計(jì)算機(jī)網(wǎng)絡(luò)：第二章 數(shù)據(jù)通信基礎(chǔ)

1ChapterTwoFundamentalsofDatacommunication2FundamentalsofDataCommunicationIntroductionofdataandsignalDatacommunicationmodelDataconvertingConnectionwayMultiplexSwitchway32.1數(shù)據(jù)通信系統(tǒng)概論作用組成：信源、傳輸信道、信宿42.2數(shù)據(jù)通信模型數(shù)據(jù)通信：實(shí)現(xiàn)在計(jì)算機(jī)網(wǎng)絡(luò)中任意兩個(gè)節(jié)點(diǎn)A和B之間傳遞有用的信息DTEDCEDTEDCE傳輸線路資源子網(wǎng)通信子網(wǎng)資源子網(wǎng)DTE/DCE接口DTE/DCE接口數(shù)據(jù)通信模型節(jié)點(diǎn)A節(jié)點(diǎn)B52.2數(shù)據(jù)通信模型數(shù)據(jù)通信系統(tǒng)的組成：DTE（DataTerminalEquipment）：數(shù)據(jù)終端設(shè)備。主機(jī)、終端、通信控制機(jī)或其他發(fā)送接收設(shè)備DCE（DataCircuit-terminatingEquipment

）：與傳輸介質(zhì)直接相連的設(shè)備。MODEM、ACEDTE/DCE接口：指DTE和DCE之間如何有效的物理連接起來(lái)，實(shí)現(xiàn)它們之間傳輸信息的同步。輸出輸入電路、連接器和電纜傳輸線路：雙絞線、光纖、同軸電纜、無(wú)線通信等6Introduction–DataandSignalsDataareentitiesthatconveymeaning(computerfile,musiconCD,resultsfromabloodgasanalysismachine)SignalsaretheelectricorelectromagneticencodingofdataComputernetworksanddata/voicecommunicationsystemstransmitsignalsDataandsignalscanbeanalogordigital2.2-Introductionofdataandsignal7FourCombinationsofDataandSignalsChapter2.1-Introductionofdataandsignal8AnalogversusDigitalChapter2.1-IntroductionofdataandsignalAnalogisacontinuouswaveform,withexamplessuchas(naturallyoccurring)musicandvoice9AnalogversusDigitalItishardertoseparatenoisefromananalogsignalthanitistoseparatenoisefromadigitalsignal(imaginethefollowingwaveformisasymphonywithnoiseembedded)Chapter2.1-Introductionofdataandsignal10AnalogversusDigitalDigitalisadiscreteornon-continuouswaveformwithexamplessuchascomputer1sand0sChapter2.1-Introductionofdataandsignal11AnalogversusDigitalNoiseinadigitalsignal.YoucanstilldiscernahighvoltagefromalowvoltageChapter2.1-Introductionofdataandsignal12AnalogversusDigitalNoiseinadigitalsignal.Toomuchnoise–youcannotdiscernahighvoltagefromalowvoltageChapter2.1-Introductionofdataandsignal13AllSignalsHaveThreeComponentsAmplitudeFrequencyPhaseChapter2.1-Introductionofdataandsignal14AmplitudeTheamplitudeofasignalistheheightofthewaveaboveorbelowagivenreferencepointChapter2.1-Introductionofdataandsignal15FrequencyThefrequencyisthenumberoftimesasignalmakesacompletecyclewithinagiventimeframe;frequencyismeasuredinHertz(Hz),orcyclespersecondSpectrum(光譜）–therangeoffrequenciesthatasignalspansfromminimumtomaximumBandwidth–theabsolutevalueofthedifferencebetweenthelowestandhighestfrequenciesofasignalChapter2.1-Introductionofdataandsignal16Chapter2.1-Introductionofdataandsignal17FrequencyForexample,consideranaveragevoiceTheaveragevoicehasafrequencyrangeofroughly300Hzto3100HzThespectrumwouldbe300–3100HzThebandwidthwouldbe2800HzChapter2.1-Introductionofdataandsignal18PhaseThephaseofasignalisthepositionofthewaveformrelativetoagivenmomentoftimeorrelativetotimezeroAchangeinphasecanbeanynumberofanglesbetween0and360degreesPhasechangesoftenoccuroncommonangles,suchas45,90,135,etc.Chapter2.1-Introductionofdataandsignal19Chapter2.1-Introductionofdataandsignal20SignalStrengthAllsignalsexperienceloss(attenuation，衰減)Attenuationisdenotedasadecibel(dB)lossDecibellosses(andgains)areadditiveChapter2.1-Introductionofdataandsignal21SignalStrengthSoifasignalloses3dB,isthatalot?A3dBlossindicatesthesignallosthalfofitspower

dB=10log10(P2/P1) -3dB=10log10(X/100) -0.3=log10(X/100) 10-0.3=X/100 0.50=X/100 X=50Chapter2.1-Introductionofdataandsignal22ConvertingAnalogDataintoAnalogSignalsInordertotransmitanalogdata,youcanmodulatethedataontoasetofanalogsignalsBroadcastradioandtelevisionaretwoverycommonexamplesofthisChapter2.3–DataConverting23Amplitudemodulation(e.g.AMradio)Chapter2.3–DataConverting24ConvertingDigitalDataintoDigitalSignalsTherearenumeroustechniquesavailabletoconvertdigitaldataintodigitalsignals.Let’sexaminefive:NRZ-L(不歸零編碼）NRZ-I（不歸零反相編碼）Manchester（曼徹斯特編碼）DifferentialManchester（差分曼徹斯特編碼）Chapter2.3–DataConverting25NRZ-LandNRZ-IEncoding

兩種不歸零極性碼26NonreturntoZero(NRZ-L)

不歸零電平編碼NRZ-L(不歸零電平編碼)Anegativevoltageisusedtorepresentonebinaryvalueandapositivevoltagetorepresenttheother.Thebitstatedeterminesthelevelofasignal.27NonreturntoZero(NRZI)

不歸零反相編碼NRZ-I(不歸零1制,NRZI)Atransition(lowtohighorhightolow)atthebeginningofabittimedenotesabinary1forthatbittime;notransitionindicatesabinary0.DifferentialencodingformorereliabilityThebitstatedeterminesthetransitionofasignalbetweenapositiveandnegativevoltage,notthevoltagesthemselves.28曼徹斯特和差分曼徹斯特編碼

ManchesterandDiff.ManchesterEncoding29ManchesterEncoding

曼徹斯特編碼Thereisatransitionatthemiddleofeachbitperiod.Themidbittransitionservesasaclockingmechanismandalsoadata:alow-to-hightransitionrepresentsa1,andahigh-to-lowtransitionrepresentsa0.30DifferentialManchesterEncoding

差分曼徹斯特編碼Themidbittransitionisusedonlytoprovideclocking.Theencodingofa0isrepresentedbythepresenceofatransitionatthebeginningofabitperiod,anda1isrepresentedbytheabsenceofatransitionatthebeginningofabitperiod.3132NotehowwithaDifferentialManchestercode,everybithasatleastonesignificantchange.Somebitshavetwosignalchangesperbit(baudrate=twicebps)Chapter2.3–DataConverting334B/5BDigitalEncodingYetanotherencodingtechniquethatconvertsfourbitsofdataintofive-bitquantities5bitscannotcontainmorethan3”0”insequence;andcannotlessthan2”1”Thefive-bitquantitiesareuniqueinthatnofive-bitcodehasmorethan2consecutivezeroesThefive-bitcodeisthentransmittedusinganNRZ-Iencodedsignal”逢一跳變，0不跳變”Chapter2.3–DataConverting34Chapter2.3–DataConverting35ConvertingDigitalDataintoAnalogSignalsThreebasictechniques:AmplitudeshiftkeyingFrequencyshiftkeyingPhaseshiftkeyingChapter2.3–DataConverting36AmplitudeShiftKeyingOneamplitudeencodesa0whileanotheramplitudeencodesa1(aformofamplitudemodulation)Chapter2.3–DataConverting37AmplitudeShiftKeyingSomesystemsusemultipleamplitudesChapter2.3–DataConverting38MultipleSignalLevelsWhyusemultiplesignallevels?Wecanrepresenttwolevelswithasinglebit,0or1Wecanrepresentfourlevelswithtwobits:00,01,10,11Wecanrepresenteightlevelswiththreebits:000,001,010,011,100,101,110,111Notethatthenumberoflevelsisalwaysapowerof2Chapter2.3–DataConverting39FrequencyShiftKeyingOnefrequencyencodesa0whileanotherfrequencyencodesa1(aformoffrequencymodulation)Chapter2.3–DataConverting40PhaseShiftKeyingOnephasechangeencodesa0whileanotherphasechangeencodesa1(aformofphasemodulation)Chapter2.3–DataConverting41QuadraturePhaseShiftKeyingFourdifferentphaseanglesused45degrees135degrees225degrees315degreesChapter2.3–DataConverting42Chapter2.3–DataConverting43QuadratureAmplitudeModulationAsanexampleofQAM,12differentphasesarecombinedwithtwodifferentamplitudesSinceonly4phaseangleshave2differentamplitudes,thereareatotalof16combinationsWith16signalcombinations,eachbaudequals4bitsofinformation(2^4=16)Chapter2.3–DataConverting44Chapter2.3–DataConverting45數(shù)據(jù)傳輸速率數(shù)據(jù)傳輸速率是指每秒能傳輸?shù)亩M(jìn)制信息位數(shù)，位/秒

S=1/T*log2NT為信號(hào)周期信號(hào)傳輸速率

B=1/TS=B*log2N46信道容量：Nyquist定理在無(wú)噪聲信道中，當(dāng)帶寬為HHz，信號(hào)電平為V級(jí)，則：數(shù)據(jù)傳輸速率=2Hlog2Vb/s（信號(hào)電平為V級(jí)，在二進(jìn)制中，僅為0、1兩級(jí)）

即：以每秒高于2H次的速率對(duì)線路采樣是無(wú)意義的，因?yàn)楦哳l分量已被濾波器慮掉無(wú)法再恢復(fù)。

47Shannon定理

在噪聲信道中，當(dāng)帶寬為HHz，信噪比為S/N，則：最大數(shù)據(jù)傳輸速率(b/s)=Hlog2(1+S/N)

很多情況下噪聲用分貝(dB)表示噪聲（dB）=10log10S/N如：噪聲為30dB，則信噪比為S/N=100048舉例：噪聲信道中的傳輸速率在噪聲信道中，當(dāng)帶寬為3kHz，信噪比為30dB（較為真實(shí)的電話信道），則：最大數(shù)據(jù)傳輸速率(b/s)=Hlog2(1+S/N)=3000log2(1+1000)=30000(b/s)最大數(shù)據(jù)傳輸速率為30kb/s，這是在噪聲信道中的傳輸速率極限，實(shí)際上是不可能達(dá)到的49舉例（續(xù)）如需達(dá)到此傳輸速率，根據(jù)Nyquist定理計(jì)算：30000=2x3000xlog2Vlog2V=5V=32即必須要把采樣信號(hào)量化為32個(gè)等級(jí)，這在模擬電話信道中是不可能的，因?yàn)榇藭r(shí)的信號(hào)變化已“湮沒(méi)”在噪聲中50HigherDataTransferRatesHowdoyousenddatafaster?Useahigherfrequencysignal(makesurethemediumcanhandlethehigherfrequencyUseahighernumberofsignallevelsInbothcases,noisecanbeaproblemChapter2.3–DataConverting51提高數(shù)據(jù)傳輸速率的方法提高相位狀態(tài)如4相位或者8相位，原來(lái)速率為1600，則新速率為？幅度+相位如采用12種相位，其中4種相位每個(gè)信號(hào)取2種幅度，則總狀態(tài)數(shù)是？原來(lái)2400的速率，現(xiàn)在是多少？52MaximumDataTransferRatesHowdoyoucalculateamaximumdatarate?UseShannon’sequation

S(f)=fxlog2(1+S/N)Wheref=signalfrequency(bandwidth),Sisthesignalpowerinwatts,andNisthenoisepowerinwattsChapter2.3–DataConverting53MaximumDataTransferRatesForexample,whatisthedatarateofa3400Hzsignalwith0.2wattsofpowerand0.0002wattsofnoise? S(f)=3400xlog2(1+0.2/0.0002) =3400xlog2(1001) =3400x9.97 =33898bpsChapter2.3–DataConverting54ConvertingAnalogDataintoDigitalSignalsToconvertanalogdataintoadigitalsignal,therearetwotechniques:Pulsecodemodulation(themorecommon)DeltamodulationChapter2.3–DataConverting55PulseCodeModulationTheanalogwaveformissampledatspecificintervalsandthe“snapshots”areconvertedtobinaryvaluesChapter2.3–DataConverting56PulseCodeModulationWhenthebinaryvaluesarelaterconvertedtoananalogsignal,awaveformsimilartotheoriginalresultsChapter2.3–DataConverting57PulseCodeModulationSincetelephonesystemsdigitizehumanvoice,andsincethehumanvoicehasafairlynarrowbandwidth,telephonesystemscandigitizevoiceintoeither128or256levelsThesearecalledquantizationlevelsIf128levels,theneachsampleis7bits(2^7=128)If256levels,theneachsampleis8bits(2^8=256)Chapter2.3–DataConverting58PulseCodeModulationHowfastdoyouhavetosampleaninputsourcetogetafairlyaccuraterepresentation?Nyquistsays2timesthehighestfrequencyThus,ifyouwanttodigitizevoice(4000Hz),youneedtosampleat8000samplespersecondChapter2.3–DataConverting59DeltaModulationAnanalogwaveformistracked,usingabinary1torepresentariseinvoltage,anda0torepresentadropChapter2.3–DataConverting60DataCodesThesetofalltextualcharactersorsymbolsandtheircorrespondingbinarypatternsiscalledadatacodeTherearethreecommondatacodesets:EBCDICASCIIUnicodeChapter2.3–DataConverting61EBCDICChapter2.3–DataConverting62ASCIIChapter2.3–DataConverting63UnicodeEachcharacteris16bitsAlargenumberoflanguages/charactersetsForexample:Tequals0000000001010100requals0000000001110010aequals0000000001100001Chapter2.3–DataConverting64HalfDuplex,FullDuplex,andSimplexConnectionsAhalfduplexconnectiontransmitsdatainbothdirectionsbutinonlyonedirectionatatimeAfullduplexconnectiontransmitsdatainbothdirectionsandatthesametimeAsimplexconnectioncantransmitdatainonlyonedirectionChapter2.4-MakingConnections65DigitalDataTransmission

數(shù)字?jǐn)?shù)據(jù)傳輸形式66Parallel

Transmission

并行傳輸Advantage:higherspeedDisadvantage:expensiveUsage:shortdistances,uptoamaximumofsay25feet(7.5m).67SerialTransmission

串行傳輸Advantage:inexpensiveDisadvantage:lowerspeedConversiondevicesarerequiredatInterfacesbetweensenderandline(parallel-to-serial)andbetweenreceiverandline(serial-to-parallel)68AsynchronousConnectionsAtypeofconnectiondefinedatthedatalinklayerTotransmitdatafromsendertoreceiver,anasynchronousconnectioncreatesaone-characterpackagecalledaframeAddedtothefrontoftheframeisaStartbit,whileaStopbitisaddedtotheendoftheframeAnoptionalparitybitcanbeaddedwhichcanbeusedtodetecterrorsChapter2.4-MakingConnections69AsynchronousConnectionsChapterFour-MakingConnections70AsynchronousConnectionsChapterFour-MakingConnections71AsynchronousConnectionsThetermasynchronousismisleadingherebecauseyoumustalwaysmaintainsynchronizationbetweentheincomingdatastreamandthereceiverAsynchronousconnectionsmaintainsynchronizationbyusingsmallframeswithaleadingstartbitChapterFour-MakingConnections72SynchronousConnectionsAsecondtypeofconnectiondefinedatthedatalinklayerAsynchronousconnectioncreatesalargeframethatconsistsofheaderandtrailerflags,controlinformation,optionaladdressinformation,errordetectioncode,anddataAsynchronousconnectionismoreelaboratebuttransfersdatainamoreefficientmannerChapterFour-MakingConnections73SynchronousConnectionsChapterFour-MakingConnections74IsochronousConnectionsAthirdtypeofconnectiondefinedatthedatalinklayerusedtosupportreal-timeapplicationsThedatamustbedeliveredatjusttherightspeed(real-time)–nottoofastandnottooslowTypicallyanisochronousconnectionmustallocateresourcesonbothendstomaintainreal-timeUSB(andFirewire)canbothsupportisochronousconnectionsChapterFour-MakingConnections75IntroductionUnderthesimplestconditions,amediumcancarryonlyonesignalatanymomentintimeFormultiplesignalstoshareamedium,themediummustsomehowbedivided,givingeachsignalaportionofthetotalbandwidthThecurrenttechniquesincludefrequencydivisionmultiplexing,timedivisionmultiplexing,andcodedivisionmultiplexingChapter2.5-MakingConnectionsEfficient:Multiplexing76FrequencyDivisionMultiplexingAssignmentofnon-overlappingfrequencyrangestoeach“user”orsignalonamedium.Thus,allsignalsaretransmittedatthesametime,eachusingdifferentfrequenciesAmultiplexoracceptsinputsandassignsfrequenciestoeachdeviceChapter2.5-MakingConnectionsEfficient:Multiplexing77FrequencyDivisionMultiplexingThemultiplexorisattachedtoahigh-speedcommunicationslineAcorrespondingmultiplexor,ordemultiplexor,isontheendofthehigh-speedlineandseparatesthemultiplexedsignalsChapter2.5-MakingConnectionsEfficient:Multiplexing78ChapterFive-MakingConnectionsEfficient:MultiplexingandCompression79FrequencyDivisionMultiplexingAnalogsignalingisusedtotransmitthedataBroadcastradioandtelevision,cabletelevision,andcellulartelephonesystemsusefrequencydivisionmultiplexingThistechniqueistheoldestmultiplexingtechniqueSinceitinvolvesanalogsignaling,itismoresusceptibletonoiseChapter2.5-MakingConnectionsEfficient:Multiplexing80TimeDivisionMultiplexingSharingofthesignalisaccomplishedbydividingavailabletransmissiontimeonamediumamongusersDigitalsignalingisusedexclusivelyTimedivisionmultiplexingcomesintwobasicforms:SynchronoustimedivisionmultiplexingStatisticaltimedivisionmultiplexingChapter2.5-MakingConnectionsEfficient:Multiplexing81SynchronousTimeDivisionMultiplexingTheoriginaltimedivisionmultiplexingThemultiplexoracceptsinputfromattacheddevicesinaround-robinfashionandtransmitsthedatainaneverendingpatternT-1andISDNtelephonelinesarecommonexamplesofsynchronoustimedivisionmultiplexingChapter2.5-MakingConnectionsEfficient:Multiplexing82Chapter2.5-MakingConnectionsEfficient:Multiplexing83SynchronousTimeDivisionMultiplexingIfonedevicegeneratesdataatafasterratethanotherdevices,thenthemultiplexormusteithersampletheincomingdatastreamfromthatdevicemoreoftenthanitsamplestheotherdevices,orbufferthefasterincomingstreamIfadevicehasnothingtotransmit,themultiplexormuststillinsertsomethingintothemultiplexedstreamChapter2.5-MakingConnectionsEfficient:Multiplexing84Chapter2.5-MakingConnectionsEfficient:Multiplexing85Chapter2.5-MakingConnectionsEfficient:Multiplexing86SynchronousTimeDivisionMultiplexingSothatthereceivermaystaysynchronizedwiththeincomingdatastream,thetransmittingmultiplexorcaninsertalternating1sand0sintothedatastreamChapter2.5-MakingConnectionsEfficient:Multiplexing87SynchronousTimeDivisionMultiplexingTheT-1multiplexorstreamisacontinuousseriesofframesChapter2.5-MakingConnectionsEfficient:Multiplexing88SynchronousTimeDivisionMultiplexingTheISDNmultiplexorstreamisaalsoacontinuousseriesofframes.EachframecontainsvariouscontrolandsyncinfoChapter2.5-MakingConnectionsEfficient:Multiplexing89SynchronousTimeDivisionMultiplexingLikewise,SONETincorporatesacontinuousseriesofframes.Chapter2.5-MakingConnectionsEfficient:Multiplexing90StatisticalTimeDivisionMultiplexingAstatisticalmultiplexortransmitsthedatafromactiveworkstationsonlyIfaworkstationisnotactive,nospaceiswastedinthemultiplexedstreamAstatisticalmultiplexoracceptstheincomingdatastreamsandcreatesaframecontainingthedatatobetransmittedChapter2.5-MakingConnectionsEfficient:Multiplexing91Chapter2.5-MakingConnectionsEfficient:Multiplexing92Toidentifyeachpieceofdata,anaddressisincludedChapter2.5-MakingConnectionsEfficient:Multiplexing93Ifthedataisofvariablesize,alengthisalsoincludedChapter2.5-MakingConnectionsEfficient:Multiplexing94Moreprecisely,thetransmittedframecontainsacollectionofdatagroupsChapter2.5-MakingConnectionsEfficient:Multiplexing95WavelengthDivisionMultiplexingWavelengthdivisionmultiplexingmultiplexesmultipledatastreamsontoasinglefiberopticlineDifferentwavelengthlasers(calledlambdas)transmitthemultiplesignalsChapter2.5-MakingConnectionsEfficient:Multiplexing96WavelengthDivisionMultiplexingEachsignalcarriedonthefibercanbetransmittedatadifferentratefromtheothersignalsDensewavelengthdivisionmultiplexingcombinesmany(30,40,50ormore)ontoonefiberCoarsewavelengthdivisionmultiplexingcombinesonlyafewlambdasChapter2.5-MakingConnectionsEfficient:Multiplexing97Chapter2.5-MakingConnectionsEfficient:Multiplexing98DiscreteMultitone(DMT)Amultiplexingtechniquecommonlyfoundindigitalsubscriberline(DSL)systemsADSL,threechannels:phone,upstream,downstreamPhone:0-4Khz;26-1.1MforInternet,multich\annels:4Khz25channelsforupstream;249fordownstream;;1Hzfor15bitsSpeedforupstream:25*4Khz*15=1.5MDMTcombineshundredsofdifferentsignals,orsubchannels,intoonestreamChapter2.5-MakingConnectionsEfficient:Multiplexing99DiscreteMultitone(DMT,ADSLModem)Eachsubchannelisquadratureamplitudemodulated(recalleightphaseangles,fourwithdoubleamplitudes)Theoretically,256subchannels,eachtransmitting60kbps,yields15.36MbpsUnfortunately,thereisnoiseChapter2.5-MakingConnectionsEfficient:Multiplexing100Chapter2.5-MakingConnectionsEfficient:Multiplexing101CodeDivisionMultiplexingAlsoknownascodedivisionmultipleaccessAnadvancedtechniquethatallowsmultipledevicestotransmitonthesamefrequenciesatthesametimeEachmobiledeviceisassignedaunique64-bitcodeChapter2.5-MakingConnectionsEfficient:Multiplexing102CodeDivisionMultiplexingTosendabinary1,amobiledevicetransmitstheuniquecodeTosendabinary0,amobiledevicestransmitstheinverseofthecodeChapter2.5-MakingConnectionsEfficient:Multiplexing103CodeDivisionMultiplexingReceivergetssummedsignal,multipliesitbyreceivercode,addsuptheresultingvaluesInterpretsasabinary1ifsumisnear+64Interpretsasabinary0ifsumisnear-64Chapter2.5-MakingConnectionsEfficient:Multiplexing104CodeDivisionMultiplexingForsimplicity,assume8-bitcodeThreedifferentmobiledevicesusethefollowingcodes:MobileA:10111001MobileB:01101110MobileC:11001101AssumeMobileAsendsa1,Bsendsa0,andCsendsa1Chapter2.5-MakingConnectionsEfficient:Multiplexing105CodeDivisionMultiplexingSignalcode:1-chip=+Nvolt;0-chip=-NvoltThreesignalstransmitted:MobileAsendsa1,or10111001,or+-+++--+MobileBsendsa0,or10010001,or+--+---+MobileCsendsa1,or11001101,or++--++-+Summedsignalreceivedbybasestation:+3,-1,-1,+1,+1,-1,-3,+3Chapter2.5-MakingConnectionsEfficient:Multiplexing106CodeDivisionMultiplexingBasestationdecodeforMobileA:Signalreceived:+3,-1,-1,+1,+1,-1,-3,+3MobileA’scode:+1,-1,+1,+1,+1,-1,-1,+1Productresult:+3,+1,-1,+1,+1,+1,+3,+3SumofProducts:+12Decoderule:Forresultnear+8,dataisbinary1Chapter2.5-MakingConnectionsEfficient:Multiplexing107CodeDivisionMultiplexingBasestationdecodeforMobileB:Signalreceived:+3,-1,-1,+1,+1,-1,-3,+3MobileB’scode:-1,+1,+1,-1,+1,+1,+1,-1Productresult:-3,-1,-1,-1,+1,-1,-3,-3SumofProducts:-12Decoderule:Forresultnear-8,dataisbinary0Chapter2.5-MakingConnectionsEfficient:Multiplexing108Chapter2.5-MakingConnectionsEfficient:Multiplexing109Chapter2.5-MakingConnectionsEfficient:Multiplexing110Chapter2.6-TransmissionMediaGuidedTransmissionMediaWirelessTransmission111Guided-wireUnguided-wirelessCharacteristicsandqualityofadatatransmissionaredeterminedbymediumandsignalForguided,themediumismoreimportantForunguided,thebandwidthproducedbytheantennaismoreimportantKeyconcernsaredatarateanddistanceChapter2.6-TransmissionMedia112Chapter2.6:DesignFactors

設(shè)計(jì)因素BandwidthHigherbandwidthgiveshigherdatarateTransmissionimpairmentsImpairment,suchasattenuation,limitthedistanceTwistedpair>coaxialcable>opticalfiberInterferenceNumberofreceiversInguidedmediaMorereceivers(multi-point)introducemoreattenuation1132.6.1導(dǎo)向傳輸媒體

GuidedTransmissionMedia1142.6.1:導(dǎo)向傳輸媒體

GuidedTransmissionMedia雙絞線同軸電纜光纜導(dǎo)向媒體1152.6.1TwistedPairCable

雙絞線116雙絞線雙絞線是由不同顏色的4對(duì)8芯線組成，每?jī)蓷l按一定規(guī)則絞織在一起，成為一個(gè)芯線對(duì)。傳輸特性：數(shù)字+模擬地理范圍：LAN10M100M連通性：點(diǎn)到點(diǎn)，點(diǎn)到多點(diǎn)抗干擾性：低頻，雙絞線高于同軸電纜價(jià)格：最便宜1172.6.1NoiseonTwisted-PairLines

雙絞線上的噪聲Twistingdoesnotalwayseliminatetheimpactofnoise,butdoessignificantlyreduceit.1182.6.1雙絞線的連接HUBRJ-45接頭(水晶頭)1192.6.1雙絞線的網(wǎng)線制作RJ-45接頭(水晶頭)壓線鉗1202.6.1雙絞線接頭的接線規(guī)則現(xiàn)行雙絞線電纜中一般包含4個(gè)雙絞線對(duì)，具體為橙/白橙、藍(lán)/白藍(lán)、綠/白綠、棕/白棕。計(jì)算機(jī)網(wǎng)絡(luò)使用1－2、3－6兩組線對(duì)分別來(lái)發(fā)送和接收數(shù)據(jù)。121122雙絞線接線標(biāo)準(zhǔn)鉸齊線頭，插入插頭用壓線鉗夾緊，使用測(cè)試儀測(cè)試接線的標(biāo)準(zhǔn)有EIA／TIAT568A和EIA／TIAT568B※1232.6.1雙絞線的特點(diǎn)其頻率范圍對(duì)于傳輸數(shù)據(jù)和語(yǔ)音都適用（現(xiàn)在5類非屏蔽雙絞線的傳輸頻率上限可以達(dá)到100MHZ以上）所以模擬傳輸（寬帶信號(hào)）/數(shù)字傳輸（基帶信號(hào)）均可使用單位價(jià)格最低，但每根只能連接一站點(diǎn)，線路總費(fèi)用增加抗高頻干擾能力較差使用雙絞線需在傳輸速率和傳輸距離之間作出選擇雙絞線（3類以下）的頻率范圍1242.6.1UTP和STP雙絞線可分為非屏蔽雙絞線（UTP）和屏蔽雙絞線（STP）兩大類，兩者的差別是在于雙絞線內(nèi)是否有一層金屬隔離膜。STP內(nèi)有一層金屬隔離膜，在數(shù)據(jù)傳輸時(shí)可減少電磁干擾，所以穩(wěn)定性較高，價(jià)格比UTP貴。UTP內(nèi)沒(méi)有這層金屬隔離膜，所以穩(wěn)定性較差，但價(jià)格相對(duì)來(lái)說(shuō)便宜多了。125UTPInterfaceStyle1262.6.1ShieldedTwisted-PairCable

屏蔽雙絞線127UnshieldedandShieldedTP

無(wú)屏蔽和屏蔽雙絞線UnshieldedTwistedPair(UTP)OrdinarytelephonewireCheapestEasiesttoinstallSuffersfromexternalEMinterferenceShieldedTwistedPair(STP)MetalbraidorsheathingthatreducesinterferenceMoreexpensiveHardertohandle(thick,heavy)128

TwistedPair-ApplicationsMostcommonmediumTelephonenetworkBetweenhouseandlocalexchange(subscriberloop)WithinbuildingsToprivatebranchexchange(PBX)Forlocalareanetworks(LAN)10Mbpsor100Mbps129傳輸特性

TransmissionCharacteristicsAnalogAmplifiersevery5kmto6kmDigitalUseeitheranalogordigitalsignalsrepeaterevery2kmor3kmLimiteddistance(sensitivetoattenuationandimpulsenoise)Limitedbandwidth(1MHz)Limiteddatarate(100Mbps,even1Gbps)Shieldingreducesinterference.Twistingreduceslow-frequencyinterferency,anddifferenttwitstlengthsinadjacentpairsreducescrosstalk.130TwistedPair-ProsandCons

雙絞線的優(yōu)缺點(diǎn)CheapEasytoworkwithLowdatarateShortrange廉價(jià)連接制作方便數(shù)據(jù)率不高近距使用131無(wú)屏蔽雙絞線的種類

UTPCategories1類線

用于≤5Mbps傳輸(適用于語(yǔ)音/低速傳輸)2類線

用于≤5Mbps傳輸(適用于語(yǔ)音/低速傳輸)3類線

用于≤16Mbps傳輸(10M以太網(wǎng)電纜,4芯)4類線

用于≤20Mbps傳輸5類線

用于≤100Mbps傳輸（8芯）超5類線

用于≤155Mbps傳輸（8芯,最常用的網(wǎng)絡(luò)電纜）6類線

用于≤250Mbps傳輸7類線用于≤600Mbps傳輸電纜上通常印有類別標(biāo)志,如cat5或category5(5類線)

標(biāo)準(zhǔn)規(guī)定計(jì)算機(jī)網(wǎng)絡(luò)常用雙絞線(UTP)每根長(zhǎng)度不宜超過(guò)100米。132近端串?dāng)_

NearEndCrosstalk近端串?dāng)_(NEXT,NearEndCrossTalk)是由一線對(duì)的信號(hào)泄漏到另一線對(duì)而引起的。當(dāng)線對(duì)不附合扭轉(zhuǎn)規(guī)格時(shí)，差分信號(hào)抑制干擾的功能就失效了。近端串?dāng)_NEXT通常是由不正確的接頭而引起的。為了盡量抵消線與線之間的磁場(chǎng)干擾，包括了抵消近場(chǎng)與遠(yuǎn)場(chǎng)的影響，所以把同一線對(duì)進(jìn)行雙絞，但是在做水晶頭時(shí)必須把雙絞拆開(kāi)，這樣就會(huì)造成1、2線對(duì)的一部分信號(hào)泄漏出來(lái)，被3、6線對(duì)所接受到。1332.6.1CoaxialCable

同軸電纜134同軸電纜（Cable）同軸電纜是由一根空心的外圓柱形的導(dǎo)體圍繞單根內(nèi)實(shí)心導(dǎo)體構(gòu)成的。同軸電纜可用于長(zhǎng)距離的電話網(wǎng)絡(luò)、有線電視信號(hào)的傳輸通道以及計(jì)算機(jī)的局域網(wǎng)。同軸電纜即可傳輸模擬信號(hào)也可傳輸數(shù)字信號(hào)。基帶同軸電纜和寬帶同軸電纜135同軸電纜（Cable）基帶電纜：數(shù)字寬帶電纜：數(shù)字+模擬連通性：點(diǎn)到點(diǎn)，點(diǎn)到多點(diǎn)抗干擾性：高頻時(shí)，同軸電纜〉雙絞線價(jià)格：稍貴1362.6.1CoaxialCable

Structure屏蔽層（銅網(wǎng)或銅箔）也是傳導(dǎo)體的一部分，以形成完整的回路137廣泛使用的同軸電纜有兩種：一種為50Ω(指沿電纜導(dǎo)體各點(diǎn)的電磁電壓對(duì)電流之比)同軸電纜，用于數(shù)字信號(hào)的傳輸，即基帶同軸電纜；另一種為75Ω同軸電纜，用于寬帶模擬信號(hào)的傳輸，即寬帶同軸電纜。■基帶傳輸型(主要用于LAN)50Ω——RG-8（粗纜，φ10mm）

RG-58（細(xì)纜，φ5mm）

93Ω——RG62型■頻帶傳輸型(主要模擬傳輸)75Ω——公用天線電視系統(tǒng)(CATV)用（CATV還有一種300Ω扁平電纜）常用同軸電纜種類138CoaxialCable-TransmissionCharacteristics

同軸電纜的傳輸特性AnalogAmplifierseveryfewkmCloserifhigherfrequencyUpto500MHzDigitalRepeaterevery1kmCloserforhigherdatarates139同軸電纜的應(yīng)用

CoaxialCableApplications用途最多的傳輸媒體電視廣播有線電視長(zhǎng)途電話傳輸可同時(shí)傳送10,000路語(yǔ)音電話正面臨光纖的激烈競(jìng)爭(zhēng)短距計(jì)算機(jī)鏈路局域網(wǎng)（帶寬10M）同軸電纜以單根銅導(dǎo)線為內(nèi)芯，外裹一層絕緣材料，外覆密集網(wǎng)狀導(dǎo)體，最外面是一層保護(hù)性塑料。金屬屏蔽層能將磁場(chǎng)反射回中心導(dǎo)體，同時(shí)也使中心導(dǎo)體免受外界干擾，故同軸電纜比雙絞線具有更高的帶寬和更好的噪聲抑制特性。140粗同軸電纜的連接收發(fā)器同軸電纜141細(xì)同軸電纜的連接Terminator–終端電阻（終結(jié)器）BNC-同軸電纜接插件142同軸電纜連接器BNC接頭T型連接器帶BNC接頭的同軸電纜143終端電阻

Terminator安裝在同軸電纜線路的兩端，用于吸收信號(hào)能量。帶鏈的（接地）不帶鏈的144同軸電纜剝線器1452.6.1OpticalFiber

光纖（光導(dǎo)纖維）146光纖3、光導(dǎo)纖維（Fiber）光纖由光纖芯、玻璃內(nèi)包層、加強(qiáng)芯外包層及保護(hù)套組成。光纖是利用光的全反射原理來(lái)實(shí)現(xiàn)傳輸?shù)?。光纖只能單向傳輸信號(hào)，因此光纜中至少包含兩條導(dǎo)芯，一個(gè)發(fā)送、一個(gè)接受。單模光纖和多模光纖單模光纖：導(dǎo)芯的直徑小到只有一個(gè)光的波長(zhǎng)，光就一直向前傳播，即直線傳播。一般使用激光作為光源，傳播距離可達(dá)10km以上，頻帶寬，價(jià)格高，較難實(shí)現(xiàn)。多模光纖：多個(gè)不同的光線在導(dǎo)芯里全反射。一般使用發(fā)光二極管作為光源。相比之下，傳播距離、頻帶等性能不如單模光纖，但是價(jià)格低，容易實(shí)現(xiàn)。147光纖物理特性玻璃封套塑料外套玻璃內(nèi)芯單芯光纜多芯光纜玻璃內(nèi)芯塑料外套玻璃封套外殼148光纖連通性：點(diǎn)到點(diǎn)/點(diǎn)到多點(diǎn)地理范圍：6-8公里不用中繼抗干擾：不受電磁干擾或噪聲的干擾價(jià)格：最貴149光是一種電磁波，它在真空中傳輸速度最快（每秒30萬(wàn)千米），但傳輸速度隨傳輸介質(zhì)密度增大而降低。相對(duì)于其他傳輸媒介，低損耗、高帶