外文翻译---6 数字数据传输：接口和调制解调器

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1、时光荏苒，岁月如白驹过隙，不经意间，我们即将从多彩的大学毕业，完成一个从大学生到社会人的转变。四年的大学生涯，让我明白了许许多多的科学知识和人生哲理，以及数不尽的感悟和成长。然只有将所学的理论知识付诸于社会的实践，才能检验和实现理论自身的价值，也只有将理论付诸于实践才能使理论得以检验。同样，一个人的价值也是通过实践活动来实现的，也只有通过实践才能锻炼人的品质，彰显人的时代风貌。而毕业实习作为大学的最后一课，也是最后一刻，我倍加珍惜，从而可以较深层次的理解社会，以便为将来能更好的胜任工作打下坚实的基础。英文资料及中文翻译6TRANSMISSIONSOFDIGITALDATA:INT

2、ERFACESANDMODEMS(FromIntroductiontoDataCommunicationsandNetWorking,BehrouzForouzan)Oncewehaveencoderourinformationintoaformatthatcanbetransmitted,thenextstepistoinvestigatethetransmissionprocessitself.Information-processingequipmentsuchasPCsgenerateencodedsignalsbutordinarilyrequireassistance

3、totransmitthosesignalsoveracommunicationlink.Forexample,aPCgeneratesadigitalsignalbutneedsanadditionaldevicetomodulateacarrierfrequencybeforeitissentoveratelephoneline.Howdowerelayencodeddatafromthegeneratingdevicetothenextdeviceintheprocess?Theanswerisabundleofwires,asortofminicommunicationl

4、ink,calledaninterface.Becauseaninterfacelinkstwodevicesnotnecessarilymadebythesamemanufacturer,itscharacteristicsmustbedefinedandstandardsmustbeestablished.Characteristicsofaninterfaceincludeitsmechanicalspecifications(howmanywiresareusedtotransportthesignal);itselectricalspecifications(thefr

5、equency,amplitude,andphaseoftheexpectedsignal);anditsfunctionalspecifications(ifmultiplewiresareused,whatdoeseachonedo?).ThesecharacteristicsarealldescribedbyseveralpopularstandardsandareincorporatedinthephysicallayeroftheOSImodel.6.1DIGITALDATATRANSMISSIONOfprimaryconcernwhenconsideringthetr

6、ansmissionofdatafromonedevicetoanotheristhewiring.Andofprimaryconcernwhenconsideringthewiringisthedatastream.Dowesendonebitatatime,ordowegroupbitsintolargergroupsand,ifso,how?Thetransmissionofbinarydataacrossalinkcanbeaccomplishedeitherinparallelmodeorserialmode.Inparallelmode,multiplebitsare

7、sentwitheachclockpulse.Inserialmode,onebitissentwitheachclockpulse.Whilethereisonlyonewaytosendparalleldata,therearetwosubclassesofserialtransmission:synchronousandasynchronous(seeFigure6-1).ParallelTransmissionBinarydata,consistingof1sand0s,