网络工程毕业外文翻译英文.docx

网络工程毕业外文翻译英文.docx

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网络工程毕业外文翻译英文

网络工程毕业外文翻译英文

WIRELESSLAN

Injustthepastfewyears,wirelessLANshavecometooccupyasignificantnicheinthelocalareanetworkmarket.Increasingly,organizationsarefindingthatwirelessLANsareanindispensableadjuncttotraditionalwiredLANs,astheysatisfyrequirementsformobility,relocation,adhocnetworking,andcoverageoflocationsdifficulttowire.Asthenamesuggests,awirelessLANisonethatmakesuseofawirelesstransmissionmedium.Untilrelativelyrecently,wirelessLANswerelittleused;thereasonsforthisincludedhighprices,lowdatarates,occupationalsafetyconcerns,andlicensingrequirements.Astheseproblemshavebeenaddressed,thepopularityofwirelessLANshasgrownrapidly.

Inthissection,wefirstlookattherequirementsforandadvantagesofwirelessLANs,andthenpreviewthekeyapproachestowirelessLANimplementation.WirelessLANsApplications

TherearefourapplicationareasforwirelessLANs:

LANextension,crossbuildinginterconnect,nomadicaccess,andadhocnetworks.Letusconsidereachoftheseinturn.

LANExtension

EarlywirelessLANproducts,introducedinthelate1980s,weremarketedassubstitutesfortraditionalwiredLANs.AwirelessLANsavesthecostoftheinstallationofLANcablingandeasesthetaskofrelocationandothermodificationstonetworkstructure.However,thismotivationforwirelessLANswasovertakenbyevents.First,asawarenessoftheneedforLANbecamegreater,architectsdesignednewbuildingstoincludeextensiveprewiringfordataapplications.Second,withadvancesindatatransmissiontechnology,therehasbeenanincreasingrelianceontwistedpaircablingforLANsand,inparticular,Category3unshieldedtwistedpair.MostolderbuildingarealreadywiredwithanabundanceofCategory3cable.Thus,theuseofawirelessLANtoreplacewiredLANshasnothappenedtoanygreatextent.

However,inanumberofenvironments,thereisaroleforthewirelessLANasan

alternativetoawiredLAN.Examplesincludebuildingswithlargeopenareas,suchasmanufacturingplants,stockexchangetradingfloors,andwarehouses;historicalbuildingswithinsufficienttwistedpairandinwhichdrillingholesfornewwiringisprohibited;andsmallofficeswhereinstallationandmaintenanceofwiredLANsisnoteconomical.Inallofthesecases,awirelessLANprovidesaneffectiveandmoreattractivealternative.Inmostofthesecases,anorganizationwillalsohaveawiredLANtosupportserversandsomestationaryworkstations.Forexample,amanufacturingfacilitytypicallyhasanofficeareathatisseparatefromthefactoryfloorbutwhichmustbelinkedtoitfornetworkingpurposes.Therefore,typically,awirelessLANwillbelinkedintoawiredLANonthesamepremises.Thus,thisapplicationareaisreferredtoasLANextension.

Cross-BuildingInterconnect

AnotheruseofwirelessLANtechnologyistoconnectLANsinnearbybuildings,betheywiredorwirelessLANs.Inthiscase,apoint-to-pointwirelesslinkisusedbetweentwobuildings.Thedevicessoconnectedaretypicallybridgesorrouters.Thissinglepoint-to-pointlinkisnotaLANperse,butitisusualtoincludethisapplicationundertheheadingofwirelessLAN.

NomadicAccess

NomadicaccessprovidesawirelesslinkbetweenaLANhubandamobiledataterminalequippedwithanantenna,suchasalaptopcomputerornotepadcomputer.Oneexampleoftheutilityofsuchaconnectionistoenableanemployeereturningfromatriptotransferdatafromapersonalportablecomputertoaserverintheoffice.Nomadicaccessisalsousefulinanextendedenvironmentsuchasacampusorabusinessoperatingoutofaclusterofbuildings.Inbothofthesecases,usersmaymovearoundwiththeirportablecomputersandmaywishaccesstotheserversonawiredLANfromvariouslocations.

AdHocNetworking

Anadhocnetworkisapeer-to-peernetwork（nocentralizedserver）setuptemporarilytomeetsomeimmediateneed.Forexample,agroupofemployees,eachwithalaptoporpalmtopcomputer,mayconveneinaconferenceroomforabusiness

orclassroommeeting.Theemployeeslinktheircomputersinatemporarynetworkjustforthedurationofthemeeting.

WirelessLANRequirements

AwirelessLANmustmeetthesamesortofrequirementstypicalofanyLAN,includinghighcapacity,abilitytocovershortdistances,fullconnectivityamongattachedstations,andbroadcastcapability.Inaddition,thereareanumberofrequirementsspecifictothewirelessLANenvironment.ThefollowingareamongthemostimportantrequirementsforwirelessLANs:

Throughput.Themediumaccesscontrolprotocolshouldmakeasefficientuseaspossibleofthewirelessmediumtomaximizecapacity.

Numberofnodes.WirelessLANsmayneedtosupporthundredsofnodesacrossmultiplecells.

ConnectiontobackboneLAN.Inmostcases,interconnectionwithstationsona

wiredbackboneLANisrequired.ForinfrastructurewirelessLANs,thisiseasilyaccomplishedthroughtheuseofcontrolmodulesthatconnecttobothtypesofLANs.Theremayalsoneedtobeaccommodationformobileusersandadhocwirelessnetworks.

Servicearea.AtypicalcoverageareaforawirelessLANmaybeuptoa300to1000footdiameter.

Batterypowerconsumption.Mobileworkersusebattery-poweredworkstationsthat

needtohavealongbatterylifewhenusedwithwirelessadapters.ThissuggeststhataMACprotocolthatrequiresmobilenodestoconstantlymonitoraccesspointsortoengageinfrequenthandshakeswithabasestationisinappropriate.Transmissionrobustnessandsecurity.Unlessproperlydesigned,awirelessLAN

maybeinterference-proneandeasilyeavesdroppedupon.ThedesignofawirelessLANmustpermitreliabletransmissioneveninanoisyenvironmentandshouldprovidesomelevelofsecurityfromeavesdropping.

Collocatednetworkoperation.AswirelessLANsbecomemorepopular,itisquite

likelyfortwoofthemtooperateinthesameareaorinsomeareawhereinterferencebetweentheLANsispossible.Suchinterferencemaythwartthenormaloperationofa

MACalgorithmandmayallowunauthorizedaccesstoaparticularLAN.License-freeoperation.UserswouldprefertobuyandoperatewirelessLAN

productswithouthavingtosecurealicenseforthefrequencybandusedbytheLAN.HandoWroaming.TheMACprotocolusedinthewirelessLANshouldenable

mobilestationstomovefromonecelltoanother.

Dynamicconfiguration.TheMACaddressingandnetworkmanagementaspectsof

theLANshouldpermitdynamicandautomatedaddition,deletion,andrelocationofendsystemswithoutdisruptiontootherusers.

PhysicalMediumSpecification

Threephysicalmediaaredefinedinthecurrent802.11standard:

Infraredat1Mbpsand2Mbpsoperatingatawavelengthbetween850and950nm.Direct-sequencespreadspectrumoperatinginthe2.4-GHzISMband.Upto7channels,eachwithadatarateof1Mbpsor2Mbps,canbeused.

Frequency-hoppingspreadspectrumoperatinginthe2.4-GHzISMband.Thedetailsofthisoptionareforfurtherstudy.

WirelessLANTechnology

WirelessLANsaregenerallycategorizedaccordingtothetransmissiontechniquethatisused.AllcurrentwirelessLANproductsfallintooneofthefollowingcategories:

Infrared（IR）LANs.AnindividualcellofanIRLANislimitedtoasingleroom,asinfraredlightdoesnotpenetrateopaquewalls.

SpreadSpectrumLANs.ThistypeofLANmakesuseofspreadspectrum

transmissiontechnology.Inmostcases,theseLANsoperateintheISM（Industrial,Scientific,andMedical）bands,sothatnoFCClicensingisrequiredfortheiruseintheU.S.

NarrowbandMicrowave.TheseLANsoperateatmicrowavefrequenciesbutdonot

usespreadspectrum.SomeoftheseproductsoperateatfrequenciesthatrequireFCClicensing,whileothersuseoneoftheunlicensedISMbands.

AsetofwirelessLANstandardshasbeendevelopedbytheIEEE802.11committee.Theterminologyandsomeofthespecificfeaturesof802.11areuniqueto

thisstandardandarenotreflectedinallcommercialproducts.However,itisusefultobefamiliarwiththestandardasitsfeaturesarerepresentativeofrequiredwirelessLANcapabilities.

ThesmallestbuildingblockofawirelessLANisabasicserviceset（BSS）,whichconsistsofsomenumberofstationsexecutingthesameMACprotocolandcompetingforaccesstothesamesharedmedium.Abasicservicesetmaybeisolated,oritmayconnecttoabackbonedistributionsystemthroughanaccesspoint.Theaccesspointfunctionsasabridge.TheMACprotocolmaybefullydistributedorcontrolledbyacentralcoordinationfunctionhousedintheaccesspoint.Thebasicservicesetgenerallycorrespondstowhatisreferredtoasacellintheliterature.Anextendedserviceset（ESS）consistsoftwoormorebasicservicesetsinterconnectedbyadistributionsystem.Typically,thedistributionsystemisawiredbackboneLAN.TheextendedservicesetappearsasasinglelogicalLANtothelogicallinkcontrol（LLC）level.Thestandarddefinesthreetypesofstations,basedonmobility:

No-transition.AstationofthistypeiseitherstationaryormovesonlywithinthedirectcommunicationrangeofthecommunicatingstationsofasingleBSS.BSS-transition.ThisisdefinedasastationmovementfromoneBSStoanotherBSSwithinthesameESS.Inthiscase,deliveryofdatatothestationrequiresthattheaddressingcapabilitybeabletorecognizethenewlocationofthestation.ESS-transition.ThisisdefinedasastationmovementfromaBSSinoneESStoaBSSwithinanotherESS.Thiscaseissupportedonlyinthesensethatthestationcanmove.Maintenanceofupper-layerconnectionssupportedby802.11cannotbeguaranteed.Infac