外文文献计算机网络.docx

外文文献计算机网络.docx

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外文文献计算机网络

英文原文：

Computernetwork

Acomputernetwork,oftensimplyreferredtoasanetwork,isacollectionofcomputersanddevicesinterconnectedbycommunicationschannelsthatfacilitatecommunicationsamongusersandallowsuserstoshareresources.Networksmaybeclassifiedaccordingtoawidevarietyofcharacteristics.Acomputernetworkallowssharingofresourcesandinformationamonginterconnecteddevices.

History：

EarlynetworksofcommunicatingcomputersincludedthemilitaryradarsystemSemi-AutomaticGroundEnvironment（SAGE）anditsrelativethecommercialairlinereservationsystemSemi-AutomaticBusinessResearchEnvironment（SABRE）,startedinthelate1950s.[1][2]

WhenRussialaunchedHisSPUTNIKSatelliteinSpaceIn1957.TheAmericanStartedAgencyNamesADVANCERESEARCHPROJECTAGENCY（ARPA）&launchedTHis1stSatelliteWithin18MonthAfterEstablishment.ThenSharingOfTheInformationInAnotherComputerTheyUseARPANET.AndThisAllResponsibilityOnAmerica'sDr.LIEDLIEDER.Thenin1969,ARPANETComesinINDIAAndINDIANSwitchedThisNameToNETWORK.Inthe1960s,theAdvancedResearchProjectsAgency（ARPA）startedfundingthedesignoftheAdvancedResearchProjectsAgencyNetwork（ARPANET）fortheUnitedStatesDepartmentofDefense.Developmentofthenetworkbeganin1969,basedondesignsdevelopedduringthe

1960s.[3]TheARPANETevolvedintothemodernInternet.

Purpose：

Computernetworkscanbeusedforavarietyofpurposes:

Facilitatingcommunications.Usinganetwork,peoplecancommunicateefficientlyandeasilyviaemail,instantmessaging,chatrooms,telephone,videotelephonecalls,andvideoconferencing.

Sharinghardware.：

Inanetworkedenvironment,eachcomputeronanetworkmayaccessandusehardwareresourcesonthenetwork,suchasprintingadocumentonasharednetworkprinter.

Sharingfiles,data,andinformation.Inanetworkenvironment,authorizedusermayaccessdataandinformationstoredonothercomputersonthenetwork.Thecapabilityofprovidingaccesstodataandinformationonsharedstoragedevicesisanimportantfeatureofmanynetworks.

Sharingsoftware.：

Usersconnectedtoanetworkmayrunapplicationprogramsonremotecomputers.

informationpreservation

Security

NetworkclassificationThefollowinglistpresentscategoriesusedforclassifyingnetworks.

Connectionmethod：

Computernetworkscanbeclassifiedaccordingtothehardwareandsoftwaretechnologythatisusedtointerconnecttheindividualdevicesinthenetwork,suchasopticalfiber,Ethernet,wirelessLAN,HomePNA,powerlinecommunicationorG.hn.

EthernetasitisdefinedbyIEEE802utilizesvariousstandardsandmediumsthatenablecommunicationbetweendevices.Frequentlydeployeddevicesincludehubs,switches,bridges,orrouters.WirelessLANtechnologyisdesignedtoconnectdeviceswithoutwiring.Thesedevicesuseradiowavesorinfraredsignalsasatransmissionmedium.ITU-TG.hntechnologyusesexistinghomewiring（coaxialcable,phonelinesandpowerlines）tocreateahigh-speed（upto1Gigabit/s）localareanetwork.

Wiredtechnologies：

Twistedpairwireisthemostwidelyusedmediumfortelecommunication.

Twisted-paircablingconsistofcopperwiresthataretwistedintopairs.Ordinarytelephonewiresconsistoftwoinsulatedcopperwirestwistedintopairs.Computernetworkingcablingconsistof4pairsofcoppercablingthatcanbeutilizedforbothvoiceanddatatransmission.Theuseoftwowirestwistedtogetherhelpstoreducecrosstalkandelectromagneticinduction.Thetransmissionspeedrangesfrom2millionbitspersecondto100millionbitspersecond.TwistedpaircablingcomesintwoformswhichareUnshieldedTwistedPair（UTP）andShieldedtwisted-pair（STP）whichareratedincategorieswhicharemanufacturedindifferentincrementsforvariousscenarios.

Coaxialcableiswidelyusedforcabletelevisionsystems,officebuildings,andotherwork-sitesforlocalareanetworks.Thecablesconsistofcopperoraluminumwirewrappedwithinsulatinglayertypicallyofaflexiblematerialwithahighdielectricconstant,allofwhicharesurroundedbyaconductivelayer.Thelayersofinsulationhelpminimizeinterferenceanddistortion.Transmissionspeedrangefrom200milliontomorethan500millionbitspersecond.

Opticalfibercableconsistsofoneormorefilamentsofglassfiberwrappedinprotectivelayers.Ittransmitslightwhichcantraveloverextendeddistances.

Fiber-opticcablesarenotaffectedbyelectromagneticradiation.Transmissionspeed

mayreachtrillionsofbitspersecond.Thetransmissionspeedoffiberopticsis

hundredsoftimesfasterthanforcoaxialcablesandthousandsoftimesfasterthana

twisted-pairwire.[citationneeded]

Wirelesstechnologies：

Terrestrialmicrowave–TerrestrialmicrowavesuseEarth-basedtransmitterandreceiver.Theequipmentlookssimilartosatellitedishes.Terrestrialmicrowavesuselow-gigahertzrange,whichlimitsallcommunicationstoline-of-sight.Pathbetweenrelaystationsspacedapprox,30milesapart.Microwaveantennasareusuallyplacedontopofbuildings,towers,hills,andmountainpeaks.

Communicationssatellites–ThesatellitesusemicrowaveradioastheirtelecommunicationsmediumwhicharenotdeflectedbytheEarth'satmosphere.Thesatellitesarestationedinspace,typically22,000miles（forgeosynchronoussatellites）abovetheequator.TheseEarth-orbitingsystemsarecapableofreceivingandrelayingvoice,data,andTVsignals.

CellularandPCSsystems–Useseveralradiocommunicationstechnologies.Thesystemsaredividedtodifferentgeographicareas.Eachareahasalow-powertransmitterorradiorelayantennadevicetorelaycallsfromoneareatothenextarea.

WirelessLANs–Wirelesslocalareanetworkuseahigh-frequencyradiotechnologysimilartodigitalcellularandalow-frequencyradiotechnology.WirelessLANsusespreadspectrumtechnologytoenablecommunicationbetweenmultipledevicesinalimitedarea.Anexampleofopen-standardswirelessradio-wavetechnologyisIEEE.

Infraredcommunication,whichcantransmitsignalsbetweendeviceswithinsmalldistancesnotmorethan10meterspeertopeeror（facetoface）withoutanybodyinthelineoftransmitting.

Scale：

Networksareoftenclassifiedaslocalareanetwork（LAN）,wideareanetwork（WAN）,metropolitanareanetwork（MAN）,personalareanetwork（PAN）,virtualprivatenetwork（VPN）,campusareanetwork（CAN）,storageareanetwork（SAN）,andothers,dependingontheirscale,scopeandpurpose,e.g.,controllerareanetwork（CAN）usage,trustlevel,andaccessrightoftendifferbetweenthesetypesofnetworks.LANstendtobedesignedforinternalusebyanorganization'sinternalsystemsandemployeesinindividualphysicallocations,suchasabuilding,whileWANsmayconnectphysicallyseparatepartsofanorganizationandmayincludeconnectionstothirdparties.

Functionalrelationship（networkarchitecture）：

Computernetworksmaybeclassifiedaccordingtothefunctionalrelationshipswhichexistamongtheelementsofthenetwork,e.g.,activenetworking,client–server,Wirelessadhocnetworkandpeer-to-peer（workgroup）architecture.

Networktopology：

Mainarticle:

NetworktopologyComputernetworksmaybeclassifiedaccordingtothenetworktopologyuponwhichthenetworkisbased,suchasbusnetwork,starnetwork,ringnetwork,meshnetwork.

Networktopologyisthecoordinationbywhichdevicesinthenetworkarearrangedintheirlogicalrelationstooneanother,independentofphysicalarrangement.Evenifnetworkedcomputersarephysicallyplacedinalineararrangementandareconnectedtoahub,thenetworkhasastartopology,ratherthanabustopology.Inthisregardthevisualandoperationalcharacteristicsofanetworkaredistinct.Networksmaybeclassifiedbasedonthemethodofdatausedtoconveythedata,theseincludedigitalandanalognetworks.

TypesofnetworksbasedonphysicalscopeCommontypesofcomputernetworksmaybeidentifiedbytheirscale.

Localareanetwork：

Alocalareanetwork（LAN）isanetworkthatconnectscomputersanddevicesinalimitedgeographicalareasuchashome,school,computerlaboratory,officebuilding,orcloselypositionedgroupofbuildings.Eachcomputerordeviceonthenetworkisanode.CurrentwiredLANsaremostlikelytobebasedonEthernettechnology,althoughnewstandardslikeITU-TG.hnalsoprovideawaytocreateawiredLANusingexistinghomewires（coaxialcables,phonelinesandpowerlines）.[4]

Typicallibrarynetwork,inabranchingtreetopologyandcontrolledaccesstoresourcesAllinterconnecteddevicesmustunderstandthenetworklayer（layer3）,becausetheyarehandlingmultiplesubnets（thedifferentcolors）.Thoseinsidethelibrary,whichhaveonly10/100Mbit/sEthernetconnectionstotheuserdeviceandaGigabitEthernetconnectiontothecentralrouter,couldbecalled"layer3switches"becausetheyonlyhaveEthernetinterfacesandmustunderstandIP.Itwouldbemorecorrecttocallthemaccessrouters,wheretherouteratthetopisadistributionrouterthat

connectstotheInternetandacademicnetworks'customeraccessrouters.

ThedefiningcharacteristicsofLANs,incontrasttoWANs（WideAreaNetworks）,includetheirhigherdatatransferrates,smallergeographicrange,andnoneedforleasedtelecommunicationlines.CurrentEthernetorotherIEEE802.3LANtechnologiesoperateatspeedsupto10Gbit/s.Thisisthedatatransferrate.IEEEhasprojectsinvestigatingthestandardizationof40and100Gbit/s.[5]

Personalareanetwork:

Apersonalareanetwork（PAN）isacomputernetworkusedforcommunicationamongcomputeranddifferentinformationtechnologicaldevicesclose