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1、Onedistinctadvantageoffiberopticcablesisnoiseimmunity.Fiberopticcablescanberoutedindiscriminatelythroughhighnoiseareaswithimpunity,althoughfireratingsshouldbeobserved.CablesthatpassthroughmultiplespacesinaPlantshouldberatedforheatingVentilationairconditioning(HVAC)Plenumswhe
2、retheycanwithstandfiresperNationalFireProtectionAssociation(NFPA)requirements・NetworkTopologiesThetermtopologyreferstothemethodusedtoconnectcomponentsonanetwoik.Themostcommontopologiesarering,bus,andstartopologiesfigure代RingNetworfdopolojyHn
3、Uii(Figures4-5,4-6,andNetworkTopo
4、logy4-7)1Hubbuttheycantakeontheappearancesofeachotherandstillmaintaintheircharacteristics.Forexample,atokenringnetworksegmentcanbewiredinastarconfiguration,wherecomponentsarecabledbacktoahubwheretheringis"inside”thehub.Thisallowsacommonwiringclosetforagivenbuildingorareawith
5、home-runcablepullsforeachcomponent.Ahubofferstheadvantagesofcentralizedmaintenanceandconfigurationcontrol.TokenBuses&RingsTheMACfunctionsoftokenringsandbusesaresimilar.ARCnet,developedbytheDatapointCorp,inthe70s,isatokenpassingprotocolthatcanbeimplementedinabusorstartopology
6、usingcoaxialorUTPcables.A"token"ispassedaroundthebusorring.Whichevernodehasthetokenisallowedtocommunicateonthemedia.ARCnetrunsat2.5Mbps,atthefollowinglengths:400feetwith10nodesusingUTP;2,000feetwithapracticallimitofupto100nodesusingRG-62coaxialcableinacoaxialstarconfiguratio
7、n(usingahuborhubs);and1,000feetwith10nodesper1,000footsegmentusingRG-62coaxialcable.ARCnetusesactiveandpassivehubsinthestarconfiguration,withnetworkcardsonthedevicesthathaveswitchesforsettingnodenumbers.Thelowestnumberednodeisthemastercontroller,givingpermissiontocommunicate
8、toeachnodebynumbe匸ARCnetisavailableina20-Mbpsversion.TheIBMtokenringprotocol,standardizedviaIEEE-802.5,runsat4or16Mbps.Nodesontheringconnecttoamulti-stationaccessunit(MAU),atypeofhub.MAUscanbeconnectedtogetherinamainring,withsegments,orlobes,fromeachMAUconnectedinastarconfig
9、urationtodeviceswithnetworkinterfacecards.Thelengthoftheringislimitedto770m
