资源描述：
《外文翻译---6数字数据传输：接口和调制解调器》由会员上传分享，免费在线阅读，更多相关内容在应用文档-天天文库。
1、英文资料及中文翻译6TRANSMISSIONSOFDIGITALDATA:INTERFACESANDMODEMS(FromIntroductiontoDataCommunicationsandNetWorking,BehrouzForouzan)Oncewehaveencoderourinformationintoaformatthatcanbetransmitted,thenextstepistoinvestigatethetransmissionprocessitself.Information-processingequipmentsuchasPCsg
2、enerateencodedsignalsbutordinarilyrequireassistancetotransmitthosesignalsoveracommunicationlink.Forexample,aPCgeneratesadigitalsignalbutneedsanadditionaldevicetomodulateacarrierfrequencybeforeitissentoveratelephoneline.Howdowerelayencodeddatafromthegeneratingdevicetothenextdevicein
3、theprocess?Theanswerisabundleofwires,asortofminicommunicationlink,calledaninterface.Becauseaninterfacelinkstwodevicesnotnecessarilymadebythesamemanufacturer,itscharacteristicsmustbedefinedandstandardsmustbeestablished.Characteristicsofaninterfaceincludeitsmechanicalspecifications(h
4、owmanywiresareusedtotransportthesignal);itselectricalspecifications(thefrequency,amplitude,andphaseoftheexpectedsignal);anditsfunctionalspecifications(ifmultiplewiresareused,whatdoeseachonedo?).Thesecharacteristicsarealldescribedbyseveralpopularstandardsandareincorporatedinthephysi
5、callayeroftheOSImodel.6.1DIGITALDATATRANSMISSIONOfprimaryconcernwhenconsideringthetransmissionofdatafromonedevicetoanotheristhewiring.Andofprimaryconcernwhenconsideringthewiringisthedatastream.Dowesendonebitatatime,ordowegroupbitsintolargergroupsand,ifso,how?Thetransmissionofbinary
6、dataacrossalinkcanbeaccomplishedeitherinparallelmodeorserialmode.Inparallelmode,multiplebitsaresentwitheachclockpulse.Inserialmode,onebitissentwitheachclockpulse.Whilethereisonlyonewaytosendparalleldata,therearetwosubclassesofserialtransmission:synchronousandasynchronous(seeFigure6
7、-1).ParallelTransmissionBinarydata,consistingof1sand0s,maybeorganizedintogroupsofnbitseach.Computersproduceandconsumedataingroupsofbitsmuchasweconceiveofandusespokenlanguageintheformofwordsratherthanletters.Bygrouping,wecansenddatanbitsatatimeinsteadofone.Thisiscalledparalleltransm
8、ission.DatatransmissionPar
