数据通信与网络编码信息

Chapter5ENCODING第五章编码5.1Digital-to-DigitalEncoding5.2Analog-to-DigitalEncoding5.3Digital-to-AnalogEncoding5.4Analog-to-AnalogEncoding5.5SummaryContentsIntroductionofEncodingTheinformationordatamustbeencodedintosignalsbeforeitcanbetransportedacrossthecommunicationmedia.Thesignalmustbemanipulatedsothatitcontainsidentifiablechangesthatarerecognizabletothesenderandreceiverasrepresentingtheinformationintended.DifferentencodingschemesEncodingDigital/digitalAnalog/digitalDigital/analogAnalog/analogIntroductionofEncodingHowinformationisencodedependsonitsoriginalformatandontheformatusedbycommunicationhardware.5.1Digital-to-digitalencodingDigital-to-digitalencodingistherepresentationofdigitalinformationbyadigitalsignal.Boththeoriginaldataandtransmitteddataaredigital.Thebinary1sand0saretranslatedintoasequenceofvoltagepulsethatcanbepropagatedoverwire.Typesofdigital-to-digitalencodingDigital/digitalencodingUnipolarPolarBipolar5.1Digital-to-digitalencoding单极性编码极化编码双极性编码5.1.1Unipolar(单极性编码)Digitaltransmissionsystemsworkbysendingvoltagepulsesalongamedialink(wireorcable).Inmosttypesofencoding,onevoltagelevelstandsforbinary0andanotherlevelstandsforbinary1.Thepolarityofpulse(脉冲极性)refertowhetheritispositiveornegative.Unipolarencodingissonamedbecauseitusesonlyonepolarity.5.1.1Unipolar(单极性编码)The1sareencodedaspositivevalue;The0sareencodedaszero,oridle.振幅时间01000111Theideaofunipolarencoding.Advantage:VerySimpleandStraightforward;inexpensivetoimplementDisadvantages:1、同步(synchronization)whenasignalisunvarying,thereceivercannotdeterminethebeginningandendingofeachbits.Therefore,asynchronizationprobleminunipolarencodingcanoccurwheneverthedatastreamincludesalonguninterruptedseriesof1sor0s.5.1.1Unipolar(单极性编码)synchronizationTocorrectlyinterpretthesignalsreceivedfromthesender,thereceiver'sbitintervalsmustcorrespondexactlytothesender'sbitintervals.Ifthereceiverclockisfasterorslower,thebitintervalsarenotmatchedandthereceivermightmisinterpretthesignals.Thesolutionofsynchronizationparallellinecarryingsynchronizationclockpulse.5.1.1Unipolar(单极性编码)Example:synchronizationGivenanexpectedbitrateof1000bps,ifthereceiverdetectsapositivevoltagelasting0.005s,itreadsone1per0.001s,orfive1s.(Propagationdelaysdistortitto0.006s)5.1.1Unipolar(单极性编码)2、DCcomponent(直流分量、直流成分)当数字信号中的电平保持一段时间的恒定时，频谱会产生很低的频率(据傅立叶分析)。这些接近于零的频率称为DCcomponent。直流分量会对不允许通过低频的系统或使用电子耦合的系统带来严重的影响。例如：电话线不能通过低于200Hz的频率；长距离链路一般会用多个变压器来隔离线路的不同部分。5.1.1Unipolar(单极性编码)5.1.2Polar(极化编码)Polarencodingusestwovoltagelevel:onepositiveandonenegative.Advantage:Byusingbothlevels,inmostpolarencodingmethodstheaveragevoltagelevelonthelineisreducedandtheDCcomponentproblemofunipolarencodingisalleviated.TypesofpolarencodingpolarNRZRZBiphaseNRZ-LNRZ-IManchesterDifferentialManchester5.1.2Polar(极化编码)INNRZencoding,thelevelofthesignalisalwayseitherpositiveornegative.※Ifthelineisidleitmeansnotransmissionisoccurringatall.Thetwomostpopularmethods:NRZ-L(非归零电平编码)NRZ-I(非归零反相编码)DisadvantageofNRZBothNRZ-LandNRZ-IhaveaDCcomponentproblem.5.1.2.1NRZ(Non-Return-to-Zero)TwomethodsofNRZ:InNRZ_L,thelevelofthesignaldependsonthetypeofbititrepresents.0spositivevoltage1snegativevoltageInNRZ_Ithesignalisinvertedifa1isencountered.无论当前处于什么状态(高电平或低电平)，只要下一位是1，则跳转到相反的状态。即电平是否反相决定了其位值。5.1.2.1NRZ(Non-Return-to-Zero)5.1.2.1NRZ(Non-Return-to-Zero)跳变，由于下一比特为1振幅时间时间NRZ-L(非归零电平编码)NRZ-I(非归零反向编码)00001111编码规则：在NRZ-L编码方式中，电平的极性具有特定的含义：正代表比特0，负代表比特1；在NRZ-I编码方式中，每段比特间隔的电平值是没有意义的，接收端是以检测每个比特开始处是否有电平的跳变来识别比特1，而在每比特开始处保持当前电平不变的则表示比特0。5.1.2.1NRZ(Non-Return-to-Zero)NRZ-I相对NRZ-L的优点：由于每遇到比特1都发生电平跃迁，因此提供了一种同步(Synchronization)机制。一串7个比特1会导致7次电平跃迁，每次跃迁都使接收方能根据信号的实际到达来对本身时钟进行重同步调整。(据统计，连续的比特1出现的几率比连续的比特0出现的几率大，因此对比特1的连续串进行同步在保持整体消息同步上更有优势。)一串连续的比特0仍会造成麻烦，但由于连续0串出现不频繁，对于解码来说其妨碍就小了许多。5.1.2.1NRZ(Non-Return-to-Zero)RZencodingusesthreevalues:positive,zero,negative.0positive(actually,negative-to-zero)1negative(actually,positive-to-zero)※halfwaythrougheachbitinterval,thesignalreturntozero.InRZ,thesignalchangesnotbetweenbits,butduringeachbit(位于位间隔之间).InRZ,thesignalchangesduringeachbitbeusedforsynchronization.5.1.2.2RZ(Return-to-Zero)encoding5.1.2.2RZ(Return-to-Zero)encoding跳变可以用于同步信号值时间00001111正跳(从负到零电平)表示0；负跳(从正到零电平)表示1。由于每位都在其位间隔中间位置归零，并保持半个位间隔时间，因此每位的开始都是由零开始跳变。若下一位是1(由高电平到零电平的跳变表示，负向跳变)，则首先由零电平跳变到高电平，持续半个位间隔(可理解为准备阶段)，然后在1/2位间隔处由高电平归零(代表1)，并保持半个位间隔时间；若下一位是0(由低电平到零电平的跳变表示，正向跳变)，则首先由零电平跳变到低电平，持续半个位间隔(可理解为准备阶段)，然后在1/2位间隔处由低电平归零(代表0)，并保持半个位间隔时间。5.1.2.2RZ(Return-to-Zero)encodingDisadvantageofRZ:requirestwosignalchangestoencodeonebit,thereforeoccupiesmorebandwidth.Usethreelevel.Advantage:Agoodencodeddigitalsignalmustcontainaprovisionforsynchronization.Thereisasignalchangeforeachbit,soprovidingsynchronization.5.1.2.2RZ(Return-to-Zero)encoding5.1.2.3Biphase(双相位编码)Inbiphaseencoding,thesignalchangesatmiddleofbitinterval,butdoesnotreturntozero.Instead,itcontinuestotheoppositepole.Twotypesofbiphaseencodingmethods:Manchester;DifferentialManchester.Thebiphaseencodingisthebestsolutiontotheproblemofsynchronization.InManchesterencoding,the