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1、英文资料及中文翻译6TRANSMISSIONSOFDIGITALDATA:INTERFACESANDMODEMS(FromIntroductiontoDataCommunicationsandNetWorking,BehrouzForouzan)Oncewehaveencoderourinformationintoaformatthatcanbetransmitted,thenextstepistoinvestigatethetransmissionprocessitself.Information-pro
2、cessingequipmentsuchasPCsgenerateencodedsignalsbutordinarilyrequireassistancetotransmitthosesignalsoveracommunicationlink.Forexample,aPCgeneratesadigitalsignalbutneedsanadditionaldevicetomodulateacarrierfrequencybeforeitissentoveratelephoneline.Howdowerela
3、yencodeddatafromthegeneratingdevicetothenextdeviceintheprocess?Theanswerisabundleofwires,asortofminicommunicationlink,calledaninterface.Becauseaninterfacelinkstwodevicesnotnecessarilymadebythesamemanufacturer,itscharacteristicsmustbedefinedandstandardsmust
4、beestablished.Characteristicsofaninterfaceincludeitsmechanicalspecifications(howmanywiresareusedtotransportthesignal);itselectricalspecifications(thefrequency,amplitude,andphaseoftheexpectedsignal);anditsfunctionalspecifications(ifmultiplewiresareused,what
5、doeseachonedo?).ThesecharacteristicsarealldescribedbyseveralpopularstandardsandareincorporatedinthephysicallayeroftheOSImodel.6.1DIGITALDATATRANSMISSIONOfprimaryconcernwhenconsideringthetransmissionofdatafromonedevicetoanotheristhewiring.Andofprimaryconcer
6、nwhenconsideringthewiringisthedatastream.Dowesendonebitatatime,ordowegroupbitsintolargergroupsand,ifso,how?Thetransmissionofbinarydataacrossalinkcanbeaccomplishedeitherinparallelmodeorserialmode.Inparallelmode,multiplebitsaresentwitheachclockpulse.Inserial
7、mode,onebitissentwitheachclockpulse.Whilethereisonlyonewaytosendparalleldata,therearetwosubclassesofserialtransmission:synchronousandasynchronous(seeFigure6-1).ParallelTransmissionBinarydata,consistingof1sand0s,maybeorganizedintogroupsofnbitseach.Computers
8、produceandconsumedataingroupsofbitsmuchasweconceiveofandusespokenlanguageintheformofwordsratherthanletters.Bygrouping,wecansenddatanbitsatatimeinsteadofone.Thisiscalledparalleltransmission.DatatransmissionPar
