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1、IntroductiontoWireless NetworkingConcepts1-无线网络概念简介WirelessLocal-AreaNetworks(无线局域网)SirWilliamHerschel(1738to1822)madeadiscoverythatinfraredlight(红外线)existedThediscoveryofinfraredlightledthewaytotheelectromagneticwave（电磁波）theoryelectromagneticwavestravelatthespeedoflightandthatelectricit
2、ycanbecarriedonthesewaveshowtheyrelatetowirelesslocal-areanetworks(WLANs)？InstandardLANs,dataispropagatedoverwiressuchasanEthernetcable（以太网线路）,intheformofelectricalsignals（电信号）.Thediscoveryopenstheairwaystotransferthesamedata,aselectricalsignals,withoutwires.ThisisaccomplishedbywayofRadi
3、oFrequencies(RF)（射频）.WithRF,thegoalistosendasmuchdataasfaraspossibleandasfastaspossible.Theproblemisthenumerousinfluencesonradiofrequenciesthatneedtobeeitherovercomeordealtwith.Oneoftheseproblemsisinterference（干扰）HowBandwidthIsAchievedfromRFSignalsToachievebandwidth（带宽）fromRFsignals,youn
4、eedtosenddataaselectricalsignalsusingsometypeofemissionmethod（发送方式）.OnesuchemissionmethodisknownasSpreadSpectrum（扩展频谱）.In1986,theFCCagreedtoallowtheuseofspreadspectruminthecommercialmarketusingwhatisknownastheindustry,scientific,andmedical(ISM)frequencybands.ToplacedataontheRFsignals,you
5、useamodulationtechnique（调制技术）.Modulationistheadditionofdatatoacarriersignal.Youareprobablyfamiliarwiththisalready.Tosendmusic,news,orspeechovertheairwaves,youusefrequencymodulation(FM)（调频）oramplitudemodulation(AM)（调幅）.Bandwidth（带宽）Inwirelessnetworking,thewordbandwidthcanmeantwodifferentt
6、hings.Inonesenseoftheword,itcanrefertodatarates（数据率）.Inanothersenseoftheword,itcanrefertothewidthofanRFchannel（RF频道宽度）.ThisbookusesthetermbandwidthtorefertothewidthoftheRFchannelandnottodatarates.thestandardunitofmeasureistheHertz(Hz)（赫兹）例如：FMradioisgenerallyahigherquality,usingabout175k
7、Hzofbandwidth.Comparethattoatelevisionsignal,whichsendsbothvoiceandvideoovertheair.TheTVsignalyoureceiveusesalmost4500kHzofbandwidth.AsFigure1-1illustrates,theelectromagneticspectrumspansfromExtremelyLowFrequency(ELF)（极低频）toExtremelyHighFrequency(EHF)（极高频）.Infact,thedatay
