本地異步通信技術(shù)（RS-232）

ComputerNetworksandInternets《計(jì)算機(jī)網(wǎng)絡(luò)與因特網(wǎng)》課件2004-02-61PARTIDataTransmissionChapter6Long-DistanceCommunication(Carriers,Modulation,andModems)遠(yuǎn)程通信(載波，調(diào)制和調(diào)制解調(diào)器)26.1IntroductionComputersusebinarydigits(bits位串)torepresentdata.Transmittingdataacrossanetworkfromonecomputertoanothermeanssendingbitsthroughtheunderlyingtransmissionmedium.Communicationsystemsuseelectriccurrent,radiowaves,orlighttotransferinformation.36.2SendingSignalsAcrossLongDistancesAnelectriccurrentcannotbepropagated(傳播)anarbitrarydistanceovercopperwirebecausethecurrentbecomesweakerasittravels.Engineerstermtheproblemsignalloss(信號損耗).Suchlossoccursbecauseresistanceinthewirecausessmallamountsoftheelectricalenergytobeconvertedtoheat.4Acontinuousoscillating(振蕩)signalwillpropagatefartherthanothersignals.Insteadoftransmittinganelectriccurrentthatonlychangeswhenthevalueofabitchanges,longdistancecommunicationsystemssendacontinuouslyoscillatingsignal,usuallyasinewave(正弦波),calledacarrier(載波).5Thecarrieroscillatescontinuously,evenwhennodataisbeingsent.Tosenddata,atransmittermodifiesthecarrierslightly.Suchmodificationarecalledmodulation(調(diào)制).Thetransmittergeneratesacontinuouslyoscillatingcarriersignal,whichitmodulatesaccordingtothedatabeingsent.6Thereceiveronalong-distancecommunicationlinkmustbeconfiguredtorecognizethecarrierthatthesenderuses.Thereceivermonitorstheincomingcarrier,detectsmodulation,reconstructstheoriginaldata,anddiscardsthecarrier.7Networktechnologiesuseavarietyofmodulationtechniques,including：

amplitudemodulation(AM調(diào)幅),frequencymodulation(FM調(diào)頻),phaseshiftmodulation(PM調(diào)相).89106.3ModemHardwareUsedForModulationAndDemodulationAhardwarecircuitthatacceptsasequenceofdatabitsandappliesmodulationtoacarrierwaveaccordingtothebitsiscalledamodulator(調(diào)制器).Ahardwarecircuitthatacceptsamodulatedcarrierwaveandrecreatesthesequenceofdatabitsthatwasusedtomodulatethecarrieriscalledademodulator(解調(diào)器).11Transmissionofdataacrossalongdistancerequiresamodulatoratoneendofthetransmissionlineandademodulatorattheother.Mostnetworksystemsarefullduplex,eachlocationneedsbothamodulatorandademodulator.manufacturescombinebothcircuitsintoasingledevicecalledamodem(調(diào)制解調(diào)器).12136.4LeasedAnalogDataCircuits

租用模擬數(shù)據(jù)線路Whenthecircuitconnectstwolocationatasinglesite,thecompanycaninstallthenecessarywiresitself.Privatecompaniescannotinstallcircuitsacrosslongdistance.Thenecessarywiringcanbeobtainedfromatelephonecompany.Telephonecompaniesallowcompaniestoleaseacircuitbetweenanytwolocations.14Onceaconnectionhasbeenleasedfromthephonecompany,amodemmustbeinstalledateachendbeforecommunicationispossible.Afterthat,theleasedlineisavailabletosenddata.Advantage:datacanbesentatanytime.Disadvantage:thelimitedconnectivityandcost.156.5Optical,RadioFrequency,andDialupModemsInadditiontodedicatedwires,modemsarealsousedwithothermedia,includingRFtransmission,glassfibers,andconventionaltelephoneconnections.Adialupmodemcontainscircuitrythatmimicsatelephone----themodemcansimulateliftingthehandset,dialing,orhangingupthephone.16176.6CarrierFrequencyAndMultiplexing

載波頻率和多路復(fù)用Eachtelevisionstationisassignedachannelnumberonwhichitbroadcastsasignal.Achannelnumberismerelyshorthandforthefrequencyatwhichthestation’scarrieroscillates.Thesignalforonechanneldoesnotinterferewiththesignalforanother.18Computernetworksusetheprincipleofseparatechannelstopermitmultiplecommunicationtoshareasingle,physicalconnection.Eachsendertransmitsasignalusingaparticularcarrierfrequency.Multiplecarrierscanpassoverthesamewireatthesametimewithoutinterference.19Frequencydivisionmultiplexing(FDM頻分多路復(fù)用)：multiplecarrierfrequenciestoallowindependentsignaltotravelthroughamedium.FDMallowsmultiplepairsofsendersandreceiverstocommunicationoverasharedmediumsimultaneously.Thecarrierusedbyeachpairsoperatesatauniquefrequencythatdoesnotinterferewiththeothers.20216.7BasebandandBroadbandtechnology

基帶和寬帶技術(shù)TheprimarymotivationforusingFrequencydivisionmultiplexing(FDM)arisesfromthedesireforhighthroughput(吞吐量).Toachievehigherthroughput,theunderlyinghardwareusesalargerpartoftheelectromagneticspectrum(頻譜)(i.e.,alargerbandwidth)22Thetermbroadband(寬帶)technologyisusedtocharacterizesuchtechnologies.Anytechnologythatusesasmallpartoftheelectromagneticspectrumandsendsonlyonesignalatatimeoverthemedium,isknownasabaseband(基帶)technology.23簡單說，所謂基帶信號就是將數(shù)字信號1或0直接用兩種不同的電壓來表示，然后送到線路上去傳輸。寬帶信號是將基帶信號進(jìn)行調(diào)制后形成的頻分復(fù)用模擬信號?；鶐盘栠M(jìn)行調(diào)制后，其頻譜被移到較高的頻率處，每一路基帶信號的頻譜被移到不同的頻段，因此合在一起不會互相干擾。因此，一條電纜中可以同時(shí)傳送多路的數(shù)字信號，從而提高了線路的利用率。246.8WaveDivisionMultiplexing

波分多路復(fù)用Technically,opticalFDMisknownasWaveDivisionMultiplexing(WDM).WDMoperatesbysendingmultiplelightwavesacrossasingleopticalfiber.Atthereceivingend,anopticalprismisusedtoseparatethefrequencies.25EDAF:光纖放大器，直接對光信號進(jìn)行放大。26 由于光載波的頻率很高，因此習(xí)慣上用波長表示光載波。 現(xiàn)在可以在一根光纖上復(fù)用80路或更多路數(shù)的光載波信號，即密集波分復(fù)用DWDM（densewavelengthdivisionmultiplexing）276.9Spread