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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