Physical layer

honggarae 23/12/2021 1149

Mainfunctions

Themainproblemstobesolvedbythephysicallayer:

(1)Thephysicallayershouldshieldasmuchaspossiblethephysicalequipmentandtransmissionmedia,andthecommunicationmeansDifferent,makethedatalinklayernotperceivethesedifferences,onlyconsidertocompletetheagreementandserviceofthislayer.

(2)Giveitsserviceusers(datalinklayer)theabilitytotransmitandreceivebitstreams(usuallyseriallytransmittedbitstreams)onaphysicaltransmissionmedium.Forthisreason,Thephysicallayershouldsolvetheproblemofestablishing,maintaining,andreleasingphysicalconnections.(3)Uniquelyidentifydatacircuitsbetweentwoadjacentsystems.

Themainfunctionofthephysicallayer:toprovidedatatransmissionchannelsanddatatransmissionfordataterminalequipment.

1.Provideadatatransmissionpathforthedataterminaldevice.Thedatapathcanbeaphysicalmediumoraconnectionofmultiplephysicalmedia.Acompletedatatransferincludesactivatingthephysicalconnection,transferringdata,andterminatingthephysicalconnection.Theso-calledactivationmeansthatnomatterhowmanyphysicalmediaareinvolved,itmustbeconnectedbetweentwodataterminaldevicesincommunicationtoformapath.

2.Totransmitdata,thephysicallayermustformanentitysuitablefordatatransmissiontoservedatatransmission.Oneistoensurethatdatacanpassthroughitcorrectly,andtheotheristoprovidesufficientbandwidth(bandwidthreferstothenumberofbits(BIT)thatcanpasspersecond)toreducecongestiononthechannel.Thewayoftransmittingdatacanmeettheneedsofpoint-to-point,point-to-multipoint,serialorparallel,half-duplexorfull-duplex,synchronousorasynchronoustransmission.

3.Completesomemanagementworkonthephysicallayer.

Components

Themediaofthephysicallayerincludesoverheadopenwires,balancedcables,opticalfibers,wirelesschannels,etc.TheinterconnectionequipmentforcommunicationreferstotheinterconnectionequipmentbetweenDTEandDCE.DTEisdataterminalequipment,alsoknownasphysicalequipment,suchascomputers,terminals,etc.areincluded.DCEisadatacommunicationdeviceorcircuitconnectiondevice,suchasamodem.DatatransmissionusuallygoesthroughDTE──DCEandthenthroughDCE──DTE.InterconnectionequipmentreferstodevicesthatconnectDTEandDCE,suchasvariousplugsandsockets.Variousthickandthincoaxialcables,T-typeconnectors,plugs,receivers,transmitters,repeaters,etc.intheLANareallmediaandconnectorsonthephysicallayer.

Importantcontent

Characteristicsofthephysicallayerinterface

(1)Mechanicalcharacteristics

SpecifytheshapeandSize,numberandarrangementofleads,fixingandlockingdevices,etc.

(2)Electricalcharacteristics

Specifythevoltagerangethatappearsoneachlineoftheinterfacecable.

(3)Functionalcharacteristics

Specifywhatacertainlevelofvoltageappearsonacertainlinemeans.

(4)ProcedurecharacteristicsSpecifytheorderofoccurrenceofvariouspossibleeventsfordifferentfunctions.

Themainfeaturesofthephysicallayer:

(1)BecausemanyphysicalregulationsorprotocolshavebeenformulatedbeforeOSI,andinthefieldofdatacommunication,thesephysicalregulationshavebeenManycommercialequipmentuses,inaddition,thephysicallayerprotocolinvolvesawiderange,sothereisnosetofnewphysicallayerprotocolsaccordingtotheabstractmodelofOSI,buttheexistingphysicalproceduresareusedtodeterminethephysicallayerasadescriptionThemechanical,electrical,functionalandregulatorycharacteristicsoftheinterfacewiththetransmissionmedia.

(2)Sincetherearemanyphysicalconnectionmethodsandtherearemanytypesoftransmissionmedia,thespecificphysicalprotocolisquitecomplicated.

Thetransmissionofsignalsisinseparablefromthetransmissionmedium,andtheremustbeinterfacesatbothendsofthetransmissionmediumforsendingandreceivingsignals.Therefore,sincethephysicallayerismainlyconcernedwithhowtotransmitsignals,themaintaskofthephysicallayeristospecifysomecharacteristicsrelatedtothetransmissionofvarioustransmissionmediaandinterfaces.

1.Mechanicalcharacteristics

Alsocalledphysicalcharacteristics,theyindicatethemechanicalcharacteristicsofthehardwareconnectioninterfacebetweencommunicationentities,suchastheshapeandsizeoftheconnectorusedintheinterface,thenumberandarrangementofleads,andfixingandlockingdevices.Thisisverysimilartotheusualpowerplugsofvariousspecifications,andtheirdimensionsarestrictlyregulated.

Thegeometricsize,numberofjackcoresandarrangementoftheDCEinterfacehavebeenstandardizedbyISO.

DTE(DataTerminalEquipment,dataterminalequipment,equipmentusedtosendandreceivedata,suchastheuser'scomputer)connectorcommonlyusedpinform,itsgeometricsizeand.DCE(DataCircuit-terminatingEquipment,datacircuitterminationequipment,usedtoconnectDTEanddatacommunicationnetworkequipment,suchasModemmodem)connector,thenumberofpinsandarrangementaremirroredandsymmetricalwiththeDCEconnector.

2.Electricalcharacteristics

Specifythephysicalconnection,theelectricalconnectionofwiresandthecharacteristicsofrelatedcircuits,generallyincluding:descriptionofreceiverandtransmittercircuitcharacteristics,signalidentification,descriptionofmaximumtransmissionrate,andTherulesrelatedtotheinterconnectioncable,theoutputimpedanceofthetransmitter,theinputimpedanceofthereceiverandotherelectricalparameters,etc.

3.Functionalcharacteristics

Specifythepurpose(usage)ofeachsignallineofthephysicalinterface,including:thestipulationmethodoftheinterfacelinefunction,thefunctionclassificationoftheinterfacesignalline-datasignalline,controlsignalline,timingsignallineAnd4typesofgroundingwire.

4.Procedurecharacteristics

Specifytheentireprocessofusingtheinterfacetotransmitbitstreamsandthelegalsequenceofeventsusedfortransmission,includingtheexecutionsequenceoftheeventsandthedatatransmissionmethod,thatis,theestablishment,maintenanceandexchangeofphysicalconnectionsInformation,thesequenceofactionsofbothDTE/DCEontheirrespectivecircuits.

Theabovefourcharacteristicsimplementtherequirementsforsignals,interfacesandtransmissionmediawhenthephysicallayertransmitsdata.

Importantstandards

SomestandardsandprotocolsofthephysicallayerhavebeenformulatedandappliedbeforetheestablishmentoftheOSI/TC97/C16subcommittee,

OSIhasalsoformulatedsomestandardsandadoptedsomeexistingresults.Someimportantstandardsarelistedbelowforreaderstoreferto.

ISO2110:Called"DataCommunication----25-pinDTE/DCEInterfaceConnectorandPinAssignment".Itisbasicallycompatiblewith"RS-232-C"ofEIA(ElectronicIndustriesAssociationofAmerica).

ISO2593:Itiscalled"DataCommunication----34-pinDTE/DCE----InterfaceConnectorandPinAssignment".

ISO4902:called"DataCommunication----37-pinDTE/DEC----InterfaceConnectorandPinAssignment".CompatiblewithEIARS-449.

CCITTV.24:Itiscalled"thedefinitiontableofinterfacecircuitbetweendataterminalequipment(DTE)anddatacircuitterminatingequipment".Itsfunction

CanbecompatiblewithEIARS-232-CandRS-449on100seriallines.

Characteristics

The4characteristicsofthephysicalinterfacereflectedinthephysicalinterfaceprotocolaremechanicalcharacteristics,electricalcharacteristics,functionalcharacteristicsandregulatorycharacteristics.:

(1)Mechanicalcharacteristics,specifytheshapeandsizeoftheconnectorusedintheinterface,thenumberandarrangementofleads,fixingandlockingdevices,etc.Thisisverysimilartotheusualcommonspecificationsforthesizeofpowerplugsthathavestrictregulations.

(2)Electricalcharacteristics,indicatingtherangeofvoltagesappearingoneachlineoftheinterfacecable.

Theelectricalcharacteristicsofthephysicallayerstipulatethelimitsofthesignalvoltage,impedancematching,transmissionrateanddistanceonthelinewhenthebinarybitstreamistransmittedonthephysicalconnection.Earlierelectricalcharacteristicsstandardsdefinedtheelectricalcharacteristicsattheboundarypointsofphysicalconnections,whilenewerelectricalcharacteristicsstandardsdefinedtheelectricalcharacteristicsoftransmittersandreceivers,andalsoprovidedrelevantregulationsforinterconnectingcables.Incomparison,newerstandardsaremoreconducivetotheintegrationofsendingandreceivinglines.Theelectricalcharacteristicsofthephysicallayerinterfacearemainlydividedintothreecategories:unbalanced,newunbalancedandnewbalanced.

Theunbalancedsignaltransmitterandreceiverworkinanunbalancedway,eachsignalistransmittedbyawire,andallsignalsshareagroundwire.Thesignallevelis+5V～+15V,whichmeansbinary"0",and-5V～-15V,whichmeansbinary"1".Thesignaltransmissionrateislimitedtolessthan20Kbps,andthewirelengthislimitedtolessthan15M.Sincethesignallineisasingleline,theinterferencebetweenthelinesislarge,andtheexternalinterferenceinthetransmissionprocessisalsogreat.

Inthenewunbalancedstandard,thetransmitterworksinanunbalancedway.Thereceiverworksinabalancedway(thatis,adifferentialreceiver).Eachsignalistransmittedbyawire.Allsignalssharetwogroundwires,thatis,onegroundwireineachdirection.Thesignalleveluses+4v~+6vtorepresentbinary"0",and-4Vto-6Vrepresentsbinary"1".Whenthetransmissiondistancereaches1000M,thesignaltransmissionrateisbelow3kbps.Asthetransmissionrateincreases,thetransmissiondistancewillbeshortened.Intheshort-distancesituationwithin10M,thetransmissionratecanreach300kbps.Becausethereceiveradoptsthedifferentialwaytoreceive,andeachdirectionusesthesignalgroundindependently,sothelineinterferenceandexternalinterferencearereduced.

Thenewbalancedstandardstipulatesthatboththetransmitterandthereceiverworkindifferentialmode.Eachsignalistransmittedbytwowires.Theentireinterfacecanworknormallywithoutsharingthesignal.ThesignallevelisdividedbytwoThedifferenceofthesignalonthewireisexpressed.Relativetoacertainwire,thedifferencebetween+4V～+6Vmeansbinary"0",andthedifferencebetween-4V～-6Vmeansbinary"1".Whenthetransmissiondistancereaches1000M,thesignaltransmissionrateisbelow100kbps;whentheshort-distancetransmissioniswithin10m,theratecanreach10Mbps.Sinceeachsignalusestwo-wiretransmission,inter-lineinterferenceandexternalinterferencearegreatlyweakened,andithasahighresistancetocommonmodeinterference.

Physical layer

(3)Functionalcharacteristics,whichstipulatethesourceandfunctionoftheinterfacesignalandtherelationshipbetweenothersignals.Thatis,thefunctionallocationandexactdefinitionofeachsignallineonthephysicalinterface.Thephysicalinterfacesignallinesaregenerallydividedintodatalines,controllines,timinglinesandgroundlines.

ThefunctionalcharacteristicsoftheDTE/DCEstandardinterfacearemainlytomaketheexactfunctiondefinitionofeachinterfacesignallineanddeterminethemutualoperationrelationship.Thedefinitionofeachinterfacesignallineusuallyadoptstwomethods:onemethodistheone-lineone-definitionmethod,thatis,eachsignallineisdefinedasafunction,CCITTV24,EIARS-232-C,EIARS-449,etc.Usethismethod;theothermethodistheone-linepolysemousmethod,whichmeansthateachsignallineisdefinedasmultiplefunctions.Thismethodhelpsreducethenumberofinterfacesignallines.ItisCCITTX.21adopted.

Interfacesignallinescangenerallybedividedintogroundinglines,datalines,controllines,timinglinesandothertypesaccordingtotheirfunctions.Thenamingofeachsignallineusuallyadoptsthreeformsofnumbers,lettercombinationsorEnglishabbreviations.Forexample,EIARS-232-Cadoptslettercombinations,EIARS-449adoptsEnglishabbreviations,andCCITTV.24isnamedafteranumber.InCCITTV.Inthesuggestion24,thenameoftheDTE/DCEinterfacesignallinestartswith1,soitisusuallycalledthe100seriesinterfaceline,andthenameofthesignallineusedfortheDTE/ACEinterfacestartswith2,soitiscalledthe200seriesinterfaceSignalline.

(4)Thefeatureoftheregulationdefinesasetofoperationproceduresforbinarybitstreamtransmissiononthesignalline,includingtheworkingsequenceandtimingofeachsignalline,sothatthebitstreamtransmissioncanbecompleted.

ThespecificationcharacteristicsofDTE/DCEstandardinterfacestipulatetherelationshipbetweenthesignallinesoftheDTE/DCEinterface,thesequenceofactions,andthemaintenanceandtestoperations.Thecharacteristicsoftheregulationsreflectvariouspossibleeventsthatmayoccurbetweenthetwopartiesinthedatacommunicationprocess.Becausethesequenceofthesepossibleeventsisnotthesame,andtherearemultiplecombinations,thecharacteristicsoftheproceduresareoftenmorecomplicated.Abetterwaytodescribethecharacteristicsofproceduresistousestatetransitiondiagrams.Becausethestatetransitiondiagramreflectsthetransitionprocessofthesystemstate,andthesystemstatetransitionisdeterminedbythecurrentstateandtheeventsthatoccurred(referringtothecontrolsignalsthatoccurredatthetime).

Differentphysicalinterfacestandardsdifferintheabovefourimportantcharacteristics.Inactualnetworks,physicalinterfacestandardssuchasEIA-232-E,EIARS-449andCCITTXarewidelyused.21recommendations.EIARS-232Cisstillthemostcommonlyusedcomputerasynchronouscommunicationinterface.

Interfaceprotocol

Telephonenetworkmodems-V.92
IRDAphysicallayer
USBphysicallayer
EIARS-232,EIA-422,EIA-423,RS-449,RS-485
EthernetphysicallayerIncluding10BASE-T,10BASE2,10BASE5,100BASE-TX,100BASE-FX.100BASE-T,1000BASE-T,1000BASE-SXandothertypes
Varietiesof802.11Wi-Fiphysicallayer
DSL
ISDN
T1andotherT-carrierlinks,andE1andotherE-carrierlinks
SONET/SDH
OpticalTransportNetwork(OTN)
GSMUmairinterfacephysicallayer
Bluetoothphysicallayer
ITURecommendations:seeITU-T
IEEE1394interface
TransferJetphysicallayer
Etherloop
ARINC818AvionicsDigitalVideoBus
G.hn/G.9960physicallayer
CANbus(controllerareanetwork)physicallayer

communicationhardware

Commondevicesatthephysicallayerinclude:networkcardfiber,CAT-5cable(RJ-45connector),hubwithfullwavefunction,Repeatertostrengthensignal,serialport,parallelport,etc.

Communicationhardwareincludescommunicationadapter(alsocalledcommunicationinterface),modem(MODEM)andcommunicationline.Inprinciple,thephysicallayeronlysolvesthebitstreamtransmissionbetweenDTEandDCE.Althoughthecommunicationcontroldeviceasthemaincomponentofthenetworknodeequipment,itsconnotationliesinthephysicallayer,datalinklayer,andevenhigherlayers.Theupperboundaryisnotveryclear,butthefunctionssuchastheMODEMinterface,bitsamplingandsending,andbitbufferingincludedinitbelongexactlytothecategoryofthephysicallayer.InordertorealizethecommunicationbetweenaPCandamodemorotherserialdevices,electroniccircuitsmustbeusedtoconverttheparalleldatainthePCintoabitstreamcompatiblewiththesedevices.Inadditiontothetransmissionofthebitstream,technicalissuessuchashowmanybitsacharacterconsistsofandhowtoextractcharactersfromthebitstreammustbesolved,whichrequirestheuseofcommunicationadaptation.Thecommunicationadaptercanbeconsideredasacircuitusedtocompletetheserialandparallelconversionofbinarydataandotherrelatedfunctions.Accordingtothecommunicationregulations,communicationadapterscanbedividedintothreetypes:TTY(TeleTypeWriter),BSC(BirarySynchronousCommuication)andHDLC(High-levelDatalinkControl).

IBMPCAsynchronousCommunicationAdapter:AsynchronouscommunicationadaptationusingTTYprotocoladoptsRS-232Cinterfacestandard.ThiskindofcommunicationadaptercannotonlybeusedforPConlinecommunication,butalsocanconnectvariousexternaldeviceswithRS-232Cinterface.Forexample,itcanbeconnectedtoinputdevicessuchasamouseanddigitizerusingRS-232Cinterface;itcanbeconnectedtovariousoutputdevicessuchasprinters,plottersandCRTmonitorsusingRS-232Cinterface.Itcanbeseenthattheuseofasynchronouscommunicationadaptersisveryextensive.Theasynchronouscommunicationprocedureregardseachcharacterasanindependentmessage.Thecharacterscanappearinthebitstreamsequentially.Theintervaltimebetweencharactersisarbitrary(thatis,asynchronoustimingisusedbetweencharacters),buteachbitinthecharacterisfixedTheclockfrequencyistransmitted.Asynchronoustimingbetweencharactersandsynchronoustimingbetweenbitsinacharacterarecharacteristicsofasynchronoustransmissionprocedures.

Eachcharacterintheasynchronoustransmissionprotocoliscomposedoffourparts:
1startbit:logically"0"meansthatthecommunicationiscalled"SPACE".
5~8databits:thecontenttobetransmitted.
1bitodd/evencheckbit:usedforerrordetection.
1~2stopbit:representedbylogic"1",usedasaspacebetweencharacters.Inthistransmissionmode,eachcharacterisseparatedbyastartbitandastopbit,soitisalsocalled"start-stop"transmission.Theserialportfirstconvertseachparallelcharacterinthedatatobesentintoaserialbitstring,andaddsastartbitbeforethestring,andacheckbitandastopbitafterthestring,andthensendsitout.Thereceivingendguaranteestheintegrityofthebitstringinthereceivedcharacterbydetectingthestartbit,checkbitandstopbit,andfinallyconvertsitintoaparallelcharacter.Theserialasynchronouscommunicationadapteritselfislikeamicrocomputer,andtheabovefunctionsareperformedtransparentlybyit,withoutuserintervention.Theearlyasynchronouscommunicationadapterwasmadeintoaseparateplug-inboard,whichcanbedirectlyinsertedintothesystemexpansionslotofthePCforuse.Later,mostoftheasynchronouscommunicationadaptersandotheradapters(suchasprinters,diskdrives,etc.)arebuilttogether.Aplug-inboardcalledamulti-functionboard.Therearealsosomehigh-endmicrocomputersthathavemadeasynchronouscommunicationadaptersonthesystemmotherboardasaregularcomponentofthemicrocomputersystem.

Programmingmethod

ThecontentoftheasynchronousserialcommunicationprogrammingmethodofthePCincludesDOS,WINDOWSandBIOS-levelPCcommunication,andasynchronouscommunicationanddevice-basedSystemPCcommunicationandcommunicationprogrammingmethod.

DOScommunication

PCsusuallyhavetwoasynchronousserialports,calledCOM1andCOM2respectively,andtheyallconformtotheRS-232Cstandard.IntheDOSoperatingsystem,COM1andCOM2aremanagedasI/Odevices,andCOM1andCOM2aretheirlogicaldevicenames.Accordingly,DOScanrealizeasynchronousserialcommunicationbyoperatingCOM1andCOM2.TheMODEcommandofDOScanbeusedtosettheparametersoftheasynchronousserialport,andtheCOPYcommandofDOSallowstheasynchronousserialporttobeusedasaspecial"file"fordatatransmission.HereisanexampleofusingtheMODEandCOPYcommandsofDOStotransfercharactersenteredbythedual-computerkeyboard.TheformatoftheMODEcommandisasfollows:

MODEportname:rate,checkmethod,numberofdatabits,numberofstopbits

wheretheportnameisCOM1orCOM2;thetransmissionrateisoptional110,150,300,600,1200,2400,4800or9600bps;thecheckmethodisE(evencheck),(oddcheck)orN(nocheck);thenumberofdatabitsis7or8;stopThenumberofdigitsis1or2digits.Theparameterssetbybothcommunicationpartiesshouldbeconsistent.Forexample,bothpartiesenterthefollowingcommands:MODECOM1:1200,E,7,1meansthatbothpartiesuseCOM1astheasynchronouscommunicationportwith1200bps,evencalibration,7databits,1stopbitTocommunicatewiththesettingparameters.ThereisastandardconsoleCOMinDOS.Infact,wheninputting,COMiskeyboard,andwhenoutputting,COMisdisplay.

ThePCreadytosendexecutesthefollowingcommand:COPYCON:COOM1:meansthattheinformationreceivedfromthekeyboardissentthroughtheCOM1serialport.

ThePCreadytoreceiveexecutesthefollowingcommand:COPYCOM1:CON:meansthatitwillreceivetheinformationfromtheCOM1serialportanddisplayitonthedisplay.

AfterthetwoPCsrespectivelyexecutetheabovecommands,thecharactersenteredonthesender’skeyboardwillbedisplayedonthereceiver’sdisplay.WhatwasintroducedaboveisthesimplestPCcommunicationrealizedwithDOS'sMODEandCOPPPYcommands.InthehigherversionofMS-DOS(suchasMS-DOSV6.0)alsoprovidesacommandcalledINTERLNK,whichisactuallyacommunicationprogram.UsingtheINTERLNKcommandandacableconnectingtheserialportsoftwoPCs,onePCcanaccessdataandrunprogramsfromthediskdriveoftheotherPCwithoutusingafloppydisktocopyfiles.ThePCusedtoentercommandsiscalledtheclient,andthePCconnectedtotheclientiscalledtheserver.Theclientusestheserver'sdriverandprinter,andtheserverdisplaystheconnectionstatusofthetwoPCs.

WhentwoPCsareconnectedbyINTERLNK,thedriveontheserverwillbemappedtotheclientintheformofanexpandeddrive.IfthetwoPCsoriginallyhadthreedrivesA,B,andC,Afterconnection,inadditiontoitsownthreedrives,theclienthasthreeextendeddrives,E,F,andG(serverdriveimage).Theclientcanusetheseextendeddrivesasifitwereitsowndrive.WhenusingINTERLNK,theremustbeatleastonefreeserialportoneachPC,andaNullMODEMserialcableofline3orline7,atleast16Kfreememoryontheclient,andtheserverThereisatleast130Koffreememoryonit.

Intheclient'sCONFIGsystemconfigurationfile.AddthefollowingcommandtoSYS:devive=c:\dos\interlnk.exe/drives:5

ThenrestarttheclienttoloadINTERLNK.Interlnkisassumedhere.TheexeisstoredintheDOSsubdirectoryoftheCdriveoftheclient.The/drives:5parameterisusedtomap5serverdrives.Bydefault,itis3drives.TheCONFIGisnotneededtostartINTERLNKontheserver.TomakeanychangestoSYS,justtypeintersvrattheDOScommandprompt.Atthistime,alineofstatusinformationappearsatthebottomofthescreen,showingtheconnectionstatusofINTERLNK.

PCcommunication

TheMicrosoftWindowsapplicationterminalallowstheuser'sPCtoconnectwithothercomputersandexchangedata,anditcanalsoemulatetheterminalrequiredbytheremotecomputerthatwillexchangedatawithittype.ThefollowingshowsthecommunicationprocessofaPCusingWINDOWSTerminaltoreadfilesfromaremotesystemwithconnectedservices.

Opentheterminal-usetheSettingsmenutosetparameters-viewfiles-usetheTransfersmenutoreceiveafile-offlinewiththeremotecomputer-usethephonemenutohangupthemodem-—UsetheFilemenutostorefiles—Exittheterminal

BIOS

Theinterrupt14Hinthebasicinputoutputsystem(BIOS)ofthePCprovidestheasynchronousserialportTheservicefunction,throughthefourfunctionsprovidedbyINT14H,canaccesstheserialcommunicationportandrealizethecommunicationwiththecomputer.TheserialportfunctionofINT14His.