatively insecure handshaking to public-keyfrom relatively insecure handshaking to public-key
encryptionencryption
 802.11 requires mutually acceptable, successful802.11 requires mutually acceptable, successful
authentication before associationauthentication before association

54. Access and Privacy ServicesAccess and Privacy Services
Deauthentication & PrivacyDeauthentication & Privacy
 DeauthenticationDeauthentication

invokedinvoked whenever an existing authentication is to bewhenever an existing authentication is to be
terminatedterminated
 PrivacyPrivacy

used to prevent messages being read by otherused to prevent messages being read by otherss

802.11802.11 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 alternativessubsequently 802.11i and WPA alternatives
evolved giving better securityevolved giving better security

55. 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