《物聯(lián)網(wǎng)專業(yè)英語教程》課件第7章

Unit7WiFiandBluetooth[117]

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HowWiFiWorks?[125]

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Bluetooth[133]參考譯文WiFi是如何工作的？

Ifyou'vebeenatanairport,inacoffeeshop,alibraryorahotelrecently,chancesareyou'vebeenrightinthemiddleofawirelessnetwork.Manypeoplealsousewirelessnetworking,alsocalledWiFior802.11networking,toconnecttheircomputersathome,andsomecitiesaretryingtousethetechnologytoprovidefreeorlow-costInternetaccesstoresidents.HowWiFiWorks？Inthenearfuture,wirelessnetworkingmaybecomesowidespreadthatyoucanaccesstheInternetjustaboutanywhereatanytime,withoutusingwires.(SeeFigure7.1)

WirelessnetworksmakeiteasytoconnecttotheInternet.

Figure7.1WiFiworkswithoutusingwires

WiFihasalotofadvantages.Wirelessnetworksareeasytosetupandinexpensive.They'realsounobtrusive—unlessyou'reonthelookoutforaplacetouseyourlaptop,youmaynotevennoticewhenyou'reinahotspot.Inthisarticle,we'lllookatthetechnologythatallowsinformationtotravelovertheair.We'llalsoreviewwhatittakestocreateawirelessnetworkinyourhome.

1.WhatIsWiFi?

Awirelessnetworkusesradiowaves,justlikecellphones,televisionsandradiosdo.Infact,communicationacrossawirelessnetworkisalotliketwo-wayradiocommunication.Here'swhathappens:

·Acomputer'swirelessadaptertranslatesdataintoaradiosignalandtransmitsitusinganantenna.

·

Awirelessrouterreceivesthesignalanddecodesit.TheroutersendstheinformationtotheInternetusingaphysical,wiredEthernetconnection.

Theprocessalsoworksinreverse,withtherouterreceivinginformationfromtheInternet,translatingitintoaradiosignalandsendingittothecomputer'swirelessadapter.

TheradiosusedforWiFicommunicationareverysimilartotheradiosusedforwalkietalkies,cellphonesandotherdevices.Theycantransmitandreceiveradiowaves,andtheycanconvert1sand0sintoradiowavesandconverttheradiowavesbackinto1sand0s.ButWiFiradioshaveafewnotabledifferencesfromotherradios:

·Theytransmitatfrequenciesof2.4GHzor5GHz.Thisfrequencyisconsiderablyhigherthanthefrequenciesusedforcellphones,walkie-talkiesandtelevisions.Thehigherfrequencyallowsthesignaltocarrymoredata.

·

Theyuse802.11networkingstandards,whichcomeinseveralflavors:802.11atransmitsat5GHzandcanmoveupto54megabitsofdatapersecond.ItalsousesOrthogonalFrequency-DivisionMultiplexing(OFDM),amoreefficientcodingtechniquethatsplitsthatradiosignalintoseveralsub-signalsbeforetheyreachareceiver.Thisgreatlyreducesinterference.802.11bistheslowestandleastexpensivestandard.Forawhile,itscostmadeitpopular,butnowit'sbecominglesscommonasfasterstandardsbecomelessexpensive.802.11btransmitsinthe2.4GHzfrequencybandoftheradiospectrum.Itcanhandleupto11megabitsofdatapersecond,anditusesComplementaryCodeKeying(CCK)modulationtoimprovespeeds.802.11gtransmitsat2.4GHzlike802.11b,butit'salotfaster—itcanhandleupto54megabitsofdatapersecond.802.11gisfasterbecauseitusesthesameOFDMcodingas802.11a.802.11nistheneweststandardthatiswidelyavailable.Thisstandardsignificantlyimprovesspeedandrange.Forinstance,although802.11gtheoreticallymoves54megabitsofdatapersecond,itonlyachievesreal-worldspeedsofabout24megabitsofdatapersecondbecauseofnetworkcongestion.802.11n,however,reportedlycanachievespeedsashighas140megabitspersecond.

·

Other802.11standardsfocusonspecificapplicationsofwirelessnetworks,likeWideAreaNetworks(WANs)insidevehiclesortechnologythatletsyoumovefromonewirelessnetworktoanotherseamlessly.

·WiFiradioscantransmitonanyofthreefrequencybands.Or,theycan"frequencyhop"rapidlybetweenthedifferentbands.Frequencyhoppinghelpsreduceinterferenceandletsmultipledevicesusethesamewirelessconnectionsimultaneously.

Aslongastheyallhavewirelessadapters,severaldevicescanuseoneroutertoconnecttotheInternet.Thisconnectionisconvenient,virtuallyinvisibleandfairlyreliable;however,iftherouterfailsoriftoomanypeopletrytousehigh-bandwidthapplicationsatthesametime,userscanexperienceinterferenceorlosetheirconnections.

2.WiFiHotspots

IfyouwanttotakeadvantageofpublicWiFihotspotsorstartawirelessnetworkinyourhome,thefirstthingyou'llneedtodoismakesureyourcomputerhastherightgear.Mostnewlaptopsandmanynewdesktopcomputerscomewithbuilt-inwirelesstransmitters.Ifyourlaptopdoesn't,youcanbuyawirelessadapterthatplugsintothePCcardslotorUSBport(SeeFigure7.2).DesktopcomputerscanuseUSBadapters,oryoucanbuyanadapterthatplugsintothePCIslotinsidethecomputer'scase.Manyoftheseadapterscanusemorethanone802.11standard.

Wirelessadapterscanplugintoa

computer'sPCcardslotorUSBport.

Figure7.2Wirelessadapters

Onceyou'veinstalledyourwirelessadapterandthedriversthatallowittooperate,yourcomputershouldbeabletoautomaticallydiscoverexistingnetworks.ThismeansthatwhenyouturnyourcomputeroninaWiFihotspot,thecomputerwillinformyouthatthenetworkexistsandaskwhetheryouwanttoconnecttoit.Ifyouhaveanoldercomputer,youmayneedtouseasoftwareprogramtodetectandconnecttoawirelessnetwork.

BeingabletoconnecttotheInternetinpublichotspotsisextremelyconvenient.Wirelesshomenetworksareconvenientaswell.Theyallowyoutoeasilyconnectmultiplecomputersandtomovethemfromplacetoplacewithoutdisconnectingandreconnectingwires.

3.BuildingaWirelessNetwork

Ifyoualreadyhaveseveralcomputersnetworkedinyourhome,youcancreateawirelessnetworkwithawirelessaccesspoint.Ifyouhaveseveralcomputersthatarenotnetworked,orifyouwanttoreplaceyourEthernetnetwork,you'llneedawirelessrouter.Thisisasingleunitthatcontains:

·AporttoconnecttoyourcableorDSLmodem;

·Arouter;

·AnEthernethub;

·Afirewall;

·Awirelessaccesspoint.

AwirelessrouterallowsyoutousewirelesssignalsorEthernetcablestoconnectyourcomputerstooneanother,toaprinterandtotheInternet.Mostroutersprovidecoverageforabout100feet(30.5meters)inalldirections,althoughwallsanddoorscanblockthesignal.Ifyourhomeisverylarge,youcanbuyinexpensiverangeextendersorrepeaterstoincreaseyourrouter'srange.

Aswithwirelessadapters,manyrouterscanusemorethanone802.11standard.802.11broutersareslightlylessexpensive,butbecausethestandardisolder,they'reslowerthan802.11a,802.11gand802.11nrouters.Mostpeopleselectthe802.11goptionforitsspeedandreliability.

Onceyoupluginyourrouter,itshouldstartworkingatitsdefaultsettings(SeeFigure7.3).MostroutersletyouuseaWebinterfacetochangeyoursettings.Youcanselect:

·

Thenameofthenetwork,knownasitsServiceSetIDentifier(SSID)—Thedefaultsettingisusuallythemanufacturer'sname.

·Thechannelthattherouteruses—Mostroutersusechannel6bydefault.Ifyouliveinanapartmentandyourneighborsarealsousingchannel6,youmayexperienceinterference.Switchingtoadifferentchannelshouldeliminatetheproblem.

Awirelessrouterusesanantennatosendsignalsto

wirelessdevicesandawiretosendsignalstotheInternet.

Figure7.3Wirelessrouter

·Yourrouter'ssecurityoptions—Manyroutersuseastandard,publiclyavailablesign-on,

soit'sagoodideatosetyourownusernameandpassword.

Securityisanimportantpartofahomewirelessnetwork,aswellaspublicWiFihotspots.Ifyousetyourroutertocreateanopenhotspot,anyonewhohasawirelesscardwillbeabletouseyoursignal.Mostpeoplewouldratherkeepstrangersoutoftheirnetwork,though.Doingsorequiresyoutotakeafewsecurityprecautions.

It'salsoimportanttomakesureyoursecurityprecautionsarecurrent.TheWiredEquivalencyPrivacy(WEP)securitymeasurewasoncethestandardforWANsecurity.TheideabehindWEPwastocreateawirelesssecurityplatformthatwouldmakeanywirelessnetworkassecureasatraditionalwirednetwork.ButhackersdiscoveredvulnerabilitiesintheWEPapproach,andtodayit'seasytofindapplicationsandprogramsthatcancompromiseaWANrunningWEPsecurity.

Tokeepyournetworkprivate,youcanuseoneofthefollowingmethods:

·WiFiProtectedAccess(WPA)isastepupfromWEPandisnowpartofthe802.11iwirelessnetworksecurityprotocol.ItusesTemporalKeyIntegrityProtocol(TKIP)encryption.AswithWEP,WPAsecurityinvolvessigningonwithapassword.MostpublichotspotsareeitheropenoruseWPAor128-bitWEPtechnology,thoughsomestillusethevulnerableWEPapproach.

·MediaAccessControl(MAC)addressfilteringisalittledifferentfromWEPorWPA.It

doesn'tuseapasswordtoauthenticateusers—itusesacomputer'sphysicalhardware.EachcomputerhasitsownuniqueMACaddress.MACaddressfilteringallowsonlymachineswithspecificMACaddressestoaccessthenetwork.Youmustspecifywhichaddressesareallowedwhenyousetupyourrouter.Thismethodisverysecure,butifyoubuyanewcomputerorifvisitorstoyourhomewanttouseyournetwork,you'llneedtoaddthenewmachines'MACaddressestothelistofapprovedaddresses.Thesystemisn'tfoolproof.AcleverhackercanspoofaMACaddress—thatis,copyaknownMACaddresstofoolthenetworkthatthecomputerheorsheisusingbelongsonthenetwork.

Wirelessnetworksareeasyandinexpensivetosetup,andmostrouters'Webinterfacesarevirtuallyself-explanatory.

Bluetoothisaproprietaryopenwirelesstechnologystandardforexchangingdataovershort

distances(usingshortwavelengthradiotransmissionsintheISMbandfrom2400-2480MHz)

fromfixedandmobiledevices,creatingPersonalAreaNetworks(PANs)withhighlevelsofsecurity.CreatedbytelecomsvendorEricssonin1994,itwasoriginallyconceivedasawirelessBluetooth

alternativetoRS-232datacables.Itcanconnectseveraldevices,overcomingproblemsofsynchronization.(Seefigure7.4)

Figure7.4BluetoothLogo

1.Implementation

Bluetoothusesaradiotechnologycalledfrequency-hoppingspreadspectrum,whichchopsupthedatabeingsentandtransmitschunksofitonupto79bands(1MHzeach;centeredfrom2402to2480MHz)intherange2,400-2,483.5MHz(allowingforguardbands).ThisrangeisinthegloballyunlicensedIndustrial,ScientificandMedical(ISM)2.4GHzshort-rangeradiofrequencyband.

OriginallyGaussianFrequency-ShiftKeying(GFSK)modulationwastheonlymodulationschemeavailable;subsequently,sincetheintroductionofBluetooth2.0+EDR,π/4-DQPSKand8DPSKmodulationmayalsobeusedbetweencompatibledevices.DevicesfunctioningwithGFSKaresaidtobeoperatinginBasicRate(BR)modewhereaninstantaneousdatarateof1Mbit/sispossible.ThetermEnhancedDataRate(EDR)isusedtodescribeπ/4-DPSKand8DPSKschemes,eachgiving2and3Mbit/srespectively.Thecombinationofthese(BRandEDR)modesinBluetoothradiotechnologyisclassifiedasa"BR/EDRradio".

Bluetoothisapacket-basedprotocolwithamaster-slavestructure.Onemastermaycommunicatewithupto7slavesinapiconet;alldevicessharethemaster'sclock.Packetexchangeisbasedonthebasicclock,definedbythemaster,whichticksat312.5μsintervals.Twoclockticksmakeupaslotof625μs;twoslotsmakeupaslotpairof1250μs.Inthesimplecaseofsingle-slotpacketsthemastertransmitsinevenslotsandreceivesinoddslots;theslave,conversely,receivesinevenslotsandtransmitsinoddslots.Packetsmaybe1,3or5slotslongbutinallcasesthemastertransmitwillbegininevenslotsandtheslavetransmitinoddslots.

Bluetoothprovidesasecurewaytoconnectandexchangeinformationbetweendevicessuch

asfaxes,mobilephones,telephones,laptops,personalcomputers,printers,GlobalPositioningSystem(GPS)receivers,digitalcameras,andvideogameconsoles.

AmasterBluetoothdevicecancommunicatewithamaximumofsevendevicesinapiconet(anad-hoccomputernetworkusingBluetoothtechnology),thoughnotalldevicessupportthislimit.Thedevicescanswitchroles,byagreement,andtheslavecanbecomethemaster(forexample,aheadsetinitiatingaconnectiontoaphonewillnecessarilybeginasmaster,asinitiatoroftheconnection;butmaysubsequentlyprefertobeslave).

TheBluetoothCoreSpecificationprovidesfortheconnectionoftwoormorepiconetstoformascatternet,inwhichcertaindevicessimultaneouslyplaythemasterroleinonepiconetandtheslaveroleinanother.

Atanygiventime,datacanbetransferredbetweenthemasterandoneotherdevice(exceptforthelittle-usedbroadcastmode).Themasterchooseswhichslavedevicetoaddress;typically,itswitchesrapidlyfromonedevicetoanotherinaround-robinfashion.Sinceitisthemasterthatchooseswhichslavetoaddress,whereasaslaveis(intheory)supposedtolistenineachreceiveslot,beingamasterisalighterburdenthanbeingaslave.Beingamasterofsevenslavesispossible;beingaslaveofmorethanonemasterisdifficult.Thespecificationisvagueastorequiredbehaviourinscatternets.

2.Uses

Bluetoothisastandardwire-replacementcommunicationsprotocolprimarilydesignedforlowpowerconsumption,withashortrange(power-class-dependent,buteffectiverangesvaryinpractice;seetablebelow)basedonlow-costtransceivermicrochipsineachdevice.Becausethedevicesusearadio(broadcast)communicationssystem,theydonothavetobeinvisuallineofsightofeachother,howeveraquasiopticalwirelesspathmustbeviable.

Theeffectiverangevariesduetopropagationconditions,materialcoverage,productionsamplevariations,antennaconfigurationsandbatteryconditions.Inmostcasestheeffectiverangeofclass2devicesisextendediftheyconnecttoaclass1transceiver,comparedtoapureclass2network.ThisisaccomplishedbythehighersensitivityandtransmissionpowerofClass1devices.

WhiletheBluetoothCoreSpecificationdoesmandateminimumsforrange,therangeofthetechnologyisapplicationspecificandisnotlimited.Manufacturersmaytunetheirimplementationstotherangeneededtosupportindividualusecases.

2.1Bluetoothprofiles

TouseBluetoothwirelesstechnology,adevicehastobeabletointerpretcertainBluetoothprofiles,whicharedefinitionsofpossibleapplicationsandspecifygeneralbehaviorsthatBluetoothenableddevicesusetocommunicatewithotherBluetoothdevices.Theseprofilesincludesettingstoparameterizeandtocontrolthecommunicationfromstart.Adherencetoprofilessavesthetimefortransmittingtheparametersanewbeforethebi-directionallinkbecomeseffective.ThereareawiderangeofBluetoothprofilesthatdescribemanydifferenttypesofapplicationsorusecasesfordevices.

2.2Listofapplications

AtypicalBluetoothmobilephoneheadset.

·Wirelesscontrolofandcommunicationbetweenamobilephoneandahandsfreeheadset.Thiswasoneoftheearliestapplicationstobecomepopular.

·WirelesscontrolofandcommunicationbetweenamobilephoneandaBluetoothcompatiblecarstereosystem.

·WirelessBluetoothheadsetandIntercom.

·WirelessnetworkingbetweenPCsinaconfinedspaceandwherelittlebandwidthisrequired.

·WirelesscommunicationwithPCinputandoutputdevices,themostcommonbeingthemouse,keyboardandprinter.

·Transferoffiles,contactdetails,calendarappointments,andremindersbetweendeviceswithOBEX.

·ReplacementofpreviouswiredRS-232serialcommunicationsintestequipment,GPSreceivers,medicalequipment,barcodescanners,andtrafficcontroldevices.

·Forcontrolswhereinfraredwasoftenused.

·ForlowbandwidthapplicationswherehigherUSBbandwidthisnotrequiredandcable-freeconnectiondesired.

·SendingsmalladvertisementsfromBluetooth-enabledadvertisinghoardingstoother,discoverable,Bluetoothdevices.

·WirelessbridgebetweentwoIndustrialEthernetnetworks.

·Threeseventh-generationgameconsoles,Nintendo'sWiiandSony'sPlayStation3and

PSPGo,useBluetoothfortheirrespectivewirelesscontrollers.

·Dial-upinternetaccessonpersonalcomputersorPDAsusingadata-capablemobilephoneasawirelessmodem.

·Shortrangetransmissionofhealthsensordatafrommedicaldevicestomobilephone,set-topboxordedicatedtelehealthdevices.

·AllowingaDECTphonetoringandanswercallsonbehalfofanearbycellphone.

·Real-TimeLocationSystems(RTLS),areusedtotrackandidentifythelocationofobjectsinreal-timeusing"Nodes"or"tags"attachedto,orembeddedintheobjectstracked,and"Readers"thatreceiveandprocessthewirelesssignalsfromthesetagstodeterminetheirlocations.

·Personalsecurityapplicationonmobilephonesforpreventionoftheftorlossofitems.

TheprotecteditemhasaBluetoothmarker(e.g.atag)thatisinconstantcommunicationwiththephone.Iftheconnectionisbroken(themarkerisoutofrangeofthephone)thenanalarmisraised.

2.3Bluetoothvs.WiFi(IEEE802.11)

BluetoothandWiFi(thebrandnameforproductsusingIEEE802.11standards)havesomesimilarapplications:settingupnetworks,printing,ortransferringfiles.WiFiisintendedasareplacementforcablingforgenerallocalareanetworkaccessinworkareas.ThiscategoryofapplicationsissometimescalledWirelessLocalAreaNetworks(WLAN).Bluetoothwasintendedforportableequipmentanditsapplications.ThecategoryofapplicationsisoutlinedastheWirelessPersonalAreaNetwork(WPAN).Bluetoothisareplacementforcablinginavarietyofpersonallycarriedapplicationsinanysettingandcanalsosupportfixedlocationapplicationssuchassmartenergyfunctionalityinthehome(thermostats,etc.).

WiFiisawirelessversionofacommonwiredEthernetnetwork,andrequiresconfigurationtosetupsharedresources,transmitfiles,andtosetupaudiolinks(forexample,headsetsandhands-freedevices).WiFiusesthesameradiofrequenciesasBluetooth,butwithhigherpower,resultinginhigherbitratesandbetterrangefromthebasestation.ThenearestequivalentsinBluetootharetheDUNprofile,whichallowsdevicestoactasmodeminterfaces,andthePANprofile,whichallowsforad-hocnetworking.

2.4Devices

ABluetoothUSBdonglewitha100mrange.TheMacBookPro,shown,alsohasabuiltinBluetoothadaptor.

Bluetoothexistsinmanyproducts,suchastheiPodTouch,LegoMindstormsNXT,PlayStation3,PSPGo,telephones,theNintendoWii,andsomehighdefinitionheadsets,modems,andwatches.Thetechnologyisusefulwhentransferringinformationbetweentwoormoredevicesthatareneareachotherinlow-bandwidthsituations.Bluetoothiscommonlyusedtotransfersounddatawithtelephones(i.e.,withaBluetoothheadset)orbytedatawithhand-heldcomputers(transferringfiles).

Bluetoothprotocolssimplifythediscoveryandsetupofservicesbetweendevices.

如果你已經(jīng)去過機場、咖啡廳、圖書館或賓館，你很可能已經(jīng)身在無線網(wǎng)絡(luò)之中。許多人在家里使用無線網(wǎng)絡(luò)(被稱為“WiFi或802.11聯(lián)網(wǎng)”)來連接計算機。一些城市也使用該技術(shù)為市民提供免費或低價的因特網(wǎng)訪問。在不久的將來，無線網(wǎng)絡(luò)的應(yīng)用將會非常廣泛，你可以不用網(wǎng)線而隨時隨地訪問因特網(wǎng)。WiFi是如何工作的？

WiFi有許多優(yōu)點：無線網(wǎng)絡(luò)容易建立，資費也便宜；它們不引人注意—除非你用筆記本刻意尋找，否則當(dāng)你在熱區(qū)使用時也感覺不到它的存在。在本文中，我們將研究這種讓信息在空中傳輸?shù)募夹g(shù)，也會討論在你家中建立無線網(wǎng)絡(luò)所需的條件。

1.什么是WiFi？

無線網(wǎng)絡(luò)使用無線電波傳播，就如同手機、電視和收音機一樣。實際上，通過無線網(wǎng)絡(luò)的通信與雙向無線電通信很像，過程如下：

計算機的無線適配器把數(shù)據(jù)轉(zhuǎn)換為無線電信號并用天線發(fā)射出去。

無線路由器接收到這個信號并解碼，然后路由器把信息發(fā)送給使用物理的、有線的以太網(wǎng)連接的因特網(wǎng)。

這個過程也可以反過來，路由器從因特網(wǎng)接收信息，然后轉(zhuǎn)換為無線電信號并發(fā)送給計算機的無線適配器。用于WiFi通信的無線電與用于步話機、手機和其他設(shè)備的無線電幾乎一樣，它們可以發(fā)射和接收無線電波，并可以把一系列的1和0轉(zhuǎn)換為無線電波，也能把無線電波反過來轉(zhuǎn)換為一系列的1和0。但WiFi無線電通信與其他無線電也有一些明顯的不同：

它們以2.4GHz或5GHz的頻率傳輸。這個頻率比手機、步話機和電視所用的頻率高很多。這種較高的頻率允許信號攜帶更多的數(shù)據(jù)。

它們使用802.11聯(lián)網(wǎng)標(biāo)準(zhǔn)，802.11聯(lián)網(wǎng)標(biāo)準(zhǔn)有以下幾個部分：802.11a以5GHz的頻率傳輸并且每秒可以傳輸高達(dá)54兆位的數(shù)據(jù)；它也使用正交頻分復(fù)用—一種更高效的編碼技術(shù)—該技術(shù)在無線電信號到達(dá)接收器之前把它們分為幾個子信號，這極大地減少了沖突。802.11b是最慢和最便宜的標(biāo)準(zhǔn)，一度因為便宜而流行。但是，因為更快的標(biāo)準(zhǔn)現(xiàn)在也變得便宜，它就不那么常用了。802.11b以2.4GHz頻率的無線電射頻波段傳輸，每秒可以處理11