光纤通信系统Optical Fiber Communications 英文资料及中文翻译.docx
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光纤通信系统Optical Fiber Communications 英文资料及中文翻译.docx
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光纤通信系统OpticalFiberCommunications英文资料及中文翻译
光纤通信系统
OpticalFiberCommunications
英文资料及中文翻译
Communicationmaybebroadlydefinedasthetransferofinformationfromonepointtoanother.Whentheinformationistobeconveyedoveranydistanceacommunicationsystemisusuallyrequired.Withinacommunicationsystemtheinformationtransferisfrequentlyachievedbysuperimposingormodulatingtheinformationontoanelectromagneticwavewhichactsasacarrierfortheinformationsignal.Thismodulatedcarrieristhentransmittedtotherequireddestinationwhereitisreceivedandtheoriginalinformationsignalisobtainedbydemodulation.Sophisticatedtechniqueshavebeendevelopedforthisprocessbyusingelectromagneticcarrierwavesoperatingatradiorequitesaswellasmicrowaveandmillimeterwavefrequencies.
Thecarriermaybemodulatedbyusingeitheropticalananalogdigitalinformationsignal..Analogmodulationinvolvesthevariationofthelightemittedfromtheopticalsourceinacontinuousmanner.Withdigitalmodulation,however,discretechangesinthelengthintensityareobtained(i.e.on-offpulses).Althoughoftensimplertoimplement,analogmodulationwithanopticalfibercommunicationsystemislessefficient,requiringafarhighersignaltonoiseratioatthereceiverthandigitalmodulation.Also,thelinearityneededforanalogmodulationismotalwaysprovidedbysemiconductoropticalsource,especiallyathighmodulationfrequencies.Forthesereasons,analogopticalfibercommunicationslinkaregenerallylimitedtoshorterdistancesandlowerbandwidthsthandigitallinks.
Initially,theinputdigitalsignalfromtheinformationsourceissuitablyencodedforopticaltransmission.Thelaserdrivecircuitdirectlymodulatestheintensityofthesemiconductorlastwiththeencodeddigitalsignal.Henceadigitalopticalsignalislaunchedintotheopticalfibercable.Theavalanchephotodiodedetector(APD)isfollowedbyafront-endamplifierandequalizerorfiltertoprovidegainaswellaslinearsignalprocessingandnoisebandwidthreduction.Finally,thesignalobtainedisdecodedtogivetheoriginaldigitalinformation.
GeneratingaSerialSignal
Althoughaparallelinput-outputschemecanprovidefastdatatransferandissimpleinoperation,ithasthedisadvantageofrequiringalargenumberofinterconnections.Asanexampletypical8bitparalleldataportuses8datalines,plusoneortwohandshakelinesandoneormoregroundreturnlines.Itisfairlycommonpracticetoprovideaseparategroundreturnlineforeachsignalline,soan8bitportcouldtypicallyusea20coreinterconnectioncable.Whilstsuchamultiwaycableisquiteacceptableforshortdistancelinks,uptoperhapsafewmeters,itbecomestooexpensiveforlongdistancelinkswhere,inadditiontothecostofthemultiwordcable,separatedriverandreceivercircuitsmayberequiredoneachofthe10signallines.Wherepartofthelinkistobemadeviaaradiolink,perhapsthroughaspacesatellite,separateradiofrequencychannelswouldberequiredforeachdatabitandthisbecomesunacceptable.
Analternativetotheparalleltransferofdataisaserialinwhichthestatesoftheindividualdatabitsaretransmittedinsequenceoverasinglewirelink.Eachbitisallocatedafixedtimeslot.Atthereceivingendtheindividualbitstatesaredetectedandstoredinseparateflip-flopstages,sothatthedatamaybereassembledtoproduceaparalleldataword.Theadvantageofthisserialmethodoftransmissionisthatitrequiresonlyonesignalwireandagroundreturn,irrespectiveofthenumberofbitsinthedatawordbeingtransmitted.Themaindisadvantageisthattherateatwhichdatacanbetransferredisreducedincomparisonwithaparalleldatatransfer,sincethebitsaredealtwithinsequenceandthelargerthenumberofbitsintheword,theslowerthemaximumtransferspeedbecomes.Formostapplicationshowever,aserialdatastreamcanprovideaperfectlyadequatedatatransferrate.Thistypeofcommunicationsystemiswellsuitedforradioortelephonelinelinks,sinceonlyonecommunicationchannelisrequiredtocarrythedata.
WehaveseenthatintheCPUsystemdataisnormallytransferredinparallelacrossthemaindatabus,soiftheinput-outputdataistobeinserialform,thenaparalleltoserialdataconversionprocessisrequiredbetweentheCPUdatabusandtheexternalI/Oline.Theconversionfromparalleldatatotheserialformcouldbeachievedbysimplyusingamultiplexedswitch,whichselectseachdatabitinturnandconnectsittotheoutputlineforafixedtimeperiod.Amorepracticaltechniquemakesuseofashiftregistertoconverttheparalleldataintoserialform.
AshiftregisterconsistsofaseriesofDtypeflip-flopsconnectedinachain,withtheQoutputofoneflip-flopdrivingtheDinputofthenextinthechain.Alloftheflip-flopsateclockedsimultaneouslybyacommonclockpulse,whentheclockpulseoccursthedatastoredineachflip-flopistransferredtothenextflip-floptotherightinthechain.Thusforeachclockpulsethedatawordiseffectivelysteppedalongtheshiftregisterbyonestage,Attheendofthechainthestateoftheoutputflip-flopwillsequencethroughthestatesofthedatabitsoriginallystoredintheregister.Theresultisaserialstreamofdatapulsesfromtheendoftheshiftregister.
Inatypicalparalleltoserialconversionarrangementtheflip-flopsmakinguptheshiftregisterhavetheirDinputswitchable.InitiallytheDinputsaresetupinawaysothatdatacanbetransferredinparallelfromtheCPUdatabusintotheregisterstages.OncethedatawordhasbeenloadedintotheregistertheDinputsareswitchedsothattheflip-flopsfromashiftregister.Nowforeachsuccessiveclockpulsethedatapatternisshiftedthroughtheregisterandcomesoutinserialformattherighthandendoftheregister.
Atthereceivingendtheserialdatawillusuallyhavetobeconvertedbackintotheparallelformbeforeitcanbeused.Theserialtoparallelconversionprocesscanalsobeachievedbyusingashiftregister.InthiscasetheserialsignalisappliedtotheDinputofthestageatthelefthandendoftheregister.Aseachserialbitisclockedintotheregisterthedatawordagainmovesstepbysteptotheright,andafterthelastbithasbeenshiftedinthecompletedatawordwillbeassembledwithintheregister.AtthispointtheparalleldatamayberetrievedbysimplyreadingoutthedatafromindividualregisterstagesinparallelItisimportantthatthenumberofstagesintheshiftregistershouldmatchthenumberofbitsinthedataword,ifthedataistobeproperlyconvertedintoparallelform.
Toachieveproperoperationofthereceivingendofaserialdatalink,itisimportantthattheclockpulseisappliedtothereceiveshiftregisteratatimewhenthedatalevelontheseriallineisstable.Itispossibletohavetheclockgeneratedateitherendofthelink,butaconvenientschemeistogeneratetheclocksignalatthetransmittingend(parallel-serialconversion)asthemastertimingsignal.Toallowforsettlingtimeanddelaysalongtheline,theactiveedgeoftheclockpulseatthereceiveendisdelayedrelativetothatwhichoperatesthetransmitregister.Iftheclockisasquarewavethesimplesapproachmightbetoarrangethatthetransmitregisteroperatesontherisingedgeoftheclockwave,andthereceiveregisteronthefallingedge,sothatthereceiveroperateshalfaclockperiodbehindthetransmitter.Ifbothregistersoperateonarisingedge,theclocksignalfromthetransmittercouldbeinvertedbeforebeingusedtodrivethereceiveshiftyregister.
Foran8bitsystemasequenceof8clockpulseswouldbeneededtosendtheserialdataword.Atthereceivingendtheclockpulsescouldbecountedandwhentheeighthpulseisreacheditmightbeassumedthatthedatainthereceiveregisteriscorrectlypositioned,andmaybereadoutasparalleldataword.Oneproblemhereisthat,ifforsomereasonthereceiveregistermissedaclockpulse,itsdatapatternwouldgetoutofstepwiththetransmitteddataanderrorswouldresult.Toovercomethisproblemafurthersignalisrequiredwhichdefinesthetimeatwhichthereceivedwordiscorrectlypositionedinthereceiveshiftregisterandreadyforparalleltransferfromtheregister.Onepossibilityistoaddafurthersignalwirealongwhichapulseissentwhenthelastdatabitisbeingtransmitted,sothatthereceiverknowswhenthedatawordiscorrectlysetupinitsshiftregister.Anotherschememightbetosendclockpulsesonlywhendatabitsarebeingsentandtoleaveatiminggapbetweenthegroupsofbitsforsuccessivedatawords.Thelackoftheclocksignalcouldthenbedetectedandusedtoresetthebitcounter,sothatitalwaysstartsatzeroatthebeginningofeachnewdataword.
SerialandParallelDatalionisprocessed.Serialindicatesthattheinformationishandledsequentially,similartoagroupofsoldiersmarchinginsinglefile.InparalleltransmissiontheinfoThetermsserialandparallelareoftenusedindescriptionsofdatatransmissiontechniques.Bothrefertothemethodbywhichinformationisdividedintocharacters,words,orblockswhicharetransmittedsimultaneously.Thiscouldbecomparedtoaplatoonofsoldiersmarchinginranks.
Theoutputofacommontypeofbusinessmachineisoneight—levelpunchedpapertape,oreightbitsofdataatatimeoneightseparateoutputs.Eachparallelsetofeightbitscomprisesacharacter,andtheoutputisreferredtoasparallelbybit,serialbycharacter.Thechoiceofcitherserialorparalleldatatransmissionspeedrequirement
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