机械毕业翻译输电线路和开关元件.docx

机械毕业翻译输电线路和开关元件.docx

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机械毕业翻译输电线路和开关元件

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.

9.2.4WirelessNetworks

Mobilecomputers,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.Someuniversitiesarealreadyinstallingantennasallovercampustoallowstudentstositunderthetreesandconsultthelibrary’scardcatalog.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:

“Mobilewirelesscomputersarelikemobilepipelessbathrooms—portapotties.Theywillbecommononvehicles,andatconstructionsites,androckconcerts.Myadviceistowireupyourhomeandstaythere.”WillmostpeoplefollowMetcalfe’sadvice?

Timewilltell.

9.2.5Internetworks

Manynetworksexistintheworld,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,butthereislittleagreementintheindustryo