数据与计算机通信第八章

WilliamStallings

DataandComputerCommunications

7thEditionChapter8Multiplexing主要内容8.1Frequency-DivisionMultiplexing8.2SynchronousTime-DivisionMultiplexing8.3StatisticalTime-DivisionMultiplexing8.4AsymmetricDigitalSubscriberLine8.5xDSL8.6RecommededReadingandWebSitesMultiplexing8.1FrequencyDivisionMultiplexingFDMUsefulbandwidthofmediumexceedsrequiredbandwidthofchannelEachsignalismodulatedtoadifferentcarrierfrequencyCarrierfrequenciesseparatedsosignalsdonotoverlap(guardbands)e.g.broadcastradioChannelallocatedevenifnodataFrequencyDivisionMultiplexing

DiagramFDMSystemFDMofThreeVoicebandSignalsAnalogCarrierSystemsAT&T(USA)Hierarchy(层次)ofFDMschemesGroup12voicechannels(4kHzeach)=48kHzRange60kHzto108kHzSupergroup60channelFDMof5groupsignalsoncarriersbetween420kHzand612kHzMastergroup10supergroupsWavelengthDivisionMultiplexingMultiplebeamsoflightatdifferentfrequencyCarriedbyopticalfiberAformofFDMEachcoloroflight(wavelength)carriesseparatedatachannel1997BellLabs100beamsEachat10GbpsGiving1terabitpersecond(Tbps)Commercialsystemsof160channelsof10GbpsnowavailableLabsystems(Alcatel)256channelsat39.8Gbpseach10.1TbpsOver100kmWDMOperationSamegeneralarchitectureasotherFDMNumberofsourcesgeneratinglaserbeamsatdifferentfrequenciesMultiplexerconsolidatessourcesfortransmissionoversinglefiberOpticalamplifiersamplifyallwavelengthsTypicallytensofkmapartDemuxseparateschannelsatthedestinationMostly1550nmwavelengthrangeWas200MHzperchannelNow50GHzDenseWavelengthDivisionMultiplexingDWDMNoofficialorstandarddefinitionImpliesmorechannelsmorecloselyspacedthatWDM200GHzorless8.2SynchronousTimeDivisionMultiplexingDatarateofmediumexceeds(超过)datarateofdigitalsignaltobetransmittedMultipledigitalsignalsinterleaved(交叉)intimeMaybeatbitlevelofblocksTimeslotspreassignedtosourcesandfixedTimeslotsallocatedevenifnodataTimeslotsdonothavetobeevenlydistributedamongstsourcesTimeDivisionMultiplexingTDMSystemTDMLinkControlNoheadersandtrailersDatalinkcontrolprotocolsnotneededFlowcontrolDatarateofmultiplexedlineisfixedIfonechannelreceivercannotreceivedata,theothersmustcarryonThecorrespondingsourcemustbequenched(约束)ThisleavesemptyslotsErrorcontrolErrorsaredetectedandhandledbyindividualchannelsystemsDataLinkControlonTDMFramingNoflagorSYNCcharactersbracketingTDMframesMustprovidesynchronizingmechanismAddeddigitframingOnecontrolbitaddedtoeachTDMframeLookslikeanotherchannel-“controlchannel”Identifiablebitpatternusedoncontrolchannele.g.alternating01010101…unlikelyonadatachannelCancompareincomingbitpatternsoneachchannelwithsyncpatternPulseStuffing(脉冲填充)Problem-SynchronizingdatasourcesClocksindifferentsourcesdriftingDataratesfromdifferentsourcesnotrelatedbysimplerationalnumberSolution-PulseStuffingOutgoing(输出)datarate(excludingframingbits)higherthansumofincomingratesStuffextradummy(额外的)bitsorpulsesintoeachincomingsignaluntilitmatcheslocalclockStuffedpulsesinsertedatfixedlocationsinframeandremovedatdemultiplexerTDMofAnalogandDigitalSourcesDigitalCarrierSystemsHierarchyofTDMUSA/Canada/JapanuseonesystemITU-Tuseasimilar(butdifferent)systemUSsystembasedonDS-1formatMultiplexes24channelsEachframehas8bitsperchannelplusoneframingbit193bitsperframeT1载波DigitalCarrierSystems(2)T1载波也叫一次群,它把24路话音信道按时分多路的原理复合在一条1.544Mb/s的高速信道上。

CCITT有一个类似1.544Mb/s的标准。

CCITT还有一个2.048Mb/s脉冲编码调制载波标准。

CCITTT1载波还可以多路复用到更高级的载波上，如图所示。图8B.25贝尔系统的T1载波图8B.26多路复用DigitalCarrierSystems(2)DigitalCarrierSystems(2)Forvoiceeachchannelcontainsonewordofdigitizeddata(PCM,8000samplespersec)Datarate8000x193=1.544MbpsFiveoutofsixframeshave8bitPCMsamplesSixthframeis7bitPCMwordplussignalingbitSignalingbitsformstreamforeachchannelcontainingcontrolandroutinginfoSameformatfordigitaldata23channelsofdata7bitsperframeplusindicatorbitfordataorsystemscontrol24thchannelissyncMixedDataDS-1cancarrymixedvoiceanddatasignals24channelsusedNosyncbyteCanalsointerleaveDS-1channelsDs-2isfourDS-1giving6.312MbpsE1载波:23个8位一组的数据样本组装成125us的基本帧,其中30个子信道用于传送数据,2个子信道用于传送控制信令,每4帧能提供64个控制位,速率为2.048Mb/s.ISDNUserNetworkInterfaceISDNallowsmultiplexingofdevicesoversingleISDNlineTwointerfaces(2B+D)BasicISDNInterfacePrimaryISDNInterfaceBasicISDNInterface(1)Digitaldataexchangedbetweensubscriber(用户)andNTE(网络终端设备)-FullDuplexSeparatephysicallineforeachdirectionPseudoternary(伪三编码)codingscheme1=novoltage,0=positiveornegative750mV+/-10%Datarate192kbpsBasicaccessistwo64kbpsBchannelsandone16kbpsDchannelThisgives144kbpsmultiplexedover192kbpsRemainingcapacityusedforframingandsyncBasicISDNInterface(2)BchannelisbasicuserchannelDataPCMvoiceSeparatelogical64kbpsconnectionsotherdifferentdestinationsDchannelusedforcontrolordataLAPDframesEachframe48bitslongOneframeevery250

sPrimaryISDNPointtopointTypicallysupportingPBX1.544MbpsBasedonUSDS-1UsedonT1services23BplusoneDchannel2.048MbpsBasedonEuropeanstandards30BplusoneDchannelLinecodingisAMIusingHDB3DS-1TransmissionFormatSONET(同步光纤网络)/SDH(同步数字体系)SynchronousOpticalNetwork(ANSI)SynchronousDigitalHierarchy(ITU-T)CompatibleSignalHierarchy(信号体系)SynchronousTransportSignallevel1(STS-1)orOpticalCarrierlevel1(OC-1)51.84MbpsCarryDS-3orgroupoflowerratesignals(DS1DS1CDS2)plusITU-Trates(e.g.2.048Mbps)MultipleSTS-1combinedintoSTS-NsignalITU-Tlowestrateis155.52Mbps(STM-1)SONETFrameFormatSONETSTS-1OverheadOctets8.3StatisticalTDMInSynchronousTDMmanyslotsarewastedStatisticalTDMallocatestimeslotsdynamicallybasedondemandMultiplexerscansinputlinesandcollectsdatauntilframefullDatarateonlinelowerthanaggregateratesofinputlinesStatisticalTDMFrameFormatsPerformanceOutputdataratelessthanaggregateinputratesMaycauseproblemsduringpeakperiodsBufferinputsKeepbuffersizetominimumtoreducedelayBufferSize

andDelayCableModemOutlineTwochannelsfromcableTVproviderdedicatedtodatatransferOneineachdirectionEachchannelsharedbynumberofsubscribersSchemeneededtoallocatecapacityStatisticalTDMCableModemOperationDownstreamCableschedulerdeliversdatainsmallpacketsIfmorethanonesubscriberactive,eachgetsfractionofdownstreamcapacityMayget500kbpsto1.5MbpsAlsousedtoallocateupstreamtimeslotstosubscribersUpstreamUserrequeststimeslotsonsharedupstreamchannelDedicatedslotsforthisHeadendschedulersendsbackassignmentoffuturetmeslotstosubscriberCableModemScheme8.4AsymmetricalDigitalSubscriberLineADSLLinkbetweensubscriberandnetworkLocalloopUsescurrentlyinstalledtwistedpaircableCancarrybroaderspect