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Defineclassification

Inbroadterms

Computernetworkisalsocalledcomputercommunicationnetwork.Thesimplestdefinitionofacomputernetworkis:acollectionofautonomouscomputersthatareconnectedtoeachotherforthepurposeofsharingresources.Accordingtothisdefinition,theearlyterminal-orientednetworkscannotberegardedascomputernetworks,butcanonlybecalledonlinesystems(becausemanyterminalsatthattimecannotberegardedasautonomouscomputers).However,withthedeclineinhardwareprices,manyterminalshaveacertaindegreeofintelligence,so"terminals"and"autonomouscomputers"graduallylosetheirstrictboundaries.Ifamicrocomputerisusedasaterminal,accordingtotheabovedefinition,theearlyterminal-orientednetworkcanalsobecalledacomputernetwork.

Inaddition,fromtheperspectiveoflogicalfunction,acomputernetworkisacollectionofcomputersystemsthatconnectmultiplecomputerswithcommunicationlinesforthebasicpurposeoftransmittinginformation.Acomputernetworkconsistsoftransmissionmediaandcommunicationequipment..

Fromtheperspectiveofusers,computernetworksaredefinedasfollows:Thereisanetworkoperatingsystemthatcanbeautomaticallymanagedforusers.Itcallsandcompletestheresourcescalledbytheuser,andtheentirenetwork,likealargecomputersystem,istransparenttotheuser.

Amoregeneraldefinitionis:theuseofcommunicationlinestoconnectgeographicallydispersedcomputersystemsandcommunicationequipmentwithindependentfunctionsindifferentforms,andrealizeresourcesharingwithcompletenetworksoftwareandprotocolsAndinformationtransmissionsystem.

Onthewhole,acomputernetworkisalarge-scaleandpowerfulsystemthatconnectscomputersdistributedindifferentgeographicalareaswithspecializedexternalequipmentusingcommunicationlines,sothatmanycomputerscaneasilycommunicatewitheachother.Transferinformationandshareresourcessuchashardware,software,dataandinformation.Simplyput,acomputernetworkisacollectionofmanyautonomouslyworkingcomputersconnectedtoeachotherbycommunicationlines.

Thesimplestcomputernetworkhasonlytwocomputersandonelinkconnectingthem,thatis,twonodesandonelink.

Connectbyconnection

Computernetworkisacollectionofautonomouscomputersinterconnectedbywires.Tobeprecise,itisacollectionofcomputerswithindependentworkingabilitydistributedindifferentgeographicallocations.Theterminalanditsancillaryequipmentareconnectedwithcommunicationequipmentandcommunicationlines,andnetworksoftwareisconfiguredtorealizeasystemofcomputerresourcesharing.

Ondemand

Computernetworkiscomposedofalargenumberofindependentbutinterconnectedcomputerstocompletecomputertaskstogether.Thesesystemsarecalledcomputernetworks.

DevelopmentHistory

Sincetheemergenceofcomputernetworks,thespeedofitsdevelopmentandtheextentofitsapplicationshavebeenamazing.Lookingatthedevelopmentofcomputernetworks,ithasroughlygonethroughthefollowingfourstages:

Birthstage

Thefirstgenerationofcomputernetworksbeforethemid-1960swascenteredonasinglecomputerThetypicalapplicationoftheremoteonlinesystemisanairplaneticketbookingsystemconsistingofacomputerandmorethan2,000terminalsacrosstheUnitedStates.Theterminalisacomputerperipheraldevice,includingamonitorandakeyboard,withoutCPUandmemory.Withtheincreaseofremoteterminals,afrontendmachine(FEP)isaddedinfrontofthehost.Atthattime,peopledefinedacomputernetworkas"asystemthatisconnectedforthepurposeoftransmittinginformationtorealizeremoteinformationprocessingorfurtherachieveresourcesharing."Suchacommunicationsystemhastheembryonicformofanetwork.

Formationstage

Thesecond-generationcomputernetworkfromthemid-1960stothe1970swasinterconnectedbymultiplehoststhroughcommunicationlinestoprovideservicesforusers.Itemergedinthe1960sInthelaterperiod,thetypicalrepresentativewasARPANETdevelopedwiththeassistanceoftheAdvancedResearchProjectsAgencyoftheUSDepartmentofDefense.Thehostsarenotdirectlyconnectedbylines,butareinterconnectedafterbeingtransferredbytheInterfaceMessageProcessor(IMP).IMPandtheirinterconnectedcommunicationlinesareresponsibleforthecommunicationtasksbetweenhosts,formingacommunicationsubnet.Thehostsconnectedbythecommunicationsubnetareresponsibleforrunningprograms,providingresourcesharing,andformingaresourcesubnet.Duringthisperiod,thenetworkconceptwas"acollectionofcomputerswithindependentfunctionsinterconnectedforthepurposeofbeingabletoshareresourceswitheachother",formingthebasicconceptofcomputernetworks.

Interconnectionstage

Thethird-generationcomputernetworkfromthelate1970stothe1990swasanopenandstandardizednetworkwithaunifiednetworkarchitectureandcompliancewithinternationalstandards.AftertheriseofARPANET,computernetworkshavedevelopedrapidly,andmajorcomputercompanieshavesuccessivelyintroducedtheirownnetworkarchitecturesandsoftwareandhardwareproductsthatimplementthesestructures.Becausethereisnounifiedstandard,itisdifficulttointerconnectproductsofdifferentmanufacturers.Peopleurgentlyneedanopen,standardizedandpracticalnetworkenvironment.Thishasledtotheemergenceoftwomostimportantinternationalarchitectures,namelytheTCP/IPsystem.StructureandtheOSIsystemstructureoftheInternationalOrganizationforStandardization.

High-speednetworktechnologystage

Thefourth-generationcomputernetworkfromthe1990stothepresent,duetothematuredevelopmentoflocalareanetworktechnology,theemergenceofopticalfiberandhigh-speednetworktechnology,theentirenetworkislikeauserThelargetransparentcomputersystemdevelopedintotheInternetrepresentedbytheInternet.

Composition

Theclassificationofcomputernetworksisthesameasthegeneralclassificationofthings,whichcanbeclassifiedaccordingtothedifferentnatureandcharacteristicsofthings(thatis,theattributesofthings).Generallyspeaking,acomputernetworkiscomposedofmultiplecomputers(orothercomputernetworkdevices)thatarephysically(orlogically)connectedthroughtransmissionmediaandsoftware.Generallyspeaking,thecompositionofacomputernetworkbasicallyincludesfourparts:acomputer,anetworkoperatingsystem,atransmissionmedium(itcanbetangibleorintangible,suchasthetransmissionmediumofawirelessnetworkisspace)andcorrespondingapplicationsoftware.

Function

Datacommunication

Datacommunicationisoneofthemostimportantfunctionsofcomputernetworks.Datacommunicationisacommunicationmethodandcommunicationservicethatusesdatatransmissiontechnologytotransferdatainformationbetweentwoterminalsinaccordancewithacertaincommunicationprotocol.Itcanrealizethetransferofdataandinformationbetweencomputersandcomputers,computersandterminals,andterminalsandterminals.Itisthethirdlargestcommunicationserviceaftertelegraphandtelephoneservices.Theinformationtransmittedindatacommunicationisexpressedintheformofbinarydata.Anotherfeatureofdatacommunicationisthatitisalwaysconnectedwithremoteinformationprocessing,whichisabroadinformationprocessingincludingscientificcalculation,processcontrol,informationretrieval,andsoon.

Resourcesharing

Resourcesharingisoneofthemainpurposesforpeopletobuildcomputernetworks.Computerresourcesincludehardwareresources,softwareresourcesanddataresources.Thesharingofhardwareresourcescanimprovetheutilizationofequipmentandavoidrepeatedinvestmentinequipment,suchasusingcomputernetworkstobuildnetworkprinters;thesharingofsoftwareresourcesanddataresourcescanmakefulluseofexistinginformationresources,reducelaborinthesoftwaredevelopmentprocess,andavoidRepetitiveconstructionoflargedatabases.

Centralizedmanagement

Thedevelopmentandapplicationofcomputernetworktechnologyhascausedchangesinmodernofficemethodsandmanagement.Atpresent,therearemanymanagementinformationsystems,officeautomationsystems,etc.,throughwhichcentralizedmanagementofdailyworkcanberealized,workefficiencycanbeimproved,andeconomicbenefitscanbeincreased.

Realizedistributedprocessing

Thedevelopmentofnetworktechnologymakesdistributedcomputingpossible.Forlarge-scaletopics,itcanbedividedintomanysmalltopics,whicharecompletedbydifferentcomputers,andthenconcentratedtosolvetheproblem.

Loadbalancing

Loadbalancingmeansthatworkisevenlydistributedtoeachcomputersystemonthenetwork.Thenetworkcontrolcenterisresponsiblefordistributionanddetection.Whenacomputerisoverloaded,thesystemwillautomaticallytransfertheloadtoalightercomputersystemforprocessing.

Itcanbeseenthatcomputernetworkscangreatlyexpandthefunctionsofcomputersystems,expandtheirapplicationscope,improvereliability,andprovideconvenienceforusers,whilealsoreducingcostsandimprovingperformance-priceratios.

Classification

Althoughtherearevariousstandardsfortheclassificationofnetworktypes,theclassificationofgeographicalareasisauniversalnetworkclassificationstandardthateveryonerecognizes.Accordingtothisstandard,variousnetworktypescanbedividedintofourtypes:localareanetwork,metropolitanareanetwork,wideareanetworkandInternet.Generallyspeaking,alocalareanetworkcanonlybeinasmallarea.Ametropolitanareanetworkisanetworkinterconnectionbetweendifferentareas.However,onepointtobeexplainedhereisthatthenetworkdivisionheredoesnothaveastrictgeographicaldistinction,butcanonlybeaqualitativeone.theconceptof.Thefollowingbrieflyintroducesthesekindsofcomputernetworks.

LocalAreaNetwork

(LocalAreaNetwork;LAN)Usuallyourcommon"LAN"referstothelocalareanetwork,whichisourmostcommonandwidelyusednetwork.Withthedevelopmentandimprovementoftheentirecomputernetworktechnology,thelocalareanetworkhasbeenfullyappliedandpopularized.Almosteveryunithasitsownlocalareanetwork,andsomeevenhavetheirownsmalllocalareanetworkinthefamily.Obviously,theso-calledlocalareanetworkisanetworkwithinalocalarea,anditcoversarelativelysmallarea.Therearenottoomanyrestrictionsonthenumberofcomputersinthelocalareanetwork.Generallyspeaking,intheenterpriseLAN,thenumberofworkstationsisabouttenstotwohundred.Generallyspeaking,thegeographicdistanceinvolvedinthenetworkcanbewithinafewmetersto10kilometers.Thelocalareanetworkisgenerallylocatedinabuildingoraunit,thereisnopathfindingproblem,anddoesnotincludetheapplicationofthenetworklayer.

Thecharacteristicsofthiskindofnetworkare:narrowconnectionrange,fewusers,easyconfiguration,andhighconnectionspeed.Atpresent,thefastestrateofLANisnow10GEthernet.IEEE’s802standardcommitteedefinesavarietyofmajorLANnetworks:Ethernet,TokenRing,FiberDistributedInterfaceNetwork(FDDI),AsynchronousTransferModeNetwork(ATM)andthelatestwirelessLAN(WLAN).Thesewillbedescribedindetaillater.

MetropolitanAreaNetwork

(MetropolitanAreaNetwork;MAN)Thiskindofnetworkisgenerallyconnectedtocomputersinacitybutnotinthesamegeographicarea.Theconnectiondistanceofthiskindofnetworkcanbe10~100kilometers,itadoptsIEEE802.6standard.ComparedwithLAN,MANhasalongerextensiondistanceandalargernumberofconnectedcomputers,whichcanbesaidtobeanextensionofLANnetworkintermsofgeographicscope.Inalargecityormetropolitanarea,aMANnetworkisusuallyconnectedtomultipleLANnetworks.SuchastheLANthatconnectsgovernmentagencies,theLANofhospitals,theLANoftelecommunications,theLANofcompaniesandenterprises,andsoon.Duetotheintroductionofopticalfiberconnection,high-speedLANinterconnectioninMANbecomespossible.

ManymetropolitanareanetworksuseATMtechnologyasthebackbonenetwork.ATMisahigh-speednetworktransmissionmethodfordata,voice,video,andmultimediaapplications.ATMincludesaninterfaceandaprotocol,whichcanswitchbetweentheconstantandvaryingtrafficvolumeofthebitrateonaconventionaltransmissionchannel.ATMalsoincludeshardware,software,andmediaconsistentwithATMprotocolstandards.ATMprovidesascalablebackboneinfrastructuretobeabletoadapttonetworksofdifferentsizes,speeds,andaddressingtechnologies.ThebiggestdisadvantageofATMisitshighcost,soitisgenerallyusedingovernmentmetropolitanareanetworks,suchaspostalservices,banks,andhospitals.

WideAreaNetwork

(WideAreaNetwork,WAN)Thistypeofnetworkisalsocalledalong-distancenetwork,anditcoversawiderareathanametropolitanareanetwork(MAN).TheLANorMANnetworkinterconnectionbetweencities,thegeographicalrangecanbefromhundredsofkilometerstothousandsofkilometers.Becausethedistanceisfar,theinformationattenuationismoreserious,sothiskindofnetworkgenerallyneedstoleaseadedicatedline,whichisconnectedthroughtheIMP(InterfaceInformationProcessing)protocolandthelinetoformameshstructuretosolvetheproblemofrouting.Thiskindofmetropolitanareanetworkisconnectedtomanyusersandthetotalexportbandwidthislimited,sotheuserterminalconnectionrateisgenerallylow,usually9.6Kbps-45Mbps,suchas:CHINANET,CHINAPAC,andCHINADDNoftheMinistryofPostsandTelecommunications.

Theabovementionedseveralclassificationsofnetworks.Infact,inreallife,theonewereallyencountermostisthelocalareanetwork,becauseitcanbelargeorsmall,anditisrelativelyeasytoimplementinboththeworkunitandthefamily.,Theapplicationisalsothemostextensivekindofnetwork,soweneedtomakeafurtherunderstandingofthelocalareanetworkandtheaccessequipmentinthelocalareanetworkbelow.

Wirelessnetwork

WiththeincreasingpopularityanddevelopmentofportablecomputerssuchasnotebookcomputersandPersonalDigitalAssistants(PDAs),peopleoftenhavetobeontheroadAnswercalls,sendfaxesande-mails,readonlineinformation,andlogintoremotemachines.However,itisimpossibletoconnecttotheunit'snetworkthroughawiredmediuminacaroranairplane.Atthistime,youmaybeinterestedinawirelessnetwork.Althoughwirelessnetworksandmobilecommunicationsareoftenlinkedtogether,thetwoconceptsarenotexactlythesame.Forexample,whenaportablecomputerisconnectedtoatelephonejackviaaPCMCIAcard,itbecomesapartofthewirednetwork.Ontheotherhand,thelocationofsomecomputersconnectedviaawirelessnetworkmaybefixed.Forexample,betweenbuildingsthatareinconvenienttoconnectviaawiredcable,thecomputersintwobuildingscanbeconnectedviaawirelessnetwork.

Wirelessnetworks,especiallywirelesslocalareanetworks,havemanyadvantages,suchaseasyinstallationanduse.However,wirelesslocalareanetworksalsohavemanyshortcomings:forexample,itsdatatransmissionrateisgenerallylower,muchlowerthanthatofwiredlocalareanetworks;inaddition,thebiterrorrateofwirelesslocalareanetworksisalsorelativelyhigh,andmutualinterferencebetweensitesisrelativelysevere.Therearedifferentwaystorealizetheuser'swirelessnetwork.Someuniversitiesabroadhaveinstalledmanyantennasontheircampuses,allowingstudentstositunderatreeandviewlibrarymaterials.Thissituationisachievedthroughdigitalcommunicationbetweentwocomputersdirectlythroughawirelesslocalareanetwork.Anotherpossiblewayistouseatraditionalanalogmodemtocommunicatethroughacellularphonesystem.Manycitiesabroadhavebeenabletoprovidecellulardigitalpacketdata(CellularDigitalPacketData,CDPD)services,soawirelesslocalareanetworkcanbeestablisheddirectlythroughtheCDPDsystem.Wirelessnetworkisthecurrentresearchhotspotathomeandabroad,andtheresearchofwirelessnetworkisdrivenbyhugemarketdemand.Thecharacteristicofthewirelessnetworkisthattheusercanaccessthecomputernetworkatanytimeandanyplace,andthischaracteristicmakesithaveastrongapplicationprospect.Manywirelessnetwork-basedproductshaveemerged,suchasPersonalCommunicationSystem(PCS)phones,wirelessdataterminals,portablevideophones,andpersonaldigitalassistants(PDAs).Thedevelopmentofwirelessnetworksdependsonthesupportofwirelesscommunicationtechnology.Wirelesscommunicationsystemsmainlyinclude:low-powercordlesstelephonesystems,analogcellularsystems,digitalcellularsystems,mobilesatellitesystems,wirelessLANandwirelessWAN,etc.

Performance

Theperformanceofacomputernetworkgenerallyreferstoseveralimportantperformanceindicators.Butinadditiontotheseimportantperformanceindicators,therearesomenon-performancefeatures,whichalsohaveagreatimpactontheperformanceofcomputernetworks.

1.Performanceindicatorsofcomputernetworks

Performanceindicatorsmeasuretheperformanceofcomputernetworksfromdifferentaspects.

(1)Rate

Thesignalssentbythecomputerareindigitalform.Bitistheunitofdatavolumeincomputers,anditisalsotheunitofinformationvolumeusedininformationtheory.TheEnglishwordbitcomesfrombinarydigit,whichmeansa"binarydigit",soabitisa1or0inabinarydigit.Therateinnetworktechnologyreferstotherateatwhichahostconnectedtoacomputernetworktransmitsdataonadigitalchannel.Itisalsocalleddatarateorbitrate.Speed​​isoneofthemostimportantperformanceindicatorsincomputernetworks.Theunitofrateisbit/s(bitpersecond)(iebitpersecond).

(2)Bandwidth

"Bandwidth"hasthefollowingtwodifferentmeanings.

①Bandwidthoriginallyreferstothebandwidthofacertainsignal.Thebandwidthofasignalreferstothefrequencyrangeoccupiedbythevariousfrequencycomponentscontainedinthesignal.Forexample,thestandardbandwidthofatelephonesignaltransmittedonatraditionalcommunicationlineis3.1kHz(from300Hzto3.4kHz,thatis,thefrequencyrangeofthemaincomponentofvoice).Theunitofbandwidthinthissenseishertz(orkilohertz,megahertz,gigahertz,etc.).

②Inacomputernetwork,bandwidthisusedtoindicatetheabilityofthenetwork'scommunicationlinestotransmitdata.Therefore,thenetworkbandwidthindicatestheamountoftimethatcanbepassedfromonepointtoanotherpointinthenetworkinaunitoftime.Highestdatarate".The"bandwidth"generallymentionedherereferstothis.Theunitofbandwidthinthissenseis"bitspersecond",whichisrecordedasbit/s.

(3)Throughput

Throughputmeanstheamountofdatapassingthroughacertainnetwork(orchannelorinterface)inaunittime.Throughputismoreoftenusedasameasureofthenetworkintherealworldinordertoknowhowmuchdataactuallycanpassthroughthenetwork.Obviously,throughputislimitedbythebandwidthofthenetworkortheratedrateofthenetwork.Forexample,fora100Mbit/sEthernet,itsratedrateis100Mbit/s,thenthisvalueisalsotheabsoluteupperlimitoftheEthernetthroughput.Therefore,for100Mbit/sEthernet,itstypicalthroughputmaybeonly70Mbit/s.Sometimesthroughputcanalsobeexpressedbythenumberofbytesorframestransmittedpersecond.

(4)Delay

Latencyreferstothedata(amessageorpacket,orevenabit)requiredtobetransmittedfromoneendofthenetwork(orlink)totheotherendtime.Latencyisaveryimportantperformanceindicator,itissometimescalleddelayordelay.Thedelayinthenetworkiscomposedofthefollowingdifferentparts.

①Delayinsending.

Senddelayisthetimerequiredforthehostorroutertosendadataframe,thatis,thetimefromwhenthefirstbitofthedataframeissenttotheendofthelastbitoftheframe.

Sosendingdelayisalsocalledtransmissiondelay.Thecalculationformulaforsendingdelayis:

Sendingdelay=dataframelength(bit/s)/channelbandwidth(bit/s)

Itcanbeseenthatforacertainnetwork,Thetransmissiondelayisnotfixed,butisproportionaltothetransmittedframelength(inbits)andinverselyproportionaltothechannelbandwidth.

②Propagationdelay.

Propagationdelayisthetimeittakesforelectromagneticwavestotravelacertaindistanceinthechannel.Thecalculationformulaofpropagationdelayis:

Propagationdelay=channellength(m)/propagationrateofelectromagneticwaveonthechannel(m/s)

PropagationofelectromagneticwaveinfreespaceThespeedisthespeedoflight,whichis300000km/s.Thepropagationrateofelectromagneticwavesinnetworktransmissionmediaisslightlylowerthaninfreespace.

③Processingtimedelay.

Whenthehostorrouterreceivesthepacket,ittakesacertainamountoftimetoprocess,suchasanalyzingtheheaderofthepacket,extractingthedatapartfromthepacket,performingerrorcheckingorfindinganappropriateroute,etc.,whichproducesProcessingtimedelay.

④Queuingdelay.

Whenthepacketistransmittedthroughthenetwork,ithastopassthroughmanyrouters.However,afterthepacketenterstherouter,itmustbequeuedintheinputqueueforprocessing.Aftertherouterdeterminestheforwardinginterface,italsoqueuesupintheoutputqueueforforwarding.Thiscreatesqueuingdelay.

Inthisway,thetotaldelayexperiencedbythedatainthenetworkisthesumoftheabovefourkindsofdelays:

Totaldelay=transmissiondelay+propagationdelay+processingdelay+Queuingdelay

(5)Delay-bandwidthproduct

Multiplythetwomeasuresofnetworkperformancediscussedabove—propagationdelayandbandwidth,andgetanotherusefulMetric:Propagationdelaybandwidthproduct,immediatedelaybandwidthproduct=propagationdelay×bandwidth.

(6)Round-triptime(RTT)

Incomputernetworks,theround-triptimeisalsoanimportantperformanceindicator,whichmeansthatthedataissentfromthesendertothesenderreceivesThetotalelapsedtimefortheconfirmationfromthereceiver(thereceiversendstheconfirmationimmediatelyafterreceivingthedata).

Whenusingsatellitecommunications,theroundtriptime(RTT)isrelativelylong.

(7)Utilizationrate

Therearetwotypesofutilizationrate:channelutilizationrateandnetworkutilizationrate.Thechannelutilizationratereferstoafewpercentofthetimethatachannelisused(thereisdatapassing),andtheutilizationrateofacompletelyidlechanneliszero.Thenetworkutilizationistheweightedaverageofthechannelutilizationoftheentirenetwork.

2.Thenon-performancecharacteristicsofcomputernetworks

Thesenon-performancecharacteristicshaveagreatrelationshipwiththeperformanceindicatorsintroducedabove.

(1)Cost

Thepriceofthenetwork(includingthecostofdesignandimplementation).Theperformanceofthenetworkiscloselyrelatedtoitsprice.Generallyspeaking,thehigherthenetworkspeed,thehighertheprice.

(2)Quality

Thequalityofthenetworkdependsonthequalityofallthecomponentsinthenetworkandhowthesecomponentsmakeupthenetwork.Thequalityofthenetworkaffectsmanyaspects,suchasthereliabilityofthenetwork,theeaseofnetworkmanagement,andsomeoftheperformanceofthenetwork.However,theperformanceofthenetworkisnotthesameasthequalityofthenetwork.Forexample,somenetworkswithacceptableperformancewillfailafterrunningforaperiodoftimeandbecomeunabletocontinueworking,indicatingthattheirqualityisnotgood.High-qualitynetworkstendtobemoreexpensive.

(3)Standardization

Thedesignofnetworkhardwareandsoftwarecaneitherfollowgeneralinternationalstandardsorfollowspecificdedicatednetworkstandards.Itisbesttoadopttheinternationalstandarddesign,sothatyoucangetbetterinteroperability,easiertoupgradeandrepair,andeasiertogettechnicalsupport.

(4)Reliability

Reliabilityiscloselyrelatedtothequalityandperformanceofthenetwork.Networkswithhigherspeedsarenotnecessarilylessreliable.However,itisoftenmoredifficultforahigher-speednetworktooperatereliably,andatthesametime,thecostwillbehigher.

(5)ScalabilityandUpgradability

Whenconstructingthenetwork,itshouldbeconsideredthatitmayneedtobeexpandedinthefuture(thatis,scaleexpansion)andupgrade(thatis,theperformanceandversionoftheimprove).Thehighertheperformanceofthenetwork,thehigheritsexpansioncostisoften,andthedifficultywillincreaseaccordingly.

(6)Easytomanageandmaintain

Ifthenetworkisnotwellmanagedandmaintained,itisdifficulttoachieveandmaintainthedesignedperformance.

Relatedapplications

Inthe21stcentury,mankindwillfullyentertheinformationage.Theimportantcharacteristicsoftheinformationagearedigitization,networkingandinformationization.Torealizeinformatization,wemustrelyonacompletenetwork,becausethenetworkcantransmitinformationveryquickly.Therefore,thenetworkhasnowbecomethelifebloodoftheinformationsocietyandanimportantfoundationforthedevelopmentoftheknowledgeeconomy.TheInternethasexertedanimmeasurableinfluenceonmanyaspectsofsociallifeandonthedevelopmentofsocialeconomy.

Thenetworkmentionedherereferstothe"threenetworks",thatis,telecommunicationnetwork,cableTVnetworkandcomputernetwork.Thesethreetypesofnetworksprovidedifferentservicestousers.Usersoftelecommunicationsnetworkscangetservicessuchastelephone,telegraph,andfax;usersofcableTVnetworkscanwatchvariousTVprograms;computernetworkscanenableuserstoquicklytransmitdatafiles,andfindandobtainvarioususefulinformationfromthenetwork.Includingimageandvideofiles.Thesethreekindsofnetworksallplayaveryimportantroleintheprocessofinformatization,butthefastestgrowingandcoreroleisthecomputernetwork.Withthedevelopmentoftechnology,telecommunicationsnetworksandcabletelevisionnetworkshavegraduallymergedintothetechnologyofmoderncomputernetworks(alsoknownascomputercommunicationnetworks),whichgaverisetotheconceptof"networkintegration".

Sincethe1990s,thecomputernetworkrepresentedbytheInternethasdevelopedrapidly.Ithasgraduallydevelopedfromtheinitialeducationandscientificresearchnetworktoacommercialnetwork,andhasbecomesecondonlytotheglobalThesecondlargesttelephonenetworkintheworld.TheInternetischangingallaspectsofourworkandlife.Ithasbroughthugebenefitstomanycountriesandacceleratedtheprocessoftheglobalinformationrevolution.TheInternetisthebiggestchangeincommunicationsincetheinventionofprinting.Now,people'slife,work,studyandcommunicationareallinseparablefromtheInternet.

Therearetwomostimportantfunctionsprovidedbycomputernetworkstousers,namely,connectivityandsharing.

Themainreasonwhysomanycomputernetworksareestablishedisthattheuseofcomputernetworksisfavoredbyindividualsandcompanies.

First,commercialapplication.

1.Themainpurposeistorealizeresourcesharingandfinallybreakthetyrannyofgeography,andmainlyusetheclient-servermodel.

2.Provideapowerfulcommunicationmedium.Suchas:e-mail(E-mail),videoconference.

3.E-commerceactivities.Forexample,varioussupplierspurchasesubsystemsandthenassemblethesecomponents.

4.ConductvarioustransactionswithcustomersthroughtheInternet.Suchas:bookstores,audio-visualpurchasesofgoodsorservicesathome.

2.Familyuse

1.Accessremoteinformation.Forexample:browseWebpagestogetinformationaboutart,business,cooking,government,health,history,hobbies,entertainment,science,sports,travel,etc.

2.Communicationbetweenindividuals.Suchas:instantmessaging(instantmessaging)use,chatrooms,peer-to-communication(peer-to-communication).

3.Interactiveentertainment.Suchas:videoondemand,instantcommentandparticipationinactivities,onlinegames.

4.E-commerceinabroadsense.Suchas:payingbillselectronically,managingbankaccounts,andprocessinginvestments.

3.Mobileusers

Basedonwirelessnetworks.

1.Removablecomputers:notebookcomputers,PDAs,3Gmobilephones.

2.Military:AwarcannotrelyonLANequipmenttocommunicate.

3.Applicationssuchastruckfleets,taxis,andexpressvehicles.

Four.SocialIssues

ThewidespreaduseoftheInternethasledtonewsocial,ethicalandpoliticalissues.

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