data communication

Introduction

Development

Thefirststage:languageisthemainstay,andinformationistransmittedthroughprimitivemeanssuchasmanpower,horsepower,andbeacon.

Thesecondstage:text,post.(Increasethemeansofinformationdissemination)

Thethirdstage:printing.(Expandthescopeofinformationdissemination)

Thefourthstage:telegraph,telephone,broadcast.(Enteringtheeraofelectricalappliances)

Thefifthstage:Intheinformationage,inadditiontolanguageinformation,therearedataandimagehistory.

Communicationastelecommunicationsisfromthe19thcenturyItstartedinthe1930s.Faradaydiscoveredelectromagneticinductionin1831.Morseinventedthetelegraphin1837.Maxwell'selectromagneticfieldtheoryin1873.Bellinventedthetelephonein1876.Marconiinventedtheradioin1895.Openedupaneweraoftelecommunications.Theelectronictubewasinventedin1906,andanalogcommunicationwasdeveloped.TheNyquistcriterionandsamplingtheoremin1928.Shannon'stheoremin1948.Inthe1950s,semiconductorswereinventedanddigitalcommunicationsweredeveloped.Theintegratedcircuitwasinventedinthe1960s.Theconceptofageostationarysatellitewasputforwardinthe1940s,butitcouldnotberealized.Aerospacetechnologyinthe1950s.Synchronoussatellitecommunicationwasrealizedforthefirsttimein1963.Inthe1960s,thelaserwasinventedandattemptedtobeusedincommunication,butwasunsuccessful.Opticalfiberwasinventedinthe1970s,andopticalfibercommunicationwasdeveloped.

Historicalfigures

Bell(1847-1922),British,workedinLondonin1868.In1871,hewenttoworkinBoston.In1873,heservedasaprofessoratBostonUniversity.In1875,themultiplextelegraphwasinvented.In1876,thetelephonewasinvented.Hehaswonmanypatentsinhislifetime.Thewifeisdeaf.

Marconi(1874-1937),Italian,experimentedinhisfather'smanorin1894.In1896,hewenttoLondon.In1897,awirelesstelegraphcompanywasestablished.In1899,wirelesscommunicationbetweenBritainandFrancewasrealizedforthefirsttime.In1916,shortwaveradiocommunicationwasrealized.In1929,aworldwidewirelesscommunicationnetworkwasestablished.WontheNobelPrize.ParticipatedintheFascistParty.

Datacommunicationhistory

Datacommunicationstartedintheearly1950swiththedevelopmentofcomputertelematicsapplications.Earlytelematicssystemsweremostlycenteredononeorseveralcomputers,relyingondatacommunicationmeanstoconnectalargenumberofremoteterminalstoformaterminal-orientedcentralizedprocessingsystem.Inthelate1960s,startingfromthebirthoftheARPAcomputernetworkintheUnitedStates,aheterogeneouscomputercommunicationnetworkwiththepurposeofresourcesharingappeared,thusopeningupanewfieldofcomputertechnology-networkinganddistributedprocessingtechnology.Afterthe1970s,therapiddevelopmentofcomputernetworksanddistributedprocessingtechnologyhasalsopromotedthedevelopmentofdatacommunications.In1976,CCITTformallyannouncedtheX.25recommendation,animportantstandardforpacket-switcheddatanetworks.Afterthat,ithasbeenimprovedandrevisedmanytimes,layingthefoundationforthetechnologicaldevelopmentofpublicandprivatedatanetworks.Inthelate1970s,theInternationalOrganizationforStandardization(ISO)proposedtheOpenSystemInterconnection(OSI)referencemodelinordertopromotetheinterconnectionofsystemsofdifferenttypes,anditwasformallyadoptedin1984asaninternationalstandard.Sincethen,thedevelopmentofcomputernetworktechnologyandapplicationhasproceededinaccordancewiththismodel.

Classification

Classifiedbyinformation

Telephonecommunicationsystem

Datacommunicationsystem

CableTVsystem

Accordingtomodulationclassification

Basebandtransmission

Modulationtransmission

Accordingtotransmissionsignalcharacteristics

Analogcommunicationsystem

Digitalcommunicationsystem

Transmissionmeans

Cablecommunication

Twistedpair,coaxialcable,etc.Localcallsandlong-distancecommunications.Modulationmethod:SSB/FDM.Coaxial-basedPCMtimedivisionmultiplexdigitalbasebandtransmissiontechnology.Opticalfiberwillgraduallyreplacecoaxial.

Microwaverelaycommunication

Comparedwithcoaxialcable,itiseasytosetup,smallinvestmentandshortcycle.AnalogtelephonemicrowavecommunicationmainlyadoptsSSB/FM/FDMmodulation,andthecommunicationcapacityis6000channels/channel.DigitalmicrowaveadoptsBPSK,QPSKandQAMmodulationtechnology.Using64QAM,256QAMandothermulti-levelmodulationtechnologiestoincreasethemicrowavecommunicationcapacity,itcantransmit1920to7680PCMdigitaltelephonesina40Mchannel.

Opticalfibercommunication

Usingthecharacteristicsoflong-distancetransmissionoflaserinopticalfiber,ithasthecharacteristicsoflargecommunicationcapacity,longcommunicationdistanceandstronganti-interference.Itisusedforlocal,long-distance,andtrunktransmission,andgraduallydevelopsuseropticalfibercommunicationnetwork.Basedonlong-wavelasersandsingle-modeopticalfibers,thenumberofcommunicationchannelsperopticalfiberexceeds10,000,andthecommunicationfiberpoweroftheopticalfiberitselfisveryhuge.Overthepastfewdecades,opticalfibercommunicationtechnologyhasdevelopedrapidly,andtherearevariousequipmentapplications,suchasaccessequipment,photoelectricconversionequipment,transmissionequipment,switchingequipment,networkequipment,andsoon.Opticalfibercommunicationequipmentiscomposedoftwoparts:photoelectricconversionunitanddigitalsignalprocessingunit.

Satellitecommunication

Thecommunicationdistanceislong,thetransmissioncapacityislarge,thecoverageareaislarge,anditisnotrestrictedbygeographicalareaandhighreliability.Maturetechnologyusesanalogmodulation,frequencydivisionmultiplexingandfrequencydivisionmultipleaccess.Digitalsatellitecommunicationadoptsdigitalmodulation,timedivisionmultiplexingandtimedivisionmultipleaccess.

Mobilecommunication

Mobilecommunicationiscommunicationbetweenmovingbodies,orbetweenmovingbodiesandfixedbodies.Themovingbodycanbeaperson,oramovingobjectsuchasacar,train,ship,radio,etc.Themobilecommunicationsystemconsistsoftwoparts:

(1)Spacesystem

(2)Groundsystem:①Satellitemobileradiostationandantenna;②Gatewaystation,basestation.

Sinceitsbirthinthe1980s,themobilecommunicationsystemwillgenerallygothroughfivegenerationsofdevelopmentby2020,andwilltransitionfromthethirdgenerationtothefourthgeneration(4G)by2010.By4G,inadditiontocellularphonesystems,broadbandwirelessaccesssystems,millimeterwaveLANs,intelligenttransmissionsystems(ITS)andstratosphericplatform(HAPS)systemswillbeputintouse.Themostobvioustrendoffuturegenerationsofmobilecommunicationsystemsistorequirehighdatarates,highmobilityandseamlessroaming.Realizingtheserequirementswillfacegreatertechnicalchallenges.Inaddition,systemperformance(suchascellsizeandtransmissionrate)willlargelydependonthefrequency.Takingintoaccountthesetechnicalissues,somesystemswillfocusonprovidinghighdatarates,andsomesystemswillfocusonenhancingmobilityorexpandingcoverage.

Fromtheperspectiveofusers,theaccesstechnologiesthatcanbeusedinclude:cellularmobilewirelesssystems,suchas3G;cordlesssystems,suchasDECT;short-rangecommunicationsystems,suchasBluetoothandDECTdatasystems;wirelesslocalareanetworks(WLAN)System;fixedwirelessaccessorwirelesslocalloopsystem;satellitesystem;broadcastingsystem,suchasDABandDVB-T;ADSLandCableModem.

Communicationsystem

Offlinedatatransmission

Itsimplyusestelephoneorsimilarlinkstotransmitdata,excludingcomputersystems.Theequipmentusedatbothendsofsuchalinkisnotapartofthecomputer,oratleastdoesnotimmediatelyprovidethedatatothecomputerforprocessing,thatis,thedataisofflinewhenitissentorreceived.Thisdatacommunicationisrelativelycheapandsimple.

Remotebatchprocessing

Thetermappliestosuchamethod:theuseofdatacommunicationtechnologytomaketheinputandoutputofdatageographicallyawayfromthecomputerthatprocessestheminbatchmode.

Onlinedatacollection

referstothemethodofusingdatacommunicationtechnologytoprovidethecomputerwiththeinputdatajustgeneratedinrealtime.Thedataisthenstoredinacomputer(suchasonadisk)andprocessedatpredeterminedintervalsorasneeded.

Interactivesystem

Asthenameimplies,itprovidesuserswiththefunctionofextractinginformationfromthecomputer.Theinquiryfunctionispassive.Inotherwords,itdoesnotmodifythestoredinformation.Thequestioncanbeverysimple,forexample:"Retrievetherecordwiththeemployeenumber1234"oritcanbecomplicated.Suchsystemsmayuseterminalsthatcanproducehardcopiesand/orvisualdisplays.

Real-timesystem

isatypeofsysteminwhichthecomputersystemobtainsandprocessesinformationinadynamicsituationsothatthecomputercantakeactionstoaffecttheeventthatisoccurring(suchasProcesscontrolapplications)orcaninfluencepeople(operators)throughaccurateandconstantlyupdatedinformationstoredincomputers,suchasinpre-salessystems.

Basictheory

Spectrumandbandwidth

Thesignalistheelectromagneticcodeofthedata,andthesignalcontainsthedatatobetransmitted.Signalsgenerallytaketimeastheindependentvariable,andacertainparameter(amplitude,frequency,orphase)representingthemessage(ordata)asthedependentvariable.Thesignalcanbedividedintocontinuoussignalanddiscretesignalaccordingtowhetherthevalueoftheindependentvariabletimeiscontinuous;accordingtowhetherthevalueofthedependentvariableiscontinuous,itcanbedividedintoanalogsignalanddigitalsignal.

Thesignalhastwobasicmanifestationsandcharacteristicsintimedomainandfrequencydomain.Thetimedomaincharacteristicreflectsthechangeofthesignalovertime.Thefrequencydomaincharacteristicnotonlycontainsthesameamountofinformationinthetimedomainofthesignal,butalsocanclearlyunderstandthefrequencyspectrumdistributionofthesignalandthebandwidthoccupiedbythesignalspectrumanalysis.Inordertoobtaintherequirementsofthetransmittedsignalonthereceivingequipmentandchannel,itisnotenoughtounderstandthetime-domaincharacteristicsofthesignal.Itisalsonecessarytoknowthefrequencyspectrumdistributionofthesignal.Thetimedomaincharacteristicsofthesignalshowhowthesignalchangesovertime.Sincemostoftheenergyinthesignalisconcentratedinarelativelynarrowfrequencyband,wecallthebandwheremostoftheenergyisconcentratedinthesignalastheeffectivebandwidth,orbandwidthforshort.Anysignalhasbandwidth.Generallyspeaking,thelargerthesignalbandwidth,thehigherthedatatransmissionrateusingthissignal,andthelargerthebandwidthrequiredforthetransmissionmedium.

Belowwewillbrieflyintroducethefrequencyspectrumandbandwidthofcommonsignals.Thefrequencyspectrumofthesoundsignalisroughlyintherangeof20Hzto2000kHz(signalsbelow20Hzareinfrasound,signalsabove2000kHzareultrasound),butamuchnarrowerbandwidthcanproduceacceptablevoicereproductionTherefore,thestandardfrequencyspectrumofthevoicesignalis300Hzto3400Hz,anditsbandwidthis3kHz.ThefrequencyspectrumoftheTVsignalis0to4MHz,soitsbandwidthis4MHz.Asaspecialexample,thebandwidthofamonostablepulsesignalisinfinite.Forbinarysignals,thebandwidthgenerallydependsontheexactshapeofthesignalwaveformandtheorderof0and1.Thegreaterthebandwidthofthesignal,themorefaithfullyitrepresentsthedigitalsequence.

Cutofffrequencyandbandwidth

AccordingtotheFourierseries,weknowthatifallfrequencycomponentsofasignalcanbetransmittedtothereceivingendthroughthechannelcompletelyunchanged,thenatthereceivingendbyThesignalformedbythesuperpositionofthesefrequencycomponentsisexactlythesameasthesignalatthetransmittingend,thatis,thereceivingendcompletelyrestoresthesignalsentbythetransmittingend.Butintherealworld,nochannelcanpassallfrequencycomponentswithoutloss.IfallFouriercomponentsareattenuatedequally,thesignalreceivedatthereceivingendisattenuatedinamplitude,butthereisnodistortion.However,alltransmissionchannelsandequipmenthavedifferentattenuationlevelsfordifferentfrequencycomponents.Somefrequencycomponentshavealmostnoattenuation,andsomefrequencycomponentsareattenuatedalittle.Thatistosay,thechannelalsohascertainamplitudefrequencycharacteristics,whichleadstoTheoutputsignalisdistorted.Usually,theharmonicsinthefrequencyrangeof0tof_carenotattenuatedduringchanneltransmission(ortheattenuationisaverysmallconstant),andheref_cAlltheharmonicsabovethefrequencyareattenuatedgreatlyduringthetransmissionprocess.Weattenuatetheamplitudeofacertaincomponentofthesignalinthechanneltransmissionprocesstotheoriginal0.707(thatis,thepoweroftheoutputsignalisreducedbyhalf).Thatfrequencyiscalledthecut-offfrequencyofthechannel.Thecut-offfrequencyreflectstheinherentphysicalcharacteristicsofthetransmissionmediumitself.Inothercases,itisbecausepeoplehavedeliberatelyinstalledfiltersinthelinetolimitthebandwidthusedbyeachuser.Sometimes,becauseadouble-passfilterisaddedtothechannel,thechannelcorrespondstotwocut-offfrequenciesf₁andf₂,whicharecalledlowercut-offfrequenciesandUppercutofffrequency.Thedifferencebetweenthesetwocutofffrequenciesf₂-f₁iscalledthebandwidthofthechannel.Ifthebandwidthoftheinputsignalissmallerthanthebandwidthofthechannel,allfrequencycomponentsoftheinputsignalcanpassthroughthechannel,sotheoutputwaveformobtainedatthechanneloutputwillbeundistorted.However,ifthebandwidthoftheinputsignalisgreaterthanthebandwidthofthechannel,somefrequencycomponentsinthesignalcannotpassthroughthechannel,sotheoutputsignalwillbesomewhatdifferentfromthesignalsentbythetransmitter,thatis,distortionwilloccur.Inordertoensurethecorrectnessofdatatransmission,thebandwidthofthesignalmustbelimited.

2.2.3ThemaximumdatatransmissionrateofthechannelThenumberofbinarydigitsthatcanbetransmittedwithinaunittimeiscalledthedatatransmissionrate.Theincreaseofthedatatransmissionratemeansthatthetimeoccupiedbyeachbitisreduced,thatis,thecycletimeofthebinarydigitalpulsesequencewillbereduced,andofcoursethepulsewidthwillalsobereduced.Wehavealreadyknownintheprevioussectionthatevenifabinarydigitalpulsesignalpassesthroughanidealchannelwithlimitedbandwidth,waveformdistortionwilloccur,andwhenthebandwidthoftheinputsignalisconstant,thesmallerthechannelbandwidth,thegreatertheoutputwaveformdistortion.Toputitanotherway,whenthebandwidthofthechannelisconstant,thelargerthebandwidthoftheinputsignal,thegreaterthedistortionoftheoutputsignal.Therefore,whenthedatatransmissionrateincreasestoacertainextent(thesignalbandwidthincreasestoacertainextent),thechanneloutputThesignalreceiverontheabovesimplycannotrecoverthetransmitteddigitalsequencefromthedistortedoutputsignal.Thismeansthatevenforanidealchannel,thelimitedbandwidthlimitsthechanneldatatransmissionrate.

Asearlyas1924,H.Nyquist(Nyquist)recognizedtheexistenceofthisbasiclimitationandderivedaformulathatexpressedthemaximumdatatransmissionrateofanoise-freelimitedbandwidthchannel.In1948,C.ShannonextendedNyquist'sworkfurthertothesituationwherethechannelisinterferedbyrandomnoise.Herewebrieflydescribetheseresultsthatareregardedasclassicswithoutproof.Nyquistprovedthatafteranycontinuoussignalf(t)passesthroughanoise-freechannelwithbandwidthB,itsoutputsignalisatimecontinuoussignalg(t)withbandwidthB.Ifyouwanttooutputadigitalsignal,youmustalsosampleg(t)atequalintervalsatacertainrate.Samplingspeedhigherthan2Btimespersecondismeaningless,becausethehighfrequencycomponentsinthesignalhigherthanthechannelbandwidthBhavebeenattenuatedbythechannel.Ifg(t)iscomposedofVdiscretizationlevels,thatis,thepossibleresultofeachsamplingisoneofVdiscretizationlevels,thenthemaximumdatatransmissionrateofthechannelisR_maxIs:

R_max=2Blog₂V(bitspersecond)

Forexample,anoise-freebandwidthis3000HzThechannelcannottransmitbinarydigitalsignalswitharateexceeding6000bitspersecond.Earlierweonlyconsideredtheidealchannelwithoutnoise.Fornoisychannels,thesituationwillquicklydeteriorate.Thethermalnoiseinthechannelismeasuredbytheratioofsignalpowertonoisepower,andtheratioofsignalpowertonoisepoweriscalledthesignal-to-noiseratio(Signal-to-NoiseRatio).IfweuseStodenotesignalpowerandNtodenotenoisepower,thesignal-to-noiseratioshouldbeexpressedasS/N.However,peopleusuallydonotusetheabsolutevalueofthesignal-to-noiseratio,butuse10log₁₀S/Ntorepresentit,andtheunitisdecibel(dB).ForachannelwithS/Nequalto10,thesignal-to-noiseratioiscalled10dB;inthesameway,ifthechannel'sS/Nisequalto100,thesignal-to-noiseratioiscalled20dB;andsoon.Shannon’sconclusiononthemaximumdatatransmissionrateofanoisychannelis:ForachannelwithabandwidthofBHzandasignal-to-noiseratioofS/N,themaximumdatatransmissionrateR_maxis:

R_max=Blog₂(1+S/N)(bitspersecond)

Forexample,forabandwidthof3kHz,thesignaltonoiseForachannelwitharatioof30dB,nomatterhowmanyquantizationlevelsareusedorhowfastthesamplingspeedis,thedatatransmissionratecannotbegreaterthan30,000bits/sec.Shannon’sconclusionisderivedfromtheinformationtheoryandhasawiderangeofapplications.Itisalmostimpossibletosurpassthisconclusionlikeinventingaperpetualmotionmachine.ItisworthnotingthatShannon'sconclusiononlygivesatheoreticallimit,andinfact,itisquitedifficulttoapproachthislimit.

Summary

Asignalisanelectromagneticcodeofamessage(ordata),andthesignalcontainsthemessagetobetransmitted.Accordingtowhetherthevalueofthedependentvariableiscontinuous,thesignalcanbedividedintoanalogsignalanddigitalsignal.Correspondingly,communicationcanalsobedividedintoanalogcommunicationanddigitalcommunication.Fourierhasprovedthatanysignal(whetheritisananalogsignaloradigitalsignal)iscomposedofharmonicsofvariousfrequencies,andanysignalhasacorrespondingbandwidth.Moreover,anychannelwillattenuatethesignalwhenittransmitssignals.Therefore,anychannelhasadatatransmissionratelimitwhentransmittingsignals.ThisistheconclusionthatNyquistandShannon'stheoremwantstotellus.

Transmissionmediaisthemostbasiccomponentofcomputernetworksandcommunications,anditoccupiesalargeproportionofthecostoftheentirecomputernetwork.Inordertoimprovetheutilizationofthetransmissionmedium,wecanusemultiplexingtechnology.Therearethreetypesofmultiplexingtechnologies:frequencydivisionmultiplexing,wavelengthdivisionmultiplexingandtimedivisionmultiplexing,whichareusedindifferentoccasions.Dataswitchingtechnologiesincludecircuitswitching,messageswitchingandpacketswitching,eachofwhichhasadvantagesanddisadvantages.Modemisadeviceusedtotransmitbinarydataofacomputeronananalogtelephonenetwork.Modemmodulationmethodsincludeamplitudemodulation,frequencymodulation,phasemodulationandquadratureamplitudemodulation,andModemalsosupportsdatacompressionanderrorcontrol.

Synchronizationmethod

Datacommunicationsynchronizationmainlyincludesbitsynchronizationandcharactersynchronization.

Bitsynchronization:Thepurposeistoenablethereceivertocorrectlyaccepteachbinarybit.Generally,therearetwomethods:self-synchronizationandexternalsynchronization.Theself-synchronizationmethodmeansthatthereceiverdirectlyusesthecharacteristicsofthecommunicationcodetoobtainsynchronizationinformationfromthedatablock,includingusingauniquesignaltoactivatethereceivingaction,orusingthelevelconversioninthedatablocktoadjustthereceivedsamplingpulse.

Theexternalsynchronizationmethodmeansthatthesendersendsaseriesofsynchronousclocksequencestothereceiverbeforesendingdata,andthereceiverlocksthereceivingfrequencyaccordingtotheclockpulsefrequencyandtiming,sothatintheprocessofreceivingdataAlwayskeepinsyncwiththesender.

Charactersynchronizationisalsocalledgroupsynchronization,anditspurposeistoenablethereceivertorecognizethedata(oftenreferringtoacharacter)toformacompletemessage.Obviously,charactersynchronizationisbasedonbitsynchronization,andonlywhentheuniquesynchronizationpatternisrecognized,cantherealdatabereceived.

Relatedmagazines

"DataCommunication"magazineissponsoredbytheInstituteofDataCommunicationScienceandTechnologyoftheMinistryofInformationIndustry.Wirelesscommunication,networkandinformationsecurity,dataprocessing,datatransmission,testingandmaintenance.

Features

Datacommunicationhasmanycharacteristicsdifferentfromtelegraphandtelephonecommunication.Whatitrealizesismainly"human(throughterminal)-machine(computer)"communicationand"machine-machine"communication,butitalsoincludes"human(throughintelligentterminal)-human"communication.Theinformationconveyedindatacommunicationisexpressedintheformofbinarydata.Anotherimportantfeatureofdatacommunicationisthatitisalwaysconnectedwithtelematics.Informationprocessingherereferstoinformationprocessinginabroadsense,includingscientificcomputingandprocesscontrol.Duetothedifferenceininformationprocessingcontentandprocessingmethods,therequirementsfordatacommunicationarealsoverydifferent.Forexample,accordingtothedifferentapplicationsofthesystem,thatis,theinformationprocessingcontentandprocessingmethods,theterminaltype,transmissioncode,transmissionrate,transmissionmethod,systemresponsetime,informationsecurityandaccuracy,systemreliability,etc.Therequirementsarealsodifferent.Therefore,thefactorsinvolvedindatacommunicationarealsomorecomplicated.

Keytechnology

Datatransmission

Inordertorealizedatacommunication,datatransmissionmustbecarriedout,thatis,datainformationfromadatasourcelocatedinoneplaceistransmittedthroughatransmissionchannelDatareceivingequipmenttoanotherplace.Thechannelusedfordatatransmissioncanbeasolid-linebasebandcircuit,orafrequency-divisionanalogcircuitoratime-divisiondigitalcircuit.Becausethetelephonenetworkhasalonghistoryofdevelopment,largecommunicationcapacity,andwidecoverage,itiseconomicallyandtechnicallyappropriatetouseittoprovidedatatransmissionchannels,anditisacommonway.Butwhenthetelephonecircuitisusedasadatatransmissionchannel,certainmeasuresmustbetakentoadaptittotherequirementsoftransmittingdatasignals.

Inadatacommunicationsystem,onlythetransmissionfunctionofsendingdatageneratedinoneplacetoanotherplaceisoftennotenough.Inordertoimprovethetransmissionquality,reducetheerrorrate,andenablethetransmissionprocesstobecarriedouteffectively,thesystemmustalsohaveadatalinkcontrolprocedure(seedatalink).Inthistypeofregulation,alltransmissioncontrolfunctionsincludingerrorcontrolarespecifiedindetail.Therearedifferenttypesofdatalinkcontrolregulationsforactualdatacommunicationsystemsorcomputernetworks,someofwhichcomplywithinternationalstandards,andsomeareformulatedbythecountryorcompanyitself.

Dataexchangeandcommunicationprotocol

Inadatacommunicationsystemorcomputernetwork,thetransmissionchannelusedcanbefixedorprovidedbytheexchangenetwork.Therearetwomainwaysofdataexchange:circuitswitchingandpacketswitching,amongwhichpacketswitchingismorecommonlyusedinactualdatanetworks.Inadatanetworkthatusespacketswitching,inadditiontothefunctionsrequiredbythedatatransmissionanddatalinkcontrolproceduresbetweenadjacentswitchingnodes,thestorageandstorageofdatapacketsoneachswitchingnodemustbecompleted.Forwarding;routing,flowcontrol,congestioncontrol,useraccesstothenetwork,andrelatednetworkmaintenanceandmanagement.Correspondingly,certainrelatedfunctionsneedtobeimplementedintheendsystemconnectedtothedataexchangenetwork.Allthesefunctionsrelatedtotheformationofthedataexchangenetworkarespecifiedintheformofcommunicationprotocols,andtheyalsoincludetheinterfaceprotocolbetweentheendsystemandthenetwork.Theso-calledprotocolreferstotherulesandconventionsthatbothpartiesmustfollowinordertocommunicateaccuratelyandeffectively.Theyareofgreatsignificanceindatacommunication,andthedatalinkcontrolprocedurementionedaboveisactuallyadatacommunicationprotocol.

Datacommunicationprotocolscanbedividedintotwocategories.Oneistheprotocolrelatedtothedatacommunicationnetwork(fromtheperspectiveofthecompositionofthecomputernetwork,sometimescalledthecommunicationsubnet),includingtheprotocolbetweennodesandnodesinthenetwork,andbetweenthenetworkandtheendsystem.Theyareprotocolsnecessarytoformadatacommunicationnetworkandrealizedatacommunicationbetweenendsystemsthroughit.Theotheristheagreementbetweentheendsystemandtheendsystem.Theyarebasedonthefunctionsimplementedbytheprevioustypeofagreement,inordertorealizetheintercommunicationbetweentheendsystemsandachievecertainapplicationpurposes,ortobeprecise,inordertoAprotocolnecessaryforcommunicationbetweenapplicationprocessesoftwoendsystems.Allcommunicationfunctionsofadatacommunicationsystemorcomputernetworkaregenerallydividedandorganizedaccordingtoacertainhierarchicalstructure.Datacommunicationprotocolsareactuallyspecificprovisionsforthecontentandimplementationrulesofeachlayer'sfunction,sotheyaregenerallyformulatedbylayer.

Application

Asacommunicationservice,datacommunicationprovidesservicesforrealizingtelematicsinabroadsense.Withtheincreasinguseofcomputersandvariousintelligentdeviceswithprocessingfunctionsinvariousfields,theapplicationscopeofdatacommunicationisalsoexpanding.Itstypicalapplicationsinclude:filetransmission,e-mail,voicemail,visualtext,directorysearch,smartusertelegramandremotecontrol,etc.Foreachspecificapplication,thecommunicationfunctionsrelatedtotheapplicationmustbeimplementedinthetelematicssystemorcomputernetwork,andthesefunctionsarealsospecifiedintheformofalayeredprotocol.

Developmenttrend

Thedevelopmenttrendofdatacommunicationisconcentratedonthefollowing:

1.Theexpansionofapplicationscopeandapplicationscale,newapplicationservicessuchaselectronicdataexchangeExchange(EDI),multimediacommunications,etc.continuetoemerge.

2.Withtheincreaseintheamountofcommunication,theInternetisincreasinglydevelopinginthedirectionofhigh-speed,broadband,digitaltransmissionandcomprehensiveutilization.Forexample,manynewtechnologiessuchasopticalfiberhigh-speedlocalareanetwork,metropolitanareanetwork,broadbandintegratedservicedigitalnetwork,relay,fastpacketswitching,etc.havedevelopedrapidly,andsomehaveenteredthepracticalstage.

3.Inconjunctionwiththedevelopmentofmobilecommunications,mobiledatacommunicationsaregainingrapiddevelopment.

4.Withthecontinuousexpansionofthescaleofnetworksandsystems,theimportanceoftheinterconnectionofdifferenttypesofnetworksandsystems(includingtheoperationandmanagementoftheinterconnectionnetwork)hasbecomeincreasinglyprominent.

5.Therehasbeenalargeincreaseincommunicationprotocolstandards,andprotocolengineeringtechnologyhasdevelopeddaybyday.

Inclusion/Influence

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