第六章數(shù)字數(shù)據(jù)傳輸：接口與調(diào)制解調(diào)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(配線).Theprimaryconcernwhenconsideringthewiringisdatastream.

Dowesendonebitatatimeoragroupbits?6.1DigitalDataTransmissionThetransmissionofbinarydataacrossalinkcanbeaccomplishedeitherinparallelorserialmode.Thetypeofdatatransmissionmode6.1.1ParallelTransmissionBinarydatamaybeorganizedintogroupsofnbitseach,andcansenddatanbitsatatime.Thismodecalledparalleltransmission(并行傳輸).Themechanismforparalleltransmissionisusenwirestosendnbitsatonetime.

eachbithasitsownwire;allnbitsbetransmittedwitheachclockpulse.6.1.1ParallelTransmission發(fā)送方接收方011000108個比特一起發(fā)送需要8條線路一般將n根導線組成一根電纜或排線，每端配有連接頭。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(異步傳輸)Thesenderhandleseachgroupindependently,sendingitwheneverready,withoutregardtoatimer.Toalertthereceivertothearrivalofanewgroup,anextrabitisaddedtothebeginningofeachbyte(calledstartbit,usuallya0).Toletthereceiverknowthatthebyteisfinished,oneormoreadditionalbitsareappendedtotheendofthebyte(calledstopbits,usually1s).Inaddition,thetransmissionofeachbytemaythenbefollowedbyagapofvaryingduration.AsynchronousTransmission(異步傳輸)AsynchronousTransmission(異步傳輸)Thestartandstopbitsandthegap

alertthereceivertothebeginningandendofeachbyteandallowittosynchronizewiththedatastream.SenderReceiver10111000110111000110111000110111000AsynchronousTransmission(異步傳輸)在字節(jié)級別，收發(fā)雙方無需同步，即雙方不必在何時發(fā)送上達成一致，因此稱這種方式為異步傳輸。在每字節(jié)內(nèi)部，每個數(shù)據(jù)位仍然需要同步！Advantage:

cheapandeffective.Disadvantage:

Theadditionofstopandstartbitsandtheinsertionofgapsintothebitstreammakeittransmissionslower.Itisanattractivechoiceforlow-speedcommunication.AsynchronousTransmission(異步傳輸)Asynchronousheremeans“asynchronousatthebytelevel,butthebitsarestillsynchronized”

Theirdurationsarethesame.NoteAsynchronousTransmission(異步傳輸)SynchronousTransmission(同步傳輸)Insynchronoustransmissionwesendbitsoneafteranotherwithoutstart/stopbitsorgaps.Itistheresponsibilityofthereceivertogroupthebits.Ifthesenderwishestosenddatainseparatebursts,thegapsbetweenburstsmustbefilledwithaspecialsequenceof0sand1sthatmeansidle.Becausetheaccuracyofthereceivedinformationiscompletelydependentontheabilityofthereceivingdevicetokeepanaccuratecountofthebits,thereforetimingisveryimportant.SynchronousTransmission(同步傳輸)Theadvantageofsynchronoustransmissionisspeed.

Withnoextrabitsorgapstointroduceatthesendingendandremoveatthereceivingendand,thereforewithfewerbitstomoveacrossthelink.

Bytesynchronizationisaccomplishedinthedatalinklayer.SynchronousTransmission(同步傳輸)Insynchronoustransmission,

wesendbitsoneafteranotherwithoutstart/stopbitsorgaps.NoteSynchronousTransmission(同步傳輸)6.2DTE-DCEInterfaceBasicConceptDTE(DataTerminalEquipment,數(shù)據(jù)終端設(shè)備)isanydevicethatisasourceofordestinationforbinarydigitaldata.Atphysicallayer,itcanbeterminal,computer,printer,faxmachine,oranyotherdevicethatgeneratesorconsumesdigitaldata.6.2DTE-DCEInterfaceBasicConceptDCE(DataCircuit-TerminatingEquipment,數(shù)據(jù)電路終接設(shè)備)

isanydevicethattransmitsorreceivesdataintheformofananalogordigitalsignalthroughanetwork.CommonlyusedDCEatphysicallayerincludemodems.6.2DTEandDCEDTEDCEDTEDCE發(fā)送端：DTE產(chǎn)生數(shù)據(jù)，并連同必要的控制字符一同傳輸給與其相連的DCE。DCE將數(shù)據(jù)轉(zhuǎn)換為適合傳輸介質(zhì)的形式，并將信號發(fā)送到網(wǎng)絡(luò)鏈路中。接收端：相應的逆操作。6.2DTE-DCEInterface通信過程中，發(fā)送和接收信號的DCE設(shè)備必須采用同一種調(diào)制方法(如FSK)，并且需要兩個設(shè)備協(xié)調(diào)工作.兩個DTE設(shè)備不需要互相協(xié)調(diào)，但是每個DTE設(shè)備必須保持與其直接相連的DCE設(shè)備協(xié)調(diào)工作。Manystandardshavebeendevelopedtodefinetheconnection

betweenaDTEandaDCE.Standardprovidesamodelforthe

mechanical,

electrical,and

functional

characteristicsoftheconnection.6.2.1StandardsTheEIA(電子工業(yè)協(xié)會)andtheITU-T(國際電信聯(lián)盟標準化部)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(傳號)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標準中發(fā)送數(shù)據(jù)的電氣規(guī)范電壓時間允許區(qū)域允許區(qū)域未定義區(qū)域未定義區(qū)域+15+30-3-15NRZ-L編碼ElectricalSpecification1010Only4wiresoutof25availableinEIA-232interfaceareusefordatafunctions.Theremaining21areusedforfunctionslikecontrol,timing,groundingandtesting.TheelectricalspecificationofEIA-232definesthatsignalsotherthandatamustbesentusingOFF(=>lessthan-3volts)andON(=>greaterthan+3volts).ControlandTiming電壓時間允許區(qū)域允許區(qū)域未定義區(qū)域未定義區(qū)域+3-ControlandTimingEIA-232標準中控制信號的電氣規(guī)范正電壓表示開狀態(tài)；負電壓表示關(guān)狀態(tài)。

注意：控制線上沒有電壓表示設(shè)備狀態(tài)不正常，而不是線路處于關(guān)閉狀態(tài)。ElectricalSpecificationThefinalimportantfunctionofelectricalspecificationisbitrate.

EIA-232allowsformaximumbitrateof20Kbps.Note:在具體的實現(xiàn)過程中，一般都選擇更高的比特率！FunctionalSpecificationDB-25

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

connector-pin3:receive(RxD);pin2:transmit(TxD);pin7:groud-otherpinsfortransmissioncontrolFunctionalSpecificationEIA-232有兩種實現(xiàn)方式：DB-25、DB-9注意：兩種實現(xiàn)的針腳不存在必然的對應關(guān)系！DB-25實現(xiàn)方式中各針腳功能分配FunctionalSpecificationFunctionalSpecificationDB-9與DB-25針腳之間的關(guān)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設(shè)備就緒

6pin:DCE設(shè)備就緒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空調(diào)制解調(diào)器的兩端都是陰性連接頭，用于連接兩端DTE設(shè)備的陽性EIA-232接口。6.3OtherInterfaceStandardsBothdatarateandcablelength(signaldistancecapability)arerestrictedbyEIA-232,datarateto20Kbpsandcablelengthto50feet(15meters).Tomeettheneedsofuserswhorequiremorespeedand/ordistance,theEIAandtheITU-Thaveintroducedadditionalinterfacestandards:EIA-449,EIA-530andX.2l.EIA-449的機械規(guī)范定義了兩種連接頭：DB-37與DB-9，兩種組合成一種46針的配置。EIA-449的功能規(guī)范給DB-37定義了與DB-25類似的功能，但在DB-37中沒有與輔助信道相關(guān)的功能，EIA-449將這些功能單獨分離，用DB-9來提供這些功能。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(選擇備用).Pin32allowstheDTEtorequesttheuseofstandbyequipmentintheeventoffailure.Newsignal.Pin34isavailableformultiplepointapplicationswhereaprimaryDTEcontrolsseveralsecondaryDTEs.Whenactivated,pin34indicatesthatoneDTEhasfinisheditsdataexchangeandanewoneisabouttostart.DB-37CategoryIIPins:Standbyindicator(備用指示).Pin36providestheconfirmationsignalfromtheDCEinresponsetoselectstandby(pin32).Sendcommon.Pin37providesacommonsignalreturnlineforunbalancedcircuitsfromtheDTEtotheDCE.Terminalinservice(終端服務(wù)中).Pin28indicatestotheDCEwhetherornottheDTEisoperational.DB-3DB-9PinFunctionEIA-232Equivalent123456789ShieldSecondaryreceivereadySecondarysenddataSecondaryreceivedataSignalgroundReceivecommonSecondaryrequesttosendSecondarycleartosendSendcommon114167121913PinfunctionsElectricalSpecificationsEIA-449usestwostandardstodefineitselectricalspecifications:RS-423:unbalancedcircuitsRS-422:balancedcircuitsElectricalSpecificationsRS-423isanunbalancedcircuitspecification,meaningthatitdefinesonlyonelineforpropagatingitssignal.Allsignalsinthisstandarduseacommonreturntocompletethecircuit.在非平衡電路模式中，EIA-449使用每一對I類針腳的第一個針腳和所有的II類針腳。

NoteElectricalSpecificationsElectricalSpecificationsRS-422isbalancedcircuitspecification,meaningthatitdefinestwolinesforpropagationofeachsignal.Signalsagainuseacommonreturnforthereturnofthesignal.在平衡模式中，EIA-449使用了每對I類針腳但不使用II類針腳。

NoteElectricalSpecifications在平衡模式中，兩條線路承載相同的傳輸。但是它們并不傳輸相同的信號。在一條線路上的信號是另一條線路上信號的互補值。此時接收方并不監(jiān)聽其中任何一個實際的信號，而是檢測兩個信號之間的差值。

ElectricalSpecificationsAsthecomplementarysignalsarriveatthereceiver,theyareputthroughasubtracter.Thismechanismsubtractsthesecondsignalfromthefirstbeforeinterpretation.Becausethetwosignalscomplementeachother,theresultofthissubtractionisadoublingofthevalueofthefirstsignal.Ifnoiseisaddedtothetransmission,itimpactsbothsignalsinthesameway.Asaresult,thenoiseiseliminatedduringthesubtractionprocess.ElectricalSpecificationsExample:CancelingofnoiseusingbalancedmodeElectricalSpecifications例如，如果在某時刻第一個信號值為5V，第二個信號值將是-5V。所以，相減的結(jié)果是5-(-5)=10。如果在傳輸中有噪聲，則噪聲會以同樣方式影響兩個互補信號(正噪聲對信號產(chǎn)生正的影響，負噪聲對信號產(chǎn)生負的影響)。結(jié)果是在減法過程中消除噪聲。例如，假定在某點存在2V噪聲，而此時第一個信號是+5V，它的互補信號是-5V。額外的歧變使第一個信號值變?yōu)?V，第二個信號為-3V。此時7-(-3)仍然是10V，這種特性有助于中和噪聲的影響并使得高速的平衡模式傳輸成為可能。6.3.2EIA-530EIA-530:aversionofEIA-449thatusesDB-25pins.ThepinfunctionsofEIA-530areessentiallythoseofEIA-449CategoryIplusthreepinsfromCategoryII.EIA-530doesnotsupportasecondarycircuit.6.3.3X.21X.21標準是lTU-T制訂的。它的目的是為了解決EIA接口中存在的問題，同時為完全的數(shù)字通信提供解決方案。Usingdatacircuitsforcontrol在EIA接口中一大部分的電路被用于控制信息。X.21撤消了EIA中大多數(shù)控制線路，并將控制信息導向數(shù)據(jù)線路。X.21標準僅采用了較少的針腳，并且能夠在數(shù)字化遠程通信中使用。X.21設(shè)計的工作環(huán)境是64Kbps的平衡線路。X.21定義的連接頭稱為DB-15。它在EIA標準提供的比特同步之外還提供了用于控制字節(jié)同步的時鐘線。同時，它通過控制和指示線來進行初始化握手過程或開始傳輸?shù)募s定過程。PinFunctionsDB-15ConnectorBytetiming(字節(jié)時序).AdvantageofferedbyX.21isthatoftiminglinestocontrolbytesynchronizationinadditiontothebitsynchronizationprovidedbytheEIAstandards.Byaddingabytetimingpulse(pins7and14),X.21improvestheoverallsynchronizationoftransmissions.Controlandindication(控制與初始化).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貝爾調(diào)制解調(diào)器和ITU-T調(diào)制解調(diào)器：

20世紀70年代早期，貝爾電話公司生產(chǎn)了第一臺商用調(diào)制解調(diào)器。作為市場上第一家和相當長時間內(nèi)惟一的一家生產(chǎn)商，貝爾公司規(guī)定了技術(shù)的開發(fā)并為后來的廠商提供了一套事實上的標準。許多常用的調(diào)制解調(diào)器是基于ITU-T發(fā)布的標準的。這些調(diào)制解調(diào)器可以分為兩組：一組是與貝爾系列調(diào)制解調(diào)器等價的(例如，V.21與貝爾調(diào)制解調(diào)器103類似)；另一組是不同的。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(智能調(diào)制解調(diào)器):containsoftwaretosupportanumberofadditionalfunctions,suchasautomaticanswering(自動應答)anddialing(撥號).IntelligentmodemswerefirstintroducedbyHayesMicrocomputerProducts,Inc.IntelligentModems用調(diào)制解調(diào)器從一臺計算機發(fā)送數(shù)據(jù)到另一臺計算機Example:從A處到B處的數(shù)據(jù)傳輸用調(diào)制解調(diào)器從一臺計算機發(fā)送數(shù)據(jù)到另一臺計算機的情況:1、數(shù)字數(shù)據(jù)被A處的調(diào)制解調(diào)器調(diào)制；2、從調(diào)制解調(diào)器發(fā)送模擬數(shù)據(jù)并利用本地回路傳送到A處的交換局;3、在交換局，模擬信號用PCM方法轉(zhuǎn)換為數(shù)字信號;4、通過電話公司的數(shù)字網(wǎng)絡(luò)輸送數(shù)字數(shù)據(jù)，到達B處的交換局;5、在B處的交換局，數(shù)字數(shù)據(jù)用PCM方法轉(zhuǎn)換成模擬數(shù)據(jù);6、從B處的交換局發(fā)送模擬數(shù)據(jù)并利用本地回路傳送到調(diào)制解調(diào)器；7、用B處的調(diào)制解調(diào)器解調(diào)模擬數(shù)據(jù)。Example:從A處到B處的數(shù)據(jù)傳輸SUMMARYDigitaltransmissioncanbeeitherparallelorserialinmode.Inparalleltransmission,agroupofbitsissentsimultaneously,witheachbitonaseparateline.Inserialtransmission,thereisonlyonelineandthebitsaresentsequentially.Serialtransmissioncanbeeithersynchronousorasynchronous.Inasynchronousserialtransmissioneachbyte(groupofeightbits)isframedwithastartbitandastopbit.Theremaybeavariable-lengthgapbetweeneachbyte.SUMMARYInsynchronousserialtransmission,bitsaresentinacontinuousstreamwithoutstartandstopbitsandwithoutgapsbetweenbytes.Regroupingthebitsintomeaningfulbytesisther