《物聯(lián)網(wǎng)專業(yè)英語教程》課件第2章

Unit2Internet[23]TextAInternet[30]TextBNetworkDevice[37]參考譯文因特網(wǎng)

1.HowdoestheInternetWork?

EventhoughtheInternetisstillayoungtechnology,it'shardtoimaginelifewithoutitnow.Everyyear,engineerscreatemoredevicestointegratewiththeInternet.Thisnetworkofnetworkscrisscrossestheglobeandevenextendsintospace.Butwhatmakesitwork?(SeeFigure2.1)InternetTextAFigure2.1Internetarchitecture

TounderstandtheInternet,ithelpstolookatitasasystemwithtwomaincomponents.Thefirstofthosecomponentsishardware.Thatincludeseverythingfromthecablesthatcarryterabitsofinformationeverysecondtothecomputersittinginfrontofyou.

OthertypesofhardwarethatsupporttheInternetincluderouters,servers,cellphonetowers,satellites,radios,smartphonesandotherdevices.Allthesedevicestogethercreatethenetworkofnetworks.TheInternetisamalleablesystem—itchangesinlittlewaysaselementsjoinandleavenetworksaroundtheworld.SomeofthoseelementsmaystayfairlystaticandmakeupthebackboneoftheInternet.Othersaremoreperipheral.

Theseelementsareconnections.Someareendpoints—thecomputer,smartphoneorotherdeviceyou'reusingtoreadthismaycountasone.Wecallthoseendpointsclients.MachinesthatstoretheinformationweseekontheInternetareservers.Otherelementsarenodeswhichserveasaconnectingpointalongarouteoftraffic.Andthentherearethetransmissionlineswhichcanbephysical,asinthecaseofcablesandfiberoptics.

Allofthishardwarewouldn'tcreateanetworkwithoutthesecondcomponentoftheInternet:theprotocols.Protocolsaresetsofrulesthatmachinesfollowtocompletetasks.WithoutacommonsetofprotocolsthatallmachinesconnectedtotheInternetmustfollow,communicationbetweendevicescouldn'thappen.Thevariousmachineswouldbeunabletounderstandoneanotherorevensendinformationinameaningfulway.Theprotocolsprovideboththemethodandacommonlanguageformachinestousetotransmitdata.

2.AMatterofProtocols

You'veprobablyheardofseveralprotocolsontheInternet.Forexample,hypertexttransferprotocoliswhatweusetoviewWebsitesthroughabrowser—that'swhatthehttpatthefrontofanyWebaddressstandsfor.Ifyou'veeverusedanFTPserver,youreliedonthefiletransferprotocol.ProtocolsliketheseanddozensmorecreatetheframeworkwithinwhichalldevicesmustoperatetobepartoftheInternet.

TwoofthemostimportantprotocolsaretheTransmissionControlProtocol(TCP)andtheInternetProtocol(IP).Weoftengroupthetwotogether—inmostdiscussionsaboutInternetProtocolsyou'llseethemlistedasTCP/IP.

Whatdotheseprotocolsdo?Attheirmostbasiclevel,theseprotocolsestablishtherulesforhowinformationpassesthroughtheInternet.Withouttheserules,youwouldneeddirectconnectionstoothercomputerstoaccesstheinformationtheyhold.You'dalsoneedbothyourcomputerandthetargetcomputertounderstandacommonlanguage.

You'veprobablyheardofIPaddresses.TheseaddressesfollowtheInternetprotocol.EachdeviceconnectedtotheInternethasanIPaddress.Thisishowonemachinecanfindanotherthroughthemassivenetwork.

TheversionofIPmostofususetodayisIPv4,whichisbasedona32-bitaddresssystem.There'sonebigproblemwiththissystem:We'rerunningoutofaddresses.That'swhytheInternetEngineeringTaskForce(IETF)decidedbackin1991thatitwasnecessarytodevelopanewversionofIPtocreateenoughaddressestomeetdemand.TheresultwasIPv6,a128-bitaddresssystem.ThathasenoughaddressestoaccommodatetherisingdemandforInternetaccessfortheforeseeablefuture.

Whenyouwanttosendamessageorretrieveinformationfromanothercomputer,theTCP/IPprotocolsarewhatmakethetransmissionpossible.Yourrequestgoesoutoverthenetwork,hittingDomainNameServers(DNS)alongthewaytofindthetargetserver.TheDNSpointstherequestintherightdirection.Oncethetargetserverreceivestherequest,itcansendaresponsebacktoyourcomputer.Thedatamighttravelacompletelydifferentpathtogetbacktoyou.ThisflexibleapproachtodatatransferispartofwhatmakestheInternetsuchapowerfultool.

3．Packet

Inordertoretrievethisarticle,yourcomputerhastoconnectwiththeWebservercontainingthearticle'sfile.We'llusethatasanexampleofhowdatatravelsacrosstheInternet.

First,youopenyourWebbrowserandconnecttoourWebsite.Whenyoudothis,yourcomputersendsanelectronicrequestoveryourInternetconnectiontoyourInternetServiceProvider(ISP).TheISProutestherequesttoaserverfurtherupthechainontheInternet.Eventually,therequestwillhitaDomainNameServer(DNS).Thisserverwilllookforamatchforthedomainnameyou'vetypedin(suchaswww.).Ifitfindsamatch,itwilldirectyourrequesttotheproperserver'sIPaddress.Ifitdoesn'tfindamatch,itwillsendtherequestfurtherupthechaintoaserverthathasmoreinformation.

TherequestwilleventuallycometoourWebserver.Ourserverwillrespondbysendingtherequestedfileinaseriesofpackets.Packetsarepartsofafilethatrangebetween1,000and1,500bytes.Packetshaveheadersandfootersthattellcomputerswhat'sinthepacketandhowtheinformationfitswithotherpacketstocreateanentirefile.Eachpackettravelsbackupthenetworkanddowntoyourcomputer.Packetsdon'tnecessarilyalltakethesamepath—they'llgenerallytravelthepathofleastresistance.

That'sanimportantfeature.Becausepacketscantravelmultiplepathstogettotheirdestination,it'spossibleforinformationtoroutearoundcongestedareasontheInternet.Infact,aslongassomeconnectionsremain,informationcouldstilltravelfromonesectiontoanother—thoughitmighttakelongerthannormal.

Whenthepacketsgettoyou,yourdevicearrangesthemaccordingtotherulesoftheprotocols.It'skindoflikeputtingtogetherajigsawpuzzle.Theendresultisthatyouseethisarticle.

Thisholdstrueforotherkindsoffilesaswell.Whenyousendane-mail,itgetsbrokenintopacketsbeforezoomingacrosstheInternet.PhonecallsovertheInternetalsoconvertconversationsintopacketsusingtheVoiceoverInternetProtocol(VoIP).WecanthanknetworkpioneerslikeVintonCerfandRobertKahnfortheseprotocols—theirearlyworkhelpedbuildasystemthat'sbothscalableandrobust.

That'showtheInternetworksinanutshell.Asyoulookcloseratthevariousdevicesandprotocols,you'llnoticethatthepictureisfarmorecomplexthantheoverviewwe'vegiven.

1.?Firewall

Afirewallisasetofrelatedprogramslocatedatanetworkgatewayserver.Itprotectstheresourcesofaprivatenetworkfromusersfromothernetworks.Thetermalsoimpliesthesecuritypolicythatisusedwiththeprograms.NetworkDeviceAnenterprisewithanintranetthatallowsitsworkerstohaveaccesstothewiderInternetinstallsafirewalltopreventoutsidersfromaccessingitsownprivatedataresourcesandforcontrollingwhatoutsideresourcesitsownusershaveaccessto.

Basically,afirewall,workingcloselywitharouterprogram,examineseachnetworkpackettodeterminewhethertoforwardittowarditsdestination.Afirewallalsoincludesorworkswithaproxyserverthatmakesnetworkrequestsonbehalfofworkstationusers.

Afirewallisofteninstalledinaspeciallydesignatedcomputerseparatefromtherestofthenetworksothatnoincomingrequestcangetdirectlyatprivatenetworkresources.

Thereareanumberoffirewallscreeningmethods.Asimpleoneistoscreenrequeststomakesuretheycomefromacceptable(previouslyidentified)domainnameandInternetProtocoladdresses.Formobileusers,firewallsallowremoteaccessintotheprivatenetworkbytheuseofsecurelogonproceduresandauthenticationcertificates.

Anumberofcompaniesmakefirewallproducts.Featuresincludeloggingandreporting,automaticalarmsatgiventhresholdsofattack,andagraphicaluserinterfaceforcontrollingthefirewall.

Computersecurityborrowsthistermfromfirefighting,whereitoriginated.Infirefighting,afirewallisabarrierestablishedtopreventthespreadoffire.

2.Gateway

Agatewayisanetworkpointthatactsasanentrancetoanothernetwork.OntheInternet,anodeorstoppingpointcanbeeitheragatewaynodeorahost(end-point)node.BoththecomputersofInternetusersandthecomputersthatservepagestousersarehostnodes.Thecomputersthatcontroltrafficwithinyourcompany'snetworkoratyourlocalInternetServiceProvider(ISP)aregatewaynodes.

Inthenetworkforanenterprise,acomputerserveractingasagatewaynodeisoftenalsoactingasaproxyserverandafirewallserver.Agatewayisoftenassociatedwithbotharouter,whichknowswheretodirectagivenpacketofdatathatarrivesatthegateway,andaswitch,whichfurnishestheactualpathinandoutofthegatewayforagivenpacket.

3.Router

Inpacket-switchednetworkssuchastheInternet,arouterisadeviceor,insomecases,softwareinacomputerthatdeterminesthenextnetworkpointtowhichapacketshouldbeforwardedtowarditsdestination.Therouterisconnectedtoatleasttwonetworksanddecideswhichwaytosendeachinformationpacketbasedonitscurrentunderstandingofthestateofthenetworksitisconnectedto.Arouterislocatedatanygatewaywhereonenetworkmeetsanother,includingeachpoint-of-presenceontheInternet.Arouterisoftenincludedaspartofanetworkswitch.

Aroutermaycreateormaintainatableoftheavailableroutesandtheirconditionsandusethisinformationalongwithdistanceandcostalgorithmstodeterminethebestrouteforagivenpacket.Typically,apacketmaytravelthroughanumberofnetworkpointswithroutersbeforearrivingatitsdestination.RoutingisafunctionassociatedwiththeNetworklayer(layer3)intheOpenSystemsInterconnection(OSI)model.Alayer-3switchisaswitchthatcanperformroutingfunctions.

AnedgerouterisarouterthatinterfaceswithanAsynchronousTransferMode(ATM)[1]network.Abrouterisanetworkbridgecombinedwitharouter.

4.Bridge

Intelecommunicationnetworks,abridgeisaproductthatconnectsaLocalAreaNetwork(LAN)toanotherlocalareanetworkthatusesthesameprotocol(forexample,EthernetorTokenRing[1]).Youcanenvisionabridgeasbeingadevicethatdecideswhetheramessagefromyoutosomeoneelseisgoingtothelocalareanetworkinyourbuildingortosomeoneonthelocalareanetworkinthebuildingacrossthestreet.AbridgeexamineseachmessageonaLAN,"passes"thoseknowntobewithinthesameLAN,andforwardsthoseknowntobeontheotherinterconnectedLAN(orLANs).

Inbridgingnetworks,computerornodeaddresseshavenospecificrelationshiptolocation.Forthisreason,messagesaresentouttoeveryaddressonthenetworkandacceptedonlybytheintendeddestinationnode.Bridgeslearnwhichaddressesareonwhichnetworkanddevelopalearningtablesothatsubsequentmessagescanbeforwardedtotherightnetwork.

Bridgingnetworksaregenerallyalwaysinterconnectedlocalareanetworkssincebroadcastingeverymessagetoallpossibledestinationswouldfloodalargernetworkwithunnecessarytraffic.Forthisreason,routernetworkssuchastheInternetuseaschemethatassignsaddressestonodessothatamessageorpacketcanbeforwardedonlyinonegeneraldirectionratherthanforwardedinalldirections.

Abridgeworksatthedata-link(physicalnetwork)levelofanetwork,copyingadataframefromonenetworktothenextnetworkalongthecommunicationspath.

Abridgeissometimescombinedwitharouterinaproductcalledabrouter.

5.Hub

Ingeneral,ahubisthecentralpartofawheelwherethespokescometogether.Thetermisfamiliartofrequentflierswhotravelthroughairport"hubs"tomakeconnectingflightsfromonepointtoanother.Indatacommunications,ahubisaplaceofconvergencewheredataarrivesfromoneormoredirectionsandisforwardedoutinoneormoreotherdirections.Ahubusuallyincludesaswitchofsomekind.Andaproductthatiscalleda"switch"couldusuallybeconsideredahubaswell.Thedistinctionseemstobethatthehubistheplacewheredatacomestogetherandtheswitchiswhatdetermineshowandwheredataisforwardedfromtheplacewheredatacomestogether.Regardedinitsswitchingaspects,ahubcanalsoincludearouter.

Indescribingnetworktopologies,ahubtopologyconsistsofabackbone(maincircuit)towhichanumberofoutgoinglinescanbeattached("dropped"),

eachprovidingoneormoreconnectionportfordevicetoattachto.ForInternetusersnotconnectedtoalocalareanetwork,thisisthegeneraltopologyusedbyyouraccessprovider.Othercommonnetworktopologiesarethebusnetworkandtheringnetwork.(Eitherofthesecouldpossiblyfeedintoahubnetwork,usingabridge.)

Asanetworkproduct,ahubmayincludeagroupofmodemcardsfordial-inusers,agatewaycardforconnectionstoalocalareanetwork(forexample,anEthernetoraTokenRing),andaconnectiontoaline.

6.Switch

Inatelecommunicationsnetwork,aswitchisadevicethatchannelsincomingdatafromanyofmultipleinputportstothespecificoutputportthatwilltakethedatatowarditsintendeddestination.Inthetraditionalcircuit-switchedtelephonenetwork,oneormoreswitchesareusedtosetupadedicatedthoughtemporaryconnectionorcircuitforanexchangebetweentwoormoreparties.OnanEthernetlocalareanetwork(LAN),aswitchdeterminesfromthephysicaldevice(MediaAccessControlorMAC)addressineachincomingmessageframewhichoutputporttoforwardittoandoutof.Inawideareapacked-switchednetworksuchastheInternet,aswitchdeterminesfromtheIPaddressineachpacketwhichoutputporttouseforthenextpartofitstriptotheintendeddestination.

IntheOpenSystemsInterconnection(OSI)communicationsmodel,aswitchperformsthelayer2orData-Linklayerfunction.Thatis,itsimplylooksateachpacketordataunitanddeterminesfromaphysicaladdress(the"MACaddress")whichdeviceadataunitisintendedforandswitchesitouttowardthatdevice.However,inwideareanetworkssuchastheInternet,thedestinationaddressrequiresalook-upinaroutingtablebyadeviceknownasarouter.Somenewerswitchesalsoperformroutingfunctions(layer3ortheNetworklayerfunctionsinOSI)andaresometimescalledIPswitches.

Onlargernetworks,thetripfromoneswitchpointtoanotherinthenetworkiscalledahop.Thetimeaswitchtakestofigureoutwheretoforwardadataunitiscalleditslatency.Thepricepaidforhavingtheflexibilitythatswitchesprovideinanetworkisthislatency.Switchesarefoundatthebackboneandgatewaylevelsofanetworkwhereonenetworkconnectswithanotherandatthesubnetworklevelwheredataisbeingforwardedclosetoitsdestinationororigin.Theformerareoftenknownascoreswitchesandthelatterasdesktopswitches.

Inthesimplestnetworks,aswitchisnotrequiredformessagesthataresentandreceivedwithinthenetwork.Forexample,alocalareanetworkmaybeorganizedinaTokenRingorbusarrangementinwhicheachpossibledestinationinspectseachmessageandreadsanymessagewithitsaddress.

1.因特網(wǎng)是如何工作的？

即使因特網(wǎng)仍然是年輕的技術(shù)，也難以想象現(xiàn)在沒有因特網(wǎng)的生活景象。每年，工程師都會(huì)設(shè)計(jì)更多與因特網(wǎng)結(jié)合的設(shè)備。這個(gè)網(wǎng)中之網(wǎng)已經(jīng)跨越了地球甚至擴(kuò)展到太空中。但是是什么使它工作的呢？網(wǎng)絡(luò)的結(jié)構(gòu)圖如圖2.1所示。

要理解因特網(wǎng)，首先應(yīng)把它看做是有兩個(gè)主要部分的系統(tǒng)。第一部分是硬件，包括從每秒可以攜帶兆兆位字節(jié)的電纜到你面前的計(jì)算機(jī)。因特網(wǎng)其他支持因特網(wǎng)的硬件包括路由器、服務(wù)器、蜂窩電話塔、衛(wèi)星、無線電、智能電話和其他設(shè)備。所有這些設(shè)備共同建立了這個(gè)網(wǎng)中之網(wǎng)。因特網(wǎng)是一個(gè)可延展性的系統(tǒng)—部件的增加和減少對(duì)其影響不大。這些部件中的一些保持不變，是構(gòu)成因特網(wǎng)的骨干。除此之外，還有其他的一些外圍部件。

這些外圍部件是可連接的。一些是終點(diǎn)—計(jì)算機(jī)、