计算机网络与互联网英语chap4

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1、Chapter4Signals,Media,AndDataTransmissionTransmissionofInformationWell-understoodbasicsFromphysicsEnergyElectromagneticwavepropagationFrommathematicsCodingtheoryTransmissionMediaTransmissionMediaCopperwireNeedtwowiresPossibilitiesTwistedpairCoaxialcableOp

2、ticalfiberFlexibleLight“staysin”Air/spaceUsedforelectromagnetictransmissionTwistedpairTwoinsulatedcopperwiresaretwistedaroundeachotherTwistedpairsoftenarebundledtogetherandwrappedinaprotectivecoatingthatallowsthebundledcabletobeburiedTwistingminimizeselec

3、tromagneticinterferencebetweenpairsBandwidthupto250kHzforanalogsignalsDigitalsignaldataratesvarywithdistance100Mbpsoverasegmentlengthof100m2400bpsforupto10milesTerminologyAnalogdatatakeoncontinuousvaluesinagiveninterval,e.g.audio(humanspeech)orvideo.Digit

4、aldatatakeondiscretevalues,e.g.textorintegers.Signalsareelectromagneticrepresentationsofdata.Signalingisthephysicalpropagationofthesignalalongasuitablemedium.Transmissionisthecommunicationofdatabythepropagationandprocessingofsignals.BandwidthMaximumtimesp

5、ersecondthesignalcanchangeTerminologyAnalogsignalisacontinuouslyvaryingelectromagneticwave.Usedinearlytelephonesystems.Analogsignalshadthedrawbackthattheyattenuate(weaken)overlongdistances.Neededamplifierstoboostthesignals.However,amplifiersdistortthesign

6、alandintroducenoise.Digitalsignalisasequenceofbinaryvoltagepulses(0’sand1’s).Digitaltransmissionavoidsthenoiseproblembyencodingtheanalogsignalintodigitalform.Thedigitizedversionisthensentacrossthenetwork.Shieldedtwisted-paircable(STP)UTPcableUTPCategories

7、Cat3upto16MHzVoicegradefoundinmostofficesTwistlengthof7.5cmto10cmCat4upto20MHzCat5upto100MHzCommonlypre-installedinnewofficebuildingsTwistlength0.6cmto0.85cmUTPcableThefollowingarethetypesofcableconnectionsusedbetweeninternetworkdevices:straight-throughca

8、blecrossovercablestraight-throughcable白橙橙白绿蓝白蓝绿白棕棕白橙橙白绿蓝白蓝绿白棕棕Useforthefollowingcabling:SwitchtorouterSwitchtoPCorserverHubtoPCorserverCrossovercable白橙橙白绿蓝白蓝绿白棕棕白橙橙白绿蓝白蓝绿白棕棕Useforthefollowingcabling:SwitchtoswitchSw