本地异步通信技术（RS-232）.ppt

ComputerNetworksandInternets 计算机网络与因特网 课件林坤辉2004 02 6 PARTIDataTransmission Chapter6Long DistanceCommunication Carriers Modulation andModems 远程通信 载波 调制和调制解调器 6 1Introduction Computersusebinarydigits bits位串 torepresentdata Transmittingdataacrossanetworkfromonecomputertoanothermeanssendingbitsthroughtheunderlyingtransmissionmedium Communicationsystemsuseelectriccurrent radiowaves orlighttotransferinformation 6 2SendingSignalsAcrossLongDistances Anelectriccurrentcannotbepropagated 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 3ModemHardwareUsedForModulationAndDemodulation Ahardwarecircuitthatacceptsasequenceofdatabitsandappliesmodulationtoacarrierwaveaccordingtothebitsiscalledamodulator 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 ifinterferencedamagesoneormoreofthecarriers t