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1、Unit11 MobileCommunications11-1TheMobileTelephoneSystemWirelesstelephonescomeintwobasicvarieties:cordlessphonesandmobilephones(sometimescalledcellphones).Cordlessphonesaredevicesconsistingofabasestationandhandsetsoldasasetforusewithinthehome.Theseareneverusedfornetworking.无绳电话手机移
2、动电话电话听筒11-1TheMobileTelephoneSystemMobilephoneshavegonethroughthreedistinctgenerations,withdifferenttechnologies:AnalogvoiceDigitalvoiceDigitalvoiceanddata(Internet,e-mail,etc.)(1G)(2G)(3G)11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceThissystemusedasingle
3、largetransmitterontopofatallbuildingandhadasinglechannel,usedforbothsendingandreceiving.Totalk,theuserhadtopushabuttonthatenabledthetransmitteranddisabledthereceiver.Suchsystems,knownaspush-to-talksystems,wereinstalledinseveralcitiesbeginninginthelate1950s.使……工作使……不工作11-1TheMobil
4、eTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceInthe1960s,IMTS(ImprovedMobileTelephoneSystem)wasinstalled.Itusedtwofrequencies,oneforsendingandoneforreceiving,sothepush-to-talkbuttonwasnolongerneeded.…IMTSsupported23channelsspreadoutfrom150MHzto450MHz.Duetothesmallnumb
5、erofchannels,usersoftenhadtowaitalongtimebeforegettingadialtone.Allinall,thelimitedcapacitymadethesystemimpractical.不实用11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceAdvancedMobilePhoneSystem(AMPS)AllthatchangedwithAMPS(AdvancedMobilePhoneSystem),inventedby
6、BellLabsandfirstinstalledintheUnitedStatesin1982.11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceAdvancedMobilePhoneSystem(AMPS)Inallmobilephonesystems,ageographicregionisdividedupintocells,whichwhythedevicesaresometimescalledcellphones.InAMPS,thecellsaretyp
7、ically10to20kmacross;indigitalsystems,thecellsaresmaller.地理的Whythemobilephonesarealsocalledcellphones?11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceAdvancedMobilePhoneSystem(AMPS)Eachcellusessomesetoffrequenciesnotusedbyanyofitsneighbors.Thekeyideathatgive
8、scellularsystemsfarmorecapacitythanprevi
