计算机专业英语第5章课件

1第5章ComputerNetworkTechnology

5.1ComputerNetworkBasics5.2InformationSecurity5.3WirelessNetworks5.4专业英语应用模块25.1ComputerNetworkBasics

Agroupofcomputersandotherdevicesconnectedtogetheriscalledanetwork,andthetechnicalconceptofconnectedcomputerssharingresourcesiscallednetworking.

Computersthatarepartofanetworkcansharedata,messages,graphics,printers,faxmachines,modems,CD-ROMs,harddisks,andotherdatastorageequipment.

3Fig.5-1Compositionofcomputernetwork4MainContentsoftheTextNetworkstructure

Networkadministrator

Topologies

StarnetworkGridnetwork

Busnetwork

Ringnetwork

Hybridnetwork

5Networkscomeinavarietyofsizesandcomplexities.Generally,thebigger,themorecomplex,andthemoredifficulttomanage.（1）LocalAreaNetwork（2）BridgedLAN（3）SwitchedLAN（4）CampusAreaNetwork（5）RoutedNetwork（6）WideAreaNetwork6TheInternetisahugeworldwideWANconsistingofthousandsofLANandmillionsofroutersandservers(Fig.5-2).7Networkadministratorhasfullrightsandpermissionstoallresourcesonanetwork.Theadministratorisusuallyresponsibleforinstalling,managing,andcontrollingserversandnetworkingcomponents.Administratorscanalsomodifythepropertiesofuseraccountsandthemembershipofgroups,createandmanageprinters,installprinters,shareresourcesandassignpermissionstothoseresources.8Thephysicaltopologydescribesthelayoutofthenetwork,justlikeamapshowsthelayoutofvariousroads,andthelogicaltopologydescribeshowthedataissentacrossthenetworkorhowthecarsareabletotravel(thedirectionandspeed)ateveryroadonthemap.Logicaltopologyisthemethodusedtopasstheinformationbetweenthecomputers.

（1）Starnetwork

（2）Gridnetwork

（3）Busnetwork

（4）Ringnetwork

（5）Hybridnetwork9KeyWords

accurate正确的，精确的annulus环backbone骨干，支柱contractor合同人，承包商diagnose诊断fashion风尚，时尚hierarchical分层的hybrid混合的merge使合并neighbor邻居，邻接10KeyWords

permission许可，允许regular常规的，合格的router路由器spreadsheet电子表格token令牌，记号topology拓扑，结构upgrade升级，上升unreliable不可靠的11Notes

[1]Withoutanetwork,thedocumentshavetobeprintedoutsothatotherscaneditthemorusethem.译文：没有网络，就必须将文档打印出来才能供他人编辑或使用。本句的“Withoutanetwork”是条件状语，“sothat”引导的是目的状语从句。12Notes

[2]Iftheuserweretoconnecthiscomputertoothercomputers,hecouldsharedataonothercomputers,includinghigh-qualityprinters.译文：如果用户能把他们的计算机与其他计算机联在一起的话，他们就可以共享其他计算机上的数据，包括高性能的打印机。本句由“If”引导条件状语从句，“includinghigh-qualityprinters”是补语。13Notes

[3]Arouterknowswherestuffis,evenifit’sseveralroutersaway.译文：路由器知道信息的位置，即使要经过几个路由器的距离。本句的“wherestuffis”是宾语从句，“evenifit’sseveralroutersaway”是让步状语从句。14Notes

[4]Agridnetworkisakindofcomputernetworkconsistingofanumberofcomputersystemsconnectedinagridtopology.译文：网状网络是一种计算机网络，它以网状拓扑结构将许多计算机系统连接在一起。本句的“consistingofanumberofcomputersystems”是现在分词短语作定语，修饰“network”，而“connectedinagridtopology”是过去分词短语作定语，修饰“computersystems”。15Notes

[5]Actually,thereareusuallytwocablessoadapterscanloopbacktoremarktheringifthereisabreak.译文：实际上，通常会有两条电缆，这样适配器就会在网络中出现故障时，可以沿环路返回重新构成环。本句的“soadapterscanloopbacktoremarkthering”是目的状语，而“ifthereisabreak”是条件状语。16ReadingMaterial——OpticalCommunicationOpticalcommunicationsystemsusehighcarrierfrequenciesinthevisibleornear-infraredregionoftheelectromagneticspectrum.Theyaresometimescalledlight-wavesystemstodistinguishthemfrommicrowavesystems,whosecarrierfrequencyistypicallysmallerbyfiveordersofmagnitude(1GHz).Opticalcommunicationsystemsdifferinprinciplefrommicrowavesystemsonlyinthefrequencyrangeofthecarrierwaveusedtocarrytheinformation.17Theapplicationofopticalfibercommunicationsisingeneralpossibleinanyareathatrequirestransferofinformationfromoneplacetoanother.However,fiber-opticcommunicationsystemshavebeendevelopedmostlyfortelecommunicationsapplications.Thetelecommunicationapplicationscanbebroadlyclassifiedintotwocategories,long-haulandshort-haul,dependingonwhethertheopticalsignalistransmittedoverrelativelylongorshortdistancescomparedwithtypicalintercitydistances(100km).185.2InformationSecurity

Securitythreatstoanetworkcanbedividedintothosethatinvolvesomesortofunauthorizedaccessandallothers.Oncesomeonegainsunauthorizedaccesstothenetwork,therangeofthingstheycandoislarge.Activeattacksincludealteringmessagecontents,masqueradingassomeoneelse,denialofservice,andplantingviruses.19MainContentsoftheTextManagement’sresponsibilityThenetworksecuritypolicyismanagement’sstatementoftheimportanceandtheircommitmenttonetworksecurity.Thepolicyneedstodescribeingeneraltermswhatwillbedone,butdoesnotdealwiththewaytheprotectionistobeachieved.20Thenetworksecuritypolicyneedstoclearlystatemanagement’spositionabouttheimportanceofnetworksecurityandtheitemsthataretobeprotected.Managementmustunderstandthatthereisnosuchthingasaperfectlysecurenetwork.Furthermore,networksecurityisaconstantlymovingtargetbecauseofadvancesintechnologyandthecreativityofpeoplewhowouldliketobreakintoanetworkoritsattachedcomputers.21AttackpreventionDifferentsecuritymechanismscanbeusedtoenforcethesecuritypropertiesdefinedinagivensecuritypolicy.Attackpreventionisaclassofsecuritymechanismsthatcontainswaysofpreventingordefendingagainstcertainattacksbeforetheycanactuallyreachandaffectthetarget.Animportantelementinthiscategoryisaccesscontrol,amechanismwhichcanbeappliedatdifferentlevelssuchastheoperatingsystem,thenetwork,ortileapplicationlayer.22Accesscontrollimitsandregulatestheaccesstocriticalresources.Thisisdonebyidentifyingorauthenticatingthepartythatrequestsaresourceandcheckingitspermissionsagainsttherightsspecifiedforthedemandedobject.Afirewallisanimportantaccesscontrolsystematthenetworklayer.Thefirewallisacentralchokepointthatallowsenforcementofaccesscontrolforservicesthatmayrunattheinsideoroutside.23AttackavoidanceSecuritymechanismsinthiscategoryassumethatanintrudermayaccessthedesiredresourcebuttheinformationismodifiedinawaythatmakesitunusablefortheattacker.Themostimportantmemberinthiscategoryiscryptography,whichisdefinedasthescienceofkeepingmessagesecure.24Theoriginalmessageiscalledplaintext.Theprocessofconvertingthismessagethroughtheapplicationofsometransformationrulesintoaformatthathidesitssubstanceiscalledencryption.Thecorrespondingdisguisedmessageisdenotedasciphertext,andtheoperationofturningitbackintoplaintextiscalleddecryption.Itisimportanttonoticethattheconversionfromplaintociphertexthastobelosslessinordertobeabletorecovertheoriginalmessageatthereceiverunderallcircumstances.25Thetransformationrulesaredescribedbyacryptographicalgorithm.Thefunctionofthisalgorithmisbasedontwomainprinciples:substitutionandtransposition.Mostsystemsinvolvemultiplestepsoftranspositionandsubstitutiontobemoreresistantagainstcryptanalysis.26KeyWords

anticipate预料，预测brute暴力的，粗暴的choke要塞，拥塞ciphertext密文creativity创造力，创新cryptographic加密的，密码的decode译码denial否认，拒绝denote表示，意味着disguise伪装，借口eavesdrop偷听，窃听27KeyWords

encryption加密hostile敌人，敌对分子interference冲突，干扰intruder闯入者，侵入者lossless无损的observation观察，观测overt公开的，公然的，明显的plaintext明文separation分离，隔离，隔开upgrade升级，提高28Notes[1]Thatrequiresfollowthroughandcommunicationwithallemployeessothattheyunderstandtheemphasisandimportanceseniormanagementisplacingonsecurity.译文：这需要后续通过与所有员工的沟通，使他们认识到高层管理人员在安全上布置的重点和重要性。本句的“sothat”引导目的状语从句。29Notes

[2]

Themostcommonformofaccesscontrolusedinmulti-usercomputersystemsareaccesscontrollistsforresourcesthatarebasedontheuserandgroupidentityoftheprocessthatattemptstousethem.译文：在多用户计算机系统中，最常用的访问控制形式是资源访问控制清单。清单是建立在进程中想要使用这些资源的用户和群的身份基础上的。本句的“usedinmulti-usercomputersystems”作定语，修饰“accesscontrol”，“thatarebased…”是修饰“accesscontrollists”的定语。30Notes

[3]

Whenaresourceshouldbeaccessed,thesystemlooksuptheuserandgroupintheaccesscontrollistandgrantsordeniesaccessasappropriate.译文：当一个资源被请求访问时，系统会在访问控制清单中查找用户和群并适当授权访问或拒绝访问。本句的“When”引导时间状语从句，“intheaccesscontrollist”作定语。31Notes

[4]

Thefirewallpreventsattacksfromtheoutsideagainstthemachinesintheinsidenetworkbydenyingconnectionattemptsfromunauthorizedpartieslocatedoutside,inaddition,afirewallmayalsobeutilizedtopreventusersbehindthefirewallfromusingcertainservicesthatareoutside.译文：防火墙通过拒绝来自外部的未授权方的连接企图，从而防止来自外部的对内部网络中的机器的攻击。除此之外，防火墙还可以用来阻止墙内用户使用外部服务。这是一个长句，由“inaddition”连接起两个并列子句，“bydenyingconnectionattemptsfromunauthorizedpartieslocatedoutside”是方式状语，“behindthe…”作定语。32Notes

[5]

Theinformationispreprocessedatthesenderbeforeitistransmittedoverthecommunicationchannelandpost-processedatthereceiver.译文：信息在通信渠道中传播之前由发送者进行了预处理，并且接收者收到信息后也要进行处理。本句采用被动语态，主语是“Theinformation”，谓语为“preprocessed”和“post-processed”，构成并列句，“beforeitistransmittedoverthecommunicationchannel”是时间状语。33ReadingMaterial——TechniquesforInternetSecurity

NetworkFirewallsThepurposeofanetworkfirewallistoprovideashellaroundthenetworkthatwillprotectthesystemsconnectedtothenetworkfromvariousthreats.Thetypesofthreatsafirewallcanprotectagainstinclude:UnauthorizedaccesstonetworkresourcesDenialofserviceMasquerading34Fig.5-3Networkstructureexample35DigitalCertificateandAuthenticationDigitalcertificateisanidentitycardcounterpartinthecomputersociety.Authenticationisanimportantpartofeverydaylife.Thelackofstrongauthenticationhasinhibitedthedevelopmentofelectroniccommerce.36DigitalSignatureAdigitalsignatureisanelectronicsignaturethatcanbeusedtoauthenticatetheidentityofthesenderofamessageorthesignerofadocument,andpossiblytoensurethattheoriginalcontentofthemessageordocumentthathasbeensentisunchanged.Adigitalsignatureisastringofbitsattachedtoanelectronicdocument,whichcouldbeawordprocessingfileoranE-mailmessage.375.3WirelessNetworks

Wirelesstechnologies,inthesimplestsense,enableoneormoredevicestocommunicatewithoutphysicalconnections—withoutrequiringnetworkorperipheralcabling.Wirelessnetworksserveasthetransportmechanismbetweendevicesandamongdevicesandthetraditionalwirednetworks(enterprisenetworksandtheInternet).38MainContentsoftheTextWirelesslocalareanetworks(WLAN)BluetoothWirelessnetworkhavemanyusessecureyourwirelessnetworkWirelessaccesspoints39Anaccesspointcommunicateswithdevicesequippedwithwirelessnetworkadaptors;itconnectstoawiredEthernetLANviaanRJ-45port.Accesspointdevicestypicallyhavecoverageareasofupto300feet(approximately100meters).Thiscoverageareaiscalledacellorrange.Usersmovefreelywithinthecellwiththeirlaptoporothernetworkdevice.Accesspointcellscanbelinkedtogethertoallowuserstoeven“roam”withinabuildingorbetweenbuildings.40Inrecentyears,wirelessnetworkinghasbecomemoreavailable,affordable,andeasytouse.Wirelessnetworkhavemanyuses.Forexample,youcansendE-mail,receivetelephonecallsandfaxesandreadremotedocumentsinyourtravel.Ifyouareusingwirelesstechnology,orconsideringmakingthemovetowireless,youshouldknowaboutthesecuritythreatsyoumayencounter.41So,ifyourneighborhoodiscloselysettled,orifyouliveinanapartmentorcondominium,failuretosecureyourwirelessnetworkcouldpotentiallyopenyourInternetconnectiontoasurprisingnumberofusers.Doingsoinvitesanumberofproblems:ServiceviolationsBandwidthshortagesAbusebymalicioususersMonitoringofyouractivityDirectattackonyourcomputer42Wirelessaccesspointscanannouncetheirpresencetowireless-enabledcomputers.Thisisreferredtoas“identifierbroadcasting”.Manypublicaccesspointsarenotsecured,andthetraffictheycarryisnotencrypted.Thiscanputyoursensitivecommunicationsortransactionsatrisk.43KeyWords

abuse欺骗affordable负担得起Bluetooth蓝牙cellular蜂窝的，多孔的cordless无绳的，不用电线的coverage范围，规模disaster灾害，天灾diverse多种多样的，不同的fashion方式，风格headphone耳机inability无能，无能为力44KeyWords

infrastructure基站，基础设施laptop微型便携式（电脑）fleet船队，车队malicious有恶意的，蓄意的nickname诨名，绰号repairperson维修人员roam漫游savvy精明的，有见识的unpredictable无法预言的violation侵害，违犯wavelength波长wireless无线的，无线电的45Notes

[1]Wirelesstechnologiesrangefromcomplexsystems,suchaswirelesslocalareanetworks(WLAN)andcellphonestosimpledevicessuchaswirelessheadphones,microphones,andotherdevicesthatdonotprocessorstoreinformation.译文：无线电技术的范围从复杂的系统如无线局域网和手机到简单的设备如无线耳机、麦克风以及其他不用来处理或储存信息的设备。本句有两处由“suchas”引导的同位语结构，“thatdo…”作定语。46Notes

[2]UnlikeatraditionalLAN,whichrequiresawiretoconnectauser’scomputertothenetwork,aWLANconnectcomputersandothercomponentstothenetworkusinganaccesspointdevice.译文：与需要一根线把用户的计算机与网络连接的传统局域网不同，无线局域网使用一个接入点装置把计算机和其他组成部分连接到网络。本句中“UnlikeatraditionalLAN”作状语，“which”引导的是非限定性定语从句，修饰“atraditionalLAN”，“usinganaccesspointdevice”作方式状语。47Notes

[3]Ifyoufailtosecureyourwirelessnetwork,anyonewithawireless-enabledcomputerwithinrangeofyourwirelessaccesspointcanhopafreerideontheInternetoveryourwirelessconnection.译文：如果你不能确保你的无线网络的安全，任何拥有一台能激活无线网络的计算机的人只要在你的无线接口范围之内就能通过你的无线连接免费上互联网。本句中“If”引导的是条件状语从句，“withawireless-enabledcomputerwithinrangeofyourwirelessaccesspoint”作定语，修饰主语“anyone”。48Notes

[4]Whenyouuseawirelessrouteroraccesspointtocreateahomenetwork,youtradewiredconnectivityforconnectivitydeliveredviaaradiosignal.译文：当你使用一个无线路由器或接入点去建立一个家庭网络时，你用有线连接交换到由无线电信号传送的连接。本句由“When”引导时间状语从句。49Notes

[5]Becauseyourconnectionisbeingtransmitted“intheclear”,malicioususerscanuse“sniffing”toolstoobtainsensitiveinformationsuchaspasswords,bankaccountnumbers,andcreditcardnumbers.译文：由于你的连接被畅通无阻地传送出去，恶意用户能够使用“嗅探”工具来获得敏感信息，诸如密码、银行帐号以及信用卡号。本句由“Because”引导原因状语从句，“suchas”后面的部分作为“sensitiveinformation”的同位语。50ReadingMaterial——InternetApplications

E-mailreferstothetransmissionofmessagesovercommunicationsnetworks.E-mailusestheofficememoparadigminwhichamessagecontainsaheaderthatspecifiesthesender,recipients,andsubject,followedbyabodythatcontainsthetextofthemessage.AnE-mailaddressisastringdividedintotwopartsbythe@character(pronouncedas“at”).Thefirstpartisamailboxidentifier,andthesecondstandsforthenameofthecomputeronwhichthemailboxresides.51Asearchtoolisacomputerprogramthatperformssearches.AsearchmethodisthewayasearchtoolrequestsandretrievesinformationfromitsWebsite.Asearchbeginsataselec