1第5章ComputerNetworkTechnology

5.1ComputerNetworkBasics5.2InformationSecurity5.3WirelessNetworks5.4專業(yè)英語(yǔ)應(yīng)用模塊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正確的，精確的annulus環(huán)backbone骨干，支柱contractor合同人，承包商diagnose診斷fashion風(fēng)尚，時(shí)尚hierarchical分層的hybrid混合的merge使合并neighbor鄰居，鄰接10KeyWords

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(hào)topology拓?fù)?，結(jié)構(gòu)upgrade升級(jí)，上升unreliable不可靠的11Notes

[1]Withoutanetwork,thedocumentshavetobeprintedoutsothatotherscaneditthemorusethem.譯文：沒(méi)有網(wǎng)絡(luò)，就必須將文檔打印出來(lái)才能供他人編輯或使用。本句的“Withoutanetwork”是條件狀語(yǔ)，“sothat”引導(dǎo)的是目的狀語(yǔ)從句。12Notes

[2]Iftheuserweretoconnecthiscomputertoothercomputers,hecouldsharedataonothercomputers,includinghigh-qualityprinters.譯文：如果用戶能把他們的計(jì)算機(jī)與其他計(jì)算機(jī)聯(lián)在一起的話，他們就可以共享其他計(jì)算機(jī)上的數(shù)據(jù)，包括高性能的打印機(jī)。本句由“If”引導(dǎo)條件狀語(yǔ)從句，“includinghigh-qualityprinters”是補(bǔ)語(yǔ)。13Notes

[3]Arouterknowswherestuffis,evenifit’sseveralroutersaway.譯文：路由器知道信息的位置，即使要經(jīng)過(guò)幾個(gè)路由器的距離。本句的“wherestuffis”是賓語(yǔ)從句，“evenifit’sseveralroutersaway”是讓步狀語(yǔ)從句。14Notes

[4]Agridnetworkisakindofcomputernetworkconsistingofanumberofcomputersystemsconnectedinagridtopology.譯文：網(wǎng)狀網(wǎng)絡(luò)是一種計(jì)算機(jī)網(wǎng)絡(luò)，它以網(wǎng)狀拓?fù)浣Y(jié)構(gòu)將許多計(jì)算機(jī)系統(tǒng)連接在一起。本句的“consistingofanumberofcomputersystems”是現(xiàn)在分詞短語(yǔ)作定語(yǔ)，修飾“network”，而“connectedinagridtopology”是過(guò)去分詞短語(yǔ)作定語(yǔ)，修飾“computersystems”。15Notes

[5]Actually,thereareusuallytwocablessoadapterscanloopbacktoremarktheringifthereisabreak.譯文：實(shí)際上，通常會(huì)有兩條電纜，這樣適配器就會(huì)在網(wǎng)絡(luò)中出現(xiàn)故障時(shí)，可以沿環(huán)路返回重新構(gòu)成環(huán)。本句的“soadapterscanloopbacktoremarkthering”是目的狀語(yǔ)，而“ifthereisabreak”是條件狀語(yǔ)。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預(yù)料，預(yù)測(cè)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eavesdrop偷聽(tīng)，竊聽(tīng)27KeyWords

encryption加密hostile敵人，敵對(duì)分子interference沖突，干擾intruder闖入者，侵入者lossless無(wú)損的observation觀察，觀測(cè)overt公開(kāi)的，公然的，明顯的plaintext明文separation分離，隔離，隔開(kāi)upgrade升級(jí)，提高28Notes[1]Thatrequiresfollowthroughandcommunicationwithallemployeessothattheyunderstandtheemphasisandimportanceseniormanagementisplacingonsecurity.譯文：這需要后續(xù)通過(guò)與所有員工的溝通，使他們認(rèn)識(shí)到高層管理人員在安全上布置的重點(diǎn)和重要性。本句的“sothat”引導(dǎo)目的狀語(yǔ)從句。29Notes

[2]

Themostcommonformofaccesscontrolusedinmulti-usercomputersystemsareaccesscontrollistsforresourcesthatarebasedontheuserandgroupidentityoftheprocessthatattemptstousethem.譯文：在多用戶計(jì)算機(jī)系統(tǒng)中，最常用的訪問(wèn)控制形式是資源訪問(wèn)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(yǔ)，修飾“accesscontrol”，“thatarebased…”是修飾“accesscontrollists”的定語(yǔ)。30Notes

[3]

Whenaresourceshouldbeaccessed,thesystemlooksuptheuserandgroupintheaccesscontrollistandgrantsordeniesaccessasappropriate.譯文：當(dāng)一個(gè)資源被請(qǐng)求訪問(wèn)時(shí)，系統(tǒng)會(huì)在訪問(wèn)控制清單中查找用戶和群并適當(dāng)授權(quán)訪問(wèn)或拒絕訪問(wèn)。本句的“When”引導(dǎo)時(shí)間狀語(yǔ)從句，“intheaccesscontrollist”作定語(yǔ)。31Notes

[4]

Thefirewallpreventsattacksfromtheoutsideagainstthemachinesintheinsidenetworkbydenyingconnectionattemptsfromunauthorizedpartieslocatedoutside,inaddition,afirewallmayalsobeutilizedtopreventusersbehindthefirewallfromusingcertainservicesthatareoutside.譯文：防火墻通過(guò)拒絕來(lái)自外部的未授權(quán)方的連接企圖，從而防止來(lái)自外部的對(duì)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(lái)阻止墻內(nèi)用戶使用外部服務(wù)。這是一個(gè)長(zhǎng)句，由“inaddition”連接起兩個(gè)并列子句，“bydenyingconnectionattemptsfromunauthorizedpartieslocatedoutside”是方式狀語(yǔ)，“behindthe…”作定語(yǔ)。32Notes

[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(dòng)語(yǔ)態(tài)，主語(yǔ)是“Theinformation”，謂語(yǔ)為“preprocessed”和“post-processed”，構(gòu)成并列句，“beforeitistransmittedoverthecommunicationchannel”是時(shí)間狀語(yǔ)。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欺騙affordable負(fù)擔(dān)得起B(yǎng)luetooth藍(lán)牙cellular蜂窩的，多孔的cordless無(wú)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fashion方式，風(fēng)格headphone耳機(jī)inability無(wú)能，無(wú)能為力44KeyWords

infrastructure基站，基礎(chǔ)設(shè)施laptop微型便攜式（電腦）fleet船隊(duì)，車隊(duì)malicious有惡意的，蓄意的nickname諢名，綽號(hào)repairperson維修人員roam漫游savvy精明的，有見(jiàn)識(shí)的unpredictable無(wú)法預(yù)言的violation侵害，違犯wavelength波長(zhǎng)wireless無(wú)線的，無(wú)線電的45Notes

[1]Wirelesstechnologiesrangefromcomplexsystems,suchaswirelesslocalareanetworks(WLAN)andcellphonestosimpledevicessuchaswirelessheadphones,microphones,andotherdevicesthatdonotprocessorstoreinformation.譯文：無(wú)線電技術(shù)的范圍從復(fù)雜的系統(tǒng)如無(wú)線局域網(wǎng)和手機(jī)到簡(jiǎn)單的設(shè)備如無(wú)線耳機(jī)、麥克風(fēng)以及其他不用來(lái)處理或儲(chǔ)存信息的設(shè)備。本句有兩處由“suchas”引導(dǎo)的同位語(yǔ)結(jié)構(gòu)，“thatdo…”作定語(yǔ)。46Notes

[2]UnlikeatraditionalLAN,whichrequiresawiretoconnectauser’scomputertothenetwork,aWLANconnectcomputersandothercomponentstothenetworkusinganaccesspointdevice.譯文：與需要一根線把用戶的計(jì)算機(jī)與網(wǎng)絡(luò)連接的傳統(tǒng)局域網(wǎng)不同，無(wú)線局域網(wǎng)使用一個(gè)接入點(diǎn)裝置把計(jì)算機(jī)和其他組成部分連接到網(wǎng)絡(luò)。本句中“UnlikeatraditionalLAN”作狀語(yǔ)，“which”引導(dǎo)的是非限定性定語(yǔ)從句，修飾“atraditionalLAN”，“usinganaccesspointdevice”作方式狀語(yǔ)。47Notes

[3]Ifyoufailtosecureyourwirelessnetwork,anyonewithawireless-enabledcomputerwithinrangeofyourwirelessaccesspointcanhopafreerideontheInternetoveryourwirelessconnection.譯文：如果你不能確保你的無(wú)線網(wǎng)絡(luò)的安全，任何擁有一臺(tái)能激活無(wú)線網(wǎng)絡(luò)的計(jì)算機(jī)的人只要在你的無(wú)線接口范圍之內(nèi)就能通過(guò)你的無(wú)線連接免費(fèi)上互聯(lián)網(wǎng)。本句中“If”引導(dǎo)的是條件狀語(yǔ)從句，“withawireless-enabledcomputerwithinrangeofyourwirelessaccesspoint”作定語(yǔ)，修飾主語(yǔ)“anyone”。48Notes

[4]Whenyouuseawirelessrouteroraccesspointtocreateahomenetwork,youtradewiredconnectivityforconnectivitydeliveredviaaradiosignal.譯文：當(dāng)你使用一個(gè)無(wú)線路由器或接入點(diǎn)去建立一個(gè)家庭網(wǎng)絡(luò)時(shí)，你用有線連接交換到由無(wú)線電信號(hào)傳送的連接。本句由“When”引導(dǎo)時(shí)間狀語(yǔ)從句。49Notes

[5]Becauseyourconnectionisbeingtransmitted“intheclear”,malicioususerscanuse“sniffing”toolstoobtainsensitiveinformationsuchaspasswords,bankaccountnumbers,andcreditcardnumbers.譯文：由于你的連接被暢通無(wú)阻地傳送出去，惡意用戶能夠使用“嗅探”工具來(lái)獲得敏感信息，諸如密碼、銀行帳號(hào)以及信用卡號(hào)。本句由“Because”引導(dǎo)原因狀語(yǔ)從句，“suchas”后面的部分作為“sensitiveinformation”的同位語(yǔ)。50ReadingMaterial——InternetApplications

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