外文翻译 - 网络

外文原文Thenetworks1BirthoftheNetTheInternethashadarelativelybrief,butexplosivehistorysofar.Itgrewoutofanexperimentbeguninthe1960sbytheU.S.DepartmentofDefense.TheDoDwantedtocreateacomputernetworkthatwouldcontinuetofunctionintheeventofadisaster,suchasanuclearwar.Ifpartofthenetworkweredamagedordestroyed,therestofthesystemstillhadtowork.ThatnetworkwasARPANET,whichlinkedU.S.scientificandacademicresearchers.ItwastheforerunneroftodaysInternet.In1985,theNationalScienceFoundation(NSF)createdNSFNET,aseriesofnetworksforresearchandeducationcommunication.BasedonARPANETprotocols,theNSFNETcreatedanationalbackboneservice,providedfreetoanyU.S.researchandeducationalinstitution.Atthesametime,regionalnetworkswerecreatedtolinkindividualinstitutionswiththenationalbackboneservice.NSFNETgrewrapidlyaspeoplediscovereditspotential,andasnewsoftwareapplicationswerecreatedtomakeaccesseasier.CorporationssuchasSprintandMCIbegantobuildtheirownnetworks,whichtheylinkedtoNSFNET.AscommercialfirmsandotherregionalnetworkprovidershavetakenovertheoperationofthemajorInternetarteries,NSFhaswithdrawnfromthebackbonebusiness.NSFalsocoordinatedaservicecalledInternet,whichregisteredalladdressesontheInternetsothatdatacouldberoutedtotherightsystem.ThisservicehasnowbeentakenoverbyNetworkSolutions,Inc.,incooperationwithNSF.2HowtheWebWorksTheWorldWideWeb,thegraphicalportionoftheInternet,isthemostpopularpartoftheInternetbyfar.OnceyouspendtimeontheWeb,youwillbegintofeellikethereisnolimittowhatyoucandiscover.TheWeballowsrichanddiversecommunicationbydisplayingtext,graphics,animation,photos,soundandvideo.Sojustwhatisthismiraculouscreation?TheWebphysicallyconsistsofyourpersonalcomputer,webbrowsersoftware,aconnectiontoanInternetserviceprovider,computerscalledserversthathostdigitaldataandroutersandswitchestodirecttheflowofinformation.TheWebisknownasaclient-serversystem.Yourcomputeristheclient;theremotecomputersthatstoreelectronicfilesaretheservers.Hereshowitworks:LetssayyouwanttopayavisittothetheLouvremuseumwebsite.FirstyouentertheaddressorURLofthewebsiteinyourwebbrowser(moreaboutthisshortly).ThenyourbrowserrequeststhewebpagefromthewebserverthathoststheLouvressite.TheLouvresserversendsthedataovertheInternettoyourcomputer.Yourwebbrowserinterpretsthedata,displayingitonyourcomputerscreen.TheLouvreswebsitealsohaslinkstothesitesofothermuseums,suchastheVaticanMuseum.Whenyouclickyourmouseonalink,youaccessthewebserverfortheVaticanMuseum.ThegluethatholdstheWebtogetheriscalledhypertextandhyperlinks.ThisfeatureallowelectronicfilesontheWebtobelinkedsoyoucaneasilyjumpbetweenthem.OntheWeb,younavigatethroughpagesofinformationbasedonwhatinterestsyouatthatparticularmoment,commonlyknownasbrowsingorsurfingtheNet.ToaccesstheWebyouneedwebbrowsersoftware,suchasNetscapeNavigatororMicrosoftInternetExplorer.HowdoesyourwebbrowserdistinguishbetweenwebpagesandotherfilesontheInternet?WebpagesarewritteninacomputerlanguagecalledHypertextMarkupLanguageorHTML.3SomeWebHistoryTheWorldWideWeb(WWW)wasoriginallydevelopedin1990atCERN,theEuropeanLaboratoryforParticlePhysics.ItisnowmanagedbyTheWorldWideWebConsortium,alsoknownastheWorldWideWebInitiative.TheWWWConsortiumisfundedbyalargenumberofcorporatemembers,includingAT&T,AdobeSystems,Inc.,MicrosoftCorporationandSunMicro-systems,Inc.ItspurposeistopromotethegrowthoftheWebbydevelopingtechnicalspecificationsandreferencesoftwarethatwillbefreelyavailabletoeveryone.TheConsortiumisrunbyMITwithINRIA(TheFrenchNationalInstituteforResearchinComputerScience)actingasEuropeanhost,incollaborationwithCERN.TheNationalCenterforSuper-computingApplications(NCSA)attheUniversityofIllinoisatUrban-Champaign,wasinstrumentalinthedevelopmentofearlygraphicalsoftwareutilizingtheWorldWideWebfeaturescreatedbyCERN.NCSAfocusesonimprovingtheproductivityofresearchersbyprovidingsoftwareforscientificmodeling,analysis,andvisualization.TheWorldWideWebwasanobviouswaytofulfillthatmission.NCSAMosaic,oneoftheearliestwebbrowsers,wasdistributedfreetothepublic.ItleddirectlytothephenomenalgrowthoftheWorldWideWeb.UnderstandingWebAddressesYoucanthinkoftheWorldWideWebasanetworkofelectronicfilesstoredoncomputersallaroundtheworld.Hypertextlinkstheseresourcestogether.UniformResourceLocatorsorURLsaretheaddressesusedtolocatethesefiles.TheinformationcontainedinaURLgivesyoutheabilitytojumpfromonewebpagetoanotherwithjustaclickofyourmouse.WhenyoutypeaURLintoyourbrowserorclickonahypertextlink,yourbrowserissendingarequesttoaremotecomputertodownloadafile.WhatdoesatypicalURLlooklike?Herearesomeexamples:/Thehomepageforstudyenglish./pub/AdirectoryoffilesatMIT*availablefordownloading.news:rec.gardens.rosesAnewsgrouponrosegardening.ThefirstpartofaURL(beforethetwoslashes*tellsyouthetypeofresourceormethodofaccessatthataddress.Forexample:http-ahypertextdocumentordirectorygopher-agopherdocumentormenuftp-afileavailablefordownloadingoradirectoryofsuchfilesnews-anewsgrouptelnet-acomputersystemthatyoucanlogintoovertheInternetWAIS*-adatabaseordocumentinaWideAreaInformationSearchdatabasefile-afilelocatedonalocaldrive(yourharddrive)Thesecondpartistypicallytheaddressofthecomputerwherethedataorserviceislocated.Additionalpartsmayspecifythenamesoffiles,theporttoconnectto,orthetexttosearchforinadatabase.YoucanentertheURLofasitebytypingitintotheLocationbarofyourwebbrowser,justunderthetoolbar.MostbrowsersrecordURLsthatyouwanttouseagain,byaddingthemtoaspecialmenu.InNetscapeNavigator,itscalledBookmarks.InMicrosoftExplorer,itscalledFavorites.OnceyouaddaURLtoyourlist,youcanreturntothatwebpagesimplybyclickingonthenameinyourlist,insteadofretypingtheentireURL.MostoftheURLsyouwillbeusingstartwithhttpwhichstandsforHypertextTransferProtocol*.httpisthemethodbywhichHTMLfilesaretransferredovertheWeb.HerearesomeotherimportantthingstoknowaboutURLs:4AURLusuallyhasnospaces.AURLalwaysusesforwardslashes(/).IfyouenteraURLincorrectly,yourbrowserwillnotbeabletolocatethesiteorresourceyouwant.Shouldyougetanerrormessageorthewrongsite,makesureyoutypedtheaddresscorrectly.YoucanfindtheURLbehindanylinkbypassingyourmousecursoroverthelink.ThepointerwillturnintoahandandtheURLwillappearinthebrowsersstatusbar,usuallylocatedatthebottomofyourscreen.DomainNamesWhenyouthinkoftheInternet,.JustwhatdothosethreelettersattheendofaWorldWideWebaddressmean?EverycomputerthathostsdataontheInternethasauniquenumericaladdress.Forexample,thenumericaladdressfortheWhiteHouseis00.Butsincefewpeoplewanttorememberlongstringsofnumbers,theDomainNameSystem(DNS)*wasdeveloped.DNS,acriticalpartoftheInternetstechnicalinfrastructure*,correlates*anumericaladdresstoaword.ToaccesstheWhiteHousewebsite,youcouldtypeitsnumberintotheaddressboxofyourwebbrowser.B.Inthiscase,.Ingeneral,thethree-letterdomainnamesuffix*isknownasagenerictop-leveldomainanddescribesthetypeoforganization.Inthelastfewyears,thelineshavesomewhatblurred*-business(commercial).-governmentAdomainnamealwayshastwoormorepartsseparatedbydotsandtypicallyconsistsofsomeformofanorganizationsnameandthethree-lettersuffix.Forexample,thedomainnameforIBM;theUnitedN.Ifadomainnameisavailable,andprovideditdoesnotinfringe*onanexistingtrademark,anyonecanregisterthenamefor$35ayearthroughNetworkSolutions,Inc.,,.domains.Youcanusetheboxbelowtoseeifanameisaavailable.Dnameyouwantisalreadytaken,however.Oftheover8milliondomainnames,85%domains.ICANN,theInternetCorporationforAssignedNamesandNumbers,managestheDomainNameSystem.Asofthiswriting,thereareplanstoaddadditionaltop-leveldomains,suchas.weband.store.Whenthatwillactuallyhappenisanybodysguess.Tocheckfor,orregisteradomainname,typeitintothesearchbox.Itshouldtakethisform:Inadditiontothegenerictop-leveldomains,244nationaltop-leveldomainswereestablishedforcountriesandterritories*,forexample:.au-Australia.ca-Canada.fr-France.de-Germany.uk-UnitedKingdomForUS$275pername,youcanalsoregisteraninternationaldomainnamewithNetNames.Beawarethatsomecountrieshaverestrictionsforregisteringnames.Ifyouplantoregisteryourowndomainname,ornot,keepthesetipsinmind:Theshorterthename,thebetter.(Butitshouldreflectyourfamilyname,interestorbusiness.)Thenameshouldbeeasytoremember.Itshouldbeeasytotypewithoutmakingmistakes.Remember,theInternetisglobal.Ideally,adomainnamewillreadinalanguageotherthanEnglish.5AboutmodemTelephonelinesweredesignedtocarrythehumanvoice,notelectronicdatafromacomputer.Modemswereinventedtoconvertdigitalcomputersignalsintoaformthatallowsthemtotraveloverthephonelines.Thosearethescratchysoundsyouhearfromamodemsspeaker.Amodemontheotherendofthelinecanunderstanditandconvertthesoundsbackintodigitalinformationthatthecomputercanunderstand.Bytheway,thewordmodemstandsforModulator/Demodulator.Buyingandusingamodemusedtoberelativelyeasy.Nottoolongago,almostallmodemstransferreddataatarateof2400Bps(bitspersecond).Today,modemsnotonlyrunfaster,theyarealsoloadedwithfeatureslikeerrorcontrolanddatacompression.So,inadditiontoconvertingandinterpretingsignals,modemsalsoactliketrafficcops,monitoringandregulatingtheflowofinformation.Thatway,onecomputerdoesntsendinformationuntilthereceivingcomputerisreadyforit.Eachofthesefeatures,modulation,errorcontrol,anddatacompression,requiresaseparatekindofprotocolandthatswhatsomeofthosetermsyouseelikeV.32,V.32bis,V.42bisandMNP5referto.Ifyourcomputerdidntcomewithaninternalmodem,considerbuyinganexternalone,becauseitismucheasiertoinstallandoperate.Forexample,whenyourmodemgetsstuck(notanunusualoccurrence),youneedtoturnitoffandontogetitworkingproperly.Withaninternalmodem,thatmeansrestartingyourcomputer-awasteoftime.Withanexternalmodemitsaseasyasflippingaswitch.Heresatipforyou:inmostareas,ifyouhaveCallWaiting,youcandisableitbyinserting*70infrontofthenumberyoudialtoconnecttotheInternet(oranyonlineservice).Thiswillpreventanincomingcallfromaccidentallykickingyouofftheline.Thistableillustratestherelativedifferenceindatatransmissionspeedsfordifferenttypesoffiles.Amodemsspeedismeasuredinbitspersecond(bps).A14.4modemsendsdataat14,400bitspersecond.A28.8modemistwiceasfast,sendingandreceivingdataatarateof28,800bitspersecond.Untilnearlytheendof1995,theconventionalwisdomwasthat28.8Kbpswasaboutthefastestspeedyoucouldsqueezeoutofaregularcoppertelephoneline.Today,youcanbuy33.6Kbpsmodems,andmodemsthatarecapableof56Kbps.Thekeyquestionforyou,isknowingwhatspeedmodemsyourInternetserviceprovider(ISP)has.IfyourISPhasonly28.8Kbpsmodemsonitsendoftheline,youcouldhavethefastestmodemintheworld,andonlybeabletoconnectat28.8Kbps.Beforeyouinvestina33.6Kbpsora56Kbpsmodem,makesureyourISPsupportsthem.6SpeedItUpTherearefasterwaystotransmitdatabyusinganISDNorleasedline.InmanypartsoftheU.S.,phonecompaniesareofferinghomeISDNatlessthan$30amonth.ISDNrequiresaso-calledISDNadapterinsteadofamodem,andaphonelinewithaspecialconnectionthatallowsittosendandreceivedigitalsignals.Youhavetoarrangewithyourphonecompanytohavethisequipmentinstalled.FormoreaboutISDN,visitDanKegelsISDNPage.AnISDNlinehasadatatransferrateofbetween57,600bitspersecondand128,000bitspersecond,whichisatleastdoubletherateofa28.8Kbpsmodem.Leasedlinescomeintwoconfigurations:T1andT3.AT1lineoffersadatatransferrateof1.54millionbitspersecond.UnlikeISDN,aT-1lineisadedicatedconnection,meaningthatitispermanentlyconnectedtotheInternet.ThisisusefulforwebserversorothercomputersthatneedtobeconnectedtotheInternetallthetime.ItispossibletoleaseonlyaportionofaT-1lineusingoneoftwosystems:fractionalT-1orFrameRelay.Youcanleasetheminblocksrangingfrom128Kbpsto1.5Mbps.Thedifferencesarenotworthgoingintoindetail,butfractionalT-1willbemoreexpensiveatthesloweravailablespeedsandFrameRelaywillbeslightlymoreexpensiveasyouapproachthefullT-1speedof1.5Mbps.AT-3lineissignificantlyfaster,at45millionbitspersecond.ThebackboneoftheInternetconsistsofT-3lines.LeasedlinesareveryexpensiveandaregenerallyonlyusedbycompanieswhosebusinessisbuiltaroundtheInternetorneedtotransfermassiveamountsofdata.ISDN,ontheotherhand,isavailableinsomecitiesforaveryreasonableprice.NotallphonecompaniesofferresidentialISDNservice.Checkwithyourlocalphonecompanyforavailabilityinyourarea.CableModemsArelativelynewdevelopmentisadevicethatprovideshigh-speedInternetaccessviaacableTVnetwork.Withspeedsofupto36Mbps,cablemodemscandownloaddatainsecondsthatmighttakefiftytimeslongerwithadial-upconnection.BecauseitworkswithyourTVcable,itdoesnttieupatelephoneline.Bestofall,itsalwayson,sothereisnoneedtoconnect-nomorebusysignals!ThisserviceisnowavailableinsomecitiesintheUnitedStatesandEurope.Thedownloadtimesinthetableabovearerelativeandaremeanttogiveyouageneralideaofhowlongitwouldtaketodownloaddifferentsizedfilesatdifferentconnectionspeeds,underthebestofcircumstances.Manythingscaninterferewiththespeedofyourfiletransfer.Thesecanrangefromexcessivelinenoiseonyourtelephonelineandthespeedofthewebserverfromwhichyouaredownloadingfiles,tothenumberofotherpeoplewhoaresimultaneouslytryingtoaccessthesamefileorotherfilesinthesamedirectory.7DSLDSL(DigitalSubscriberLine)isanotherhigh-speedtechnologythatisbecomingincreasinglypopular.DSLlinesarealwaysconnectedtotheInternet,soyoudontneedtodial-up.Typically,datacanbetransferredatratesupto1.544Mbpsdownstreamandabout128Kbpsupstreamoverordinarytelephonelines.SinceaDSLlinecarriesbothvoiceanddata,youdonthavetoinstallanotherphoneline.YoucanuseyourexistinglinetoestablishDSLservice,providedserviceisavailableinyourareaandyouarewithinthespecifieddistancefromthetelephonecompanyscentralswitchingoffice.DSLservicerequiresaspecialmodem.Pricesforequipment,DSLinstallationandmonthlyservicecanvaryconsiderably,socheckwithyourlocalphonecompanyandInternetserviceprovider.Thegoodnewsisthatpricesarecomingdownascompetitionheatsup.Websites.AnatomyofaWebPage.(AwebpageisanelectronicdocumentwritteninacomputerlanguagecalledHTML,shortforHypertextMarkupLanguage.)Eachwebpagehasauniqueaddress,calledaURL*orUniformResourceLocator,whichidentifiesitslocationonthenetwork.Awebsitehasoneormorerelatedwebpages,dependingonhowitsdesigned.Webpagesonasitearelinkedtogetherthroughasystemofhyperlinks*,enablingyoutojumpbetweenthembyclickingonalink.OntheWeb,younavigatethroughpagesofinformationaccordingtoyourinterests.8HomeSweetHomePageWhenyoubrowsetheWorldWideWebyoullseethetermhomepageoften.Thinkofahomepageasthestartingpointofawebsite.Likethetableofcontentsofabookormagazine,thehomepageusuallyprovidesanoverviewofwhatyoullfindatthewebsite.Asitecanhaveonepage,manypagesorafewlongones,dependingonhowitsdesigned.Ifthereisntalotofinformation,thehomepagemaybetheonlypage.Butusuallyyouwillfindatleastafewotherpages.Webpagesvarywildlyindesignandcontent,butmostuseatraditionalmagazineformat.Atthetopofthepageisamasthead*orbannergraphic*,thenalistofitems,suchasarticles,oftenwithabriefdescription.Theitemsinthelistusuallylinktootherpagesonthewebsite,ortoothersites.Sometimestheselinksarehighlighted*wordsinthebodyofthetext,orarearrangedinalist,likeanindex.Theycanalsobeacombination*ofboth.Awebpagecanalsohaveimagesthatlinktoothercontent.Howcanyoutellwhichtextarelinks?Textlinksappearinadifferentcolorfromtherestofthetext-typicallyinblueandunderlined.Whenyoumoveyourcursoroveratextlinkoroveragraphiclink,itwillchangefromanarrowtoahand.Thehypertextwordsoftenhint*atwhatyouwilllinkto.Whenyoureturntoapagewithalinkyouvealreadyvisited,thehypertextwordswilloftenbeinadifferentcolor,soyouknowyouvealreadybeenthere.Butyoucancertainlygothereagain.Dontbesurprisedthough,ifthenexttimeyouvisitasite,thepagelooksdifferentandtheinformationhaschanged.TheWebisadynamic*medium.Toencouragevisitorstoreturntoasite,somewebpublisherschangepagesoften.ThatswhatmakesbrowsingtheWebsoexciting9AHome(Page)ofYourOwnInthe60s,peopleaskedaboutyourastrological*sign.Inthe90s,theywanttoknowyourURL.Thesedays,havingawebaddressisalmostasimportantasastreetaddress.Yourwebsiteisanelectronicmeetingplaceforyourfamily,friendsandpotentially*,millionsofpeoplearoundtheworld.Buildingyourdigitaldomaincanbeeasierthanyoumaythink.Bestofall,youmaynothavetospendacent.TheWebbrimswithallkindsoffreeservices,fromtoolstohelpyoubuildyoursite,tofreegraphics,animationandsitehosting.Allittakesissometimeandcreativity.Thinkofyourhomepageasthestartingpointofyourwebsite.Likethetableofcontentsofabookormagazine,thehomepageisthefrontdoor.Yoursitecanhaveoneormorepages,dependingonhowyoudesignit.Ifthereisntalotofinformationjustyet,yoursitewillmostlikelyhaveonlyahomepage.Butthesiteissuretogrowovertime.Whilewebpagesvarydramatically*intheirdesignandcontent,mostuseatraditionalmagazinelayout.Atthetopofthepageisabannergraphic.Nextcomesagreetingandashortdescriptionofthesite.Pictures,text,andlinkstootherwebsitesfollow.Ifthesitehasmorethanonepage,therestypicallyalistofitems-similartoanindex-oftenwithabriefdescription.Theitemsinthelistlinktootherpagesonthewebsite.Sometimestheselinksarehighlightedwordsinthebodyofthetext.Itcanalsobeacombinationofboth.Additionally,awebpagemayhaveimagesthatlinktoothercontent.Beforeyoustartbuildingyoursite,dosomeplanning.Thinkaboutwhomthesiteisforandwhatyouwanttosay.Next,gatherupthematerialthatyouwanttoputonthesite:writethecopy,scanthephotos,designorfindthegraphics.Drawaroughlayoutonasheetofpaper.Whiletherearenorulesyouhavetofollow,thereareafewthingstokeepinmind:Startsimply.Ifyouaretooambitiousatthebeginning,youmaynevergetthesiteofftheground.Youcanalwaysaddtoyoursite.Lessisbetter.Mostpeopledontliketoreadalotoftextonline.Breakitintosmallchunks.Userestraint.Althoughyoucanusewildcolorsandimagesforthebackgroundofyourpages,makesureyourvisitorswillbeabletoreadthetexteasily.Smallerisbetter.MostpeopleconnecttotheInternetwithamodem.Sinceitcantakealongtimetodownloadlargeimagefiles,keepthefilesizessmall.Havetherights.Dontputanymaterialonyoursiteunlessyouaresureyoucandoitlegally.ReadLearntheNetscopyrightarticleformoreaboutthis.StakeYourClaimNowitstimetorollupyoursleevesandstartbuild