现代计算机网络讲义2(英语)+物理层

21:42:5112.ThePhysicalLayerDefinesthemechanical,electrical,functionalandprocedural(timing)interfaceTransmissionmediaExamplesofcommunicationsystems21:42:5122.1TheTheoreticalBasisforDataCommunicationFourierAnalysisBandwidth-LimitedSignalsMaximumDataRateofaChannel21:42:5132.1.1FourierAnalysisAmplitudesFrequencyAdatasignalcanbeimaginedthatitrepeatstheentirepatternoverandoverforever)2cos()2sin(21)(11nftbnftactgnnnnAnyperiodicfunctiong(t)withperiodT21:42:5142.1.2Bandwidth-LimitedSignals(1)21:42:5152.1.2Bandwidth-LimitedSignals(2)21:42:5162.1.2Bandwidth-LimitedSignals(3)Relationbetweendatarateandharmonicsonvoice-gradeline(3000Hzbandwidth)Bandwidthlimiteddatarate21:42:5172.1.3TheMaximumDataRateofachannelNyquist’stheoremmaximumdatarate=2Hlog2Vbits/secShannon’stheoremmaximumdatarate=Hlog2(1+S/N)bits/secExample(Modem,TelephoneLine):S/N=1000,maximumdatarate≈7H=28kbps21:42:5182.2GuidedTransmissionMediaMagneticMediaTwistedPairthetwistedwireradiateslesseffectivelyCoaxialCableFiberOptics21:42:519FiberOpticsThreecomponents:Lightsource,transmissionmedium,detectorApulseoflightindicatesa1bitandtheabsenceoflightindicatesa0bitLightthroughfiberMultimode&singlemodefiber21:42:5110Attenuationoflightthroughfiberintheinfraredregion21:42:5111InformationWaveCarriedFrequencyandwavelengthΛf=ctofinitedifferences1.30-micronband,λ=1.3x10-6δλ=0.17x10-6δfisabout3*1013Hz=30THz8bits/Hz,weget240Tbps.AfewbitsperHertzatlowfrequenciesOften8bitsathighfrequencies21:42:5112FiberOpticNetworks(1)*Afiberopticringwithactiverepeaters21:42:5113FiberOpticNetworks(2)*WDM(WavelengthDivisionMultiplexing)DWDM(DenseWavelengthDivisionMultiplexing)OXC(OpticalCross-Connects)OpticalandElectricaltransitionFiberCross-Connect,AutomaticlayoutWavelengthSelectiveCross-ConnectOpticalStorage?21:42:51142.3WirelessTransmissionTheElectromagneticSpectrumRadioTransmissionMicrowaveTransmissionInfraredWavesLightwaveTransmission21:42:51152.3.1TheElectromagniticSpectrum21:42:5116FrequenciesandregulationsEuropeUSAJapanMobilephonesNMT453-457MHz,463-467MHz;GSM890-915MHz,935-960MHz;1710-1785MHz,1805-1880MHzAMPS,TDMA,CDMA824-849MHz,869-894MHz;TDMA,CDMA,GSM1850-1910MHz,1930-1990MHz;PDC810-826MHz,940-956MHz;1429-1465MHz,1477-1513MHzCordlesstelephonesCT1+885-887MHz,930-932MHz;CT2864-868MHzDECT1880-1900MHzPACS1850-1910MHz,1930-1990MHzPACS-UB1910-1930MHzPHS1895-1918MHzJCT254-380MHzWirelessLANsIEEE802.112400-2483MHzHIPERLAN15176-5270MHzIEEE802.112400-2483MHzIEEE802.112471-2497MHz21:42:51172.4CommunicationSatellites*Amplifiesincomingsignal,rebroadcastsatanotherfrequencyGeostationarySatellites(GEO)Medium-EarthOrbitSatellites(MEO)Low-EarthOrbitSatellites(LEO)21:42:5118Satellitesandproperties*AltitudeabovetheearthRound-tripdelaytimeNumberofsatellitesforglobalcoverage.21:42:5119VSAT(VerySmallApertureTerminal)*1-meterorsmallerantennas1wattofpowerTheuplinkis19.2kbps,thedownlinkis512kbpsormore.HubwithalargeantennatorelaytrafficbetweenVSATs.21:42:5120SatellitesversusFiberFiberswithinthetelephonesystemtohandlelongdistancecalls.SatellitesHigherbandwidth.MobilecommunicationBroadcastingMilitarycommunicationDifficulttolayfiber21:42:51212.5ThePublicSwitchTelephoneNetwork(PSTN)StructureoftheTelephoneSystemTheLocalLoop:Modems,ADSLandWirelessTrunksandMultiplexingSwitching21:42:51222.5.1StructureoftheTelephoneSystemPointtoPoint(a)Fully-interconnectednetwork.(b)Centralizedswitch.(c)Two-levelhierarchy21:42:5123AtypicalcircuitrouteLocalloops(analogtwistedpairs)Trunks(digitalfiberoptics)SwitchingofficesAnalog,lowbandwidth21:42:51242.5.3TheLocalLoop:Modems,ADSL,andwirelessTheanalogtransmissions,modemThedigitaltransmissions,codecs21:42:5125InfluenceonsignaltransferAttenuationLossofenergyDependsonfrequencyanddistanceDistortionDifferentFouriercomponentatdifferentspeedNoiseThermalnoise,random,unavoidableCrosstalkImpulse21:42:5126SomeconceptsBandwidth,therangeoffrequencySymbol,onepieceinfo.,asampleBaudrate,numberofsample/sec,symbolrateBitrate,numberofsymbol/sectimesnumberofbits/symbol21:42:5127Modems(1)AbinarysignalAmplitudemodulationFrequencymodulationPhasemodulation21:42:5128Modems(2)(a)QPSK.(b)QAM-16.(c)QAM-6421:42:5129Modems(3)(b)V.32for9600bps,2400Baud*(5bit-1)(c)V32bisfor14,400bps,2400Baud*(7bit-1)V.90providesa33.6-kbpsupstreamchannel(usertoISP),buta56kbpsdownstreamchannel(ISPtouser),accordingthelimitsofShannonandNyquisttheorem.21:42:5130DigitalSubscriberLines(1)*MakingtheentirecapacityofthelocalloopavailableFactors:length,thickness,generalqualityBandwidthversusdistancedovercategory3UTPforDSL.21:42:5131DigitalSubscriberLines(2)*Overcategory3UTPDon’taffecttelephonesMuchfastthan56kbpsAlwaysOnADSLusingdiscretemultitonemodulation:frequencydivisionmultiplexing21:42:5132AtypicalADSLequipmentconfiguration*NID:NetworkinterfacedeviceDSLAM:Digitalsubscriberlineaccessmultiplexer21:42:5133WirelessLocalLoops:WLL*Fixedwireless,2-5kmArchitectureofanLMDSsystemLocalMultipointDistributionServiceAsymmetricband