外文翻译--子网络.doc

原文部分Inmostwideareanetworks,thesubnetconsistsoftwodistinctcomponents:transmissionlinesandswitchingelements.Transmissionlines(alsocalledcircuits,channels,ortrunks)movebitsbetweenmachines.Theswitchingelementsarespecializedcomputersusedtoconnecttwoormoretransmissionlined.Whendataarriveonanincomingline,theswitchingelementmustchooseanoutgoinglinetoforwardthemon.Unfortunately,thereisnostandardterminologyusedtonamethesecomputers.Theyarevariouslycalledpacketswitchingexchanges,amongotherthings.Asagenerictermfortheswitchingcomputers,willuseexistshere.Inthismodel,showninfig.9.5,eachhostisgenerallyconnectedtoaLANonarouter.Thecollectionofcommunicationlinesandrouters(butnotthehosts)formthesubnet.Anasideabouttheterm“subnet”isworthmaking.Originally,itsonlymeaningwasthecollectionofroutersandcommunicationlinesthatmovedpacketsfromthesourcehosttothedestinationhost.However,networkaddressing.Hencethetermhasacertainambiguityaboutit.Unfortunately,nowidely-usedalternativeexistsforitsinitialmeaning,sowithsomehesitationwewilluseitinbothsenses.Fromthecontext,itwillalwaysbeclearwhichismeant.InmostWANs,thenetworkcontainsnumerouscablesortelephonelines,eachoneconnectingapairofrouters.Iftworoutersthatdonotshareacableneverthelesswishtocommunicate,theymustdothisindirectly,viaotherrouters.Whenapacketissentfromoneroutertoanotherviaoneormoreintermediaterouters,thepacketisreceivedateachintermediaterouterinitsentirety,storedthereuntiltherequiredoutputlineisfree,andthenforwarded.Asubnetusingthisprincipleiscalledapoint-to-point,store-and-forward,orpacket-switchedsubnet.Nearlyallwideareanetworks(exceptthoseusingsatellites)havestore-and-forwardsubnets.Whenthepacketsaresmallandallthesamesize,theyareoftencalledcells.Whenapoint-to-pointsubnetisused,animportantdesignissueiswhattherouterinterconnectiontopologyshouldlooklike.Fig.9.6showsseveralpossibletopologies.Localnetworksthatweredesignedassuchusuallyhaveasymmetrictopology.Incontrast,wideareanetworkstypicallyhaveirregulartopologies.AsecondpossibilityforaWANisasatelliteorgroundradiosystem.Eachrouterhasanantennathroughwhichitcansendandreceive.Allrouterscanheartheoutputfromthesatellite,andinsomecasestheycanalsoheartheupwardtransmissionsoftheirfellowrouterstothesatelliteaswell.Sometimestheroutersareconnectedtoasubstantialpoint-to-pointsubnet,withonlysomeofthemhavingasatelliteantenna.Satellitenetworksareinherentlybroadcastandaremostusefulwhenthebroadcastpropertyisimportant.9.2.4WirelessNetworksMobilecomputers,suchasnotebookcomputersandpersonaldigitalassistants(PDA),arethefastest-growingsegmentofthecomputerindustry.ManyoftheownersofthesecomputershavedesktopmachinesonLANsandWANsbackattheofficeandwanttobeconnectedtotheirhomebaseevenwhenawayfromhomeoraroute.Sincehavingawiredconnectionisimpossibleincarsandairplanes,thereisalotofinterestinwirelessnetworks.Onthissectionwewillbrieflyintroducethistopic.Actually,digitalwirelesscommunicationisnotanewidea.Asearlyas1901,theItalianphysicistGuglielmoMarconidemonstratedaship-to-shorewirelesstelegraphusingMorseCode(dotsanddashesrebinary,afterall).Modemdigitalwirelesssystemshavebetterperformance,butthebasicideaisthesame.Wirelessnetworkshavemanyuses.Acommononeistheportableoffice.Peopleontheroadoftenwanttousetheirportableelectronicequipmenttosendandreceivetelephonecalls,faxes,andelectronicmail,readremotefiles,loginonremotemachines,andsoon,anddothisfromanywhereonland,sea,orair.Wirelessnetworksarelfgreatvaluetofleetsoftrucks,taxis,buses,andrepairpersonsforkeepingincontactwithhome.Anotheruseisforrescueworkersatdisastersites(fires,floods,earthquakes,etc.)wherethetelephonesystemhasbeendestroyed.Computerstherecansendmessages,keeprecords,andsoon.Finally,wirelessnetworksareimportanttothemilitary.Ifyouhavetobeablefightawaranywhereonearthonshortnotice,countingonusingthelocalnetworkinginfrastructureisprobablynotagoodidea.Itisbettertobringyourown.Althoughwirelessnetworkingandmobilecomputingareoftenrelated,theyarenotidentical,asfig.9.7shows.Portablecomputersaresometimeswired.Forexample,ifatravelerplugsaplugsaportablecomputerintothetelephonejackinahotel,wehavemobilitywithoutawirelessnetwork.Anotherexampleissomeonecarryingaportablecomputeralongasheinspectsatrainfortechnicalproblems.Herealongcordcantrailalongbehind(vacuumcleanermodel).Ontheotherhand,somewirelesscomputersarenotportable.Animportantexamplehereisacompanythatownsanolderbuildingthatdoesnothavenetworkcablinginstalledandwantstoconnectitscomputers.InstallingawirelessLANmayrequirelittlemorethanbuyingasmallboxwithsomeelectronicsandsettingupsomeantennas.Thissolutionmaybecheaperthanwiringthebuilding.AlthoughwirelessLANsareeasytoinstall,theyalsohavesomedisadvantages.Typicallytheyhaveacapacityof1to2Mbps,whichismuchslowerthanwiredLANs.Theerrorratesareoftenmuchhigher,too,andthetransmissionsfromdifferentcomputerscaninterferewithoneanother.Butofcourse,therearealsothetruemobile,wirelessapplications,rangingfromtheportableofficetopeoplewalkingaroundastorewithaPDAdoinginventory.Atmanybusyairports,carrentalreturnclerksworkoutintheparkinglotwithwirelessportablecomputers.Theytypeinabuilt-inprinter,callsthemaincomputer,getstherentalinformation,andprintsoutthebillonthespot.Wirelessnetworkscomeinmanyforms.Someuniversitiesarealreadyinstallingantennasallovercampustoallowstudentstositunderthetreesandconsultthelibraryscardcatalog.HerethecomputerscommunicatedirectlywiththewirelessLANindigitalform.Anotherpossibilityisusingacellular(i.e.portable)telephonewithatraditionalanalogmodem.Directdigitalcellarservice,calledCDPD(CellularDigitalPacketData)isbecomingavailableinmanycities.Finally,itispossibletohavedifferentcombinationsofwiredandwirelessnetworking.Forexample,infig.9.8(a),wedepictanairplanewithanumberofpeopleusingmodemsandseat-backtelephonestocalltheoffice.Eachcallisindependentoftheotherones.Amuchmoreefficientoption,however,istheflyingLANofFig.9.8(b).HereeachseatcomesequippedwithanEthernetconnectorintowhichpassengerscanplugtheircomputers.Asinglerouterontheaircraftmaintainssradiolinkwithsomerouterontheground,changingroutersasitfliesalong.ThisconfigurationisjustatraditionalLAN,exceptthatitsconnectiontotheoutsideworldhappenstobearadiolinkinsteadofahardwiredline.Whilemanypeoplebelievethatwirelessportablecomputersarethewaveofthefuture,atleastonedissentingvoicehasbeenheard.BobMetcalfe,theinventorofEthernet,haswritten:“Mobilewirelesscomputersarelikemobilepipelessbathroomsportapotties.Theywillbecommononvehicles,andatconstructionsites,androckconcerts.Myadviceistowireupyourhomeandstaythere.”WillmostpeoplefollowMetcalfesadvice?Timewilltell.9.2.5InternetworksManynetworksexistintheworld,oftenwithdifferenthardwareandsoftware.Peopleconnectedtoonenetworkoftenwanttocommunicatewithpeopleattachedtoadifferentone.Thisdesirerequiresconnectingtogetherdifferent,andfrequentlyincompatiblenetworks,sometimesbyusingmachinescalledgatewaystomaketheconnectionandprovidethenecessarytranslation,bothintermsofhardwareandsoftware.Acollectionofinterconnectednetworksiscalledaninternetworkorjustinternet.AcommonformofinternetisacollectionofLANsconnectedbyaWAN.Infact,ifweweretoreplacethelabel“subnet”inFig.9.5by“WAN”,nothingelseinthefigurewouldhavetochange.TheonlyrealdistinctionbetweenasubnetandaWANinthiscaseiswhetherornothostsarepresent.Ifitcontainsbothroutersandhostswiththeirownusers,itisaWAN.Toavoidconfusion,pleasenotethattheword“internet”willalwaysbeusedinagenericsense.Incontrast,theInternet,theInternet(noteuppercaseI)meansaspecificworldwideinternetthatiswidelyusedtoconnectuniversities,governmentoffices,companies,andoflate,privateindividuals.Subnets,networks,andinternetworksareoftenconfused.Subnetmakesthemostsenseinthecontextofawideareanetwork,whereitreferstothecollectionofroutersandcommunicationlinesownedbythenetworkoperator,forexample,companieslikeAmericaOnlineandCompuServe.Asananalogy,thetelephonesystemconsistsoftelephoneswitchingofficesconnectedtoeachotherbyhigh-speedlines.Theselinesandequipment,ownedandmanagedbythetelephonecompany,formthesubnetofthetelephonesystem.Thetelephonesthemselves(thehostsinthisanalogy)arenotpartofthesubnet.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.Afive-layernetworkisillustratedinFig.9.9.Theentitiescomprisingthecorrespondinglayersondifferentmachinesarecalledpeers.Inotherwords,itisthepeersthatcommunicateusingtheprotocol.Inreality,nodataaredirectlytransferredfromlayernononemachinetolayernonanothermachine.Instead,eachlayerpassesdataandcontrolinformationtothelayerimmediatelybelowit,untilthelowestlayerisreached.Belowlayer1isthephysicalmediumthroughwhichactualcommunicationoccurs.InFig.9.9,virtualcommunicationisshownbydottedlinesandphysicalcom