通信及基础工程30

Unit15TextAHowWiFiWorksHowWiFiWorksIfyou'vebeeninanairport,coffeeshop,libraryorhotelrecently,chancesareyou'vebeenrightinthemiddleofawirelessnetwork.Manypeoplealsousewirelessnetworking,alsocalledWiFior802.11networking[1],toconnecttheircomputersathome,andsomecitiesaretryingtousethetechnologytoprovidefreeorlow-costInternetaccesstoresidents.Inthenearfuture,wirelessnetworkingmaybecomesowidespreadthatyoucanaccesstheInternetjustaboutanywhereatanytime,withoutusingwires.TextTextWordsNotesDiscussionHomeWiFihasalotofadvantages.Wirelessnetworksareeasytosetupandinexpensive.They'realsounobtrusive--unlessyou'reonthelookoutforaplacetowatchstreamingmoviesonyourtablet,youmaynotevennoticewhenyou'reinahotspot.Inthisarticle,we'lllookatthetechnologythatallowsinformationtotravelovertheair.We'llalsoreviewwhatittakestocreateawirelessnetworkinyourhome.TextTextWordsNotesDiscussionHome1.WhatIsWiFi?Awirelessnetworkusesradiowaves,justlikecellphones,televisionsandradiosdo.Infact,communicationacrossawirelessnetworkisalotliketwo-wayradiocommunication.Here'swhathappens:1)Acomputer'swirelessadaptertranslatesdataintoaradiosignalandtransmitsitusinganantenna.2)Awirelessrouterreceivesthesignalanddecodesit.TheroutersendstheinformationtotheInternetusingaphysical,wiredEthernetconnection.TextTextWordsNotesDiscussionHomeTheprocessalsoworksinreverse,withtherouterreceivinginformationfromtheInternet,translatingitintoaradiosignalandsendingittothecomputer'swirelessadapter.TextTextWordsNotesDiscussionHomeTheradiosusedforWiFicommunicationareverysimilartotheradiosusedforwalkie-talkies,cellphonesandotherdevices.Theycantransmitandreceiveradiowaves,andtheycanconvert1sand0sintoradiowavesandconverttheradiowavesbackinto1sand0s.ButWiFiradioshaveafewnotabledifferencesfromotherradios:TextTextWordsNotesDiscussionHome1)Theytransmitatfrequenciesof2.4GHzor5GHz.Thisfrequencyisconsiderablyhigherthanthefrequenciesusedforcellphones,walkie-talkiesandtelevisions.Thehigherfrequencyallowsthesignaltocarrymoredata.2)Theyuse802.11networkingstandards,whichcomeinseveralflavors:TextTextWordsNotesDiscussionHome①802.11atransmitsat5GHzandcanmoveupto54megabitsofdatapersecond.ItalsousesOrthogonalFrequency-DivisionMultiplexing(OFDM),amoreefficientcodingtechniquethatsplitsthatradiosignalintoseveralsub-signalsbeforetheyreachareceiver.Thisgreatlyreducesinterference.②802.11bistheslowestandleastexpensivestandard.Forawhile,itscostmadeitpopular,butnowit'sbecominglesscommonasfasterstandardsbecomelessexpensive.802.11btransmitsinthe2.4GHzfrequencybandoftheradiospectrum.Itcanhandleupto11megabitsofdatapersecond,anditusesComplementaryCodeKeying(CCK)modulationtoimprovespeeds.TextTextWordsNotesDiscussionHome③802.11gtransmitsat2.4GHzlike802.11b,butit'salotfaster--itcanhandleupto54megabitsofdatapersecond.802.11gisfasterbecauseitusesthesameOFDMcodingas802.11a.④802.11nisthemostwidelyavailableofthestandardsandisbackwardcompatiblewitha,bandg.Itsignificantlyimprovedspeedandrangeoveritspredecessors.Forinstance,although802.11gtheoreticallymoves54megabitsofdatapersecond,itonlyachievesreal-worldspeedsofabout24megabitsofdatapersecondbecauseofnetworkcongestion.802.11n,however,reportedlycanachievespeedsashighas140megabitspersecond.802.11ncantransmituptofourstreamsofdata,eachatamaximumof150megabitspersecond,butmostroutersonlyallowfortwoorthreestreams.TextTextWordsNotesDiscussionHome⑤802.11acistheneweststandardasofearly2013.Ithasyettobewidelyadopted,andisstillindraftformattheInstituteofElectricalandElectronicsEngineers(IEEE),butdevicesthatsupportitarealreadyonthemarket.802.11acisbackwardcompatiblewith802.11n(andthereforetheothers,too),withnonthe2.4GHzbandandaconthe5GHzband.Itislesspronetointerferenceandfarfasterthanitspredecessors,pushingamaximumof450megabitspersecondonasinglestream,althoughreal-worldspeedsmaybelower.Like802.11n,itallowsfortransmissiononmultiplespatialstreams--uptoeight,optionally.Itissometimescalled5GWiFibecauseofitsfrequencyband,sometimesGigabitWiFibecauseofitspotentialtoexceedagigabitpersecondonmultiplestreamsandsometimesVeryHighThroughput(VHT)forthesamereason.TextTextWordsNotesDiscussionHome⑥Other802.11standardsfocusonspecificapplicationsofwirelessnetworks,likewideareanetworks(WANs)insidevehiclesortechnologythatletsyoumovefromonewirelessnetworktoanotherseamlessly.3)WiFiradioscantransmitonanyofthreefrequencybands.Or,theycan"frequencyhop"rapidlybetweenthedifferentbands.Frequencyhoppinghelpsreduceinterferenceandletsmultipledevicesusethesamewirelessconnectionsimultaneously.TextTextWordsNotesDiscussionHomeAslongastheyallhavewirelessadapters,severaldevicescanuseoneroutertoconnecttotheInternet.Thisconnectionisconvenient,virtuallyinvisibleandfairlyreliable;however,iftherouterfailsoriftoomanypeopletrytousehigh-bandwidthapplicationsatthesametime,userscanexperienceinterferenceorlosetheirconnections.Althoughnewer,fasterstandardslike802.11accouldhelpwiththat.Next,we'lllookathowtoconnecttotheInternetfromaWiFihotspot.TextTextWordsNotesDiscussionHome2.WiFiHotspotsAWiFihotspotissimplyanareawithanaccessiblewirelessnetwork.Thetermismostoftenusedtorefertowirelessnetworksinpublicareaslikeairportsandcoffeeshops.Somearefreeandsomerequirefeesforuse,butineithercasetheycanbehandywhenyouareonthego.Youcanevencreateyourownmobilehotspotusingacellphoneoranexternaldevicethatcanconnecttoacellularnetwork.AndyoucanalwayssetupaWiFinetworkathome.TextTextWordsNotesDiscussionHomeIfyouwanttotakeadvantageofpublicWiFihotspotsoryourownhome-basednetwork,thefirstthingyou'llneedtodoismakesureyourcomputerhastherightgear.Mostnewlaptopsandmanynewdesktopcomputerscomewithbuilt-inwirelesstransmitters,andjustaboutallmobiledevicesareWiFienabled.Ifyourcomputerisn'talreadyequipped,youcanbuyawirelessadapterthatplugsintothePCcardslotorUSBport.DesktopcomputerscanuseUSBadapters,oryoucanbuyanadapterthatplugsintothePCIslotinsidethecomputer'scase.Manyoftheseadapterscanusemorethanone802.11standard.TextTextWordsNotesDiscussionHomeOnceyou'veinstalledawirelessadapterandthedriversthatallowittooperate,yourcomputershouldbeabletoautomaticallydiscoverexistingnetworks.ThismeansthatwhenyouturnyourcomputeroninaWiFihotspot,thecomputerwillinformyouthatthenetworkexistsandaskwhetheryouwanttoconnecttoit.Ifyouhaveanoldercomputer,youmayneedtouseasoftwareprogramtodetectandconnecttoawirelessnetwork.TextTextWordsNotesDiscussionHomeBeingabletoconnecttotheInternetinpublichotspotsisextremelyconvenient.Wirelesshomenetworksareconvenientaswell.Theyallowyoutoeasilyconnectmultiplecomputersandtomovethemfromplacetoplacewithoutdisconnectingandreconnectingwires.Inthenextsection,we'lllookathowtocreateawirelessnetworkinyourhome.TextTextWordsNotesDiscussionHome3.BuildingaWirelessNetworkIfyoualreadyhaveseveralcomputersnetworkedinyourhome,youcancreateawirelessnetworkwithawirelessaccesspoint.Ifyouhaveseveralcomputersthatarenotnetworked,orifyouwanttoreplaceyourEthernetnetwork,you'llneedawirelessrouter.Thisisasingleunitthatcontains:·AporttoconnecttoyourcableorDSLmodem·

Arouter·

AnEthernethub[2]·

Afirewall·

AwirelessaccesspointTextTextWordsNotesDiscussionHomeAwirelessrouterallowsyoutousewirelesssignalsorEthernetcablestoconnectyourcomputersandmobiledevicestooneanother,toaprinterandtotheInternet.Mostroutersprovidecoverageforabout100feet(30.5meters)inalldirections,althoughwallsanddoorscanblockthesignal.Ifyourhomeisverylarge,youcanbuyinexpensiverangeextendersorrepeaterstoincreaseyourrouter'srange.TextTextWordsNotesDiscussionHomeOnceyoupluginyourrouter,itshouldstartworkingatitsdefaultsettings.MostroutersletyouuseaWebinterfacetochangeyoursettings.Youcanselect:·Thenameofthenetwork,knownasitsservicesetidentifier(SSID)--Thedefaultsettingisusuallythemanufacturer'sname.TextTextWordsNotesDiscussionHome·

Thechannelthattherouteruses--Mostroutersusechannel6bydefault.Ifyouliveinanapartmentandyourneighborsarealsousingchannel6,youmayexperienceinterference.Switchingtoadifferentchannelshouldeliminatetheproblem.·

Yourrouter'ssecurityoptions--Manyroutersuseastandard,publiclyavailablesign-on,soit'sagoodideatosetyourownusernameandpassword.TextTextWordsNotesDiscussionHomeSecurityisanimportantpartofahomewirelessnetwork,aswellaspublicWiFihotspots.Ifyousetyourroutertocreateanopenhotspot,anyonewhohasawirelesscardwillbeabletouseyoursignal.Mostpeoplewouldratherkeepstrangersoutoftheirnetwork,though.Doingsorequiresyoutotakeafewsecurityprecautions.TextTextWordsNotesDiscussionHomeIt'salsoimportanttomakesureyoursecurityprecautionsarecurrent.TheWiredEquivalencyPrivacy(WEP)securitymeasurewasoncethestandardforWANsecurity.TheideabehindWEPwastocreateawirelesssecurityplatformthatwouldmakeanywirelessnetworkassecureasatraditionalwirednetwork.ButhackersdiscoveredvulnerabilitiesintheWEPapproach,andtodayit'seasytofindapplicationsandprogramsthatcancompromiseaWANrunningWEPsecurity.ItwassucceededbythefirstversionofWiFiProtectedAccess(WPA),whichusesTemporalKeyIntegrityProtocol(TKIP)[3]encryptionandisastepupfromWEP,butisalsonolongerconsideredsecure.TextTextWordsNotesDiscussionHomeTokeepyournetworkprivate,youcanuseoneorbothofthefollowingmethods:·

WiFiProtectedAccessversion2(WPA2)isthesuccessortoWEPandWPA,andisnowtherecommendedsecuritystandardforWiFinetworks.·

MediaAccessControl(MAC)addressfilteringisalittledifferentfromWEP,WPAorWPA2.Itdoesn'tuseapasswordtoauthenticateusers--itusesacomputer'sphysicalhardware.TextTextWordsNotesDiscussionHomeWirelessnetworksareeasyandinexpensivetosetup,andmostrouters'Webinterfacesarevirtuallyself-explanatory.TextTextWordsNotesDiscussionHomeunobtrusiveadj.不突出的，不引人注目的notableadj.值得注意的；显著的flavorn.特点；韵味；味handlevt.处理；操作；运用；买卖improvevt.改善；提高…的价值significantlyadv.显著地；相当数量地WordsWordsTextWordsNotesDiscussionHomepredecessorn.前身，原有事物；前任routern.路由器congestionn.阻塞；拥挤，堵车

extendern.扩充器；延长器repeatern.[通信]转发器adaptern.适配器；改编者；接合器WordsTextWordsNotesDiscussionHomedefaultn.系统默认值；违约；缺乏hotspotn.热点；热区equivalencyn.等价；相等WordsTextWordsNotesDiscussionHome[1]802.11协议簇是国际电工电子工程学会（IEEE）为无线局域网络制定的标准。虽然Wi-Fi使用了802.11的媒体访问数据链路层（DLL）和物理层（PHY），但是两者并不完全一致。在以下标准中，使用最多的应该是802.11n标准，工作在2.4GHz或5GHz频段，可达600Mbps（理论值）。IEEE最初制定的一个无线局域网标准，主要用于解决办公室局域网和校园网中用户与用户终端的无线接入，业务主要限于数据存取，速率最高只能达到2Mbps。由于它在速率和传输距离上都不能满足人们的需要，因此，IEEE小组又相继推出了802.11b和802.11a两个新标准。TextWordsNotesDiscussionNotesHome[2]hub是一个多端口的转发器，在以hub为中心设备时，即使网络中某条线路产生了故障，并不影响其它线路的工作。所以hub在局域网中得到了广泛的应用。大多数的时候它用在星型与树型网络拓扑结构中，以RJ45接口与各主机相连（也有BNC接口），hub按照不同的说法有很多种类。hub按照对输入信号的处理方式上，可以分为无源hub、有源hub、智能hub。TextWordsNotesDiscussionHome[3]TemporalKeyIntegrityProtocol（TKIP）是IEEE802.11i规范中负责处理无线安全问题的加密协议。在IEEE802.11i规范中，TKIP负责处理无线安全问题的加密部分。TKIP实现了密钥混合功能，结合秘密与根密钥初始化向量传递给它的RC4初始化之前。TKIP是包裹在已有WEP密码外围的一层“外壳”。TKIP由WEP使用的同样的加密引擎和RC4算法组成。TextWordsNotesDiscussionHomeQuestionsfordiscussion1.WhatisWiFi?2.WhatdoesWiFiHotspotsmean?3.HowtosetupaWiFiconnectioninyourhome?TextWordsNotesDiscussionHomeAnswerstoquestionsfordiscussionTextWordsNotesDiscussionHome1.WhatisWiFi?WiFireferstoawirelessnetwork,whichusesradiowavesjustlikecellphones,televisionsandradiosdo.Infact,communicationacrossawirelessnetworkisalotliketwo-wayradiocommunication.Here'swhathappens:Tostart,acomputer'swirelessadaptertranslatesdataintoaradiosignalandtransmitsitusinganantenna.Andthen,awirelessrouterreceivesthesignalanddecodesit.TheroutersendstheinformationtotheInternetusingaphysical,wiredEthernetconnection.TextWordsNot