计算机专业英语Unit-6-Network

UnitSixNetworkPassageOne

NetworkFoundationandTopologyTrainingtarget:

Inthispart,ourtargetistotrainyourreadingcomprehension.Wehavemadetheflexiblesentencesstrongblackandmarkedthesubject,predicateandhaveobjectofthem.Trytograspthemainideaofthesesentences.主语、谓语、宾语PassageOne

NetworkFoundationandTopologyComputernetworksaredatacommunicationsystemsmadeupofhardwareandsoftware.Anetwork

is

acollectionofcomputersanddevices

connectedtogetherviacommunicationdevicessuchascabletelephonelines,modems,orothermeans.Sometimesanetworkiswireless;thatis,itusesnophysicallinesorwires.Whenyourcomputerisconnectedtocomputersnetworkedtogether,users

canshare

resources,suchashardwaredevices,softwareprograms,data,andinformation.

Sharingresourcessavestimeandmoney.Forexample,insteadofpurchasingoneprinterforeverycomputerviaanetwork,thenetworkenablesallofthecomputerstoaccessthesameprinter.communicationn.通信viaprep.通过,经过cablen.电缆wirelessadj.无线的physicaladj.物理的PassageOne

NetworkFoundationandTopologyclassifyv.分类geographicaladj.地理的localadj.地方的,当地的Ethernetn.以太网FDDI光纤数据分布接口metropolitanadj.首都的,大城市1.TypesofNetworkNetworksareoftenclassifiedaccordingtotheirgeographicalextent:LAN,MAN,WAN.

LocalAreaNetwork(LAN)AtypicalLANspansasmallarealikeasinglebuildingorasmallcampusandoperatesbetween10Mbpsand2Gbps.BecauseLANtechnologiescovershortdistances,theyprovidethehighestspeedconnectionsamongcomputers.EthernetandFDDIareexamplesofstandardLANs.

MetropolitanAreaNetwork(MAN)MANisabiggerversionofaLANinacity,itissmallerthanaWANbutlargerthanaLAN.MANisapublichigh-speednetwork,andrunsataspeedof100Mbpsorevenfaster,capableofvoiceanddatatransmissionoveradistanceofupto80kilometers(50miles).PassageOne

NetworkFoundationandTopologytopologyn.拓扑结构peer-to-peer

对等网络privilegen.特权

WideAreaNetwork(WAN)WANissometimescalledlonghaulnetworks,providecommunicationoverlongdistance.Itcanspanmorethanonegeographicalarea,oftenacountryorcontinent.UsuallyWANsoperateatslowerspeedthanLANs,andhavemuchgreaterdelaybetweenconnections.TypicalspeedsforaWANrangefrom56Kbpsto155Mbps.TheInternetcanbecorrectlyregardedasthelargestWANinexistence.2.Topology

Anetworkarchitecturecanbedescribedintwoways:client-serverandpeer-to-peer.Client-servernetworkisanetworkcomprisedofseveralworkstationsandoneormoreservers.Inclient-servernetwork,anadministratorcancontroltheprivilegesofeachuser.Peer-to-PeerNetworkisatypeofnetworkwhereallcomputersonthenetworkhavethepotentialtoshareresourcesthattheyhavecontrolPassageOne

NetworkFoundationandTopologypotentialn.潜能,潜力meshn.网孔,网丝

over.Allcomputersonthenetworkcanpotentiallyactasbothaclientandaserver.Becauseofthisfact,thereisnocentralcontrolofthenetworkandthereforethistypeofnetworkstructureisconsideredtobelesssecureandhardertomanagethantheclient-serverarchitecture.Topologydefinesthestructureofthenetwork.Therearefivemajortopologiesinusetoday:Bus,Ring,Star,Tree,andMesh.Eachisusedforspecificnetworktypes,althoughsomenetworktypescanusemorethanonetopology.

Bus—ThesimplesttopologyistheBus(Pic6.1).InaBus,allthedevicesonthenetworkareconnectedtoacommoncable.Normally,thiscableisterminatedateitherend,andcanneverbeallowedtoformaclosedloop.PassageOne

NetworkFoundationandTopologyPic6.1Busnetwork

Ring—ARingtopology(Pic6.2)isverysimilartoaBus.InaRing,allthedevicesonthenetworkareconnectedtoacommoncablewhichloopsfrommachinetomachine.Afterthelastmachineonthenetwork,thecablethenreturnstothefirstdevicetoformaclosedloop.

Pic6.2Ringtopology PassageOne

NetworkFoundationandTopology

Star—Astartopology(Pic6.3)iscompletelydifferentfromeitheraBusoraRing.InaStar,eachdevicehasitsowncablerunthatconnectsthedevicetoacommonhuborconcentrator.Onlyonedeviceispermittedtouseeachportonthehub.

Pic6.3StartopologyPassageOne

NetworkFoundationandTopologyHierarchicaladj.分等级的

Tree—ATreetopology(Pic6.4)canbethoughtofasbeinga“StarofStars”network.InaTreenetwork,eachdeviceisconnectedtoitsownportonaconcentratorinthesamemannerasinaStar.However,concentratorsareconnectedtogetherinahierarchicalmanner.

Pic6.4TreenetworkPassageOne

NetworkFoundationandTopologygeometricaladj.几何的,几何学的

Mesh—AMeshtopologyconsistsofanetworkwhereeverydeviceonthenetworkisphysicallyconnectedtoeveryotherdeviceonthenetwork.Thisprovidesagreatdealofperformanceandreliability,howeverthecomplexityanddifficultyofcreatingoneincreasesgeometricallyasthenumberofnodesonthenetworkincreases.Pic6.5showsanexampleofafour-nodeMeshnetwork.

Pic6.5afour-nodeMeshnetworkPassageOne

NetworkFoundationandTopologynetwork网络 cable电缆wireless无线的 LAN局域网MAN城域网 WAN广域网Ethernet以太网 FDDI光纤数据分布接口client-server客户-服务器 peertopeernetwork对等网络topology拓扑结构 Busnetwork总线型网络Ringnetwork环型网络 Starnetwork星型网络Treenetwork树型网络 Meshnetwork网型网络KeyWordsPassageTwo

NetworkDevicesTrainingtarget:

Inthispartourtargetistotrainyourreadingspeed.Youshouldpaymoreattentionto“keywords”.Wehavemarkedkeywordsinsomeparagraphs.Youcanquicklygraspthemainideaofthesentencesandparagraphsbythesekeywords,therestcanbepracticedbyyourself.Itwillraiseyourreadingspeed.

关键词PassageTwo

NetworkDevicesNetworkdevicesincludeallcomputers,media,interfacecardsandotherequipmentsneededtoperformdata-processingandcommunicationswithinthenetwork.Let’slookatsometypicalnetworkdevices.

NetworkInterfaceCard(NIC)Thenetworkinterfacecardprovidesthephysicalconnectionbetweenthenetworkandthecomputerworkstation.MostNICsareinternal,withthecardfittingintoanexpansionslotinsidethecomputer.NICisamajorfactorindeterminingthespeedandperformanceofanetwork.Itisagoodideatousethefastestnetworkcardavailableforthetypeofworkstationyouareusing.NICsareconsideredLayer2devicesbecauseeachindividualNICthroughouttheworldcarriesauniquecode,calledaMediaAccessControl(MAC)address.Thisaddressisusedtocontroldatacommunicationforthehostonthenetwork.expansionn.扩充,开展,扩展uniqueadj.唯一的PassageTwo

NetworkDevices

RepeaterOneofthedisadvantagesofthetypeofcablethatweprimarilyuse,CAT5UTP,iscablelength.ThemaximumlengthforUTPcableinanetworkis100meters(approximately333feet).Ifyouneedtoextendbeyondthenetworklimit,youmustaddadevicetoyournetwork.Thisdeviceiscalledarepeater.Therepeaterelectricallyamplifiesthesignalitreceivesandrebroadcastsit.RepeatersarenetworkingdevicesthatexitatLayer1,thephysicallayer,oftheOSIreferencemodel.

HubGenerallyspeaking,hubisusedwhenreferringtothedevicethatservesasthecenterofanetwork,asshowninPic6.6.Thepurposeofahubisto

repeatern.中继器（重发器）amplifyv.放大,增强PassageTwo

NetworkDevices

regenerateandretimenetworksignals.Youwillnoticethecharacteristicsofahubaresimilartotherepeater’s,whichiswhyahubisalsoknownasmulti-portrepeater.Thedifferenceisthenumberofcablesthatconnecttothedevice.Wherearepeatertypicallyhasonlytwoports,ahubgenerallyhasfromfourtotwentyormoreports.Whereasarepeaterreceivessignalononeportandrepeatsitontheother,ahubreceivessignalononeportandtransmitsittoallotherports.

Pic6.6hubasthecenterofanetworkregeneratev.再生PassageTwo

NetworkDevices

BridgeAbridgeisaLayer2devicethatallowsyoutosegmentalargenetworkintotwosmaller,moreefficientnetworks.Bridgescanbeusedtoconnectdifferenttypesofcable,orphysicaltopologies.Theymust,however,beusedbetweennetworkswiththesameprotocol.Bridgesarestore-and-forwarddevices.EverynetworkingdevicehasauniqueMACaddressontheNIC.BridgesfilternetworkingtrafficbyonlylookingattheMACaddress.Therefore,theycanrapidlyforwardtrafficrepresentinganynetworklayerprotocol.BecausebridgeslookonlyatMACaddresses,theyarenotconcernedwithnetworklayerprotocols.Consequently,bridgesareconcernedonlywithpassingornotpassingframes,basedontheirdestinationMACaddresses.segmentv.分割store-and-forward存储转发filtern.过滤framen.帧PassageTwo

NetworkDevices

SwitchAswitchisaLayer2devicejustasabridgeis.Infact,aswitchissometimescalledamulti-portbridge,justlikeahubiscalledamulti-portrepeater.Switches,atfirstglance,oftenlooklikehubs.Bothhubsandswitcheshavemanyconnectionports.Thedifferencebetweenahubandaswitchiswhathappensinsidethedevice.SwitchesmakeaLANmuchmoreefficient.Theydothisby“switching”dataonlyouttheporttowhichtheproperhostisconnected.Incontrast,ahubsendsthedataoutallitsportssothatallthehostshavetoseeandprocessallthedata.Pic6.7showsaswitchandthesymbolforaswitch.

Pic6.7aswitchandthesymbolforaswitchmulti-portn.多端口PassageTwo

NetworkDevices

RouterTherouteristhefirstdeviceyouworkwiththatisattheOSInetworklayer,otherwiseknownasLayer3.Theroutermakedecisionsbasedonnetworkaddressesasopposedtoindividuallayer2MACaddresses.Thepurposeofarouteristoexamineincomingpackets,choosethebestpathforthemthroughthenetwork,andthenswitchthemtotheproperoutgoingport.Routersarethemostimportanttrafficregulatingdevicesonlargenetworks.Theyenablevirtuallyanytypeofcomputertocommunicatewithanyothercomputeranywhereintheworld!Pic6.8showsarouterandthesymbolforarouter.

Pic6.8arouterandthesymbolforarouterPassageTwo

NetworkDevices

GatewayAgatewaycantranslateinformationbetweendifferentnetworkdataformatsornetworkarchitectures.ItcantranslateTCP/IPtoAppleTalk,sothatcomputerssupportingTCP/IPcancommunicatewithApplebrandcomputers.Mostgatewaysoperateattheapplicationlayer.

TransmissionMediaTransmissionmediacanbetransferredinwiredorwirelessmethod.Thebasicwiredmediaaretwistedpair,coaxialcable,andopticalfiber.Wirelessmediahaveterrestrialmicrowave,satellitemicrowaveandbroadcastradio.NetworkingmediaareconsideredLayer1componentsofOSImodel.twistedpairn.双绞线coaxialadj.同轴的fibern.光纤opticaln.光学terrestrialadj.陆地的PassageTwo

NetworkDevicesrepeatern.中继器 hubn.集线器bridgen.网桥 switchn.交换机routern.路由器 cablen.电缆framen.帧 segmentn.段wirelessn.无线的 gatewayn.网关transmissionMedian.传输介质 twistedpairn.双绞线coaxialcablen.同轴电缆 opticalfibern.光纤NIC（NetworkInterfaceCard）网络接口卡MAC（MediaAccessControl）address媒体访问控制地址(物理地址)OSI（OpenSystemInterconnect）开放式系统互联参考模型STP（ShieldedTwisted-Pair）屏蔽双绞线UTP（UnshieldedTwisted-Pair）非屏蔽双绞线KeyWordsReadingTCP/IPProtocolTrainingtarget:

Readthefollowingreadingmaterialsandusethereadingskillsmentionedinthepassageabove.Theremaybesomenewwordsinthereadingmaterials,justtrytoguessthemeaningandputyourdictionaryaside.ReadingTCP/IPProtocolAwidevarietyofnetworkprotocolsmodelsexist,whicharedefinedbymanystandardorganizationsworldwideandtechnologyvendorsoveryearsoftechnologyevolutionanddevelopment.OneofthemostpopularnetworkprotocolmodelisTCP/IP,whichistheheartofInternetworkingcommunications.ThenameTCP/IPreferstoasuiteofdatacommunicationprotocols.ThenameismisleadingbecauseTCPandIPareonlytwoofdozensofprotocolsthatcomposethesuite.Itsnamecomesfromtwoofthemoreimportantprotocolsinthesuite,theTransmissionControlProtocol(TCP)andtheInternetProtocol(IP).TCP/IPoriginatedoutoftheinvestigativeresearchintonetworkingprotocolsthattheDepartmentofDefense(DoD)initiatedin1969.Intheearly1980s,theTCP/IPprotocolsweredeveloped.In1983,theybecamestandardprotocolsforARPANET.BecauseofthehistoryoftheTCP/IPprotocolsuite,itisoftenreferredtoastheDoDprotocolsuiteortheInternetprotocolsuite.TCP/IPprotocolsuiteincludemorethan100protocols,nowlet’sseesometypicalprotocols.ReadingTCP/IPProtocol

HypertextTransferProtocol(HTTP)HTTPisanapplication-levelprotocolfordistributed,collaborative,hypermediainformationsystems.IthasbeeninusebytheWorld-WideWebglobalinformationinitiativesince1990.Itisastatelessprotocolwhichcanbeusedformanytasksbeyonditsuseforhypertext,suchasnameserversanddistributedobjectmanagementsystems,throughextensionofitsrequestmethods,errorcodesandheaders.AfeatureofHTTPisthetypingandnegotiationofdatarepresentation,allowingsystemstobebuiltindependentlyofthedatabeingtransferred.

FileTransferProtocol(FTP)FTPenablesfilesharingbetweenhosts.FTPusesTCPtocreateavirtualconnectionforcontrolinformationandthencreatesaseparateTCPconnectionfordatatransfers.ThecontrolconnectionusesanimageoftheTELNETprotocoltoexchangecommandsandmessagesbetweenhosts.ReadingTCP/IPProtocol

TransmissionControlProtocol(TCP)TCPsupportsthenetworkatthetransportlayer.TCPspecifiestheformatofthedataandacknowledgementthattwocomputersexchangetoachieveareliabletransfer,aswellastheprocedurethecomputersusetoensurethatthedataarrivescorrectly.ItspecifieshowTCPsoftwaredistinguishesamongmultipledestinationsonagivenmachine,andhowcommunicatingmachinesrecoverformerrorslikelostorduplicatedpackets.TheprotocolalsospecifieshowtwocomputersinitiateaTCPstreamtransferandhowtheyagreewhenitiscomplete.BecauseTCPassumeslittleabouttheunderlyingcommunicationsystem,TCPcanbeusedwithavarietyofpacketdeliverysystems,includingtheIPdatagramdeliveryservice.Infact,thelargevarietyofdeliverysystemsTCPcanuseisoneofitsstrengths.ReadingTCP/IPProtocol

UserDatagramProtocol(UDP)UDPsupportsthenetworkatthetransportlayer.UDPprovidestheprimarymechanismthatapplicationprogramsusetosenddatagramstootherapplicationprograms.UDPprovidesprotocolportsusedtodistinguishamongmultipleprogramsexecutingonasinglemachine.UDPusestheunderlyingIPtotransportamessagefromonemachinetoanother,andprovidesthesameunreliable,connectionlessdatagramdeliverysemanticsasIP.Itdoesnotuseacknowledgementtomakesuremessagesarrive,itdoesnotorderincomingmessages,anditdoesnotprovidefeedbacktocontroltherateatwhichinformationflowsbetweenthemachines.

InternetProtocol(IP)IPprovidessupportatthenetworklayer.IPprovidesthreeimportantdefinitions.FirsttheIPprotocoldefinesthebasicunitofdatatransferusedthroughoutaTCP/IPinternet.Thus,itspecifiestheexactformatofalldataasitpassesacrossaTCP/IPinternet.Second,IPsoftwareperformstheroutingfunction,choosingapathoverwhichdatawillbesent.Third,IPincludesasetofrulesthatembodytheideaofunreliablepacketdelivery,meaningthereisnoguaranteethatthedatawillreachtheintendedhost.Thedatagrammaybedamageduponarrival,outoforde