西电初试专业课讲义计算机网络17-wirelesslans

COMPUTERNETWORKSChapter17WirelessLANsOverviewAwirelessLANuseswirelesstransmissionmediumUsedtohavehighprices,lowdatarates,occupationalsafetyconcerns,andlicensingrequirementsSavesinstallationofLANcablingEasesrelocationandothermodificationstonetworkstructureSingleCellMultiCellWLANTopologiesPeer-to-peernetworkSetuptemporarilytomeetsomeimmediateneedE.g.groupofemployees,eachwithlaptoporpalmtop,inbusinessorclassroommeetingNetworkfordurationofmeetingInfrastructureWirelessLANAdHocLanWLANRequirementsSameasanyLANHighcapacity,shortdistances,fullconnectivity,broadcastcapabilityThroughput:efficientusewirelessmediumNumberofnodes:HundredsofnodesacrossmultiplecellsConnectiontobackboneLAN:UsecontrolmodulestoconnecttobothtypesofLANsServicearea:100to300mLowpowerconsumption:NeedlongbatterylifeonmobilestationsTransmissionrobustnessandsecurity:InterferenceproneandeasilyeavesdroppedCollocatednetworkoperation:TwoormorewirelessLANsinsameareaLicense-freeoperationHandoff/roaming:MovefromonecelltoanotherDynamicconfiguration:Addition,deletion,andrelocationofendsystemswithoutdisruptiontousersWLANTechnologyInfrared(IR)LANs:IndividualcellofIRLANlimitedtosingleroom.IRlightdoesnotpenetrateopaquewallsDirected-beamIROmnidirectionalDiffusedconfigurationSpreadspectrumLANs:MostlyoperateinISM(industrial,scientific,andmedical)bandsNoFederalCommunicationsCommission(FCC)licensingisrequiredinUSANarrowbandmicrowave:MicrowavefrequenciesbutnotusespreadspectrumSomerequireFCClicensingSpreadSpectrumLANConfigureHubConfigurationUsuallyusemultiple-cellarrangementAdjacentcellsusedifferentcenterfrequenciesHubistypicallymountedonceilingConnectedto

wiredLANConnecttostationsattachedtowiredLANandinothercellsMayalsocontrolaccessIEEE802.11pointcoordinationfunctionMayalsoactasmultiportrepeaterStationstransmittohubandreceivefromhubStationsmaybroadcastusinganomnidirectionalantennaLogicalbusconfigurationPeer-to-PeerConfigurationNohubMACalgorithmsuchasCSMAusedtocontrolaccessAdhocLANsSpreadSpectrumLANTransmissionLicensingregulationsdifferfromonecountrytoanotherUSAFCCauthorizedtwounlicensedapplicationswithintheISMband:Spreadspectrum-upto1wattVerylowpowersystems-upto0.5wattsUSAFCCauthorizedthreeunlicensedbands902-928MHz(915-MHzband)2.4-2.4835GHz(2.4-GHzband)5.725-5.825GHz(5.8-GHzband)NarrowbandMicrowaveLicensedNarrowbandRFUseofcellconfigurationAdjacentcellsusenon-overlappingfrequencybandsAlltransmissionsareencryptedLicensednarrowbandLANguaranteesinterference-freecommunicationUnlicensedNarrowbandRF1995,RadioLANintroducednarrowbandwirelessLANusingunlicensedISMspectrumUsedfornarrowbandtransmissionatlowpower0.5wattsorlessOperatesat10Mbps,5.8-GHzband50minsemiopenofficeand100minopenofficePeer-to-peerconfigurationElectsonenodeasdynamicmasterMastercanchangeautomaticallyasconditionschangeIEEE802.11ArchitectureMACprotocolandphysicalmediumspecificationforwirelessLANsSmallestbuildingblockisbasicserviceset(BSS)NumberofstationsSameMACprotocolCompetingforaccesstosamesharedwirelessmediumMaybeisolatedorconnecttobackbonedistributionsystem(DS)throughaccesspoint(AP)APfunctionsasbridgeMACprotocolmaybedistributedorcontrolledbycentralcoordinationfunctioninAPDScanbeswitch,wirednetwork,orwirelessnetworkBSS&ESSBasicServiceSet-BSSEachstationbelongstosingleBSSWithinrangeonlyofotherstationswithinBSSCanhavetwoBSSsoverlapStationcouldparticipateinmorethanoneBSSAssociationbetweenstationandBSSdynamicStationsmayturnoff,comewithinrange,andgooutofrangeExtendedServiceSet-ESSTwoormoreBSSinterconnectedbyDSAppearsassinglelogicalLANtoLLCAccessPoint(AP)LogicwithinstationthatprovidesaccesstoDSProvidesDSservicesinadditiontoactingasstationTointegrateIEEE802.11architecturewithwiredLAN,portalusedPortallogicimplementedindevice

thatispartofwiredLANandattachedtoDSE.g.BridgeorrouterIEEE802.11ServicesServiceProviderCategoryAssociationDistributionsystemMSDUdeliveryAuthenticationStationLANaccessandsecurityDeauthenticationStationLANaccessandsecurityDissassociationDistributionsystemMSDUdeliveryDistributionDistributionsystemMSDUdeliveryIntegrationDistributionsystemMSDUdeliveryMSDUdeliveryStationMSDUdeliveryPrivacyStationLANaccessandsecurityReassocationDistributionsystemMSDUdeliveryIEEE802.11

ProtocolMAClayercoversthreefunctionalareasReliabledatadeliveryAccesscontrolSecurityReliableDataDelivery802.11physicalandMAClayerssubjecttounreliabilityNoise,interference,andotherpropagationeffectsresultinlossofframesEvenwitherror-correctioncodes,framesmaynotsuccessfullybereceivedCanbedealtwithatahigherlayer,suchasTCPHowever,retransmissiontimersathigherlayerstypicallyorderofsecondsMoreefficienttodealwitherrorsattheMAClevel802.11includesframeexchangeprotocolStationreceivingframereturnsacknowledgment(ACK)frameExchangetreatedasatomicunitNotinterruptedby

anyotherstationIfnoACKwithinshortperiodoftime,retransmitAD-HOCFourFrameExchange无线传输的“范围”传输范围(TX_range)：可以成功接收帧的通信范围，取决于发送能量和无线电波传输特性物理层侦听范围(PCS_range)：可以检测到该传输的范围，取决于接收器灵敏度和无线电波传输特性。干扰范围(IF_range)：在此范围内的节点如果发送不相关的帧，将干扰接收端的接收并导致丢帧。AD-HOC模式下的“隐藏”终端问题C不能发送,却发送了,从而造成干扰AD-HOC模式下的“暴露”终端问题C能发送,却没有发送,从而造成资源浪费用RTS-CTS解决隐藏终端问题IEEE802.11MACDistributedwirelessfoundationMAC(DWFMAC)DistributedaccesscontrolmechanismOptionalcentralizedcontrolDistributedCoordinationFunction(DCF)ContentionalgorithmtoprovideaccesstoalltrafficAsynchronoustrafficPointcoordinationfunction(PCF)CentralizedMACalgorithmContentionfreeBuiltontopofDCFDCFCSMA/CA(CollisionAvoidance)Algorithm:StationwithframesensesmediumIfidle,waittoseeifremainsidleforoneIFS.Ifso,maytransmitimmediatelyIfbusy(eitherinitiallyoresbusyduringIFS)stationdeferstransmissionContinuetomonitoruntilcurrenttransmissionisoverOncecurrenttransmissionover,delayanotherIFSIfremainsidle,backoffrandomtimeandagainsenseIfmediumstillidle,stationmaytransmitDuringbackofftime,ifesbusy,backofftimerishaltedandresumeswhenmediumesidleToensurestability,binaryexponentialbackoffusedIFSUsethreevaluesforIFSSIFS(shortIFS):ShortestIFSForallimmediateresponseactions(ACK,CTS)PIFS(pointcoordinationfunctionIFS):MiddlelengthIFSUsedbythecentralizedcontrollerinPCFschemewhenissuingpollsDIFS(distributedcoordinationfunctionIFS):LongestIFSUsedasminimumdelayforasynchronousframescontendingforaccessPCFAlternativeaccessmethodimplementedontopofDCFPollingbycentralizedpollingmaster(pointcoordinator)UsesPIFSwhenissuingpollsPointcoordinatorpollsinround-robintostationsconfiguredforpollingWhenpollissued,polledstationmayrespondusingSIFSIfpointcoordinatorreceivesresponse,itissuesanotherpollusingPIFSIfnoresponseduringexpectedturnaroundtime,coordinatorissuespollSuperframePointcoordinatorwouldlockoutasynchronoustrafficbyissuingpollsSuperframeintervaldefinedAtbeginningofsuperframe,pointcoordinatormayseizecontrolandissuepollsforgivenperiodRestofsuperframeavailableforcontention-basedaccessAtendofsuperframeinterval,pointcoordinatorcontendsforaccessusingPIFSIfidle,pointcoordinatorgainsimmediateaccessFullsuperframeperiodfollowsIfbusy,pointcoordinatormustwaitforidletogainaccessResultsinforeshortenedsuperframeperiodfornextcycleIEEE802.11MACFrameFormatFrameControl:TypeofframeControl,management,ordataProvidescontrolinformationDuration/ConnectionID:Ifusedasdurationfield,indicatestime(ins)channelwillbeallocatedforsuccessfultransmissionofMACframeInsomecontrolframes,containsassociationorconnectionidentifierAddresses(AP,src,dst)SequenceControlFrameBodyFrameCheckSequenceIEEE802.11MACFramesControlFramesPowerSave-Poll(PS-Poll)RequesttoSend(RTS)CleartoSend(CTS)Acknowledgment(ACK)Contention-Free(CF)-endCF-End+CF-AckDataFramesCarryingdataDataData+CF-AckData+CF-PollData+CF-Ack+CF-PollCarryingnodataNullFunctionCF-Ack,CF-Poll,CF-Ack+CF-PollManagementFramesRequests,response,reassociation,dissociation,authenticationOriginalIEEE802.11PhysicalLayerDSSS2.4GHzISMbandat1Mbpsand2MbpsUpto7channels,each1Mbpsor2Mbps,canbeusedEachchannelbandwidth5MHzEncodingschemeDBPSKfor1-MbpsandDQPSKfor2-MbpsFHSS2.4GHzISMband

at1Mbpsand2Mbps1-MHzchannelsareusedTwo-levelGaussianFSKmodulationfor1-MbpsFor2Mbps,four-levelGFSKusedInfraredOmnidirectional850-950nmRangeupto20m1Mbpsused16-PPM(pulsepositionmodulation)2MbpsusedintensitymodulationIEEE802.11a物理层采用OFDM（正交频分复用）技术工作频段是5GHz的ISM数据速率为54Mbps采用了52个频率48个用于数据4个用于同步控制BPSK,QPSK,16QAM,64QAMIEEE802.11b物理层采用HR-DSSS(高速率直接序列扩频)技术工作频段是2.4GHz数据速率为5.5Mbps,11MbpsCCK(补码键控)I