网络chapter 6.docx

网络chapter 6.docx

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网络chapter6

Chapter6Multiplexing

Wheneverthetransmissioncapacityofamediumlinkingtwodevicesisgreaterthanthetransmissionneedsofthedevices,thelinkcanbeshared,muchasalargewaterpipecancarrywatertoseveralseparatehouses,atonce.Multiplexingisthesetoftechniquesthatallowsthesimultaneoustransmissionofmultiplesignalsacrossasingledatalink.

Asdataandtelecommunicationsusageincreases,sodoestraffic.Wecanaccommodatethisincreasebycontinuingtoaddindividuallineseachtimeanewchannelisneeded,orwecaninstallhighercapacitylinksanduseeachtocarrymultiplesignals.AsdescribedinChapter5,today'stechnologyincludeshigh-bandwidthmediasuchascoaxialcable,opticalfiber,andterrestrialandsatellitemicrowaves.Eachofthesehasacarryingcapacityfarinexcessofthatneededfortheaveragetransmissionsignal.Ifthetransmissioncapacityofalinkisgreaterthanthetransmissionneedsofthedevicesconnectedtoit,theexcesscapacityiswasted.Anefficientsystemmaximizestheutilizationofallfacilities.Inaddition,theexpensivetechnologyinvolvedoftenbecomescost-effectiveonlywhenlinksareshared.

Figure6.1showstwopossiblewaysoflinkingfourpairsofdevices.InFigure6.la,eachpairhasitsownlink.Ifthefullcapacityofeachlinkisnotbeingutilized,aportionofthatcapacityisbeingwasted.InFigure6.lb,transmissionsbetweenthepairsaremultiplexed;thesamefourpairssharethecapacityofaSinglelink.

6.1Manytoone/onetomany

Inamultiplexedsystem,ndevicessharethecapacityofonelink.Figure6.lbshowsthebasicformatofamultiplexedsystem.Thefourdevicesontheleftdirecttheirtransmissionstreamstoamultiplexer（MUX）whichcombinesthemintoasinglestream（manytoone）.Atthereceivingend,thatstreamisfedintoade-multiplexer（DEMUX）,whichseparatesthestreambackintoitscomponenttransmissions（onetomany）anddirectsthemtotheirintendedreceivingdevices.

Figure6.1Multiplexingversusnomultiplexing

InFigure6.lbthewordpathreferstothephysicallink.Thewordchannelreferstoaportionofapaththatcarriesatransmissionbetweena.givenpairofdevices.Onepathcanhavemany（n）channels.

8.2Typeofmultiplexing

Signalsaremultiplexedusingthreebasictechniques:

frequency-divisionmultiplexing（FDM）,time-divisionmultiplexing（TDM）andwavelength-divisionmultiplexing（WDM）.TDMisfurthersubdividedintosynchronousTDM（usuallyjustcalledTDM）andasynchronousTDMalsocalledstatisticalTDMorconcentrator.

Frequency-DivisionMultiplexing（FDM）

Frequency-divisionmultiplexing（FDM）isananalogtechniquethatcanbeappliedwhenthebandwidthofalinkisgreaterthanthecombinedbandwidthsofthesignalstobetransmitted.InFDM,signalsgeneratedbyeachsendingdevicemodulatedifferentcarrierfrequencies.Thesemodulatedsignalsarethencombinedintoasinglecompositesignalthatcanbetransportedbythelink.Carrierfrequenciesareseparatedbyenoughbandwidthtoaccommodatethemodulatedsignal.Thesebandwidthrangesarethechannelsthroughwhichthevarioussignalstravel.Channelsmustbeseparatedbystripsofunusedbandwidth（guardbands）topreventsignalsfromoverlapping.Inaddition,carrierfrequenciesmustnotinterferewiththeoriginaldatafrequencies.Failuretoadheretoeitherconditioncanresultinun-recoverabilityoftheoriginalsignals.

Figure6.2givesaconceptualviewofFDM.Inthisillustration,thetransmissionpathisdividedintothreeparts,eachrepresentingachanneltocarryonetransmission.Asananalogy,imagineapointwherethreenarrowstreetsmergetoformathree-lanehighway.Eachofthethreestreetscorrespondstoalaneofthehighway.Eachcarmergingontothehighwayfromoneofthestreetsstillhasitsownlaneandcantravelwithoutinterferingwithcarsinotherlanes.

Figure6.2FDM

KeepinmindthatalthoughFigure6.2showsthepathasdividedspatiallyintoseparatechannels,actualchanneldivisionsareachievedbyfrequencyratherthanbyspace.

TheFDMProcess

Figure6.3isaconceptualtime-domainillustrationofthemultiplexingprocess.FDMisananalogprocessandweshowithereusingtelephonesastheinputandoutputdevices.Eachtelephonegeneratesasignalofasimilarfrequencyrange.Insidethemultiplexer,thesesimilarsignalsaremodulatedontodifferentcarrierfrequencies（f1,f2,andf3）.Theresultingmodulatedsignalsarethencombinedintoasinglecompositesignalthatissentoutoveramedialinkthathasenoughbandwidthtoaccommodateit.

Figure6.3FDMmultiplexingprocess,time-domain

Figure6.4isthefrequency-domainillustrationforthesameconcept.（Notethatthehorizontalaxisofthisfiguredenotesfrequency,nottime.Allthreecarrierfrequenciesexistatthesametimewithinthebandwidt