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原文部分Inmostwideareanetworks,thesubnetconsistsoftwodistinctcomponentstransmissionlinesandswitchingelements.Transmissionlinesalsocalledcircuits,channels,ortrunksmovebitsbetweenmachines.Theswitchingelementsarespecializedcomputersusedtoconnecttwoormoretransmissionlined.Whendataarriveonanincomingline,theswitchingelementmustchooseanoutgoinglinetoforwardthemon..Unfortunately,thereisnostandardterminologyusedtonamethesecomputers.Theyarevariouslycalledpacketswitchingexchanges,amongotherthings.Asagenerictermfortheswitchingcomputers,willuseexistshere.Inthismodel,showninfig.9.5,eachhostisgenerallyconnectedtoaLANonarouter.Thecollectionofcommunicationlinesandroutersbutnotthehostsformthesubnet.Anasideaboutthetermsubnetisworthmaking.Originally,itsonlymeaningwasthecollectionofroutersandcommunicationlinesthatmovedpacketsfromthesourcehosttothedestinationhost.However,networkaddressing.Hencethetermhasacertainambiguityaboutit.Unfortunately,nowidelyusedalternativeexistsforitsinitialmeaning,sowithsomehesitationwewilluseitinbothsenses.Fromthecontext,itwillalwaysbeclearwhichismeant.InmostWANs,thenetworkcontainsnumerouscablesortelephonelines,eachoneconnectingapairofrouters.Iftworoutersthatdonotshareacableneverthelesswishtocommunicate,theymustdothisindirectly,viaotherrouters.Whenapacketissentfromoneroutertoanotherviaoneormoreintermediaterouters,thepacketisreceivedateachintermediaterouterinitsentirety,storedthereuntiltherequiredoutputlineisfree,andthenforwarded.Asubnetusingthisprincipleiscalledapointtopoint,storeandforward,orpacketswitchedsubnet.Nearlyallwideareanetworksexceptthoseusingsatelliteshavestoreandforwardsubnets.Whenthepacketsaresmallandallthesamesize,theyareoftencalledcells.Whenapointtopointsubnetisused,animportantdesignissueiswhattherouterinterconnectiontopologyshouldlooklike.Fig.9.6showsseveralpossibletopologies.Localnetworksthatweredesignedassuchusuallyhaveasymmetrictopology.Incontrast,wideareanetworkstypicallyhaveirregulartopologies.AsecondpossibilityforaWANisasatelliteorgroundradiosystem.Eachrouterhasanantennathroughwhichitcansendandreceive.Allrouterscanheartheoutputfromthesatellite,andinsomecasestheycanalsoheartheupwardtransmissionsoftheirfellowrouterstothesatelliteaswell.Sometimestheroutersareconnectedtoasubstantialpointtopointsubnet,withonlysomeofthemhavingasatelliteantenna.Satellitenetworksareinherentlybroadcastandaremostusefulwhenthebroadcastpropertyisimportant.9.2.4WirelessNetworksMobilecomputers,suchasnotebookcomputersandpersonaldigitalassistantsPDA,arethefastestgrowingsegmentofthecomputerindustry.ManyoftheownersofthesecomputershavedesktopmachinesonLANsandWANsbackattheofficeandwanttobeconnectedtotheirhomebaseevenwhenawayfromhomeoraroute.Sincehavingawiredconnectionisimpossibleincarsandairplanes,thereisalotofinterestinwirelessnetworks.Onthissectionwewillbrieflyintroducethistopic.Actually,digitalwirelesscommunicationisnotanewidea.Asearlyas1901,theItalianphysicistGuglielmoMarconidemonstratedashiptoshorewirelesstelegraphusingMorseCodedotsanddashesrebinary,afterall.Modemdigitalwirelesssystemshavebetterperformance,butthebasicideaisthesame.Wirelessnetworkshavemanyuses.Acommononeistheportableoffice.Peopleontheroadoftenwanttousetheirportableelectronicequipmenttosendandreceivetelephonecalls,faxes,andelectronicmail,readremotefiles,loginonremotemachines,andsoon,anddothisfromanywhereonland,sea,orair.Wirelessnetworksarelfgreatvaluetofleetsoftrucks,taxis,buses,andrepairpersonsforkeepingincontactwithhome.Anotheruseisforrescueworkersatdisastersitesfires,floods,earthquakes,etc.wherethetelephonesystemhasbeendestroyed.Computerstherecansendmessages,keeprecords,andsoon.Finally,wirelessnetworksareimportanttothemilitary.Ifyouhavetobeablefightawaranywhereonearthonshortnotice,countingonusingthelocalnetworkinginfrastructureisprobablynotagoodidea.Itisbettertobringyourown.Althoughwirelessnetworkingandmobilecomputingareoftenrelated,theyarenotidentical,asfig.9.7shows.Portablecomputersaresometimeswired.Forexample,ifatravelerplugsaplugsaportablecomputerintothetelephonejackinahotel,wehavemobilitywithoutawirelessnetwork.Anotherexampleissomeonecarryingaportablecomputeralongasheinspectsatrainfortechnicalproblems.Herealongcordcantrailalongbehindvacuumcleanermodel.Ontheotherhand,somewirelesscomputersarenotportable.Animportantexamplehereisacompanythatownsanolderbuildingthatdoesnothavenetworkcablinginstalledandwantstoconnectitscomputers.InstallingawirelessLANmayrequirelittlemorethanbuyingasmallboxwithsomeelectronicsandsettingupsomeantennas.Thissolutionmaybecheaperthanwiringthebuilding..AlthoughwirelessLANsareeasytoinstall,theyalsohavesomedisadvantages.Typicallytheyhaveacapacityof1to2Mbps,whichismuchslowerthanwiredLANs.Theerrorratesareoftenmuchhigher,too,andthetransmissionsfromdifferentcomputerscaninterferewithoneanother.Butofcourse,therearealsothetruemobile,wirelessapplications,rangingfromtheportableofficetopeoplewalkingaroundastorewithaPDAdoinginventory.Atmanybusyairports,carrentalreturnclerksworkoutintheparkinglotwithwirelessportablecomputers.Theytypeinabuiltinprinter,callsthemaincomputer,getstherentalinformation,andprintsoutthebillonthespot.Wirelessnetworkscomeinmanyforms.Someuniversitiesarealreadyinstallingantennasallovercampustoallowstudentstositunderthetreesandconsultthelibraryscardcatalog.HerethecomputerscommunicatedirectlywiththewirelessLANindigitalform.Anotherpossibilityisusingacellulari.e.portabletelephonewithatraditionalanalogmodem.Directdigitalcellarservice,calledCDPDCellularDigitalPacketDataisbecomingavailableinmanycities.Finally,itispossibletohavedifferentcombinationsofwiredandwirelessnetworking.Forexample,infig.9.8a,wedepictanairplanewithanumberofpeopleusingmodemsandseatbacktelephonestocalltheoffice.Eachcallisindependentoftheotherones.Amuchmoreefficientoption,however,istheflyingLANofFig.9.8b.HereeachseatcomesequippedwithanEthernetconnectorintowhichpassengerscanplugtheircomputers.Asinglerouterontheaircraftmaintainssradiolinkwithsomerouterontheground,changingroutersasitfliesalong.ThisconfigurationisjustatraditionalLAN,exceptthatitsconnectiontotheoutsideworldhappenstobearadiolinkinsteadofahardwiredline.Whilemanypeoplebelievethatwirelessportablecomputersarethewaveofthefuture,atleastonedissentingvoicehasbeenheard.BobMetcalfe,theinventorofEthernet,haswrittenMobilewirelesscomputersarelikemobilepipelessbathroomsportapotties.Theywillbecommononvehicles,andatconstructionsites,androckconcerts.Myadviceistowireupyourhomeandstaythere.WillmostpeoplefollowMetcalfesadviceTimewilltell.9.2.5InternetworksManynetworksexistintheworld,oftenwithdifferenthardwareandsoftware.Peopleconnectedtoonenetworkoftenwanttocommunicatewithpeopleattachedtoadifferentone.Thisdesirerequiresconnectingtogetherdifferent,andfrequentlyincompatiblenetworks,sometimesbyusingmachinescalledgatewaystomaketheconnectionandprovidethenecessarytranslation,bothintermsofhardwareandsoftware.Acollectionofinterconnectednetworksiscalledaninternetworkorjustinternet.AcommonformofinternetisacollectionofLANsconnectedbyaWAN.Infact,ifweweretoreplacethelabelsubnetinFig.9.5byWAN,nothingelseinthefigurewouldhavetochange.TheonlyrealdistinctionbetweenasubnetandaWANinthiscaseiswhetherornothostsarepresent.Ifitcontainsbothroutersandhostswiththeirownusers,itisaWAN.Toavoidconfusion,pleasenotethatthewordinternetwillalwaysbeusedinagenericsense.Incontrast,theInternet,theInternetnoteuppercaseImeansaspecificworldwideinternetthatiswidelyusedtoconnectuniversities,governmentoffices,companies,andoflate,privateindividuals.Subnets,networks,andinternetworksareoftenconfused.Subnetmakesthemostsenseinthecontextofawideareanetwork,whereitreferstothecollectionofroutersandcommunicationlinesownedbythenetworkoperator,forexample,companieslikeAmericaOnlineandCompuServe.Asananalogy,thetelephonesystemconsistsoftelephoneswitchingofficesconnectedtoeachotherbyhighspeedlines.Theselinesandequipment,ownedandmanagedbythetelephonecompany,formthesubnetofthetelephonesystem.Thetelephonesthemselvesthehostsinthisanalogyarenotpartofthesubnet.Thecombinationofasubnetanditshostsformsanetwork.InthecaseofaLAN,thecableandthehostsformthenetwork.Therereallyisnosubnet.Aninternetworkisformedwhendistinctnetworksareconnectedtogether.Inourview,connectingaLANandaWANorconnectingtwoLANsformsaninternetwork,butthereislittleagreementintheindustryoverterminologyinthisarea.9.3networksoftwareThefirstcomputernetworksweredesignedwiththehardwareasthemainconcernandthesoftwareasanafterthought.Thisstrategynolongerworks.Networksoftwareisnowhighlystructured.Inthefollowingsectionsweexaminethesoftwarestructuringtechniqueinsomedetail.Themethoddescribedhereformsthekeystoneoftheentirebookandwilloccurrepeatedlylateron.9.3.1ProtocolHierarchiesToreducetheirdesigncomplexity,mostnetworksareorganizedasaseriesoflayersorlevels,eachonebuiltupontheonebelowit.Thenumberoflayers,thenameofeachlayer,thecontentsofeachlayer,andthefunctionofeachlayerdifferfromnetworktonetworktonetwork.However,inallnetworks,thepurposeofeachlayeristooffercertainservicestothehigherlayers,shieldingthoselayersfromthedetailsofhowtheofferedservicesareactuallyimplemented.Layernononemachinecarriesonaconversationwithlayernonanothermachine.Therulesandconventionsusedinthisconversationarecollectivelyknownasthecommunicatingpartiesonhowcommunicationistoproceed.Asananalogy,whenawomanisintroducedtoaman,shemaychoosetostickoutherhand.He,inturn,maydecideeithertoshakeitorkissit,depending,forexample,onwhethersheisanAmericanlawyeratabusinessmeetingoraEuropeanprincessataformalball.Violatingtheprotocolwillmakecommunicationmoredifficult,ifnotimpossible.AfivelayernetworkisillustratedinFig.9.9.Theentitiescomprisingthecorrespondinglayersondifferentmachinesarecalledpeers.Inotherwords,itisthepeersthatcommunicateusingtheprotocol.Inreality,nodataaredirectlytransferredfromlayernononemachinetolayernonanothermachine.Instead,eachlayerpassesdataandcontrolinformationtothelayerimmediatelybelowit,untilthelowestlayerisreached.Belowlayer1isthephysicalmediumthroughwhichactualcommunicationoccurs.InFig.9.9,virtualcommunicationisshownbydottedlinesandphysicalcommunicationbysolidlines.Betweeneachpairofadjacentlayersthereisaninterface.Theinterfacedefineswhichprimitiveoperationsandservicesthelowerlayerofferstotheupperone.Whennetworkdesignersdecidehowmanylayerstoincludeinanetworkandwhateachoneshoulddo,oneofthemostimportantconsiderationsisdefiningcleaninterfacesbetweenthelayers.Doingso,inturn,requiresthateachlayerperformaspecificcollectionofwellunderstoodfunctions.Inadditiontominimizingtheamountofinformationthatmustbepassedbetweenlayers,cleancutinterfacesalsomakeitsimplertoreplacetheimplementationofonelayerwithacompletelydifferentimplementationofonelayerwithacompletelydifferentimplementatione.g.,allthetelephonelinesarereplacedbysatellitechannels,becauseallthatisrequiredofthenewimplementationisthatitoffersexactlythesamesetofservicestoitsupstairsneighborastheoldimplementationdid.Asetoflayersandprotocolsiscalledanetworkarchitecture.Thespecificationofanarchitecturemustcontainenoughinformationtoallowanimplementertowritetheprogramorbuildthehardwareforeachlayersothatitwillcorrectlyobeytheappropriateprotocol.Neitherthedetailsoftheimplementationnorthespecificationof
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