Úvod
Rozvoj
Thefirststage:languageisthemainstay,andinformationistransmittedthroughprimitivemeanssuchasmanpower,horsepower,andbeacon.
Druhá fáze: text, příspěvek. (Zvýšení prostředků šíření informací)
Třetí fáze: tisk. (Rozšiřte rozsah šíření informací)
Čtvrtá fáze: telegraf, telefon, vysílání. (Vstup do éry elektrických spotřebičů)
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.
Historické postavy
Bell(1847-1922),Brit, pracoval v Londýně v roce 1868. V roce 1871 šel pracovat do Bostonu. V roce 1873 sloužil jako profesor na univerzitě v Bostonu. V roce 1875 byl vynalezen mnohonásobný telegraf.
Marconi(1874-1937),Italian,experimentedinhisfather'smanorin1894.In1896,hewenttoLondon.In1897,awirelesstelegraphcompanywasestablished.In1899,wirelesscommunicationbetweenBritainandFrancewasrealizedforthefirsttime.In1916,shortwaveradiocommunicationwasrealized.In1929,aworldwidewirelesscommunicationnetworkwasestablished.WontheNobelPrize.ParticipatedintheFascistParty.
Historie datové komunikace
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.
Klasifikace
Klasifikováno podle informací
Telefonní komunikační systém
Datový komunikační systém
Systém kabelové televize
p>Podle modulační klasifikace
Přenos v základním pásmu
Modulační přenos
Podle charakteristik přenosového signálu
Analogový komunikační systém
p>Digitální komunikační systém
Přenosové prostředky
Kabelová komunikace
Kroucená dvoulinka, koaxiální kabel atd. Místní hovory a komunikace na dálku. Metoda modulace: SSB/FDM. Koaxiální PCM časové dělení technologie multiplexního digitálního základního pásma. Optické vlákno bude postupně nahrazovat koaxiální.
Mikrovlnná reléová komunikace
Comparedwithcoaxialcable,itiseasytosetup,smallinvestmentandshortcycle.AnalogtelephonemicrowavecommunicationmainlyadoptsSSB/FM/FDMmodulation,andthecommunicationcapacityis6000channels/channel.DigitalmicrowaveadoptsBPSK,QPSKandQAMmodulationtechnology.Using64QAM,256QAMandothermulti-levelmodulationtechnologiestoincreasethemicrowavecommunicationcapacity,itcantransmit1920to7680PCMdigitaltelephonesina40Mchannel.
Komunikace s optickým vláknem
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.Komunikace s optickým vláknemequipmentiscomposedoftwoparts:photoelectricconversionunitanddigitalsignalprocessingunit.
Satelitní komunikace
Thecommunicationdistanceislong,thetransmissioncapacityislarge,thecoverageareaislarge,anditisnotrestrictedbygeographicalareaandhighreliability.Maturetechnologyusesanalogmodulation,frequencydivisionmultiplexingandfrequencydivisionmultipleaccess.Digitalsatellitecommunicationadoptsdigitalmodulation,timedivisionmultiplexingandtimedivisionmultipleaccess.
Mobilní komunikace
Mobilní komunikaceiscommunicationbetweenmovingbodies,orbetweenmovingbodiesandfixedbodies.Themovingbodycanbeaperson,oramovingobjectsuchasacar,train,ship,radio,etc.Themobilecommunicationsystemconsistsoftwoparts:
(1) Vesmírný systém
(2)Pozemní systém:①Satelitní mobilní radiostanice a anténa;②Brána, základnová stanice.
Sinceitsbirthinthe1980s,themobilecommunicationsystemwillgenerallygothroughfivegenerationsofdevelopmentby2020,andwilltransitionfromthethirdgenerationtothefourthgeneration(4G)by2010.By4G,inadditiontocellularphonesystems,broadbandwirelessaccesssystems,millimeterwaveLANs,intelligenttransmissionsystems(ITS)andstratosphericplatform(HAPS)systemswillbeputintouse.Themostobvioustrendoffuturegenerationsofmobilecommunicationsystemsistorequirehighdatarates,highmobilityandseamlessroaming.Realizingtheserequirementswillfacegreatertechnicalchallenges.Inaddition,systemperformance(suchascellsizeandtransmissionrate)willlargelydependonthefrequency.Takingintoaccountthesetechnicalissues,somesystemswillfocusonprovidinghighdatarates,andsomesystemswillfocusonenhancingmobilityorexpandingcoverage.
Z hlediska fixačních zařízení mezi technologie přístupu, které lze použít, patří:mobilní mobilní bezdrátové systémy,jako je 3G;bezdrátové systémy,jako je DECT;komunikační systémy krátkého dosahu,jako jsou datové systémy BluetoothandDECT;bezdrátové sítě,C.B.
Komunikační systém
Offline přenos dat
Itsimplyusestelephoneorsimilarlinkstotransmitdata,excludingcomputersystems.Theequipmentusedatbothendsofsuchalinkisnotapartofthecomputer,oratleastdoesnotimmediatelyprovidethedatatothecomputerforprocessing,thatis,thedataisofflinewhenitissentorreceived.Thisdatacommunicationisrelativelycheapandsimple.
Vzdálené dávkové zpracování
Thetermappliestosuchamethod:theuseofdatacommunicationtechnologytomaketheinputandoutputofdatageographicallyawayfromthecomputerthatprocessestheminbatchmode.
Online sběr dat
referstothemethodofusingdatacommunicationtechnologytoprovidethecomputerwiththeinputdatajustgeneratedinrealtime.Thedataisthenstoredinacomputer(suchasonadisk)andprocessedatpredeterminedintervalsorasneeded.
Interaktivní systém
Asthenameimplies,itprovidesuserswiththefunctionofextractinginformationfromthecomputer.Theinquiryfunctionispassive.Inotherwords,itdoesnotmodifythestoredinformation.Thequestioncanbeverysimple,forexample:"Retrievetherecordwiththeemployeenumber1234"oritcanbecomplicated.Suchsystemsmayuseterminalsthatcanproducehardcopiesand/orvisualdisplays.
Systém v reálném čase
isatypeofsysteminwhichthecomputersystemobtainsandprocessesinformationinadynamicsituationsothatthecomputercantakeactionstoaffecttheeventthatisoccurring(suchasProcesscontrolapplications)orcaninfluencepeople(operators)throughaccurateandconstantlyupdatedinformationstoredincomputers,suchasinpre-salessystems.
Základní teorie
Spektrum a šířka pásma
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.
Mezní frekvence a šířka pásma
AccordingtotheFourierseries,weknowthatifallfrequencycomponentsofasignalcanbetransmittedtothereceivingendthroughthechannelcompletelyunchanged,thenatthereceivingendbyThesignalformedbythesuperpositionofthesefrequencycomponentsisexactlythesameasthesignalatthetransmittingend,thatis,thereceivingendcompletelyrestoresthesignalsentbythetransmittingend.Butintherealworld,nochannelcanpassallfrequencycomponentswithoutloss.IfallFouriercomponentsareattenuatedequally,thesignalreceivedatthereceivingendisattenuatedinamplitude,butthereisnodistortion.However,alltransmissionchannelsandequipmenthavedifferentattenuationlevelsfordifferentfrequencycomponents.Somefrequencycomponentshavealmostnoattenuation,andsomefrequencycomponentsareattenuatedalittle.Thatistosay,thechannelalsohascertainamplitudefrequencycharacteristics,whichleadstoTheoutputsignalisdistorted.Usually,theharmonicsinthefrequencyrangeof0tofcarenotattenuatedduringchanneltransmission(ortheattenuationisaverysmallconstant),andherefcAlltheharmonicsabovethefrequencyareattenuatedgreatlyduringthetransmissionprocess.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,thenthemaximumdatatransmissionrateofthechannelisRmaxIs:
Rmax=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,themaximumdatatransmissionrateRmaxis:
Rmax=Blog2(1+S/N)(bitspersecond)
Forexample,forabandwidthof3kHz,thesignaltonoiseForachannelwitharatioof30dB,nomatterhowmanyquantizationlevelsareusedorhowfastthesamplingspeedis,thedatatransmissionratecannotbegreaterthan30,000bits/sec.Shannon’sconclusionisderivedfromtheinformationtheoryandhasawiderangeofapplications.Itisalmostimpossibletosurpassthisconclusionlikeinventingaperpetualmotionmachine.ItisworthnotingthatShannon'sconclusiononlygivesatheoreticallimit,andinfact,itisquitedifficulttoapproachthislimit.
souhrn
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.
Metoda synchronizace
Datacommunicationsynchronizationmainlyincludesbitsynchronizationandcharactersynchronization.
Bitsynchronization:Thepurposeistoenablethereceivertocorrectlyaccepteachbinarybit.Generally,therearetwomethods:self-synchronizationandexternalsynchronization.Theself-synchronizationmethodmeansthatthereceiverdirectlyusesthecharacteristicsofthecommunicationcodetoobtainsynchronizationinformationfromthedatablock,includingusingauniquesignaltoactivatethereceivingaction,orusingthelevelconversioninthedatablocktoadjustthereceivedsamplingpulse.
Theexternalsynchronizationmethodmeansthatthesendersendsaseriesofsynchronousclocksequencestothereceiverbeforesendingdata,andthereceiverlocksthereceivingfrequencyaccordingtotheclockpulsefrequencyandtiming,sothatintheprocessofreceivingdataAlwayskeepinsyncwiththesender.
Charactersynchronizationisalsocalledgroupsynchronization,anditspurposeistoenablethereceivertorecognizethedata(oftenreferringtoacharacter)toformacompletemessage.Obviously,charactersynchronizationisbasedonbitsynchronization,andonlywhentheuniquesynchronizationpatternisrecognized,cantherealdatabereceived.
Související časopisy
"DataCommunication"magazineissponsoredbytheInstituteofDataCommunicationScienceandTechnologyoftheMinistryofInformationIndustry.Wirelesscommunication,networkandinformationsecurity,dataprocessing,datatransmission,testingandmaintenance.
Funkce
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.
Klíčová technologie
Přenos dat
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.
Výměna dat a komunikační protokol
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.
aplikace
Asacommunicationservice,datacommunicationprovidesservicesforrealizingtelematicsinabroadsense.Withtheincreasinguseofcomputersandvariousintelligentdeviceswithprocessingfunctionsinvariousfields,theapplicationscopeofdatacommunicationisalsoexpanding.Itstypicalapplicationsinclude:filetransmission,e-mail,voicemail,visualtext,directorysearch,smartusertelegramandremotecontrol,etc.Foreachspecificapplication,thecommunicationfunctionsrelatedtotheapplicationmustbeimplementedinthetelematicssystemorcomputernetwork,andthesefunctionsarealsospecifiedintheformofalayeredprotocol.
Vývojový trend
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.
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