会计学FivefactorsforDataTransmissionTheBasicElementsforDataTransmissionTerminology2.1DataandSignals2.SignalsSignals:ElectricorelectromagneticrepresentationsofdataAnalogsignals：continuouselectromagneticwave。Digitalsignals：aserialsofpulses,using0and1torepresentthevalue3.Transmission1.AnalogsignalsrepresentAnalogdataThefrequencyofhearingisbetween20~20KHz.Limitfrequencyrangeforvoicechannelis300-3400Hz.2.AnalogsignalsrepresentDigitaldataPublictelephonesystem(300Hzto3400Hz)usemodemtotransmitthesignalsrepresentingthedigitaldata.Theprocedurethatloadsthedigitaldatatocarrierwaveiscalledmodulation.Theproceduretogetthedigitaldatafromthecarrierwaveiscalleddemodulation.Amplitudeshiftkeying(ASK)Frequencyshiftkeying(FSK)Phaseshiftkeying(PK)3.DigitalsignalsrepresentDigitaldataDigitalsignalDiscrete,discontinuousvoltagepulsesEachpulseisasignalelementBinarydataencodedintosignalelementsTooptimizethecharacterofsignalstransmission,encodingtechnologyisused,suchasNon-returntoZero-Level,ManchesterandDifferentialManchester./4.DigitalsignalsrepresentAnalogdataPulseCodeModulation(PCM)Ifasignalissampledatregularintervalsataratehigherthantwicethehighestsignalfrequency,thesamplescontainalltheinformationoftheoriginalsignal.Voicedatalimitedtobelow4000Hz,require8000samplepersecond.NyquistSamplingTheoremPulseCodeModulation(PCM)PCMPCMExamplePCMExampleAnalogTransmissionDigitalTransmissionDigitalandanalogchannelDigitalchannel：ADSL、ISDN、DDN、ATM、LANAnalogchannel：Catv,radio,telephoneTechnicalnormsfordatatransmissionDatarateInbitspersecondbpsRateatwhichdatacanbecommunicatedBandwidthIncyclespersecondofHertzHzConstrainedbytransmitterandmediumErrorrateRateatwhicherrorsmaybeoccurredNyquist’sBandwidthBitRateandBaudRateBitRateandBaudRateTransmissionrateforTelephone（bandwidth=3KHz）Shannon’sCapacityFormulaQ:Thes/n=30dB，thebandwidthis3kHz，tocalculatethemaxdatarate。∵10lg(S/N)=30∴S/N=1000∴C=3klog2(1+1000)≈30kbps2.2AsynchronousandSynchronousTransmissionAsynchronousAsynchronous(diagram)