《计算机英语》计算机英语第七章

计算机英语Chapter7ComputerNetwork11Types2Topologies目录3ModelsCONTENT4NetworkingDevices5WirelessNetwork2LeadinComputernetworksarethecombinedresultsofcomputertechnologiesandcommunicationtechnologies.Acomputernetworkiscomposedofmultiplecomputersconnectedtogetherusingatelecommunicationsystempurposeofsharingdata,resourcesandcommunication.[1]Itprovidesfunctionssuchsharing,anddistributedprocessing.fortheasinformationexchange,resource31Types41TypesInthelightoftheirphysicalsize,networksmetropolitan,andMoreover,theconnectionoftwoormorenetworksisworldwideexampleofaninternetwork.canbedividedintolocal,networks.wideareacalledaninternetwork.TheInternetisawell-known51TypesAlocalareanetwork(LAN)isaanetworkgeographicallylimitedarea,suchasahome,anoffice,orabuilding,asFigure7-1shows.Itcomputersofficesandfactoriestoshareresourcesandexchangeinformation.Althoughsingleisgeographicallybeconnectedtodefiningtheirsmallergeographicneedforleasedtelecommunicationlines.thatconnectscomputersinLocalAreaNetworkiswidelyusedtoconnectpersonalandworkstationsincompanyLANlimited,separateLANcanformlargernetworks.TheLANsincludecharacteristicsofmuchhigherdatatransferrates,range,andlackofa61TypesMetropolitanAreaNetworkAmetropolitanareanetworkbiggerversionofa(MAN),asshowninFigurenormallyusessimilartechnology.Itextendsoveranentirecity.Itmaybeasinglenetwork,oritmaybeameansofconnectinganumberofLANsintosharedexample,acompanycanuseaMANtoinarehigh-capacityandhigh-speedfiberoptics.7-2,isbasicallyaLANandnetworksuchasacabletelevisionalargernetworksothatresourcesmaybeLAN-to-LANaswellasdevice-to-device.ForconnecttheLANsallofitsofficesthroughoutacity.BackbonesofaMAN71TypesWideAreaNetworkAsthenamesuggests,awideareanetwork(WAN)coversalargegeographicalareasuchcontinent,oreventhewholewhichdependontheirownhardwarefortransmission,WANsmaysuchtransmitdata,voice,image,andvideoinformationoverlargewidelyusedWAN.Figure7-3showswhataWANis.asacountry,aworld.IncontrasttoLANsutilizecombinedcommunicationdevicesastelephonelines,microwaves,andsatellitestogeographicalareas.TheInternetisthemost81TypesInternetworkAnetworkofnetworksiscalledaninternetwork,orsimplytheinternet.Itexistenceonthisplanet.TheinternethugelyconnectsisthelargestnetworkinallWANsanditcanhaveconnectiontoLANsandhomenetworks.IntranetsareinternalnetworksthatuseInternetandWebtechnologies.Theyworkinginuserscompany’scustomersinternetworkexample,acompany’scustomersmaybeprovidedwithaccesstosomepartsofitsintranetthuscreatinganextranet.makecompanyinformationaccessibletoemployeesandfacilitategroups.Anextranetisanintranetthatextendstotheauthorizedoutsidethecompany.ExtranetsfacilitateIncommunicationsamongaorsuppliers.thisterm,anextranetisanthatislimitedinscopetoasingleorganizationorentity.For92Topologies102TopologiesTheconfiguration,orphysicalarrangement,ofthecallednetworktopologyornetworkarchitecture.Thefivebus,ring,hybriddeviceinacommunicationnetworkisprincipalnetworktopologiesarestar,andmesh.112TopologiesStarNetworkAstarnetworkisaLANinwhicheachdevice(node)isconnectedtoacentralcomputer(hub)inconfigurationconsistingterminals.[2]Infromfurtherroutingnecessary.Thestarismicrocomputerstoamainframethatallowsaccesstoanorganization'sdatabase.astar-shaped(SeeFigure7-4),commonlyanetworkofacentralcomputersurroundedbyastarnetwork,messagespassdirectlyanodetothecentralcomputer,whichhandlesanyanothernode)thatmightbe(astoacommontopologyforlinking122TopologiesStarNetworkAstarnetworkisreliableinthesensethatanodecanfailwithoutaffectinganyothernodeonthenetwork.ItscomputerresultsinAdditionally,becauseeachnodeisthehub,cablecostscanbehigh.weakness,however,isthatfailureofthecentralashutdownoftheentirenetwork.individuallywiredto132TopologiesBusNetworkInthecaseofbustopology,alldevicessharesinglecommunicationlineorcablesendingdataaccesscontroltechnologysuchrecognizesonehostasthebusmastertobothonedirectionandassoonasitreachestheextremeend,theterminatorremovesthedatafromtheline.calledbus.Bustopologymayhaveproblemswhilemultiplehostsareatthesametime.Therefore,bustopologyeitherusesamediaascarriersensemultipleaccess(CSMA)[3]orsolvetheissue.AsFigure7-5shows,endsofthesharedchannelhavelineterminators.Thedataissentinonly142TopologiesBusNetworkThebusnetworkistypicallyusedwhenonlyafewmicrocomputersaretobelinkednetwork,theinstallandlessexpensive.However,failureofthemakeallotherdevicesstopfunctioning.together.Comparedwiththestarbusnetworkiseasiertosharedcommunicationlinecan152TopologiesRingNetworkInaringnetwork(SeeFigure7-6),eachdeviceconnectstocircularnetworktowhichisnotadjacenttoit,thedatatravelsthroughallintermediatecanbepassedtoconnectonemoredeviceintheadministratormayneedonlycable.exactlytwootherdevices,creatingastructure.Whenonedevicetriescommunicateorsendmessagetoadevicedevices.Inthisway,informationanydeviceonthenetwork.Totheexistingstructure,onemoreextra162TopologiesRingNetworkBecauseofitsstructureofaclosedloop,theringnetworknetworks.However,failureofanydeviceresultsinofthewholepointoffailure.Theremorebackupring.cancoverlargerdistancesthanstarandbusfailurering.Thus,everyconnectioninmethodswhichtheringisaareemployone172TopologiesHybridNetworkAnetworkstructurewhosedesigncontainsmoretopology.Thehybriddemeritsofalltheincorporatingtopologies.thanonetopologyiscalledthehybridtopologyinheritsmeritsandFigure7-7representsanarbitrarilyhybridtopology.attributesofstar,ringWANstopologymostlyexampleofthelargesthybridThecombiningtopologiesmaycontainandbybustopologies.Mostareconnectedmeansofdual-ringandnetworksconnectedtothemarestartopologynetworks.Internetisthebesttopology.182TopologiesMeshNetworkInthistypeoftopology,adeviceisconnectedtooneormultipledevices.Thistopologyhasdevicesinalsohavedeviceswhichareinpoint-to-pointconnectionwitheveryotherdeviceormaypoint-to-pointconnectiontoafewdevicesonly.Hence,meshtopologycomesintotwotypes.Oneisfullmesh,inwhichalldeviceshaveapoint-to-pointconnectiontoprovidesthemostreliable8representsafullmeshnetworktopology.Thoughthistopologyismoretheoretical,itcanbeimplementedmoremesh,inThistopologyexistswhereweneedtoprovidereliabilitytosomedevicesoutofall.everyotherhostinthenetwork.Itnetworkstructureamongallnetworktopologies.Figure7-easilyinwirelessnetworking.Anotheroneispartialwhichnotalldeviceshavepoint-to-pointconnectiontoeveryotherdevice.193Models203ModelsComputernetworksmaybefurtherclassifiedaccordingtothenetworklayeratwhichtheyoperateaccordingtosomebasicreferencemodelsthatareconsideredreferencemodelandthefive-layerTCP/IPmodel.[4]tobestandardsintheindustrysuchastheseven-layerOSIToeasenetworkengineering,thewholenetworkingconceptisdividedintoindependentofallotherlayers.Butasawhole,almostallnetworkingtasksdependonalloftheselayers.Layerssharedataamongthemoneachotheronlytotakeinputandsendoutput.multiplelayers.Eachlayerisinvolvedinsomeparticulartaskandisandtheydepend213ModelsNo.1OSIReferenceModelToensurenationalandworldwidedatacommunication,ISOOrganization)developedamodelforopensysteminterconnection,orOSImodel.Thisisaconceptualizedviewofhowonesystemcommunicateprotocolsdefinedin(InternationalStandardizationlayeredmodelshouldwiththeother,usingvariouseachlayer.223ModelsNo.1OSIReferenceModelInnetworking,aprotocolisasetofstandardsandrulesforcommunicationbetweennetworkdevices.Protocolsoftenprovideservices,suchase-mailsorfile-transfer.Hundredsofdifferentcomputernetworkprotocolshavebeendeveloped,eachdesignedforspecificfamilyprotocolslikeTransmissionControlProtocol(TCP),UserDatagramHyperTextTransportProtocol(HTTP),andFilewithIPormoreprotocolsandtwoconnecteddevicescanidentifyaswellasabidebythem.purposesandenvironments.TheInternetProtocol(IP)itself,higher-levelcontainsasetofrelatednetworkprotocols.BesidesIPProtocol(UDP),TransferProtocol(FTP)allintegratetoprovideadditionalcapabilities.IntheOSImodel,eachlayercontainsone233ModelsNo.1OSIReferenceModelTheOSImodelhasthefollowingsevenlayers:243ModelsNo.1OSIReferenceModelPhysicalLayer(Layer1):Thislayerdealsactualcommunicationmechanismaslength,etc.Itsfunctionistotransmissionandreceptionofrawstreamsoveraphysicalmedium.withhardwaretechnologyandsuchsignaling,voltage,cabletypeandensurethebit253ModelsNo.1OSIReferenceModelDataLinkLayer(Layer2):Thislayertakestherawtransmissiondatafromthephysicallayerandmakesdataupperlayerandviceversa.ThislayeralsochecksanytransmissionerrorsandsortsthemElectricalandElectronicsEngineersstandardsdealingwithdividesthedatalayer,whichisresponsibleforcontrollinghowdevicesintomediumwhichencapsulatingthemframes,andsendsthoseframestotheoutaccordingly.Specifically,IEEE802[5](afamilyofInstituteofLANsandMANs)linklayerintotwosublayers:MediaAccessControl(MAC)anetworkgainaccessandpermissiontotransmitit;andLogicalLinkControl(LLC)layer,isresponsibleforidentifyingnetworklayerprotocolsandthenandcontrolserrorcheckingandframesynchronization.263ModelsNo.1OSIReferenceModelNetworkLayer(Layer3):Thislayerprovidesthefunctionalandproceduralmeansanotheroftransferringvariableconnectedtomachineaddress.Thenetworklayermayhavetopacketsdatagrams.lengthdatasequences(datagrams)fromonenodetothesamenetwork.IttranslateslogicalnetworkaddressintophysicalbreaklargedatagramsintosmallerandhostreceivingthepacketwillhavetoassemblethefragmentedTransportLayer(Layer4):Thislayerprovidesend-to-enddatadeliveryamonghosts.Itsegmentsandthengivesthemtransport-layerprotocolin(TCP),usuallybuiltontopoftheInternetProtocol(IP).takesdatafromthesessionlayerandbreaksitintosmallerunitscalledtothenetworklayerfortransmission.AnexampleofathestandardInternetstackisTransmissionControlProtocol273ModelsNo.1OSIReferenceModelSessionLayer(Layer5):Thislayermaintainssessionsbetweenremotehosts.Eachhostconnectionisidentifyseparate.Itthem.Terminationhappenswhenasessionisidleforsomeperiodoftime.referredtosessionasasession.ThesessionlayerusesasessionIDtoeachuniquewhichkeepsthedatastreamfromdifferenthostsisresponsibleforsettingupcommunicationsessionsandterminatingPresentationLayer(Layer6):Thislayerisresponsibleforthetranslationofdatabetweenanetworkingserviceandanapplication,includingcharacterencoding,datacompressionandencryption/decryption.Datafromthesenderisconvertedtoon-the-wirenativerepresentationofthereceiver.data(generalstandardformat)andatthereceiver'senditisconvertedtothe283ModelsNo.1OSIReferenceModelApplicationLayer(Layer7):Thislayerisresponsibleforintegratingnetworkserviceswiththeoperatingsystem.Itprovidestheinterfacebetweentheapplicationsandtherestofthenetwork.Differentprotocolsareassociatedwiththeapplicationlayer,suchasHTTP,FTP,SMTPandmanyothers.Actually,mostoftheseprotocolsorservicescanspanmultiplelayersandtheapplicationlayeristhehighestlayerthattheseprotocolsfunctionat.[6]TheOSImodeldividesnetworkingtasksintologicallayersforeasiercomprehensionsoitaidsinallowsspecializationoffeaturesatdifferentlevels,promotesstandardsofinteroperabilitybetweennetworksanddevicesandprovidesmodularityinmeansthatdeveloperscanchangefeatureswithoutchangingtheentireapproach.networkingfeatures.Modularity293ModelsNo.1OSIReferenceModelApplicationLayer(Layer7):Thislayerisresponsibleforintegratingnetworkserviceswiththeoperatingsystem.Itprovidestheinterfacebetweentheapplicationsandtherestofthenetwork.Differentprotocolsareassociatedwiththeapplicationlayer,suchasHTTP,FTP,SMTPandmanyothers.Actually,mostoftheseprotocolsorservicescanspanmultiplelayersandtheapplicationlayeristhehighestlayerthattheseprotocolsfunctionat.[6]TheOSImodeldividesnetworkingtasksintologicallayersforeasiercomprehensionsoitaidsinallowsspecializationoffeaturesatdifferentlevels,promotesstandardsofinteroperabilitybetweennetworksanddevicesandprovidesmodularityinmeansthatdeveloperscanchangefeatureswithoutchangingtheentireapproach.networkingfeatures.Modularity303ModelsNo.1OSIReferenceModelAlthoughOSIhasallthoseadvantages,OSIlayersarefunctions.layer-to-layercorrespondencewiththerealworldfunctionsmessage.AparticularprotocolimplementationmaynotrepresenteveryOSIlayerormayspreadacrossmultiplelayers.theoreticalanddonotactuallyperformrealIndustryimplementationsrarelyhaveatheOSIlayers.Inperformdifferentprotocolsdifferentthathelpsendorreceivetheoverall313ModelsNo.2TCP/IPReferenceModelAmajorityoftheinternetusesaprotocolsuitecalledtheInternetprotocolsuitedifferentprotocolsprotocols(TransmissionsuitewithsomefeaturesfoundundertheOSImodel.AlsoitcombinesthefeaturesofsomeadjacentOSIlayersandsplitsotherlayers'parts.ThefournetworklayersdefinedbyTCP/IPmodelanditscomparisonwiththeOSImodelareasfollows:Thissuiteisacombinationofprotocolswhichencompassesanumberofthetwomajorfordifferentpurposesandneeds.BecauseinthissuiteareTCPandIP,thisiscommonlytermedasTCP/IPControlProtocol/InternetProtocol)protocolsuite.Thisprotocolhasitsownreferencemodelwhichitfollowsovertheinternet.IncontrasttheOSImodel,thismodelofprotocolscontainsfewerlayers.Itdoesomit323ModelsNo.2TCP/IPReferenceModelAsFigure7-10shows,thelinklayeristhelowestcomponentlayeroftheInternetprotocols.ItprovidesmechanismUnlikeindependentofunderlyingnetworkhardware.Asaresult,TCP/IPtopofvirtuallyThelinklayerisInternetlayerinterfacesoftwodifferenthostsonthesamelink.ofsendingandreceivingactualdata.itsOSImodelcounterpart,thislayerisarchitectureandmaybeimplementedonanyhardwarenetworkingtechnology.usedtomovepacketsbetweenthe333ModelsNo.2TCP/IPReferenceModelTheInternetlayerisasubsetoftheOSInetworklayer.Itfacilitateshostaddressingandrecognitionaswellasdefinesrouting.IPworksonthislayer.Thetransportlayerdefineshowdatashouldlayerisguaranteedeliveryofpackets.flowbetweenhosts.MajorprotocolatthisTCP.Thetransportlayerhandleshost-to-hostcommunication,butitdoesnotTheapplicationlayerdefinestheprotocolwhichenablesuserstointeractwiththenetwork.ItincludestheOSIapplicationlayer,presentationlayer,andmostofthesessionlayer.ThestrictlayeringoftheOSImodeldoesnotpresentcontradictionsinTCP/IP,asitispermissiblemodel.[7]thatprotocolusagedoesnotfollowthehierarchyimpliedinalayered344NetworkingDevices354NetworkingDevicesTointerconnectmanytypesofnetworksthatexist,varioustypesofused,includingbridges,switches,routers,networkcards,etc.communicationdevicesarehubs,repeaters,364NetworkingDevicesAHubAhub,alsocalledaconcentratorormulti-stationaccessunitpointforcablesinconnectsegmentsofaLANanditcontainsmultipleports.Whenapacketarrivesatoneport,itisthatallsegmentsoftheLANcanseeallthepackets.(MSAU),isapieceofhardwarewhichprovidesacentralanetwork.Ahubisacommonlyusedtocopiedtotheotherportsso374NetworkingDevicesBRepeaterArepeaterisadevicethatacceptsasignalfromatransmissionmedium,amplifiesit,andretransmitsitoverthemedium.Asasignaltravelslongdistance,itmayundergoareductioninattenuation.distortedbyattenuation.Analogrepeaterscanfrequentlysignalwhilequality.RepeatersoftenareusedtointerconnectsegmentsinareusedtoextendWANtransmissiononwireandwirelessmedia.overastrength,anoccurrencecalledRepeatersregenerateanalogordigitalsignalsthatcanbetheamplifyonlydigitalrepeaterscanreconstructasignaltoitsnear-originalaLAN;theyalso384NetworkingDevicesCBridgeAnetworkbridgeisaninternetworkingdeviceusedtohelpconservethebandwidthonthenetwork.Itconnectsmultiplethedatalinklayer(Layer2)oftheOSImodel.Bridgesaresimilartohubsorrepeaters,devicesthatconnectnetworksegmentsatthephysicallayer,andhowever,abridgeworksbyusingbridgingwherenetworknetworksegments.[8]ItismoreefficientandeconomicaltotocombinesthetwoseparateLANs.networksegmentsattrafficfromoneismanagedratherthansimplyrebroadcastingtoadjacentuseabridgeconnecttwoseparateLANs,insteadofcreatingonelargeLANthat394NetworkingDevicesDSwitchAswitchisanotherinternetworkingdeviceusedtomanagebandwidthconnectedtotheswitch,theswitchcansupplyeachofthecomputerswithadedicatedcomputernetworkbyusingpacketswitching[9]toforwardswitchforwardsdataonlyit,ratherthanbroadcaststhesamedataoutofeachofitsports.Switchesoperateatthedatalinklayer(Layer2)andsometimesthenetworklayer(Layer3)oftheOSImodel.onalargenetwork.Whenthecomputersaredirectlyamountofbandwidth.Itconnectsdevicestogetheronareceive,processandlessdatatothedestinationdevice.Unlikeadvancedhubs,atooneormultipledevicesthatneedtoreceive404NetworkingDevicesERouterRoutersaredevicesthatmaketheInternetpossible.Theyareconnectedto(Layer3).Whentheyreceiveapacketofinformation,theydecidewhichwaytheyshouldefficiently.Forexample,whensendingane-mail,thee-mailmessagewillfirstbebrokenupintosmallpackets.Eachpacketwillcontaindestinationnearestrouterconnectednetworkreceiver’scomputer.Thefigurebelowrepresentsarouter.twoormorenetworksatcrossingpointsandoperateatthenetworklayersendthepackettoensurethatitgetstoitsdestinationquicklyandtheIPaddressofthethecomputer.Then,thecomputerwillshipallthepacketstotomanynetworks.Therouterthendecideswhichitshouldsendthepackettosothatthepacketcanbestreachthe414NetworkingDevicesFNetworkCardAnetworkcardisanecessarycomponentofacomputerwithoutwhichconnectednetworkadapteroranetworkinterfacecard(NIC).Itprovidesphysicalaccesstoanetworkingmediumandallowsuserstoconnecttoeachothereitherbyusingcablesorwirelessly.Mostbrandedcomputershavenetworkcardspre-installed.acomputercannotbeoveranetwork.Itisalsoknownasa425WirelessNetwork435WirelessNetworkBeingwirelessmeanstransmittingsignalsusingradioWirelesstechnologiesareusedfortasksassimpleasswitchingoffthetelevisionorascomplexassupplyingthesalesforcewithinformationfromautomatedkeyboardsandhavebecomepartofourdailylife.wavesasthemediuminsteadofwires.anenterpriseapplication.Nowcordlessmice,PDAs,andmobilephones445WirelessNetwork1WirelessTransmissionTransmissionmediaareeitherphysicalorwireless.Physicaltransmissionmediacommunicationscommunicationssignalsthroughtheairorspacebyusingradio,microwaveandinfraredsignals.usewire,cable,andothertangiblematerialstosendsendsignalswhilewirelesstransmissionmediaWirelesstransmissionisaformofunguidedmedia.Wirelesscommunicationinvolvesnophysicallinkestablishedbetweentwoormoredevices,communicatingwirelessly.Wirelesssignalsarespreadoverinandarereceivedandinterpretedbyappropriateantennas.theair455WirelessNetworkWhenanantennaisattachedtoanelectricalcircuitofacomputerorwirelessdevice,itconvertsthedigitaldataintoalloverwithinthesesignalsandconvertsthem1.Radiowirelesssignalsandspreaditsfrequencyrange.Thereceiverontheotherendreceivesbacktodigitaldata.TransmissionRadiofrequency(RF)iseasiertogenerateandbecauseofitslargewaveswavelengthitcanpenetratethroughwallsandstructuresalike.RadiocanhavewavelengthfromfromFrequency).whereashigherRFcantravelinlowfrequencyradio1mm—100,000kmandhavefrequencyrangingHigh3Hz(ExtremelyLowFrequency)to300GHz(ExtremelyRadiowavesatlowerfrequenciescantravelthroughwallsstraightlineandbounceback.Thepoweroffrequencywavesdecreasessharplyastheycoverlongdistances.Highwaveshavemorepower.465WirelessNetworkForradiotransmission,atransmitterisneededtosendthebroadcastradiothereceiverhasanantennathatislocatedinnetworksusewirelessdevices.signalandareceivertoacceptit.Toreceivethebroadcastradiosignal,therangeofthesignal.Someatransceiver,whichbothsendsandreceivessignalsfromAproposedradiofrequencyspecificationthatmanyportabledeviceswilluseforshort-rangewirelesscommunicationisBluetooth,devicescancommunicatewitheachother,desktopcomputers,anetwork,ortheInternet.ToincludeatransceiverchipcalledBluetooth.WithuseBluetoothtechnology,eachdevicemustandbewithinaspecificrange.475WirelessNetwork2.MicrowaveTransmissionElectromagneticwavesabove100MHztendtotravelinastraightlineandsignalsparticularstation.Amicrowavestationisanearth-basedreflectivedishthatcontainsmicrowavebothsenderandreceivermustbealignedtobestrictlyinMultiplefrequenciestransmissiondependshighlyupontheweatherconditionsandthefrequencybeingused.overthemcanbesentbybeamingthosewavestowardsoneantennas,transceivers,andotherequipmentnecessaryforcommunications.Becausemicrowavestravelinstraightlines,line-of-sight(LOS).antennascanbealignedtoreachfarther.Microwaveshavehigherandcan’tpenetratewall-likeobstacles.Microwave485WirelessNetworkMicrowavesareusedinspacecraftcommunication,andmuchcommunicationsareofthetransmittedstationsandcommunicationssatellites.Theyarealsoemployedin3.InfraredTransmissionworld'sdata,TV,andtelephonelongdistancesbymicrowavesbetweengroundmicrowaveovensandinradartechnology.Infraredwaveliesinbetweenvisiblelightspectrumandmicrowaves.Ithaswavelengthof700nmrangesfrom300GHzto430THz.to1mmandfrequency495WirelessNetwork2WirelessLANTheWirelessLAN(WLAN)isoneofthewirelesstechnologies.Nowadays,WLANwherewiringisdifficultorcostly.WLANsystemstransmitandreceivedataovertheairusingradioconnections.hasbecomeapopularalternativetowiredLANs,especiallyatlocationsfrequencyorinfraredtechnologytominimizetheneedforwiredThesenetworksprovideaveryfastdataspeedcomparedwiththedataratesofmobilethemostwidespreadandpopularexampleaLANtechnologydesignedtoprovidein-buildingbroadbandcoverage.telecommunicationstechnology,andtheirrangeisverylimited.Wi-FiisofWLANtechnology.Wi-Fistandsfor505WirelessNetworkIngeneral,theWLANcanoperateintwodifferentmodes.communicatedirectlywitheachother.Thisiscalledanadhocwirelessnetwork.Theadhocnetworkworksinpeer-to-peermodeanditusesphysicalmeshtopologywiththatalleachother.Theadhocwirelessnetworksaretypicallyusedtohosts.Inthefirstmode,twowirelessdeviceslogicalbustopology.Themeshtopologymeansdeviceshavetocommunicatedirectlywithcreatetemporaryconnectionsbetweentwo515WirelessNetworkThesecondmodeininfrastructurewhichawirelessnetworkcanoperateisthephysicalstartopologyandalogicalbustopology.Thiswirelessnetworkusesawirelessaccesspoint(WAP).TheAPisinmiddleandcommunicateswithallstationsonthenetwork.StationsthatneedtoandtheAPthensendsthatmessagetoinfrastructurewirelessnetworkcanbeconnectedeasilywiththewirednetwork.network.TheinfrastructurenetworkusesatypeofthesenddatatootherstationsfirstsenddatatotheAP,theenddevice.Withandthethenetworknewhostscanbeaddedeasily525WirelessNetwork3IssuesAlthoughwirelessnetworksareendowedwithinherentcharacteristicssuchasmobility,simplicityandreachability,therearetwomajorissueswiththem.Oneoftheprimaryconcernsaboutwirelessdatadeliveryisthat,unliketheinadequate.LostInternetthroughwiredservices,itsqualityofservice(QoS)ispacketsprotocols.andatmosphericinterferencearerecurringproblemsofthewirelessAnotherissueisaboutthesecurityrisk.Basicnetworksecuritymechanismsliketheservicesetidentifier(SSID)andwiredequivalentprivacy(WEP)maybeadequateentitiesthatrequirestrongersecurity.forresidencesandsmallbusinesses,buttheyareinadequateforthe53airusingradio译文：OSI模型的严格分层并不与TCP/IP矛盾，因为它允许该协议的使用不遵循分_________OSImodel_____:alayeredmodelthatprovidesaconceptualizedviewTheinternethugelyconnectsisaconceptualizedviewofhowonesystemfrequencywavesdecreasessharplyAcomputernetworkisAbbreviationsHz(Extremely(RadioFrequency)purposeofsharingdata,resourcesandcommunication.apacketarrivesatoneport,itissendthepackettoMicrowaveshavehighernetworksuseWords54Wordsspare[spεə]adj.备用的；空闲的overloaded['əuvə'ləudid][dʒiə'græfikəli]adj.超负荷的geographicallyadv.地理上leased[liːst]adj.租用的backbonecontinent['bækbəun]['kɔntinənt]n.支柱n.洲microwave['maikrəuweiv]n.微波n.卫星satelliteInternet['sætəlait]['intənet]n.因特网internetwork[intə'netwəːk]n.互联网络intranet['intrənet]n.内联网n.外联网extranet['ekstrənet][tə'pɒlədʒɪ]topologyn.拓扑n.节点node[nəud]hub[hʌb]n.中心；集线器55Wordsrouting['ruːtɪŋ]n.选择路径shutdown['ʃʌtdaun]n.关闭n.电缆cable['keibl]adjacent[ə'dʒeisənt]adj.邻近的loop[luːp]n.环demerit[diː'merit]n.缺点dual-ring['djuːəl-riŋ]adj.双环的adj.无线的wireless['waiəlis]protocol['prəutəkɔl]n.协议datagram['deitəɡræm]n.数据报n.数据包packet['pækit]idle['aidl]adj.空闲的；停顿的decryption[diː'kripʃən]n.解密on-the-wire[ɔn-ðə-'waiə]adj.线上的span[spæn]v.跨越56Wordspermissible[pə'misibl]adj.许可的repeaterbridge[ri'piːtə]n.中继器[bridʒ][switʃ]n.网桥switchroutern.交换机n.路由器n.集中器['ruːtə(r)]concentrator['kɔnsəntreitə]amplify['æmplifai]v.扩大attenuation[əˌtenju'eiʃən]n.衰减distort[dis'tɔːt]v.扭曲bandwidth['bændwidθ]n.带宽rebroadcast[ˌriː'brɔːdkɑːst;-kæst]v.重播permissible[pə'misibl]adj.许可的repeaterbridge[ri'piːtə]n.中继器[bridʒ][switʃ]n.网桥switchroutern.交换机n.路由器['ruːtə(r)]57Wordscordless['kɔːdlis]adj.无线的infrared[ˌinfrə'red]n.红外线unguided['ʌn'gaidid]adj.无向导的antenna[æn'tenə]n.天线n.波长wavelength['weivleŋθ]transmitter[trænz'mitə;træns-][træns'siːvə;trænz-]n.发送器n.收发器transceiverBluetooth['bluˌtuθ]n.蓝牙n.雷达n.光谱radar['reidə]spectrum['spektrəm]adhoc[ˌæd'hɔk][ri'kəːriŋ]adj.特别的；点对点的recurringadj.复发的adj.无线的cordlessinfrared['kɔːdlis][ˌinfrə'red]n.红外线unguided['ʌn'gaidid]adj.无向导的antenna[æn'tenə]n.天线58Phrases59Phrasesinformationexchangeworkfiberopticinexistenceworkinthesensebustopologybusmasterlineterminatorworkworkworkreferencemodel信息交换有线电视网光纤现有的星型网络就……意义而言总线拓扑总线主控线路终端器环形网络混合网络网状网络参考模型60PhrasesabidebybitstreamdataframeframesynchronizationsessionlayerpresentationlayercharacterencodingreferencepointItocolsuitelinklayeranalogsignal遵守比特流数据帧帧同步会话层表示层字符编码参考点互联网协议组链路层模拟信号61Phrasesdigitalsignalpacketswitchingdestinationdevicenetworkadapterworkbouncebackelectromagneticwavemicrowaveovenwork数字信号分组交换目标设备网络适配器无线网络反弹电磁波微波炉控制网络62Abbreviations63AbbreviationsLANMAN城域网(MetropolitanAreaNetwork)WAN广域网(WideAreaNetwork)CSMA载波侦听多路访问(CarrierSenseMultipleAccess)OSIISOIPTCPUDPHTTPFTPIEEE局域网(LocalAreaNetwork)开放式系统互联(OpenSystemInterconnect)国际标准化组织(InternationalStandardizationOrganization)互联网协议(InternetProtocol)传输控制协议(TransmissionControlProtocol)用户数据报协议(UserDatagramProtocol)超文本传输协议(HyperTextTransportProtocol)文件传输协议(FileTransferProtocol)电气与电子工程师协会(InstituteofElectricalandElectronicEngineers)64AbbreviationsMACLLCSMTPTCP/IP媒体访问控制(MediaAccessControl)逻辑链路控制(LogicalLinkControl)简单邮件传输协议(SimpleMessageTransferProtocol)传输控制/网络通讯协定(Transmis