《计算机网络第五版(英文版)ppt-(2)资料讲解.ppt》由会员分享，可在线阅读，更多相关《计算机网络第五版(英文版)ppt-(2)资料讲解.ppt（98页珍藏版）》请在taowenge.com淘文阁网|工程机械CAD图纸|机械工程制图|CAD装配图下载|SolidWorks_CaTia_CAD_UG_PROE_设计图分享下载上搜索。
1、计算机网络第五版(英文版)ppt-(2)2.1TheoreticalBasisforDataCommunication2.1.1FourierAnalysisAtime-varyingsignalcanbeequivalentlyrepresentedasaseriesoffrequencycomponents(harmonics)an,bn weights of harmonics2.1.2Bandwidth-LimitedSignals(1)BandwidthAlltransmissionfacilitiesdiminishdifferentharmonicsbydifferentamou
2、nts,namely,signaldistortion.Thereiscutofffrequencyfc,atwhichthereceivedpowerhasfallenbyhalf,theamplitudesaretransmittedmostlyundiminishedfrom0uptofc.Thiswidthofthefrequencyrageiscalledthebandwidth.(analog)bandwidthvs.(digital)bandwidth2.1.2Bandwidth-LimitedSignals(2)BasebandSignalsthatrunfrom0uptoam
3、ax.frequencyPassbandSignalsthatareshiftedtoahigherrangoffrequencies2.1.2Bandwidth-LimitedSignals(3)Havinglessbandwidth(harmonics)degradesthesignalLost!Lost!Lost!BandwidthMax.datarate=Blog2(1+S/N)bits/secAnoiselesschannelNyquiststheoremNyquistsformulagivestheoreticalupperboundAnoisechannel(1)Shannons
4、theoremrelatesthedataratetothebandwidth(B)andsignalstrength(S)relativetothenoise(N),assumerandomnoiseispresent2.1.3MaximumDataRateofaChannel(1)Max.datarate=2Blog2Vbits/secHow fast signal can changeHow many levels can be seen2.1.3MaximumDataRateofaChannel(2)Anoisechannel(2)SNR(Signal-to-NoiseRatio)Th
5、eamountofthermalnoisepresentismeasuredbytheratioofthesignalpowertothenoisepowerSNRisusedtoexpressthequantity10log10(S/N)indB(namelydecibels)ShannonsformulagivestheoreticalupperboundofmaximumdatarateonthenoisechannelTherandomicityofrandomnoiseshouldbetakencareofProblems:1,2,3,42.2GuidedTransmissionMe
6、dia2.2.1TwistedPair(1)Category 5 UTP cable with four twisted pairsVerycommon;usedinLANs,telephonelinesTwistsreduceradiatedsignal(interference)ThecategoryoftwistedpairinstructureUTP(unshieldedtwistedpair)STP(shieldedtwistedpair)ThecategoryoftwistedpairinbandwidthANSI/TIA/EIA5682.2.1TwistedPair(2)cate
7、goryBandwidthCat316MHzCat5100MHzCat5e100MHzCat6250MHzCat7(STP)600MHzhttp:/www.utm.edu/staff/leeb/568/568.htmCrossover Wiring2.2.1TwistedPair(3)ThecategoryoftwistedpairinbandwidthLinkTerminologyFull-duplexlinkUsedfortransmissioninbothdirectionsatonce.e.g.,usedifferenttwistedpairsforeachdirectionHalf-
8、duplexlinkBothdirections,butnotatthesametimeSimplexlinkOnlyonefixeddirectionatalltimes2.2.1TwistedPair(3)FieldTestingofTwistedPairCablingSystem(ANSI/TIA/EIA-TSB-67)(1)ThespecificlengthABCDA+B+C+D100mA+C+D10m2.2.1TwistedPair(4)FieldTestingofTwistedPairCablingSystem(2)Somerelatedparameters(1)Attenuati
9、onNear-end crosstalk(NEXT)iscrosstalkthatappearsattheoutputofawirepairatthetransmitter(near)endofthecable.Far-end crosstalk(FEXT)isameasureoftheunwantedsignalcouplingfromatransmitteratthenear-endintoaneighboringpairmeasuredatthefar-end.2.2.1TwistedPair(5)FieldTestingofTwistedPairCablingSystem(3)Some
10、relatedparameters(2)Equal level far-end crosstalk(ELFEXT)isameasureoftheunwantedsignalcouplingfromatransmitteratthenear-endintoaneighboringpairmeasuredatthefar-endrelativetothereceivedsignallevelmeasuredonthatsamepair.Power sum equal level far-end crosstalk(PSELFEXT)isacomputationoftheunwantedsignal
11、couplingfrommultipletransmittersatthenear-endintoapairmeasuredatthefar-endrelativetothereceivedsignallevelonthatsamepair.2.2.1TwistedPair(6)FieldTestingofTwistedPairCablingSystem(4)Somerelatedparameters(3)2.2.2CoaxialCable(“Co-ax”)Bettershieldingandmorebandwidthforlongerdistancesandhigherratesthantw
12、istedpair.Basebandco-ax:50(InputImpedance)Usedforanalogtransissinco-ax,sometimealsocalledbroadbandco-ax,likeTVcable:752.2.3PowerLinesHouseholdelectricalwiringisanotherexampleofwiresConvenienttouse,buthorribleforsendingdata2.2.4FiberCables(1)CommonforhighratesandlongdistancesLongdistanceISPlinks,Fibe
13、r-to-the-Home(FTTH)Lightcarriedinverylong,thinstrandofglassLight source(LED,laser)PhotodetectorLight trapped bytotal internal reflection2.2.4FiberCables(2)LighttrappedbytotalinternalreflectionThecategoryoffibercableMultimodefiberLightcanbounce(50mcore)UsedwithLEDsforcheaper,shorterdistancelinksSingl
14、e-modefiberCoresonarrow(10m)lightcanpropagateinastraightlineUsedwithlasersforlongdistances,e.g.,100km2.2.4FiberCables(2)Fiberhasenormousbandwidth(THz)andtinysignallosshencehighratesoverlongdistances2.2.4FiberCables(3)ThestructureoffibercableSome types of fiber connector2.2.4FiberCables(4)Acomparison
15、ofsemiconductordiodesandLEDsaslightsources2.3WirelessTransmission2.3.1ElectromagneticSpectrum(1)TherelationofwavelengthandfrequencyThefundamentalrelationbetween(wavelength),f(frequency)andc(speedoflineinavacuum,3108m/sec)Ifwesolveforfanddifferentiatewithrespectto,wegetIfwegotofinitedifferenceinstead
16、ofdifferentialsandonlylookatabsolutevalues,wegetProblems:6,7,10,11,122.3.1ElectromagneticSpectrum(2)DifferentbandshavedifferentusesRadio:wide-areabroadcast;Infrared/Light:line-of-sightMicrowave:LANsand3G/4GMicrowave2.3.1ElectromagneticSpectrum(3)Spreadspectrumandultra-wideband(UWB)communication(1)Fr
17、equencyhoppingspreadspectrum(FHSS)Amethodoftransmittingradiosignalsbyrapidlyswitching(hundredstimespersecond)acarrieramongmanyfrequencychannels,usingapseudorandomsequenceknowntobothtransmitterandreceiver.2.3.1ElectromagneticSpectrum(4)Spreadspectrumandultra-wideband(UWB)communication(2)Directsequenc
18、espreadspectrum(DSSS)Thetransmissionsmultiplythedatabeingtransmittedbyanoisesignal.Thisnoisesignalisapseudorandomsequenceof1and1values,atafrequencymuchhigherthanthatoftheoriginalsignal.Forexample802.11,CDMA,GPS2.3.1ElectromagneticSpectrum(5)Spreadspectrumandultra-wideband(UWB)communication(3)UWB(Ult
19、ra-WideBand)UWBtransmissionstransmitinformationbygeneratingradioenergyatspecifictimeinstantsandoccupyinglargebandwidththusenablingapulse-positionortime-modulation.UWBisabandwidthofatleast500MHzoratleast20%ofthecenterfrequencyoftheirfrequencyband.ItisappliedinwirelessPAN802.11b/g/n802.11a/g/n2.3.1Ele
20、ctromagneticSpectrum(6)Tomanageinterference,spectrumiscarefullydivided,anditsuseregulatedandlicensed,e.g.,soldatauction.Fortunately,therearealsounlicensed(“ISM”)bands:Freeforuseatlowpower;devicesmanageinterferenceWidelyusedfornetworking;WiFi,Bluetooth,Zigbee,etc.In the HF band,radio waves bounce off
21、 the ionosphere.In the VLF,LF,and MF bands,radio waves follow the curvature of the earth2.3.2RadioTransmissionRadiosignalspenetratebuildingswellandpropagateforlongdistanceswithpathloss2.3.3MicrowaveTransmissionMicrowaveshavemuchbandwidthandarewidelyusedindoors(WiFi)andoutdoors(3G,satellites)Signalis
22、attenuated/reflectedbyeverydayobjectsStrengthvarieswithmobilityduemultipathfading,etc.2.3.4Wirelessvs.Wires/FiberWireless+Easyandinexpensivetodeploy+Naturallysupportsmobility+NaturallysupportsbroadcastTransmissionsinterfereandmustbemanagedSignalstrengthshencedataratesvarygreatlyWires/Fiber+Easytoeng
23、ineerafixeddatarateoverpoint-to-pointlinksCanbeexpensivetodeploy,esp.overdistancesDoesntreadilysupportmobilityorbroadcast2.4CommunicationSatellitesSats needed for global coverage2.4.1KindsofSatellitesSatellitesandtheirpropertiesvarybyaltitudeGeostationary(GEO),Medium-EarthOrbit(MEO),andLow-EarthOrbi
24、t(LEO)2.4.2GeostationarySatellites(1)Theprincipalsatellitebands2.4.2GeostationarySatellites(2)GEOsatellitesorbit35,000kmaboveafixedlocationVSAT(computers)cancommunicatewiththehelpofahubDifferentbands(L,S,C,Ku,Ka)intheGHzareinusebutmaybecrowdedorsusceptibletorain.The Iridium satellites form six neckl
25、aces around the earth.2.4.3Low-EarthOrbitSatellitesSystemssuchasIridiumusemanylow-latencysatellitesforcoverageandroutecommunicationsviathemProblems:13,15,162.5DigitalModulationandMultiplexingModulationschemessendbitsassignalsBasebandtransmissiondirectlyconvertsbitsintosignalItscommonforwiresPassband
26、transmissionregulatestheamplitude,phase,orfrequencyofcarriersignalItscommonforwirelessandopticalchannelsMultiplexingschemesshareachannelamongusers2.5.1Basebandtransmission(1)Linecodessendsymbolsthatrepresentoneormorebits(1)NRZisthesimplest,literallinecode(+1V=“1”,-1V=“0”)Othercodestradeoffbandwidtha
27、ndsignaltransitions2.5.1Basebandtransmission(2)Linecodessendsymbolsthatrepresentoneormorebits(2)BandwidthefficiencyUsemorethentwosignalinglevelsTherelationofbitrateandsymbolrateBitrate=SymbolrateXlog2VwhereVisnumberofsignalinglevelsBaudrateisanoldnameforsymbolrateClockRecovery(1)Todecodethesymbols,s
28、ignalsneedsufficienttransitionsOtherwiselongrunsof0s(or1s)areconfusing2.5.1Basebandtransmission(3)10 0 0 0 0 0 0 0 0 0 um,0?er,0?2.5.1Basebandtransmission(4)ClockRecovery(2)StrategiesManchestercoding,exclusive-ORoftheNRZ-encodeddataandtheclock,somixesclocksignalineverysymbol4B/5Bmaps4databitsto5code
29、dbitswith1sand0s:Theresulting5-bitcodesarethentransmittedusingtheNRZIencodingScramblerXORstx/rxdatawithpseudorandombitsDataCodeDataCodeDataCodeDataCode0000111100100010101000100101100110100001010010101010111001100111101110110010101000110011101010101101110111000011101010111011111011101111111111012.5.1
30、Basebandtransmission(4)BalancedSignalsSignalsthathavenoDCelectricalcomponentarecalledItsanadvantageBipolarencodingUsetwovoltagelevelswhicharepositiveandnegativetorepresentalogical1with0Vrepresentingalogicalzero8B/10BlinecodeClosebalancedsignalWriteShortreports(2-1)WhatisMLT-3coding?Whatisdifferenceb
31、etweenMLT-3andAMIencoding?Andinwhatsituationaretheyused?Problems:172.5.2PassbandTransmission(1)Modulatingtheamplitude,frequency,orphaseofacarriersignalsendsbitsina(non-zero)frequencyrangeNRZ signal of bitsAmplitude shift keyingFrequency shift keyingPhase shift keyingBPSK2 symbols1 bit/symbolQPSK4 sy
32、mbols2 bits/symbolQAM-1616 symbols4 bits/symbolQAM-6464 symbols6 bits/symbolQAM varies amplitude and phaseBPSK/QPSK varies only phase2.5.2PassbandTransmission(2)Constellationdiagramsareashorthandtocapturetheamplitudeandphasemodulationsofsymbols2.5.2PassbandTransmission(3)Gray-codingassignsbitstosymb
33、olssothatsmallsymbolerrorscausefewbiterrorsABCDEOverall FDM channel2.5.3FrequencyDivisionMultiplexing(1)FDM(FrequencyDivisionMultiplexing)sharesthechannelbyplacingusersondifferentfrequenciesGuard band2.5.3FrequencyDivisionMultiplexing(2)OFDM(OrthogonalFDM)isanefficientFDMtechniqueusedfor802.11,4Gcel
34、lularandothercommunicationsSubcarriersarecoordinatedtobetightlypacked2.5.4TimeDivisionMultiplexingTimedivisionmultiplexingsharesachannelovertimeUserstaketurnsonafixedschedule;thisisnotpacketswitchingorSTDM(StatisticalTDM)Widelyusedintelephone/cellularsystemsTime slot2.5.5CodeDivisionMultipleAccessCD
35、MAsharesthechannelbygivingusersacodeCodesareorthogonal;canbesentatthesametimeWidelyusedaspartof3Gnetworks2.5.5CodeDivisionMultiplexing(1)CDMAsharesthechannelbygivingusersacodeEachbittimeissubdividedintomshortintervalscalledchipsEachstationisassignedauniquem-bitcodecalledachip sequenceTotransmita1bit
36、,astationsandsitschipsequenceTotransmita0bit,astationsandsthenegationofitschipsequenceCodesareorthogonal;canbesentatthesametimeAformofspread spectrumcommunicationfrombbits/sectombchips/sec2.5.5CodeDivisionMultiplexing(2)TheprincipleofCDMAOrthogonality:Twovectorsareorthogonalifandonlyiftheirdotproduc
37、tiszero.LetSbeam-chipvector,foritsnegation,ForanydistinctSandT,thenormalizedinnerproductNotethatifST=0,thenThenormalizedinnerproductAlso,2.5.5CodeDivisionMultiplexing(3)ExampleProblems:42,46,522.6ThePublicSwitchedTelephoneNetwork(PSTN)2.6.1StructureoftheTelephoneSystemAhierarchicalsystemforcarryingv
38、oicecallsismadeofLocalloops,mostlyanalogtwistedpairstohousesTrunks,digitalfiberopticlinksthatcarrycallsSwitchingoffices,thatmovecallsamongtrunksProblems:18DigitalAnalog2.6.2Localloop(1):modems(1)Telephonemodemssenddigitaldataoveran3.3KHzanalogvoicechannelinterfacetothePOTSRates56kbps;earlywaytoconne
39、cttotheInternet2.6.2Localloop(1):modems(2)Modemscomeinmanyvarieties:telephonemodem,DSLmodem,cablemodem,wirelessmodem,andsoon.Telephonemodem(1)Mostmodemssendat2400baudUsingTCM(TrellisCodedModulation)tocorrecterror2.6.2Localloop(1):modems(3)Telephonemodem(1)ModemsstandardV.32andV.32bis:32constellation
40、,4/6databits,1checkbitV.34andV.34bis:32constellation,12/14databits,1checkbitV.90andV.92:4000Hzchannel,7bitssampling,56kbpsdownstreamand33.6kbps/48kbpsupstream2.6.2Localloop(2):DigitalSubscriberLines(1)DSLbroadbandsendsdataoverthelocallooptothelocalofficeusingfrequenciesthatarenotusedforPOTS(1)Teleph
41、one/computersattachtothesameoldphoneline2.6.2Localloop(2):DigitalSubscriberLines(2)DSLbroadbandsendsdataoverthelocallooptothelocalofficeusingfrequenciesthatarenotusedforPOTS(2)RatesvarywithlineADSL2upto12MbpsOFDMisusedupto1.1MHzforADSL2MostbandwidthdownProblems:242.6.2Localloop(3):FiberToTheHomeFTTH
42、broadbandreliesondeploymentoffiberopticcablestoprovidehighdataratescustomersOnewavelengthcanbesharedamongmanyhousesFiberispassive(noamplifiers,etc.)2.6.4TrunksandMultiplexing(1):DigitizingVoiceSignalsTDM(TimeDivisionMultiplexing)cansatisfythedemandsofdigitaltrunksCOECusePCM(PulseCodeModulation)techn
43、iquetodigitizetheanalogsignals8bitsquantizeasampleevery125secToreducetheerrorduetoquantization,itusealogarithmic scale-lawandA-lawinG.711Companding(aportmanteauofcompressingandexpanding)AmethodofmitigatingthedetrimentaleffectsofachannelwithlimitedrangeDiscussingTopics(2-1)1.Compandingvs.(compressing
44、/decompressing,suchasG.729,MP3,etc.)Problems:25 2.6.4TrunksandMultiplexing(2):TDM(1)CallsarecarrieddigitallyonPSTNtrunksusingTDMAcallisan8-bitPCMsampleeach125s(64kbps)TraditionalT1(Bellsystems)carrierhas24callchannelseach125s(1.544Mbps)withsymbolsbasedonAMI2.6.4TrunksandMultiplexing(2):TDM(2)Multipl
45、eT1tobemultiplexedintohigher-ordercarriers2.6.4TrunksandMultiplexing(2):TDM(3)ITU-TE-carrierITU-Tstandard,widelyusedinEuropean,China,etc.E1carrierhas30callchannels,2signalingchannels,8-bitdatasamplingeach125s(2.048Mbps)MultipleE1tobemultiplexedintohigher-ordercarriers128,512,2048,and8192channelscorr
46、espondingto8.848,34.304,139.264,and565.148Mbps2.6.4TrunksandMultiplexing(2):TDM(4)SONET(SynchronousOpticalNETwork)/SDH(SynchronousDigitalHierarchy)TheyareITU-TstandardSONETisusedinNorthAmericaandJapanSDHisusedinEuropeanandChinaTheyarecontrolledbyamasterclock2.6.4TrunksandMultiplexing(2):TDM(5)SONETi
47、stheworldwidestandardforcarryingdigitalsignalsonopticaltrunksKeeps125sframe;baseframeis810bytes(52Mbps)Payload“floats”withinframingforflexibility1 byte2.6.4TrunksandMultiplexing(2):TDM(6)ForSONEThierarchyat3:1(but4:1inSDH)perlevelisusedforhigherratesEachlevelalsoaddsasmallamountofframingRatesfrom50M
48、bps(STS-1)to40Gbps(STS-768)2.6.4TrunksandMultiplexing(2):TDM(7)Whenacarrierisnotmultiplexedandcarriersthedatafromonlysinglesource,thelettercisappendedtothedesignationWhenmultiplestreamsmultiplexintoonestream,theyareinterleavedbycolumn.Problems:27,29,33,512.6.4TrunksandMultiplexing(3):WDM(1)WDM(Wavel
49、engthDivisionMultiplexing),anothernameforFDM,isusedtocarrymanysignalsononefiber2.6.4TrunksandMultiplexing(3):WDM(2)WDMsystemsaredividedintodifferentwavelengthpatterns,conventional(CWDM)anddense(DWDM).CWDMsystemsprovideupto8channelsDWDMusesdenserchannelspacingAtypicalsystemwoulduse40channelsat100GHzs
50、pacingor80channelswith50GHzspacingDWDMhavebeenwidelyusedPSTN:Internet:2.6.5Switching(1)PSTNusescircuitswitching;Internetusespacketswitching2.6.5Switching(2)Circuitswitchingrequirescallsetup(connection)beforedataflowssmoothlyAlsodisconnectionatend(notshown)PacketswitchingtreatsmessagesindependentlyNo