《物联网专业英语教程》课件第2章

Unit2Internet[23]TextAInternet[30]TextBNetworkDevice[37]参考译文因特网

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.因特网是如何工作的？

即使因特网仍然是年轻的技术，也难以想象现在没有因特网的生活景象。每年，工程师都会设计更多与因特网结合的设备。这个网中之网已经跨越了地球甚至扩展到太空中。但是是什么使它工作的呢？网络的结构图如图2.1所示。

要理解因特网，首先应把它看做是有两个主要部分的系统。第一部分是硬件，包括从每秒可以携带兆兆位字节的电缆到你面前的计算机。因特网其他支持因特网的硬件包括路由器、服务器、蜂窝电话塔、卫星、无线电、智能电话和其他设备。所有这些设备共同建立了这个网中之网。因特网是一个可延展性的系统—部件的增加和减少对其影响不大。这些部件中的一些保持不变，是构成因特网的骨干。除此之外，还有其他的一些外围部件。

这些外围部件是可连接的。一些是终点—计算机、