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《计算机网络专业英语》（北邮版）ByfarthemostpopularserialinterfacebetweenacomputeranditsCRTterminalistheasynchronousserialinterface,thisinterfaceissocalledbecausethetransmitteddataandthereceiveddataarenotsynchronizedoveranyextendedperiodandthereforenospecialmeansofsynchronizingtheclocksatthetransmitterandreceiverisnecessary.Infact,theasynchronousserialdatalinkisaveryoldformofdatatransmissionsystemandhasitsoriginalintheeraofteleprinter.•CRT:CathodeRayTube•era:[iərə]时代•Serialdatatransmissionsystemshavebeenaroundforalongtimeandarefoundinthetelephone(humanspeech),Morsecode,semaphore,andeventhesmokesignalsonceusedbynativeAmericans.Thefundamentalproblemencounteredbyallserialdatatransmissionsystemsishowtosplittheincomingdatastreamintoindividualunits(i.e.,bits)andhowtogrouptheseunitsintocharacters.Forexample,inMorsecodethedotsanddashesofacharacterareseparatedbyanintersymbolspace,whiletheindividualcharactersareseparatedbyanintercharacterspace,whichisthreetimesthedurationofanintersymbolspace.Paragraph2semaphore:['seməf:]ɔ(铁路的)臂板信号；旗语Native:土著fundamental[fndəmentəl]adj.ʌ基本的;重要的,必要的duration:持续时间Paragraph3•Firstweexaminehowthedatastreamisdividedintoindividualbitsandthebitsgroupedintocharactersinanasynchronousserialdatalink.Thekeytotheoperationofthistypeoflinkisbothsimpleandingenious.Fig1-1givestheformatofdatatransmittedoversuchalink.Fig.1-1Anasynchronousserialdatalinkissaidtobecharacteroriented,asinformationistransmittedintheformofgroupsofbitscalledcharacters.Thesecharactersareinvariableunitscomprising7or8bitsof"information”plus2to4controlbitsandfrequentlycorrespondtoASCIIencodedcharacters.Initially,whennoinformationisbeingtransmitted,thelineisinanidlestate.Traditionally,theidlestateisreferredtoasthemarklevelandcorrespondstoalogical1level.•Characteroriented面向字符的，在信息传输中，还可以有面向比特的，即传输信息的Unit是BitsorBytes。•ASCII字符是由7Bits的数据位，加上1Bit的奇偶校验位组成，即7个比特的数据中，1的个数，决定了奇偶校验位的值是0还是1，当信息位中1的个数为偶数个，那么该位为0，否则该位为1，e.g.4D＝10110010（B）最后一个比特为0，是因为前面7个比特中，有4个1，为偶数个1。接收方就是边接收信息，边计算1的个数。isreferredtoas被称为…correspondto相应于…译文：异步串行数据链路被认为是面向字符的，因为信息是被称为字符的比特组的形式传送的。这些字符加上一些固定的单元，每个单元都包含7到8比特的信息比特加上2到4比特的控制比特，通常对应于ASCII字符。起初，当信息没有发送时候，线路处于闲置状态，而空闲状态被称为信号电平并对应于逻辑1电平。Whenthetransmitterwishestosenddata,itfirstplacesthelineinaspacelevelforoneelementperiod.ThiselementiscalledthestartbitandhasdurationofTseconds.Thetransmitterthensendsthecharacter,1bitatatime,byplacingeachsuccessivebitonthelinefordurationofTseconds,untilallbitshavebeentransmitted.Thenasingleparitybitiscalculatedbythetransmitterandsentafterthedatebits.Finally,thetransmittersendsastopbitatamarklevelforoneortwobitperiods.当发送方要发送数据时，它首先将线路置为空号电平，该电平持续一个单元时长，这个单元被称为起始比特，它持续T秒时间。接着发送方发送数据，一次一个比特地将相继的比特送到线路上，每个比特持续T秒，直到所有的比特被发送完毕。然后，发送方计算出一个奇偶校验值，在数据比特发送完毕之后发送，最后，发送方发送一个停止比特，其信号电平为传号电平，持续一到二个比特发送时间。Paragraph5（P.20）1bitatatime一次一个比特Successive后继的，连续的s...