本地异步通信技术（RS-232）

ComputerNetworksandInternets《计算机网络与因特网》课件林坤辉2004-02-6PARTIDataTransmissionChapter6Long-DistanceCommunication(Carriers,Modulation,andModems)远程通信(载波，调制和调制解调器)6.1IntroductionComputersusebinarydigits(bits位串)torepresentdata.Transmittingdataacrossanetworkfromonecomputertoanothermeanssendingbitsthroughtheunderlyingtransmissionmedium.Communicationsystemsuseelectriccurrent,radiowaves,orlighttotransferinformation.6.2SendingSignalsAcrossLongDistancesAnelectriccurrentcannotbepropagated(传播)anarbitrarydistanceovercopperwirebecausethecurrentbecomesweakerasittravels.Engineerstermtheproblemsignalloss(信号损耗).Suchlossoccursbecauseresistanceinthewirecausessmallamountsoftheelectricalenergytobeconvertedtoheat.Acontinuousoscillating(振荡)signalwillpropagatefartherthanothersignals.Insteadoftransmittinganelectriccurrentthatonlychangeswhenthevalueofabitchanges,longdistancecommunicationsystemssendacontinuouslyoscillatingsignal,usuallyasinewave(正弦波),calledacarrier(载波).Thecarrieroscillatescontinuously,evenwhennodataisbeingsent.Tosenddata,atransmittermodifiesthecarrierslightly.Suchmodificationarecalledmodulation(调制).Thetransmittergeneratesacontinuouslyoscillatingcarriersignal,whichitmodulatesaccordingtothedatabeingsent.Thereceiveronalong-distancecommunicationlinkmustbeconfiguredtorecognizethecarrierthatthesenderuses.Thereceivermonitorstheincomingcarrier,detectsmodulation,reconstructstheoriginaldata,anddiscardsthecarrier.Networktechnologiesuseavarietyofmodulationtechniques,including：amplitudemodulation(AM调幅),frequencymodulation(FM调频),phaseshiftmodulation(PM调相).6.3ModemHardwareUsedForModulationAndDemodulationAhardwarecircuitthatacceptsasequenceofdatabitsandappliesmodulationtoacarrierwaveaccordingtothebitsiscalledamodulator(调制器).Ahardwarecircuitthatacceptsamodulatedcarrierwaveandrecreatesthesequenceofdatabitsthatwasusedtomodulatethecarrieriscalledademodulator(解调器).Transmissionofdataacrossalongdistancerequiresamodulatoratoneendofthetransmissionlineandademodulatorattheother.Mostnetworksystemsarefullduplex,eachlocationneedsbothamodulatorandademodulator.manufacturescombinebothcircuitsintoasingledevicecalledamodem(调制解调器).6.4LeasedAnalogDataCircuits租用模拟数据线路Whenthecircuitconnectstwolocationatasinglesite,thecompanycaninstallthenecessarywiresitself.Privatecompaniescannotinstallcircuitsacrosslongdistance.Thenecessarywiringcanbeobtainedfromatelephonecompany.Telephonecompaniesallowcompaniestoleaseacircuitbetweenanytwolocations.Onceaconnectionhasbeenleasedfromthephonecompany,amodemmustbeinstalledateachendbeforecommunicationispossible.Afterthat,theleasedlineisavailabletosenddata.Advantage:datacanbesentatanytime.Disadvantage:thelimitedconnectivityandcost.6.5Optical,RadioFrequency,andDialupModemsInadditiontodedicatedwires,modemsarealsousedwithothermedia,includingRFtransmission,glassfibers,andconventionaltelephoneconnections.Adialupmodemcontainscircuitrythatmimicsatelephone----themodemcansimulateliftingthehandset,dialing,orhangingupthephone.6.6CarrierFrequencyAndMultiplexing载波频率和多路复用Eachtelevisionstationisassignedachannelnumberonwhichitbroadcastsasignal.Achannelnumberismerelyshorthandforthefrequencyatwhichthestation’scarrieroscillates.Thesignalforonechanneldoesnotinterferewiththesignalforanother.Computernetworksusetheprincipleofseparatechannelstopermitmultiplecommunicationtoshareasingle,physicalconnection.Eachsendertransmitsasignalusingaparticularcarrierfrequency.Multiplecarrierscanpassoverthesamewireatthesametimewithoutinterference.Frequencydivisionmultiplexing(FDM频分多路复用)：multiplecarrierfrequenciestoallowindependentsignaltotravelthroughamedium.FDMallowsmultiplepairsofsendersandreceiverstocommunicationoverasharedmediumsimultaneously.Thecarrierusedbyeachpairsoperatesatauniquefrequencythatdoesnotinterferewiththeothers.6.7BasebandandBroadbandtechnology基带和宽带技术TheprimarymotivationforusingFrequencydivisionmultiplexing(FDM)arisesfromthedesireforhighthroughput(吞吐量).Toachievehigherthroughput,theunderlyinghardwareusesalargerpartoftheelectromagneticspectrum(频谱)(i.e.,alargerbandwidth)Thetermbroadband(宽带)technologyisusedtocharacterizesuchtechnologies.Anytechnologythatusesasmallpartoftheelectromagneticspectrumandsendsonlyonesignalatatimeoverthemedium,isknownasabaseband(基带)technology.简单说，所谓基带信号就是将数字信号1或0直接用两种不同的电压来表示，然后送到线路上去传输。宽带信号是将基带信号进行调制后形成的频分复用模拟信号。基带信号进行调制后，其频谱被移到较高的频率处，每一路基带信号的频谱被移到不同的频段，因此合在一起不会互相干扰。因此，一条电缆中可以同时传送多路的数字信号，从而提高了线路的利用率。6.8WaveDivisionMultiplexing波分多路复用Technically,opticalFDMisknownasWaveDivisionMultiplexing(WDM).WDMoperatesbysendingmultiplelightwavesacrossasingleopticalfiber.Atthereceivingend,anopticalprismisusedtoseparatethefrequencies.EDAF:光纤放大器，直接对光信号进行放大。由于光载波的频率很高，因此习惯上用波长表示光载波。现在可以在一根光纤上复用80路或更多路数的光载波信号，即密集波分复用DWDM（densewavelengthdivisionmultiplexing）6.9SpreadSpectrum扩散频谱Atransmittersendsthesamesignalonasetofcarrierfrequencies.Areceiverisconfiguredtocheckallcarrierfrequenciesandtousewhicheverisworkingatpresent.Somemodemsselectasetofcarrierfrequenciesandusethemsimultaneously.ifinterferencedamagesoneormoreofthecarri