ComputerNetworkTechnology5.1ComputerNetworkBasics5.2InformationSecurity5.3WirelessNetworks5.4專業(yè)英語應(yīng)用模塊15.1ComputerNetworkBasicsNetworkingarosefromtheneedtosharedatainatimelyfashion.Personalcomputersarewonderfulbusinesstoolsforproducingdata,spreadsheets,graphics,andothertypesofinformation,butdonotallowyoutoquicklysharethedatayouhaveproduced.Agroupofcomputersandotherdevicesconnectedtogetheriscalledanetwork,andthetechnicalconceptofconnectedcomputerssharingresourcesiscallednetworking.2（1）LocalAreaNetworkALocalAreaNetwork(LAN)isahomogenousentity.Everypacketplacedonthenetworkisseenbyevery“node”(serverorworkstation),buteachnodeprocessesonlythepacketsaddressedtoit,exceptfornetworktestequipmentandhackertools,whichreadanypacketstheyplease.AllnodesonaLANmaybetappedintoasinglecable(oldcoaxEthernet)orwiredtoacentral“hub”.3（2）BridgedLANAbridgedLANnetworkisaLANthatgottoobiganda“bridge”hadtobeinsertedtocutdownthetraffic.Abridgeknowswhatnetworkaddressesareoneachsideofit,andpassesonlytrafficaddressedtotheotherside,thus“segmenting”thenetwork.4（3）SwitchedLANAswitchedLANhasaveryfast,multi-portbridgecalleda“switch”or“switchinghub”.AswitchedLANmaybesofinegrainedithasonlyonenodeoneachport,reducingtrafficoneachcableasfarasitcanbereduced.5（4）CampusAreaNetworkACampusAreaNetwork(CAN)willhaveatleastone“backbone”towhichtwoormoreLANsareconnectedthroughbridges,switchesorrouters.6（5）RoutedNetworkAroutednetworkisonewhereindividualLANsareconnectedthroughrouters,devicesmuchsmarterthanbridgesorswitches.Arouterknowswherestuffis,evenifit’sseveralroutersaway.7（6）WideAreaNetworkAWideAreaNetwork(WAN)isanetworkthatpassesoverlinksnotownedbytheownerofthenetwork.Generally,theselinksareownedbyatelephonecompanyorsomeothercommoncarrier.TheInternetisahugeworldwideWANconsistingofthousandsofLANandmillionsofroutersandservers.82．NetworkAdministratorNetworkadministratorhasfullrightsandpermissionstoallresourcesonanetwork.Theadministratorisusuallyresponsibleforinstalling,managing,andcontrollingserversandnetworkingcomponents.Administratorscanalsomodifythepropertiesofuseraccountsandthemembershipofgroups,createandmanageprinters,installprinters,shareresourcesandassignpermissionstothoseresources.93．TopologiesThephysicaltopologydescribesthelayoutofthenetwork,justlikeamapshowsthelayoutofvariousroads,andthelogicaltopologydescribeshowthedataissentacrossthenetworkorhowthecarsareabletotravel(thedirectionandspeed)ateveryroadonthemap.Logicaltopologyisthemethodusedtopasstheinformationbetweenthecomputers.10（1）Starnetwork（2）Gridnetwork（3）Busnetwork（4）Ringnetwork（5）Hybridnetwork11accurate

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診斷fashion

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使合并12neighbor鄰居，鄰接permission許可，允許regular 常規(guī)的，合格的router 路由器spreadsheet電子表格token 令牌，記號topology 拓?fù)洌Y(jié)構(gòu)upgrade升級，上升unreliable 不可靠的13[1]

Withoutanetwork,thedocumentshavetobeprintedoutsothatotherscaneditthemorusethem.譯文：沒有網(wǎng)絡(luò)，就必須將文檔打印出來才能供他人編輯或使用。本句的“Withoutanetwork”是條件狀語，“sothat”引導(dǎo)的是目的狀語從句。14[2]Iftheuserweretoconnecthiscomputertoothercomputers,hecouldsharedataanddevicesonothercomputers,includinghigh-qualityprinters.譯文：如果用戶能把他的計算機(jī)與其他計算機(jī)連在一起的話，他就可以共享其他計算機(jī)上的數(shù)據(jù)和設(shè)備，包括高性能的打印機(jī)。本句由“If

”引導(dǎo)條件狀語從句，“includinghigh-qualityprinters”是補(bǔ)語。15[3]

Arouterknowswherestuffis,evenifit’sseveralroutersaway.譯文：路由器知道信息的位置，即使要經(jīng)過幾個路由器的距離。本句的“wherestuffis”是賓語從句，“evenifit’sseveralroutersaway”是讓步狀語從句。16[4]Agridnetworkisakindofcomputernetworkconsistingofanumberofcomputersystemsconnectedinagridtopology.譯文：網(wǎng)狀網(wǎng)絡(luò)是一種計算機(jī)網(wǎng)絡(luò)，它以網(wǎng)狀拓?fù)浣Y(jié)構(gòu)將許多計算機(jī)系統(tǒng)連接在一起。本句的“consistingofanumberofcomputersystems”是現(xiàn)在分詞短語，作定語修飾“network”，而“connectedinagridtopology”是過去分詞短語作定語，修飾“computersystems”。17[5]Actually,thereareusuallytwocablessoadapterscanloopbacktoremarktheringifthereisabreak.譯文：實際上，通常會有兩條電纜，這樣適配器就會在網(wǎng)絡(luò)中出現(xiàn)故障時，可以沿環(huán)路返回重新構(gòu)成環(huán)。本句的“soadapterscanloopbacktoremarkthering”是目的狀語，而“ifthereisabreak”是條件狀語。18OpticalCommunicationFiber-opticcommunicationsystemsarelight-wavesystemsthatemployopticalfibersforinformationtransmission.Microwaveradioisusedinbroadcastingandtelecommunicationtransmissionsbecause,duetotheirshortwavelength,highlydirectionalantennasaresmallerandthereforemorepracticalthantheywouldbeatlongerwavelengths(lowerfrequencies).19Opticalcommunicationsystemsdifferinprinciplefrommicrowavesystemsonlyinthefrequencyrangeofthecarrierwaveusedtocarrytheinformation.Theopticalcarrierfrequenciesaretypically200THz,incontrast,themicrowavecarrierfrequenciesare1GHz.20Theapplicationofopticalfibercommunicationsisingeneralpossibleinanyareathatrequirestransferofinformationfromoneplacetoanother.However,fiber-opticcommunicationsystemshavebeendevelopedmostlyfortelecommunicationsapplications.Thisisunderstandableinviewoftheexistingworldwidetelephonenetworkswhichareusedtotransmitnotonlyvoicesignalsbutalsocomputerdataandfaxmessages.21Eachsuccessivegenerationoflight-wavesystemsiscapableofoperatingathigherbitratesandoverlongerdistances.Periodicregenerationoftheopticalsignalbyusingrepeatersisstillrequiredformostlong-haulsystems.225.2InformationSecuritySecuritythreatstoanetworkcanbedividedintothosethatinvolvesomesortofunauthorizedaccessandallothers.Oncesomeonegainsunauthorizedaccesstothenetwork,therangeofthingstheycandoislarge.23Management’sresponsibilityThenetworksecuritypolicyismanagement’sstatementoftheimportanceandtheircommitmenttonetworksecurity.Thepolicyneedstodescribeingeneraltermswhatwillbedone,butdoesnotdealwiththewaytheprotectionistobeachieved.24Management,atalllevels,needstosupportthepolicyandperiodicallyreinforceitwithemployeesinvariousways.ITandnetworkstaffmayneedtoinstalladditionalhardware,software,andprocedurestoperformautomatedsecuritychecking.25AttackpreventionDifferentsecuritymechanismscanbeusedtoenforcethesecuritypropertiesdefinedinagivensecuritypolicy.Dependingontheanticipatedattacks,differentmeanshavetobeappliedtosatisfythedesiredproperties.Attackpreventionisaclassofsecuritymechanismsthatcontainswaysofpreventingordefendingagainstcertainattacksbeforetheycanactuallyreachandaffectthetarget.26Accesscontrollimitsandregulatestheaccesstocriticalresources.Themostcommonformofaccesscontrolusedinmulti-usercomputersystemsareaccesscontrollistsforresourcesthatarebasedontheuserandgroupidentityoftheprocessthatattemptstousethem.Theidentityofauserisdeterminedbyaninitialauthenticationprocessthatusuallyrequiresanameandapassword.27Theloginprocessretrievesthestoredcopyofthepasswordcorrespondingtotheusernameandcomparesitwiththepresentedone.Whenbothmatch,thesystemgrantstheusertheappropriateuserandgroupcredentials.28Afirewallisanimportantaccesscontrolsystematthenetworklayer.Theideaofafirewallisbasedontheseparationofatrustedinsidenetworkofcomputersundersingleadministrativecontrolfromapotentialhostileoutsidenetwork.Thefirewallisacentralchokepointthatallowsenforcementofaccesscontrolforservicesthatmayrunattheinsideoroutside.29AttackavoidanceSecuritymechanismsinthiscategoryassumethatanintrudermayaccessthedesiredresourcebuttheinformationismodifiedinawaythatmakesitunusablefortheattacker.Whiletheinformationistransportedoverthecommunicationchannel,itresistsattacksbybeingnearlyuselessforanintruder.Onenotableexceptionisattacksagainsttheavailabilityoftheinformation,asanattackercouldstillinterruptthemessage.30Themostimportantmemberinthiscategoryiscryptography,whichisdefinedasthescienceofkeepingmessagesecure.Itallowsthesendertotransforminformationintowhatmayseemlikearandomdatastreamtoanattacker,butcanbeeasilydecodedbyanauthorizedreceiver.31Themostcommonattack,calledknownplaintextattack,isexecutedbyobtainingciphertexttogetherwithitscorrespondingplaintext.Theencryptionalgorithmmustbesocomplexthatevenifthecodebreakerisequippedwithplentyofsuchpairs,itisinfeasibleforheretoretrievethekey.32anticipate 預(yù)料，預(yù)測brute 暴力的，粗暴的choke 要塞，擁塞ciphertext 密文creativity 創(chuàng)造力，創(chuàng)新cryptographic 加密的，密碼的decode 譯碼denial 否認(rèn)，拒絕denote 表示，意味著disguise 偽裝，借口33eavesdrop 偷聽，竊聽encryption 加密hostile 敵人，敵對分子interference 沖突，干擾intruder 闖入者，侵入者lossless 無損的observation 觀察，觀測overt 公開的，公然的，明顯的plaintext 明文separation 分離，隔離，隔開upgrade 升級，提高34[1]

Thatrequiresfollowthroughandcommunicationwithallemployeessothattheyunderstandtheemphasisandimportanceseniormanagementisplacingonsecurity.譯文：這需要后續(xù)通過與所有員工的溝通，使他們認(rèn)識到高層管理人員在安全布置上的重點和重要性。本句的“sothat”引導(dǎo)目的狀語從句。35[2]

Themostcommonformofaccesscontrolusedinmulti-usercomputersystemsareaccesscontrollistsforresourcesthatarebasedontheuserandgroupidentityoftheprocessthatattemptstousethem.譯文：在多用戶計算機(jī)系統(tǒng)中，最常用的訪問控制形式是資源訪問控制清單。清單是建立在進(jìn)程中想要使用這些資源的用戶和群的身份基礎(chǔ)上的。本句的“usedinmulti-usercomputersystems”作定語，修飾“accesscontrol”，“thatarebased…”是修飾“accesscontrollists”的定語。36[3]

Whenaresourceshouldbeaccessed,thesystemlooksuptheuserandgroupintheaccesscontrollistandgrantsordeniesaccessasappropriate.譯文：當(dāng)一個資源被請求訪問時，系統(tǒng)會在訪問控制清單中查找用戶和群并適當(dāng)授權(quán)訪問或拒絕訪問。本句的“When”引導(dǎo)時間狀語從句，“intheaccesscontrollist”作定語。37[4]

Thefirewallpreventsattacksfromtheoutsideagainstthemachinesintheinsidenetworkbydenyingconnectionattemptsfromunauthorizedpartieslocatedoutside,inaddition,afirewallmayalsobeutilizedtopreventusersbehindthefirewallfromusingcertainservicesthatareoutside.譯文：防火墻通過拒絕來自外部的未授權(quán)方的連接企圖，防止來自外部的對內(nèi)部網(wǎng)絡(luò)中的機(jī)器的攻擊。除此之外，防火墻還可以用來阻止墻內(nèi)用戶使用外部服務(wù)。這是一個長句，由“inaddition”連接起兩個并列子句，“bydenyingconnectionattemptsfromunauthorizedpartieslocatedoutside”是方式狀語，“behindthe…”作定語。38[5]

Theinformationispreprocessedatthesenderbeforeitistransmittedoverthecommunicationchannelandpost-processedatthereceiver.譯文：信息在通信渠道中傳播之前由發(fā)送者進(jìn)行了預(yù)處理，并且接收者收到信息后也要進(jìn)行處理。本句采用被動語態(tài)，主語是“Theinformation”，謂語為“preprocessed”和“post-processed”，構(gòu)成并列句，“beforeitistransmittedoverthecommunicationchannel”是時間狀語。39TechniquesforInternetSecurity1．NetworkFirewallsThepurposeofanetworkfirewallistoprovideashellaroundthenetworkthatwillprotectthesystemsconnectedtothenetworkfromvariousthreats.Thetypesofthreatsafirewallcanprotectagainstinclude:Unauthorizedaccesstonetworkresources—anintrudermaybreakintoahostonthenetworkandagainunauthorizedaccesstofiles.40Denialofservice—anindividualfromoutsideofthenetworkcould,forexample,sendthousandsofmailmessagestoahostonthenetinanattempttofillavailablediskspaceorloadthenetworklinks.Masquerading—electronicmailappearingtohaveoriginatedfromoneindividualcouldhavebeenforgedbyanotherwiththeintenttoembarrassorcauseharm.41Afirewallcanreduceriskstonetworksystemsbyfilteringoutinherentlyinsecurenetworkservices.42DigitalCertificateandAuthenticationDigitalcertificateisanidentitycardcounterpartinthecomputersociety.Whenapersonwantstogetadigitalcertificate,hegenerateshisownkeypair,givesthepublickeyaswellassomeproofofhisidentificationtotheCertificateAuthority(CA).CAwillchecktheperson’sidentificationtoassuretheidentityoftheapplicant.43DigitalSignatureAdigitalsignatureisanelectronicsignaturethatcanbeusedtoauthenticatetheidentityofthesenderofamessageorthesignerofadocument,andpossiblytoensurethattheoriginalcontentofthemessageordocumentthathasbeensentisunchanged.44Adigitalsignatureisastringofbitsattachedtoanelectronicdocument,whichcouldbeawordprocessingfileoranE-mailmessage.Thisbitstringisgeneratedbythesigner,anditisbasedonboththedocument’sdataandtheperson’ssecretpassword.Digitalsignaturesareeasilytransportable,cannotbeimitatedbysomeoneelse,andcanbeautomaticallytime-stamped.455.3WirelessNetworksWirelesstechnologies,inthesimplestsense,enableoneormoredevicestocommunicatewithoutphysicalconnections—withoutrequiringnetworkorperipheralcabling.Wirelesstechnologiesuseradiofrequencytransmissionsasthemeansfortransmittingdata,whereaswiredtechnologiesusecables.46Wirelessnetworksserveasthetransportmechanismbetweendevicesandamongdevicesandthetraditionalwirednetworks(enterprisenetworksandtheInternet).Wirelessnetworksaremanyanddiversebutarefrequentlycategorizedintothreegroupsbasedontheircoveragerange:wirelesswideareanetworks(WWAN),WLANs,andwirelesspersonalareanetworks(WPAN).47WLANsallowgreaterflexibilityandportabilitythandotraditionalwiredlocalareanetworks.UnlikeatraditionalLAN,whichrequiresawiretoconnectauser’scomputertothenetwork,aWLANconnectcomputersandothercomponentstothenetworkusinganaccesspointdevice.48AdhocnetworkssuchasBluetootharenetworksdesignedtodynamicallyconnectremotedevicessuchascellphones,laptops,andPDAs.Thesenetworksaretermed“adhoc”becauseoftheirshiftingnetworktopologies.WhereasWLANsuseafixednetworkinfrastructure,adhocnetworksmaintainrandomnetworkconfigurations,relyingonamaster-slavesystemconnectedbywirelesslinkstoenabledevicestocommunicate.49Inrecentyears,wirelessnetworkinghasbecomemoreavailable,affordable,andeasytouse.Homeusersareadoptingwirelesstechnologyingreatnumbers.On-the-golaptopusersoftenfindfreewirelessconnectionsinplaceslikecoffeeshopsandairports.50Wirelessnetworkshavemanyuses.Forexample,youcansendE-mail,receivetelephonecallsandfaxesandreadremotedocumentsinyourtravel.Inaddition,wirelessnetworksareofgreatvaluetofleetsoftrucks,taxis,busesandrepairpersonsforkeepingincontactwithhome.51Wirelessaccesspointscanannouncetheirpresencetowireless-enabledcomputers.Thisisreferredtoas“identifierbroadcasting”.Incertainsituations,identifierbroadcastingisdesirable.Manypublicaccesspointsarenotsecured,andthetraffictheycarryisnotencrypted.Thiscanputyoursensitivecommunicationsortransactionsatrisk.52AccessingtheInternetviaapublicwirelessaccesspointinvolvesserioussecuritythreatsyoushouldguardagainst.Thesethreatsarecompoundedbyyourinabilitytocontrolthesecuritysetupofthewirelessnetwork.What’smore,youareofteninrangeofnumerouswireless-enabledcomputersoperatedbypeopleyoudon’tknow.53abuse 濫用affordable 負(fù)擔(dān)得起bluetooth 藍(lán)牙cellular 蜂窩的，多孔的cordless 無繩的，不用電線的coverage 范圍，規(guī)模disaster 災(zāi)害，天災(zāi)diverse 多種多樣的，不同的54fashion 方式，風(fēng)格headphone 耳機(jī)inability 無能，無能為力infrastructure 基站，基礎(chǔ)設(shè)施laptop 微型便攜式（計算機(jī)）fleet 船隊，車隊malicious 有惡意的，蓄意的nickname 諢名，綽號55repairperson 維修人員roam 漫游savvy 精明的，有見識的unpredictable 無法預(yù)言的violation 侵害，違犯wavelength 波長wireless 無線的，無線電的56[1]Wirelesstechnologiesrangefromcomplexsystems,suchaswirelesslocalareanetworks(WLAN)andcellphonestosimpledevicessuchaswirelessheadphones,microphones,andotherdevicesthatdonotprocessorstoreinformation.譯文：無線電技術(shù)的范圍從復(fù)雜的系統(tǒng)如無線局域網(wǎng)和手機(jī)到簡單的設(shè)備如無線耳機(jī)、麥克風(fēng)以及其他不用來處理或儲存信息的設(shè)備。本句有兩處由“suchas”引導(dǎo)的同位語結(jié)構(gòu)，“thatdo…”作定語。57[2]UnlikeatraditionalLAN,whichrequiresawiretoconnectauser’scomputertothenetwork,aWLANconnectcomputersandothercomponentstothenetworkusinganaccesspointdevice.譯文：與需要一根線把用戶的計算機(jī)與網(wǎng)絡(luò)連接的傳統(tǒng)局域網(wǎng)不同，無線局域網(wǎng)使用一個接入點裝置把計算機(jī)和其他組成部分連接到網(wǎng)絡(luò)。本句中“UnlikeatraditionalLAN”作狀語，“which”引導(dǎo)的是非限定性定語從句，修飾“atraditionalLAN”，“usinganaccesspointdevice”作方式狀語。58[3]Ifyoufailtosecureyourwirelessnetwork,anyonewithawireless-enabledcomputerwithinrangeofyourwirelessaccesspointcanhopafreerideontheInternetoveryourwirelessconnection.譯文：如果你不能確保你的無線網(wǎng)絡(luò)的安全，任何擁有一臺能激活無線網(wǎng)絡(luò)的計算機(jī)的人，只要在你的無線接口范圍之內(nèi)，就能通過你的無線連接免費上互聯(lián)網(wǎng)。本句中“If

”引導(dǎo)的是條件狀語從句，“withawireless-enabledcomputerwithinrangeofyourwirelessaccesspoint”作定語，修飾主語“anyone”。59[4]Whenyouuseawirelessrouteroraccesspointtocreateahomenetwork,youtradewiredconnectivityforconnectivitydeliveredviaaradiosignal.譯文：當(dāng)你使用一個無線路由器或接入點去建立一個家庭網(wǎng)絡(luò)時，你用有線連接交換到由無線電信號傳送的連接。本句由“When”引導(dǎo)時間狀語從句。60[5]Becauseyourconnectionisbeingtransmitted“intheclear”,malicioususerscanuse“sniffing”toolstoobtainsensitiveinformationsuchaspasswords,bankaccountnumbers,andcreditcardnumbers.譯文：由于你的連接被暢通無阻地傳送出去，惡意用戶能夠使用“嗅探”工具來獲得敏感信息，諸如密碼、銀行賬號以及信用卡號。本句由“Because”引導(dǎo)原因狀語從句，“suchas”后面的部分作為“sensitiveinformation”的同位語。61InternetApplications

E-mailE-mailreferstothetransmissionofmessagesovercommunicationsnetworks.E-mailusestheofficememoparadigminwhichamessagecontainsaheaderthatspecifiesthesender,recipients,andsubject,followedbyabodythatcontainsthetextofthemessage.AnE-mailaddressisastringdividedintotwopartsbythe@character(pronouncedas“at”).62

SearchToolsandMethodsAsearchtoolisacomputerprogramthatperformssearches.Asearchmethodisthewayasearchtoolrequestsandretrievesinformationfrom