模擬信號用連續(xù)變化的電磁波表示數(shù)據(jù)

ComputerNetworks5(2.5,2.6,2.7):DataEncodingDataandSignalsUsuallyusedigitalsignalsfordigitaldataandanalogsignalsforanalogdataCanuseanalogsignaltocarrydigitaldataModemCanusedigitalsignaltocarryanalogdataCompactDiscaudioAnalogSignalsCarryingAnalogandDigitalData模擬信號：用連續(xù)變化的電磁波表示數(shù)據(jù)DigitalSignalsCarryingAnalogandDigitalData(數(shù)字傳送器）數(shù)字信號：用電壓脈沖序列表示數(shù)據(jù)AnalogTransmissionAnalogsignaltransmittedwithoutregardtocontentMaybeanalogordigitaldataAttenuatedoverdistanceUseamplifierstoboostsignalAlsoamplifiesnoiseDigitalTransmissionConcernedwithcontentIntegrityendangeredbynoise,attenuationetc.RepeatersusedRepeaterreceivessignalExtractsbitpatternRetransmitsAttenuationisovercomeNoiseisnotamplifiedAdvantagesofDigitalTransmissionDigitaltechnology：數(shù)字技術(shù)LowcostLSI/VLSItechnologyDataintegrity：數(shù)據(jù)完整性LongerdistancesoverlowerqualitylinesCapacityutilization：容量利用率Highbandwidthlinksare

economicalHighdegreeofmultiplexingeasierwithdigitaltechniquesSecurity&Privacy：安全和保密EncryptionIntegration：一體化CantreatanaloganddigitaldatasimilarlyTransmissionImpairmentsSignalreceivedmaydifferfromsignaltransmittedAnalog-degradationofsignalqualityDigital-biterrorsCausedbyAttenuationandattenuationdistortion：衰減和失真Delaydistortion：時延失真Noise：噪聲AttenuationSignalstrengthfallsoffwithdistanceDependsonmediumReceivedsignalstrength（解決方法：使用放大器）mustbeenoughtobedetectedmustbesufficientlyhigherthannoisetobereceivedwithouterrorAttenuationisanincreasingfunctionoffrequency(解決方法：采用均衡技術(shù)或使用放大器）DelayDistortionOnlyinguidedmediaPropagationvelocityvarieswithfrequencyNoise(1)AdditionalsignalsinsertedbetweentransmitterandreceiverThermal：熱噪聲Duetothermalagitationofelectrons：電子熱運動Uniformlydistributed：均勻分布Whitenoise：白噪聲Intermodulation：互調(diào)噪聲SignalsthatarethesumanddifferenceoforiginalfrequenciessharingamediumNoise(2)Crosstalk（串?dāng)_）AsignalfromonelineispickedupbyanotherImpulse（沖擊噪聲）Irregularpulsesorspikese.g.ExternalelectromagneticinterferenceShortdurationHighamplitudeEncoderDecoderModulatorDemodulator數(shù)字或模擬數(shù)字或模擬數(shù)字模擬g(t)m(t)fcs(f)x(t)tffcg(t)m(t)x(t)s(t)無論是模擬信息還是數(shù)字信息都能編碼成模擬信號或數(shù)字信號。具體選擇何種編碼方式則取決于需要滿足的特殊要求以及可提供的媒體與通信設(shè)施。編碼和調(diào)制EncodingTechniquesDigitaldata,digitalsignal：數(shù)字?jǐn)?shù)據(jù)/數(shù)字信號Analogdata,digitalsignal：模擬數(shù)據(jù)/數(shù)字信號Digitaldata,analogsignal：數(shù)字?jǐn)?shù)據(jù)/模擬信號Analogdata,analogsignal：模擬數(shù)據(jù)/模擬信號模擬傳輸和數(shù)字傳輸話音電話模擬數(shù)字Modem模擬模擬CODEC數(shù)字?jǐn)?shù)字?jǐn)?shù)字傳輸數(shù)字模擬數(shù)據(jù),模擬信號數(shù)字?jǐn)?shù)據(jù)，模擬信號數(shù)字?jǐn)?shù)據(jù)，數(shù)字信號模擬數(shù)據(jù)，數(shù)字信號DigitalData,DigitalSignalDigitalsignalDiscrete,discontinuousvoltagepulsesEachpulseisasignalelementBinarydataaretransmittedbyencodingeachdataintosignalelementsTerms(1)Unipolar：單極性AllsignalelementshavesamesignPolar：雙極性O(shè)nelogicstaterepresentedbypositivevoltagetheotherbynegativevoltageDatarate：數(shù)據(jù)率RateofdatatransmissioninbitspersecondDurationorlengthofabit：比特持續(xù)時間TimetakenfortransmittertoemitthebitForadatarateR,thebitdurationis1/R.Terms(2)Modulationrate：調(diào)制速率RateatwhichthesignallevelchangesDependonthenatureofthedigitalencodingMeasuredinbaud=signalelementspersecondMarkandSpace：傳號和空號Binary1andBinary0respectivelyInterpretingSignalsNeedtoknowTimingofbits-whentheystartandendSignallevelsFactorsaffectingsuccessfulinterpretingofsignals（接收器成功解釋收到信號時的決定因素）SignaltonoiseratioDatarateBandwidthComparisonofEncodingSchemes(1)SignalSpectrum：信號頻譜LackofhighfrequenciesreducesrequiredbandwidthLackofdccomponentallowsaccouplingviatransformer,providingisolationConcentratepowerinthemiddleofthebandwidthClocking：時鐘同步SynchronizingtransmitterandreceiverExternalclockSyncmechanismbasedonsignalComparisonofEncodingSchemes(2)Errordetection：差錯檢測CanbebuiltintosignalencodingSignalinterferenceandnoiseimmunity：SomecodesarebetterthanothersCostandcomplexity：成本和復(fù)雜性Highersignalrate(&thusdatarate)leadtohighercostsSomecodesrequiresignalrategreaterthandatarateEncodingSchemesNonreturntoZero-Level(NRZ-L,即不歸零電平)NonreturntoZeroInverted(NRZI，即不歸零1制)Bipolar–AMI(AlternateMarkInversion)交替反轉(zhuǎn)碼Pseudoternary：偽三進(jìn)制編碼Manchester：曼徹斯特編碼DifferentialManchesterB8ZS：雙極性8零替換HDB3：高密度雙極性3零碼NonreturntoZero-Level(NRZ-L)Twodifferentvoltagesfor0and1bitsVoltageconstantduringbitintervalnotransitioni.e.noreturntozerovoltagee.g.Absenceofvoltageforzero,constantpositivevoltageforoneMoreoften,negativevoltageforonevalueandpositivefortheotherThisisNRZ-L：不歸零碼NonreturntoZeroInvertedNonreturntozeroinvertedononesConstantvoltagepulsefordurationofbitDataencodedaspresenceorabsenceofsignaltransitionatbeginningofbittimeTransition(lowtohighorhightolow)denotesabinary1Notransitiondenotesbinary0AnexampleofdifferentialencodingNRZDifferentialEncoding：差分編碼DatarepresentedbychangesratherthanlevelsMorereliabledetectionoftransitionratherthanlevelIncomplextransmissionlayoutsitiseasytolosesenseofpolarityNRZprosandconsProsEasytoengineerMakegooduseofbandwidthConsdccomponentLackofsynchronizationcapabilityUsedformagneticrecordingNotoftenusedforsignaltransmissionMultilevelBinary：多電平二進(jìn)制UsemorethantwolevelsBipolar-AMI(AlternateMarkInversion,即交替?zhèn)魈柗崔D(zhuǎn))zerorepresentedbynolinesignalonerepresentedbypositiveornegativepulseonepulsesalternateinpolarityNolossofsyncifalongstringofones(zerosstillaproblem)NonetdccomponentNarrowerbandwidthEasyerrordetectionPseudoternary：偽三進(jìn)制編碼OnerepresentedbyabsenceoflinesignalZerorepresentedbyalternatingpositiveandnegativeNoadvantageordisadvantageoverbipolar-AMIBipolar-AMIandPseudoternaryTradeOffforMultilevelBinaryNotasefficientasNRZEachsignalelementonlyrepresentsonebitIna3levelsystemcouldrepresentlog23=1.58bitsReceivermustdistinguishbetweenthreelevels

(+A,-A,0)Requiresapprox.3dBmoresignalpowerforsameprobabilityofbiterrorBiphase：雙相碼ManchesterTransitioninmiddleofeachbitperiod：每比特周期中央跳變TransitionservesasclockanddataLowtohighrepresentsoneHightolowrepresentszeroUsedbyIEEE802.3DifferentialManchesterMidbittransitionisclockingonlyTransitionatstartofabitperiodrepresentszeroNotransitionatstartofabitperiodrepresentsoneNote:thisisadifferentialencodingschemeUsedbyIEEE802.5曼徹斯特編碼001100BiphaseProsandConsConAtleastonetransitionperbittimeandpossiblytwoMaximummodulationrateistwiceNRZRequiresmorebandwidthProsSynchronizationonmidbittransition(selfclocking)NodccomponentErrordetectionAbsenceofexpectedtransitionModulationRate：調(diào)制速率Scrambling：擾碼技術(shù)UsescramblingtoreplacesequencesthatwouldproduceconstantvoltageFillingsequenceMustproduceenoughtransitionstosyncMustberecognizedbyreceiverandreplacewithoriginalSamelengthasoriginalNodccomponentNolongsequencesofzerolevellinesignalNoreductionindatarateErrordetectioncapabilityB8ZS：雙極性8零替換BipolarWith8ZerosSubstitutionBasedonbipolar-AMIIfoctetofallzerosandlastvoltagepulseprecedingwaspositiveencodeas000+-0-+Ifoctetofallzerosandlastvoltagepulseprecedingwasnegativeencodeas000-+0+-CausestwoviolationsofAMIcodeUnlikelytooccurasaresultofnoiseReceiverdetectsandinterpretsasoctetofallzerosHDB3：高密度雙極性3零碼HighDensityBipolar3ZerosBasedonbipolar-AMIStringoffourzerosreplacedwithoneortwopulsesB8ZSandHDB3HDB3的替換規(guī)則自上一次替換后雙極性脈沖（比特1）的個數(shù)前面的脈沖極性奇數(shù)偶數(shù)-+000-+00+000+-00-DigitalData,AnalogSignalPublictelephonesystem300Hzto3400HzUsemodem(modulator-demodulator)Amplitudeshiftkeying(ASK)Frequencyshiftkeying(FSK)Phaseshiftkeying(PSK)ModulationTechniquesAmplitudeShiftKeyingValuesrepresentedbydifferentamplitudesofcarrierUsually,oneamplitudeiszeroi.e.presenceandabsenceofcarrierisusedSusceptibletosuddengainchangesInefficientUpto1200bpsonvoicegradelinesUsedoveropticalfiberASKBandwidthforASKFrequencyShiftKeyingValuesrepresentedbydifferentfrequencies(nearcarrier)LesssusceptibletoerrorthanASKUpto1200bpsonvoicegradelinesHighfrequencyradioEvenhigherfrequencyonLANsusingco-axFSKBandwidthforFSKFSKonVoiceGradeLine1700PhaseShiftKeyingPhaseofcarriersignalisshiftedtorepresentdataDifferentialPSKPhaseshiftedrelativetoprevioustransmissionratherthansomereferencesignalpskPSK

ConstellationQuadraturePSKMoreefficientusebyeachsignalelementrepresentingmorethanonebite.g.shiftsof/2(90o)EachelementrepresentstwobitsCanuse8phaseanglesandhavemorethanoneamplitude9600bpsmodemuse12angles,fourofwhichhavetwoamplitudes4-PSK4-PSK

ConstellationPSK

BandwidthPerformanceofDigitaltoAnalogModulationSchemesBandwidthASKandPSKbandwidthdirectlyrelatedtobitrateFSKbandwidthrelatedtodatarateforlowerfrequencies,buttooffsetofmodulatedfrequencyfromcarrierathighfrequencies(SeeStallingsformath)Inthepresenceofnoise,biterrorrateofPSKandQPSKareabout3dBsuperiortoASKandFSKAnalogData,DigitalSignalDigitization：數(shù)字化ConversionofanalogdataintodigitaldataDigitaldatacanthenbetransmittedusingNRZ-LDigitaldatacanthenbetransmittedusingcodeotherthanNRZ-LDigitaldatacanthenbeconvertedtoanalogsignalAnalogtodigitalconversiondoneusingacodecPulsecodemodulation&DeltamodulationPulseCodeModulation(PCM)(1)Ifasignalissampledatregularintervalsataratehigherthantwicethehighestsignalfrequency,thesamplescontainalltheinformationoftheoriginalsignal(采樣定理)(Proof-Stallingsappendix4A)Voicedatalimitedtobelow4000HzRequire8000samplepersecondAnalogsamples(PulseAmplitudeModulation,PAM)Eachsampleassigneddigitalvalue

PAMPulseCodeModulation(PCM)(2)4bitsystemgives16levelsQuantizedQuantizingerrorornoiseApproximationsmeanitisimpossibletorecoveroriginalexactly8bitsamplegives256levelsQualitycomparablewithanalogtransmission8000samplespersecondof8bitseachgives64kbpsQuantizedPAMsingalPCM-8級量化PCMEntireprocessPCM轉(zhuǎn)化過程BACDABCD模擬話音采樣時鐘PAM信號PCM信號采樣電路量化和比較數(shù)字化聲音NyquistTheoremNonlinearEncodingQuantizationlevelsnotevenlyspa