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外文翻译--子网络.doc

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外文翻译--子网络.doc

原文部分INMOSTWIDEAREANETWORKS,THESUBNETCONSISTSOFTWODISTINCTCOMPONENTSTRANSMISSIONLINESANDSWITCHINGELEMENTSTRANSMISSIONLINESALSOCALLEDCIRCUITS,CHANNELS,ORTRUNKSMOVEBITSBETWEENMACHINESTHESWITCHINGELEMENTSARESPECIALIZEDCOMPUTERSUSEDTOCONNECTTWOORMORETRANSMISSIONLINEDWHENDATAARRIVEONANINCOMINGLINE,THESWITCHINGELEMENTMUSTCHOOSEANOUTGOINGLINETOFORWARDTHEMONUNFORTUNATELY,THEREISNOSTANDARDTERMINOLOGYUSEDTONAMETHESECOMPUTERSTHEYAREVARIOUSLYCALLEDPACKETSWITCHINGEXCHANGES,AMONGOTHERTHINGSASAGENERICTERMFORTHESWITCHINGCOMPUTERS,WILLUSEEXISTSHEREINTHISMODEL,SHOWNINFIG95,EACHHOSTISGENERALLYCONNECTEDTOALANONAROUTERTHECOLLECTIONOFCOMMUNICATIONLINESANDROUTERSBUTNOTTHEHOSTSFORMTHESUBNETANASIDEABOUTTHETERM“SUBNET”ISWORTHMAKINGORIGINALLY,ITSONLYMEANINGWASTHECOLLECTIONOFROUTERSANDCOMMUNICATIONLINESTHATMOVEDPACKETSFROMTHESOURCEHOSTTOTHEDESTINATIONHOSTHOWEVER,NETWORKADDRESSINGHENCETHETERMHASACERTAINAMBIGUITYABOUTITUNFORTUNATELY,NOWIDELYUSEDALTERNATIVEEXISTSFORITSINITIALMEANING,SOWITHSOMEHESITATIONWEWILLUSEITINBOTHSENSESFROMTHECONTEXT,ITWILLALWAYSBECLEARWHICHISMEANTINMOSTWANS,THENETWORKCONTAINSNUMEROUSCABLESORTELEPHONELINES,EACHONECONNECTINGAPAIROFROUTERSIFTWOROUTERSTHATDONOTSHAREACABLENEVERTHELESSWISHTOCOMMUNICATE,THEYMUSTDOTHISINDIRECTLY,VIAOTHERROUTERSWHENAPACKETISSENTFROMONEROUTERTOANOTHERVIAONEORMOREINTERMEDIATEROUTERS,THEPACKETISRECEIVEDATEACHINTERMEDIATEROUTERINITSENTIRETY,STOREDTHEREUNTILTHEREQUIREDOUTPUTLINEISFREE,ANDTHENFORWARDEDASUBNETUSINGTHISPRINCIPLEISCALLEDAPOINTTOPOINT,STOREANDFORWARD,ORPACKETSWITCHEDSUBNETNEARLYALLWIDEAREANETWORKSEXCEPTTHOSEUSINGSATELLITESHAVESTOREANDFORWARDSUBNETSWHENTHEPACKETSARESMALLANDALLTHESAMESIZE,THEYAREOFTENCALLEDCELLSWHENAPOINTTOPOINTSUBNETISUSED,ANIMPORTANTDESIGNISSUEISWHATTHEROUTERINTERCONNECTIONTOPOLOGYSHOULDLOOKLIKEFIG96SHOWSSEVERALPOSSIBLETOPOLOGIESLOCALNETWORKSTHATWEREDESIGNEDASSUCHUSUALLYHAVEASYMMETRICTOPOLOGYINCONTRAST,WIDEAREANETWORKSTYPICALLYHAVEIRREGULARTOPOLOGIESASECONDPOSSIBILITYFORAWANISASATELLITEORGROUNDRADIOSYSTEMEACHROUTERHASANANTENNATHROUGHWHICHITCANSENDANDRECEIVEALLROUTERSCANHEARTHEOUTPUTFROMTHESATELLITE,ANDINSOMECASESTHEYCANALSOHEARTHEUPWARDTRANSMISSIONSOFTHEIRFELLOWROUTERSTOTHESATELLITEASWELLSOMETIMESTHEROUTERSARECONNECTEDTOASUBSTANTIALPOINTTOPOINTSUBNET,WITHONLYSOMEOFTHEMHAVINGASATELLITEANTENNASATELLITENETWORKSAREINHERENTLYBROADCASTANDAREMOSTUSEFULWHENTHEBROADCASTPROPERTYISIMPORTANT924WIRELESSNETWORKSMOBILECOMPUTERS,SUCHASNOTEBOOKCOMPUTERSANDPERSONALDIGITALASSISTANTSPDA,ARETHEFASTESTGROWINGSEGMENTOFTHECOMPUTERINDUSTRYMANYOFTHEOWNERSOFTHESECOMPUTERSHAVEDESKTOPMACHINESONLANSANDWANSBACKATTHEOFFICEANDWANTTOBECONNECTEDTOTHEIRHOMEBASEEVENWHENAWAYFROMHOMEORAROUTESINCEHAVINGAWIREDCONNECTIONISIMPOSSIBLEINCARSANDAIRPLANES,THEREISALOTOFINTERESTINWIRELESSNETWORKSONTHISSECTIONWEWILLBRIEFLYINTRODUCETHISTOPICACTUALLY,DIGITALWIRELESSCOMMUNICATIONISNOTANEWIDEAASEARLYAS1901,THEITALIANPHYSICISTGUGLIELMOMARCONIDEMONSTRATEDASHIPTOSHOREWIRELESSTELEGRAPHUSINGMORSECODEDOTSANDDASHESREBINARY,AFTERALLMODEMDIGITALWIRELESSSYSTEMSHAVEBETTERPERFORMANCE,BUTTHEBASICIDEAISTHESAMEWIRELESSNETWORKSHAVEMANYUSESACOMMONONEISTHEPORTABLEOFFICEPEOPLEONTHEROADOFTENWANTTOUSETHEIRPORTABLEELECTRONICEQUIPMENTTOSENDANDRECEIVETELEPHONECALLS,FAXES,ANDELECTRONICMAIL,READREMOTEFILES,LOGINONREMOTEMACHINES,ANDSOON,ANDDOTHISFROMANYWHEREONLAND,SEA,ORAIRWIRELESSNETWORKSARELFGREATVALUETOFLEETSOFTRUCKS,TAXIS,BUSES,ANDREPAIRPERSONSFORKEEPINGINCONTACTWITHHOMEANOTHERUSEISFORRESCUEWORKERSATDISASTERSITESFIRES,FLOODS,EARTHQUAKES,ETCWHERETHETELEPHONESYSTEMHASBEENDESTROYEDCOMPUTERSTHERECANSENDMESSAGES,KEEPRECORDS,ANDSOONFINALLY,WIRELESSNETWORKSAREIMPORTANTTOTHEMILITARYIFYOUHAVETOBEABLEFIGHTAWARANYWHEREONEARTHONSHORTNOTICE,COUNTINGONUSINGTHELOCALNETWORKINGINFRASTRUCTUREISPROBABLYNOTAGOODIDEAITISBETTERTOBRINGYOUROWNALTHOUGHWIRELESSNETWORKINGANDMOBILECOMPUTINGAREOFTENRELATED,THEYARENOTIDENTICAL,ASFIG97SHOWSPORTABLECOMPUTERSARESOMETIMESWIREDFOREXAMPLE,IFATRAVELERPLUGSAPLUGSAPORTABLECOMPUTERINTOTHETELEPHONEJACKINAHOTEL,WEHAVEMOBILITYWITHOUTAWIRELESSNETWORKANOTHEREXAMPLEISSOMEONECARRYINGAPORTABLECOMPUTERALONGASHEINSPECTSATRAINFORTECHNICALPROBLEMSHEREALONGCORDCANTRAILALONGBEHINDVACUUMCLEANERMODELONTHEOTHERHAND,SOMEWIRELESSCOMPUTERSARENOTPORTABLEANIMPORTANTEXAMPLEHEREISACOMPANYTHATOWNSANOLDERBUILDINGTHATDOESNOTHAVENETWORKCABLINGINSTALLEDANDWANTSTOCONNECTITSCOMPUTERSINSTALLINGAWIRELESSLANMAYREQUIRELITTLEMORETHANBUYINGASMALLBOXWITHSOMEELECTRONICSANDSETTINGUPSOMEANTENNASTHISSOLUTIONMAYBECHEAPERTHANWIRINGTHEBUILDINGALTHOUGHWIRELESSLANSAREEASYTOINSTALL,THEYALSOHAVESOMEDISADVANTAGESTYPICALLYTHEYHAVEACAPACITYOF1TO2MBPS,WHICHISMUCHSLOWERTHANWIREDLANSTHEERRORRATESAREOFTENMUCHHIGHER,TOO,ANDTHETRANSMISSIONSFROMDIFFERENTCOMPUTERSCANINTERFEREWITHONEANOTHERBUTOFCOURSE,THEREAREALSOTHETRUEMOBILE,WIRELESSAPPLICATIONS,RANGINGFROMTHEPORTABLEOFFICETOPEOPLEWALKINGAROUNDASTOREWITHAPDADOINGINVENTORYATMANYBUSYAIRPORTS,CARRENTALRETURNCLERKSWORKOUTINTHEPARKINGLOTWITHWIRELESSPORTABLECOMPUTERSTHEYTYPEINABUILTINPRINTER,CALLSTHEMAINCOMPUTER,GETSTHERENTALINFORMATION,ANDPRINTSOUTTHEBILLONTHESPOTWIRELESSNETWORKSCOMEINMANYFORMSSOMEUNIVERSITIESAREALREADYINSTALLINGANTENNASALLOVERCAMPUSTOALLOWSTUDENTSTOSITUNDERTHETREESANDCONSULTTHELIBRARY’SCARDCATALOGHERETHECOMPUTERSCOMMUNICATEDIRECTLYWITHTHEWIRELESSLANINDIGITALFORMANOTHERPOSSIBILITYISUSINGACELLULARIEPORTABLETELEPHONEWITHATRADITIONALANALOGMODEMDIRECTDIGITALCELLARSERVICE,CALLEDCDPDCELLULARDIGITALPACKETDATAISBECOMINGAVAILABLEINMANYCITIESFINALLY,ITISPOSSIBLETOHAVEDIFFERENTCOMBINATIONSOFWIREDANDWIRELESSNETWORKINGFOREXAMPLE,INFIG98A,WEDEPICTANAIRPLANEWITHANUMBEROFPEOPLEUSINGMODEMSANDSEATBACKTELEPHONESTOCALLTHEOFFICEEACHCALLISINDEPENDENTOFTHEOTHERONESAMUCHMOREEFFICIENTOPTION,HOWEVER,ISTHEFLYINGLANOFFIG98BHEREEACHSEATCOMESEQUIPPEDWITHANETHERNETCONNECTORINTOWHICHPASSENGERSCANPLUGTHEIRCOMPUTERSASINGLEROUTERONTHEAIRCRAFTMAINTAINSSRADIOLINKWITHSOMEROUTERONTHEGROUND,CHANGINGROUTERSASITFLIESALONGTHISCONFIGURATIONISJUSTATRADITIONALLAN,EXCEPTTHATITSCONNECTIONTOTHEOUTSIDEWORLDHAPPENSTOBEARADIOLINKINSTEADOFAHARDWIREDLINEWHILEMANYPEOPLEBELIEVETHATWIRELESSPORTABLECOMPUTERSARETHEWAVEOFTHEFUTURE,ATLEASTONEDISSENTINGVOICEHASBEENHEARDBOBMETCALFE,THEINVENTOROFETHERNET,HASWRITTEN“MOBILEWIRELESSCOMPUTERSARELIKEMOBILEPIPELESSBATHROOMSPORTAPOTTIESTHEYWILLBECOMMONONVEHICLES,ANDATCONSTRUCTIONSITES,ANDROCKCONCERTSMYADVICEISTOWIREUPYOURHOMEANDSTAYTHERE”WILLMOSTPEOPLEFOLLOWMETCALFE’SADVICETIMEWILLTELL925INTERNETWORKSMANYNETWORKSEXISTINTHEWORLD,OFTENWITHDIFFERENTHARDWAREANDSOFTWAREPEOPLECONNECTEDTOONENETWORKOFTENWANTTOCOMMUNICATEWITHPEOPLEATTACHEDTOADIFFERENTONETHISDESIREREQUIRESCONNECTINGTOGETHERDIFFERENT,ANDFREQUENTLYINCOMPATIBLENETWORKS,SOMETIMESBYUSINGMACHINESCALLEDGATEWAYSTOMAKETHECONNECTIONANDPROVIDETHENECESSARYTRANSLATION,BOTHINTERMSOFHARDWAREANDSOFTWAREACOLLECTIONOFINTERCONNECTEDNETWORKSISCALLEDANINTERNETWORKORJUSTINTERNETACOMMONFORMOFINTERNETISACOLLECTIONOFLANSCONNECTEDBYAWANINFACT,IFWEWERETOREPLACETHELABEL“SUBNET”INFIG95BY“WAN”,NOTHINGELSEINTHEFIGUREWOULDHAVETOCHANGETHEONLYREALDISTINCTIONBETWEENASUBNETANDAWANINTHISCASEISWHETHERORNOTHOSTSAREPRESENTIFITCONTAINSBOTHROUTERSANDHOSTSWITHTHEIROWNUSERS,ITISAWANTOAVOIDCONFUSION,PLEASENOTETHATTHEWORD“INTERNET”WILLALWAYSBEUSEDINAGENERICSENSEINCONTRAST,THEINTERNET,THEINTERNETNOTEUPPERCASEIMEANSASPECIFICWORLDWIDEINTERNETTHATISWIDELYUSEDTOCONNECTUNIVERSITIES,GOVERNMENTOFFICES,COMPANIES,ANDOFLATE,PRIVATEINDIVIDUALSSUBNETS,NETWORKS,ANDINTERNETWORKSAREOFTENCONFUSEDSUBNETMAKESTHEMOSTSENSEINTHECONTEXTOFAWIDEAREANETWORK,WHEREITREFERSTOTHECOLLECTIONOFROUTERSANDCOMMUNICATIONLINESOWNEDBYTHENETWORKOPERATOR,FOREXAMPLE,COMPANIESLIKEAMERICAONLINEANDCOMPUSERVEASANANALOGY,THETELEPHONESYSTEMCONSISTSOFTELEPHONESWITCHINGOFFICESCONNECTEDTOEACHOTHERBYHIGHSPEEDLINESTHESELINESANDEQUIPMENT,OWNEDANDMANAGEDBYTHETELEPHONECOMPANY,FORMTHESUBNETOFTHETELEPHONESYSTEMTHETELEPHONESTHEMSELVESTHEHOSTSINTHISANALOGYARENOTPARTOFTHESUBNETTHECOMBINATIONOFASUBNETANDITSHOSTSFORMSANETWORKINTHECASEOFALAN,THECABLEANDTHEHOSTSFORMTHENETWORKTHEREREALLYISNOSUBNETANINTERNETWORKISFORMEDWHENDISTINCTNETWORKSARECONNECTEDTOGETHERINOURVIEW,CONNECTINGALANANDAWANORCONNECTINGTWOLANSFORMSANINTERNETWORK,BUTTHEREISLITTLEAGREEMENTINTHEINDUSTRYOVERTERMINOLOGYINTHISAREA93NETWORKSOFTWARETHEFIRSTCOMPUTERNETWORKSWEREDESIGNEDWITHTHEHARDWAREASTHEMAINCONCERNANDTHESOFTWAREASANAFTERTHOUGHTTHISSTRATEGYNOLONGERWORKSNETWORKSOFTWAREISNOWHIGHLYSTRUCTUREDINTHEFOLLOWINGSECTIONSWEEXAMINETHESOFTWARESTRUCTURINGTECHNIQUEINSOMEDETAILTHEMETHODDESCRIBEDHEREFORMSTHEKEYSTONEOFTHEENTIREBOOKANDWILLOCCURREPEATEDLYLATERON931PROTOCOLHIERARCHIESTOREDUCETHEIRDESIGNCOMPLEXITY,MOSTNETWORKSAREORGANIZEDASASERIESOFLAYERSORLEVELS,EACHONEBUILTUPONTHEONEBELOWITTHENUMBEROFLAYERS,THENAMEOFEACHLAYER,THECONTENTSOFEACHLAYER,ANDTHEFUNCTIONOFEACHLAYERDIFFERFROMNETWORKTONETWORKTONETWORKHOWEVER,INALLNETWORKS,THEPURPOSEOFEACHLAYERISTOOFFERCERTAINSERVICESTOTHEHIGHERLAYERS,SHIELDINGTHOSELAYERSFROMTHEDETAILSOFHOWTHEOFFEREDSERVICESAREACTUALLYIMPLEMENTEDLAYERNONONEMACHINECARRIESONACONVERSATIONWITHLAYERNONANOTHERMACHINETHERULESANDCONVENTIONSUSEDINTHISCONVERSATIONARECOLLECTIVELYKNOWNASTHECOMMUNICATINGPARTIESONHOWCOMMUNICATIONISTOPROCEEDASANANALOGY,WHENAWOMANISINTRODUCEDTOAMAN,SHEMAYCHOOSETOSTICKOUTHERHANDHE,INTURN,MAYDECIDEEITHERTOSHAKEITORKISSIT,DEPENDING,FOREXAMPLE,ONWHETHERSHEISANAMERICANLAWYERATABUSINESSMEETINGORAEUROPEANPRINCESSATAFORMALBALLVIOLATINGTHEPROTOCOLWILLMAKECOMMUNICATIONMOREDIFFICULT,IFNOTIMPOSSIBLEAFIVELAYERNETWORKISILLUSTRATEDINFIG99THEENTITIESCOMPRISINGTHECORRESPONDINGLAYERSONDIFFERENTMACHINESARECALLEDPEERSINOTHERWORDS,ITISTHEPEERSTHATCOMMUNICATEUSINGTHEPROTOCOLINREALITY,NODATAAREDIRECTLYTRANSFERREDFROMLAYERNONONEMACHINETOLAYERNONANOTHERMACHINEINSTEAD,EACHLAYERPASSESDATAANDCONTROLINFORMATIONTOTHELAYERIMMEDIATELYBELOWIT,UNTILTHELOWESTLAYERISREACHEDBELOWLAYER1ISTHEPHYSICALMEDIUMTHROUGHWHICHACTUALCOMMUNICATIONOCCURSINFIG99,VIRTUALCOMMUNICATIONISSHOWNBYDOTTEDLINESANDPHYSICALCOMMUNICATIONBYSOLIDLINESBETWEENEACHPAIROFADJACENTLAYERSTHEREISANINTERFACETHEINTERFACEDEFINESWHICHPRIMITIVEOPERATIONSANDSERVICESTHELOWERLAYEROFFERSTOTHE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