数据通信与网络06-数字数据传输：接口与调制解调器-3

第六章数字数据传输：接口与调制解调器Chapter6TransmissionofDigitalData:InterfacesandModems6.1DigitalDataTransmission6.2DTE-DCEInterface6.3OtherInterfaceStandards6.4Modems6.5SummaryContentsIntroductionInformation-processingdevicescangenerateencodedsignalsbutrequireassistancetotransmitthosesignalsoveracommunicationlink.Whenwewantrelayencodedatafromsourcedevicetothenextdevice,weneedabundleofwires,asortofmini-communicationlink,calledaninterface.Forintercommunication,theinterface’scharacteristicsmustbedefinedandstandardsmustbeestablished.IntroductionCharacteristicsofinterfaceinclude:

mechanicalspecifications;electricalspecifications;functionalspecifications.ThesecharacteristicsarealldescribedbyseveralpopularstandardsandareincorporatedinthephysicallayeroftheOSImodel.6.1DigitalDataTransmissionTheprimaryconcernwhenconsideringthemoveofdatafromonedevicetoanotheristhewiring(配线).Theprimaryconcernwhenconsideringthewiringisdatastream.

Dowesendonebitatatimeoragroupbits?6.1DigitalDataTransmissionThetransmissionofbinarydataacrossalinkcanbeaccomplishedeitherinparallelorserialmode.Thetypeofdatatransmissionmode6.1.1ParallelTransmissionBinarydatamaybeorganizedintogroupsofnbitseach,andcansenddatanbitsatatime.Thismodecalledparalleltransmission(并行传输).Themechanismforparalleltransmissionisusenwirestosendnbitsatonetime.

eachbithasitsownwire;allnbitsbetransmittedwitheachclockpulse.6.1.1ParallelTransmission发送方接收方011000108个比特一起发送需要8条线路一般将n根导线组成一根电缆或排线，每端配有连接头。6.1.1ParallelTransmissionAdvantage:Thetransmissionspeed.

Allelsebeingequal,paralleltransmissioncanincreasethetransferspeedbyafactorofnoverserialtransmission.

Disadvantage:Thecost.

Requirencommunicationlines.

usuallylimitedtoshortdistancestransmission.6.1.2SerialtransmissionInserialtransmissiononebitfollowsanother,soweneedonlyonecommunicationchannel.6.1.2SerialtransmissionTheadvantageofserialtransmission:Onlyonecommunicationchannelbeused,soreducesthecostoftransmissionroughlyafactorofn.Sincecommunicationwithindevicesisparallel,conversiondevicesarerequiredattheinterfacebetweenthesenderandtheline(parallel-to-serial)andbetweenthelineandthereceiver(serial-to-parallel).Serialtransmissionhavetwoways:asynchronousorsynchronous.Asynchronoustransmissionissonamedbecausethetimingofasignalisunimportant.Informationisreceivedandtranslatedbyagreed-uponpatterns.Patternsarebasedongroupingthebitstreamintobytes.Eachgroupissendalongthelinkasaunit.AsynchronousTransmission(异步传输)Thesenderhandleseachgroupindependently,sendingitwheneverready,withoutregardtoatimer.Toalertthereceivertothearrivalofanewgroup,anextrabitisaddedtothebeginningofeachbyte(calledstartbit,usuallya0).Toletthereceiverknowthatthebyteisfinished,oneormoreadditionalbitsareappendedtotheendofthebyte(calledstopbits,usually1s).Inaddition,thetransmissionofeachbytemaythenbefollowedbyagapofvaryingduration.AsynchronousTransmission(异步传输)AsynchronousTransmission(异步传输)Thestartandstopbitsandthegap

alertthereceivertothebeginningandendofeachbyteandallowittosynchronizewiththedatastream.SenderReceiver10111000110111000110111000110111000AsynchronousTransmission(异步传输)在字节级别，收发双方无需同步，即双方不必在何时发送上达成一致，因此称这种方式为异步传输。在每字节内部，每个数据位仍然需要同步！Advantage:

cheapandeffective.Disadvantage:

Theadditionofstopandstartbitsandtheinsertionofgapsintothebitstreammakeittransmissionslower.Itisanattractivechoiceforlow-speedcommunication.AsynchronousTransmission(异步传输)Asynchronousheremeans“asynchronousatthebytelevel,butthebitsarestillsynchronized”

Theirdurationsarethesame.NoteAsynchronousTransmission(异步传输)SynchronousTransmission(同步传输)Insynchronoustransmissionwesendbitsoneafteranotherwithoutstart/stopbitsorgaps.Itistheresponsibilityofthereceivertogroupthebits.Ifthesenderwishestosenddatainseparatebursts,thegapsbetweenburstsmustbefilledwithaspecialsequenceof0sand1sthatmeansidle.Becausetheaccuracyofthereceivedinformationiscompletelydependentontheabilityofthereceivingdevicetokeepanaccuratecountofthebits,thereforetimingisveryimportant.SynchronousTransmission(同步传输)Theadvantageofsynchronoustransmissionisspeed.

Withnoextrabitsorgapstointroduceatthesendingendandremoveatthereceivingendand,thereforewithfewerbitstomoveacrossthelink.

Bytesynchronizationisaccomplishedinthedatalinklayer.SynchronousTransmission(同步传输)Insynchronoustransmission,

wesendbitsoneafteranotherwithoutstart/stopbitsorgaps.NoteSynchronousTransmission(同步传输)6.2DTE-DCEInterfaceBasicConceptDTE(DataTerminalEquipment,数据终端设备)isanydevicethatisasourceofordestinationforbinarydigitaldata.Atphysicallayer,itcanbeterminal,computer,printer,faxmachine,oranyotherdevicethatgeneratesorconsumesdigitaldata.6.2DTE-DCEInterfaceBasicConceptDCE(DataCircuit-TerminatingEquipment,数据电路终接设备)

isanydevicethattransmitsorreceivesdataintheformofananalogordigitalsignalthroughanetwork.CommonlyusedDCEatphysicallayerincludemodems.6.2DTEandDCEDTEDCEDTEDCE发送端：DTE产生数据，并连同必要的控制字符一同传输给与其相连的DCE。DCE将数据转换为适合传输介质的形式，并将信号发送到网络链路中。接收端：相应的逆操作。6.2DTE-DCEInterface通信过程中，发送和接收信号的DCE设备必须采用同一种调制方法(如FSK)，并且需要两个设备协调工作.两个DTE设备不需要互相协调，但是每个DTE设备必须保持与其直接相连的DCE设备协调工作。Manystandardshavebeendevelopedtodefinetheconnection

betweenaDTEandaDCE.Standardprovidesamodelforthe

mechanical,

electrical,and

functional

characteristicsoftheconnection.6.2.1StandardsTheEIA(电子工业协会)andtheITU-T(国际电信联盟标准化部)havebeeninvolvedindevelopingDTE-DCEinterfacestandards.

TheEIAstandardsarecalledEIA-232,EIA-442,EIA-449,andsoon.TheITU-TstandardsarecalledtheVseriesandtheXseries.6.2.1Standards6.2.2EIA-232InterfaceTheEIA-232(RS-232,RecommendedStandard)definesthemechanical,electrical,functionalcharacteristicsoftheinterfacebetweenaDTEandaDCE.

Issuedin1962Themostrecentversion:EIA-232-F-199MechanicalSpecificationThemechanicalspecificationoftheEIA-232standarddefinestheinterfaceasa25-wirecablewithamaleandafemaleDB-25pinconnectorattachedtoeitherend.Thelengthofthecablemaynotexceed15meters(about50feet).MechanicalSpecificationmaleconnectorfemaleconnectorADB-25connectorisaplugwith25pinsorreceptacles,eachofwhichisattachedtoasinglewirewithaspecificfunction.ElectricalSpecificationTheelectricalspecificationofthestandarddefinesthevoltagelevelsandthetypeofsignaltobetransmittedineitherdirectionbetweentheDTEandtheDCE.EIA-232statesthatalldatamustbetransmittedaslogical1sand0s(calledmark(传号)andspace(空白))usingnon-returntozero,level(NRZ-L)encoding,with0definedasapositivevoltageand1definedasanegativevoltage.ElectricalSpecificationEIA-232define

twodistinctranges,oneforpositivevoltagesandonefornegative.Receiverrecognizesandacceptsasanintentionalsignalanyvoltagethatfallswithintheserange,butnovoltagesthatfalloutsidetheranges.

Toberecognizedasdata,theamplitudeofsignalmustfallbetween+3and+15voltsorbetween-3and-15volts.在EIA-232标准中发送数据的电气规范电压时间允许区域允许区域未定义区域未定义区域+15+30-3-15NRZ-L编码ElectricalSpecification1010Only4wiresoutof25availableinEIA-232interfaceareusefordatafunctions.Theremaining21areusedforfunctionslikecontrol,timing,groundingandtesting.TheelectricalspecificationofEIA-232definesthatsignalsotherthandatamustbesentusingOFF(=>lessthan-3volts)andON(=>greaterthan+3volts).ControlandTiming电压时间允许区域允许区域未定义区域未定义区域+3-ControlandTimingEIA-232标准中控制信号的电气规范正电压表示开状态；负电压表示关状态。

注意：控制线上没有电压表示设备状态不正常，而不是线路处于关闭状态。ElectricalSpecificationThefinalimportantfunctionofelectricalspecificationisbitrate.

EIA-232allowsformaximumbitrateof20Kbps.Note:在具体的实现过程中，一般都选择更高的比特率！FunctionalSpecificationDB-25

EIA-232:rate<64Kbps;connectionlength<15meters;25pin

connector-pin3:receive(RxD);pin2:transmit(TxD);pin7:groud-otherpinsfortransmissioncontrolFunctionalSpecificationEIA-232有两种实现方式：DB-25、DB-9注意：两种实现的针脚不存在必然的对应关系！DB-25实现方式中各针脚功能分配FunctionalSpecificationFunctionalSpecificationDB-9与DB-25针脚之间的关系Example:SynchronousFull-DuplexTransmissionTheFiguredemonstratestheprocessofEIA-232insynchronousfull-duplexmodeoveraleasedlineusingonlytheprimarychannel.TheDTEsarecomputer;TheDCEsaremodem.

Example:SynchronousFull-DuplexTransmissionStep1:preparationTwogroundingcircuits,1(shield)and7(signalground),areactivebetweenbothend.Pin1:shieldPin7:signalgroundExample:SynchronousFull-DuplexTransmissionStep2:ReadinessItensuresthatallfourdevicesarereadyfortransmission.

20pin:DTE设备就绪

6pin:DCE设备就绪Example:SynchronousFull-DuplexTransmissionStep3:SetupSetupphysicalconnectionbetweenthesendingandreceivingmodems.

Pin4:RequesttosendPin5:CleartosendPin8:ReceivedlinesignaldetectorExample:SynchronousFull-DuplexTransmissionStep4:DatatransferPin2:transmitteddataPin24:transmittersignalelementtimingPin3:receiveddataPin17:receiversignalelementtimingExample:SynchronousFull-DuplexTransmissionStep5:ClearingPin4:requesttosendPin5:cleartosendPin8:receivedlinesignaldetectorModemsarenotneededtoconnecttwocompatibledigitaldevicesdirectly.Butwedoneedaninterfacetohandletheexchange,justasEIA-232cabledose.Thesolution,providedbytheEIAstandards,iscalledanullmodem.AnullmodemprovidestheDTE-DCE/DCE-DTEinterfacewithouttheDCEs.NullModemNullModemPin2:transmitteddataPin3:receiveddataCrossingConnectionsFortransmissiontooccur,thewiresmustbecrossed.

Pin2ofthefirstDTEconnectstopin3ofthesecondDTE;andpin2ofthesecondDTEconnectstopin3ofthefirst.Severalotherpinsalsoneedreconnection.AnullmodemisanEIA-232interfacethatcompletesthenecessarycircuitstofooltheDTEsateitherendintobelievingthattheyhaveDCEsandanetworkbetweenthem.NullModemNullmodempinconnectionsNullModem空调制解调器的两端都是阴性连接头，用于连接两端DTE设备的阳性EIA-232接口。6.3OtherInterfaceStandardsBothdatarateandcablelength(signaldistancecapability)arerestrictedbyEIA-232,datarateto20Kbpsandcablelengthto50feet(15meters).Tomeettheneedsofuserswhorequiremorespeedand/ordistance,theEIAandtheITU-Thaveintroducedadditionalinterfacestandards:EIA-449,EIA-530andX.2l.EIA-449的机械规范定义了两种连接头：DB-37与DB-9，两种组合成一种46针的配置。EIA-449的功能规范给DB-37定义了与DB-25类似的功能，但在DB-37中没有与辅助信道相关的功能，EIA-449将这些功能单独分离，用DB-9来提供这些功能。6.3.1EIA-449DB-37andDB-9connectors6.3.1EIA-449CategoryIincludesthosepinswhosefunctionsarecompatiblewiththoseofEIA-232.ForeachcategoryIpin,EIA-449definestwopins.CategoryIIpinsarethosethathavenoequivalentinEIA-232orhavebeenredefined.DB-376.3.1EIA-449ForeachcategoryIpin,EIA-449definestwopins.

Pin4andpin22:senddataPin5andpin23:sendtimingPin6andpin24:receivedataDB-37PinFunctionsPinFunctionsCategoryPinFunctionsCategory12345678910111213141516171819ShieldSignalrateindicatorUnassignedSenddataSendtimingReceivedataRequesttosendReceivetimingCleartosendLocalloopbackDatamodeTerminalreadyReceivereadyRemoteloopbackIncomingcallSelectfrequencyTerminaltimingTestmodeSignalground202122232425262728293031323334353637ReceiveCommonUnassignedSenddataSendtimingReceivedataRequesttosendReceivetimingCleartosendTerminalinserviceDatamodeTerminalreadyReceivereadySelectreadySignalqualityNewsignalTerminaltimingStandbyindicatorSendcommonIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIDB-37CategoryIIPins:Localloopback.Pin10isusedforlocalloopbacktesting.Remoteloopback.Pin14isusedforremoteloopbacktesting.Selectfrequency.Pin16isusedtochoosebetweentwodifferentfrequencyrates.Receivecommon.Pin20providesacommonsignalreturnlineforunbalancedcircuitsfromtheDCEtotheDTE.CategoryIIPins:Selectstandby(选择备用).Pin32allowstheDTEtorequesttheuseofstandbyequipmentintheeventoffailure.Newsignal.Pin34isavailableformultiplepointapplicationswhereaprimaryDTEcontrolsseveralsecondaryDTEs.Whenactivated,pin34indicatesthatoneDTEhasfinisheditsdataexchangeandanewoneisabouttostart.DB-37CategoryIIPins:Standbyindicator(备用指示).Pin36providestheconfirmationsignalfromtheDCEinresponsetoselectstandby(pin32).Sendcommon.Pin37providesacommonsignalreturnlineforunbalancedcircuitsfromtheDTEtotheDCE.Terminalinservice(终端服务中).Pin28indicatestotheDCEwhetherornottheDTEisoperational.DB-3DB-9PinFunctionEIA-232Equivalent123456789ShieldSecondaryreceivereadySecondarysenddataSecondaryreceivedataSignalgroundReceivecommonSecondaryrequesttosendSecondarycleartosendSendcommon114167121913PinfunctionsElectricalSpecificationsEIA-449usestwostandardstodefineitselectricalspecifications:RS-423:unbalancedcircuitsRS-422:balancedcircuitsElectricalSpecificationsRS-423isanunbalancedcircuitspecification,meaningthatitdefinesonlyonelineforpropagatingitssignal.Allsignalsinthisstandarduseacommonreturntocompletethecircuit.在非平衡电路模式中，EIA-449使用每一对I类针脚的第一个针脚和所有的II类针脚。

NoteElectricalSpecificationsElectricalSpecificationsRS-422isbalancedcircuitspecification,meaningthatitdefinestwolinesforpropagationofeachsignal.Signalsagainuseacommonreturnforthereturnofthesignal.在平衡模式中，EIA-449使用了每对I类针脚但不使用II类针脚。

NoteElectricalSpecifications在平衡模式中，两条线路承载相同的传输。但是它们并不传输相同的信号。在一条线路上的信号是另一条线路上信号的互补值。此时接收方并不监听其中任何一个实际的信号，而是检测两个信号之间的差值。

ElectricalSpecificationsAsthecomplementarysignalsarriveatthereceiver,theyareputthroughasubtracter.Thismechanismsubtractsthesecondsignalfromthefirstbeforeinterpretation.Becausethetwosignalscomplementeachother,theresultofthissubtractionisadoublingofthevalueofthefirstsignal.Ifnoiseisaddedtothetransmission,itimpactsbothsignalsinthesameway.Asaresult,thenoiseiseliminatedduringthesubtractionprocess.ElectricalSpecificationsExample:CancelingofnoiseusingbalancedmodeElectricalSpecifications例如，如果在某时刻第一个信号值为5V，第二个信号值将是-5V。所以，相减的结果是5-(-5)=10。如果在传输中有噪声，则噪声会以同样方式影响两个互补信号(正噪声对信号产生正的影响，负噪声对信号产生负的影响)。结果是在减法过程中消除噪声。例如，假定在某点存在2V噪声，而此时第一个信号是+5V，它的互补信号是-5V。额外的歧变使第一个信号值变为7V，第二个信号为-3V。此时7-(-3)仍然是10V，这种特性有助于中和噪声的影响并使得高速的平衡模式传输成为可能。6.3.2EIA-530EIA-530:aversionofEIA-449thatusesDB-25pins.ThepinfunctionsofEIA-530areessentiallythoseofEIA-449CategoryIplusthreepinsfromCategoryII.EIA-530doesnotsupportasecondarycircuit.6.3.3X.21X.21标准是lTU-T制订的。它的目的是为了解决EIA接口中存在的问题，同时为完全的数字通信提供解决方案。Usingdatacircuitsforcontrol在EIA接口中一大部分的电路被用于控制信息。X.21撤消了EIA中大多数控制线路，并将控制信息导向数据线路。X.21标准仅采用了较少的针脚，并且能够在数字化远程通信中使用。X.21设计的工作环境是64Kbps的平衡线路。X.21定义的连接头称为DB-15。它在EIA标准提供的比特同步之外还提供了用于控制字节同步的时钟线。同时，它通过控制和指示线来进行初始化握手过程或开始传输的约定过程。PinFunctionsDB-15ConnectorBytetiming(字节时序).AdvantageofferedbyX.21isthatoftiminglinestocontrolbytesynchronizationinadditiontothebitsynchronizationprovidedbytheEIAstandards.Byaddingabytetimingpulse(pins7and14),X.21improvestheoverallsynchronizationoftransmissions.Controlandindication(控制与初始化).Pins3and5oftheDB-15connectorareusedfortheinitialhandshake,oragreementtobegintransmitting.(Pin3istheequivalentofrequesttosend.Pin5istheequivalentofcleartosend.)PinFunctionsDB-15pinsPinFunction12345678Pin9101112131415FunctionShieldTransmitdataorcontrolControlReceivedataorcontrolIndicationSignalelementtimingByetimingSignalgroundTransmitdataorcontrolControlReceivedataorcontrolIndicationSignalelementtimingBytetimingReservedPinFunctions6.4MODEMSThetermmodemisacompositewordthatreferstothetwofunctionalentitiesthatmakeupthedevice:asignalmodulatorandasignaldemodulator.Amodulatorconvertsadigitalsignaltoananalogsignal.Ademodulatorconvertsananalogsignaltoadigitalsignal.ModemConcept6.4MODEMSEachDCEmustbecompatiblewithbothitsownDTEandwithotherDCEs.6.4.1TransmissionRateBandwidthEverylinehasanupperlimitandalowerlimitonthefrequenciesofthesignalsitcancarry,Thislimitedrangeiscalledthebandwidth.ModemSpeed

ThespeedofmodemisdependuponitssignalmodulatingmethodsASKFSKPSKandQAMBandwidthTraditionaltelephonelinescancarryfrequenciesbetween300Hzand3300Hz,givingthemabandwidthof3000Hz.Theeffectivebandwidthofatelephonelinebeingusedfordatatransmissionis2400Hz,coveringtherangefrom600Hzto3000Hz.ModemSpeedThebandwidthrequiredforASKtransmissionisequaltothebaudrateofthesignal.①

Baudrareinsimplexorhalf-duplexASK②Baudrareinfull-duplexASKModemSpeed③Baudrateinhalf-duplexFSKModemSpeed④Baudrateinfull-duplexFSKModemSpeedPSKandQAMTheminimumbandwidthrequiredforPSKorQAMtransmissionisthesameasthatrequiredforASKtransmission.Thebitratecanbegreaterdependingonthenumberofbitsthatcanberepresentedbyeachsignalunit.ModemSpeedTheoreticalbitratesfarmodernsEncodingHalf-DuplexFull-DuplexASK,FSK,2-PSK4-PSK,4-QAM8-PSK,8-QAM16-QAM32-QAM64-QAM128-QAM256-QAM240048007200960012,00014,40016,80019,20012002400360048006000720084009600ModemSpeed贝尔调制解调器和ITU-T调制解调器：

20世纪70年代早期，贝尔电话公司生产了第一台商用调制解调器。作为市场上第一家和相当长时间内惟一的一家生产商，贝尔公司规定了技术的开发并为后来的厂商提供了一套事实上的标准。许多常用的调制解调器是基于ITU-T发布的标准的。这些调制解调器可以分为两组：一组是与贝尔系列调制解调器等价的(例如，V.21与贝尔调制解调器103类似)；另一组是不同的。6.4.2ModemStandards6.4.2ModemStandardsBellModemsITU-TModem103/113202212201208209V.22bitsV.32v.32bitsV.32terboV.33V.34TwomodemstandardsBellModemsITU-TModemITU-T/BellcompatibilityITU-TBellBaudRateBitRateModulationV.21V.22V.23V.26V.27V.29103212202201208209300600120012001600240030012001200240048009600FSK4-PSKFSK4-PSK8-PSK16-QAMModemStandardsTheIntelligentmodems(智能调制解调器):containsoftwaretosupportanumberofadditionalfunctions,suchasautomaticanswering(自动应答)anddialing(拨号).IntelligentmodemswerefirstintroducedbyHayesMicrocomputerProducts,Inc.IntelligentModems用调制解调器从一台计算机发送数据到另一台计算机Example:从A处到B处的数据传输用调制解调器从一台计算机发送数据到另一台计算机的情况:1、数字数据被A处的调制解调器调制；2、从调制解调器发送模拟数据并利用本地回路传送到A处的交换局;3、在交换局，模拟信号用PCM方法转换为数字信号;4、通过电话公司的数字网络输送数字数据，到达B处的交换局;5、在B处的交换局，数字数据用PCM方法转换成模拟数据;6、从B处的交换局发送模拟数据并利用本地回路传送到调制解调器；7、用B处的调制解调器解调模拟数据。Example:从A处到B处的数据传输SUMMARYDigitaltransmissioncanbeeitherparallelorserialinmode.Inparalleltransmission,agroupofbitsissentsimultaneously,witheachbitonaseparateline.Inserialtransmission,thereisonlyonelineandthebitsaresentsequentially.Serialtransmissioncanbeeithersynchronousorasynchronous.Inasynchronousserialtransmissioneachbyte(groupofeightbits)isframedwithastartbitandastopbit.Theremaybeavariable-lengthgapbetweeneachbyte.SUMMARYInsynchronousserialtransmission,bitsaresentinacontinuousstreamwithoutstartandstopbitsandwithoutgapsbetweenbytes.Regroupingthebitsintomeaningfulbytesisther