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Chapter2ThePhysicalLayerCollegeofComputerScienceChongqingUniversityContentThePhysicalLayerTheoreticalBasisforDataCommunicationsTransmissionMediaATaxonomy分类ofCommunicationNetworksExampleNetworksFromSignalstoPackets001011100010100010101011100101010101011101110000001111010101110101010101101Header/BodyHeader/BodyHeader/BodyReceiverSenderApplicationPresentationSessionTransportNetworkDatalinkPhysicalFromSignalstoPacketsalogSignaltal”SignalStream00101110001ckets0100010101011100101010101011101110000001111010101110101010101101Header/BodyHeader/BodyHeader/BodyReceiverSenderketnsmissionThePhysicalLayerPhysicalLinkNetworkTransportApplication•ThePhysicalLayerprovidesthemechanical,electrical,functionalandproceduralcharacteristicstoestablish,（机械特性，电气特性，功能特性，过程特性）maintain,andreleasephysicalconnectionsbetweendatalinkentities.•Aphysical-connectionmayinvolveintermediateopensystems,eachrelayingbittransmissionwithinthePhysicalLayer.•PhysicalLayerentitiesareinterconnected连通的bymeansofaphysicalmedium.Fourcharacteristics•Mechanicalcharacteristics:mechanicalspecificationofelectricalconnectorsandcables,forexamplemaximumcablelength•Electricalcharacteristics:electricalspecificationoftransmissionlinesignallevelandimpedance阻抗•Functionalcharacteristics:thefunctionlogicofelectricsignalsgeneratedoncertainwiresinthephysicalmedia•Proceduralcharacteristics:theproceduresofprotocolsthatcontrolthesequenceofeventsRS-232•RS-232(RecommendedStandard232)isthetraditionalnameforaseriesofstandardsforserialbinarysingle-endeddataandcontrolsignalsconnectingbetweenaDTE(DataTerminalEquipment)andaDCE(DataCommunicationsEquipment).Itiscommonlyusedincomputerserialports.•MechanicalcharacteristicsDB25:maleandfemaleconnectorDB9:maleandfemaleconnector•Electricalcharacteristics−dataratesofupto20kbps,overcablesofupto15metres−negative,bipolar（两极的）logicinwhichanegativevoltageisusedtorepresentalogic‘1’,andapositivevoltagerepresentsalogic‘0’•Functionalcharacteristics•...