Telephony分析.docx
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Telephony分析.docx
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Telephony分析
MobilePhoneHandheldHardwareHardwareRickRogersJohnLombardoO'ReillyMedia,Inc.O'ReillyMediaAndroidApplicationDevelopment,1stEdition
AppendixA.WirelessProtocols
Ifyou'renew tomobiledevelopment,theplethoraofwirelesstelephonyacronymscanbeconfusingatfirst.Thegoodnewsisthat,forthemostpart,youcanignorethembecauseyoudon'tknowexactlywhichenvironmentyourapplicationwillrunin.Thebadnewsisthatyourapplicationshouldbepreparedtorunin all oftheenvironments.
Tohelpyoufollowthedebates,standards,anddiscussionsthatinevitablyarisewhendiscussingcellularandwirelesstechnologies,thisappendixintroducesthemainprotocolsinhistoricalorder.
MobilePhoneHandheldHardwareHardwareRickRogersJohnLombardoO'ReillyMedia,Inc.O'ReillyMediaAndroidApplicationDevelopment,1stEdition
A.1.Prehistory
Whenmobilephoneswerefirstinventedinthe1940s,theywerejustanalogradiosdrivenfromacarbattery.Thesystemwasaptlynamed MobileTelephoneSystem(MTS),anditwaswoefullyinadequate.Inspiteofthehighcostofservice,waitingliststoobtaintheservicewerelongbecauseMTSofferedonlyafewchannelsinanygeography.An"improved"versioncalledIMTS,introducedinthe1960s,helpedsome,butwasstillfarshortofthedemand.
Thefirstanalogcellularradiomobilephonesystemsstartedtoappearin1969andtheearly1970s—withphonesstillthesizeofabriefcase.ThevariouscellulartechnologiesinNorthAmericaconvergedaroundtheAdvancedMobilePhoneService(AMPS)standard, stillanalogtechnologybutnowbasedoncellularradiosthatcouldreusethefrequencyspectrumandwerestandardizedacrossmanufacturers.AtthistimeEuropehadnolessthanninedifferentanalogmobilephonetechnologystandards,oneforeachmajorregionandcountryinthecontinent.
MobilePhoneHandheldHardwareHardwareRickRogersJohnLombardoO'ReillyMedia,Inc.O'ReillyMediaAndroidApplicationDevelopment,1stEdition
A.2.TheDawnofSecondGeneration(2G)DigitalCellular
RoaminginEurope wasobviouslyimpossible.Partlytoalleviatethisproblem,the European operatorsdecidedtostandardizethenextgenerationofmobilephonesbyformingthe EuropeanTelecommunicationsStandardsInstitute(ETSI).Intheearly1980s,ETSIdevelopedadigitalmobilephonestandardknownasGSM(originally GroupeSpecialMobile,later GlobalSystemforMobileCommunications).TheGSMstandardincludedsomethingtermedShortMessageService(SMS),whichusedsparebandwidthonthecontrolchanneltosendandreceiveshort160-bytemessages.
TheGSMsystemandsomeotherdigitalcellularstandards(suchasthedigitalsuccessortoAMPSinNorthAmerica,D-AMPS,orIS-54)multiplexdifferentvoicecallersonacommonradiofrequencybyusingtimedivisionmultiplexing(TimeDivisionMultipleAccess,orTDMA). Essentially,thesignalfromeachuserisrapidlysampled,andsamplesfromdifferentusersareinterleavedandbroadcastinanassignedtimeslot.Thesampledspeechisreassembledatthereceivingendofthesignal,andinthiswaymultipleuserscanshareasingleradiochannel.
Thecellularprotocolsareactuallyquiteabitmorecomplexthanthissimpleexplanationwouldimply.Atthesametimetheradiosignalisbeingsampledanddesampled,itisalsohoppingaroundtoapresetsequenceoffrequencies,andsamplesarebeingreorderedintime,allinordertoreducemobileeffectssuchasinterference,jitter,dropouts,andmultipathdistortion.
Intheverylate1980s,QualcommintroducedanewdigitalsystemintheU.S.termedCDMA,forCodeDivisionMultipleAccess (lateralsocalledIS-95andstilllatercdmaOne).Insteadofdividingeachvoicesignalintotime-baseddivisions,CDMAtransmittedallofthesignalsonmultipleradiofrequenciesatthesametime.
Buthowtokeepthesignalsfrominterferingwitheachother?
InCDMA,thesignalsmakeuseoforthogonal"codes"thatdefinewhichofthefrequenciesareusedforwhichsignal.Thesignalistransmittedonanumberoffrequenciesdefinedbythecode,andcanbeextractedonthereceivingendbysamplingonlythosefrequenciesassignedtothisparticularcode.Theothersignalsonthosesamefrequenciesareaveragedoutasnoisebecausetheydon'tappearconsistentlyinmostofthefrequencies.CDMAprovedtobemuchmoreefficientatspectrumusethanTDMA,butGSMhadalreadytakenhold,andwasthemorepopularstandardworldwide.
The2Gmobileprotocolsweremainlydesignedforvoice,butalsoprovidedthefirstrealchannelsfordata.Atfirstthedatarateswereslow,thecoveragespotty,andthetechnologyinefficientinitsuseoftheavailablebandwidthbecauseitwasbasedoncircuitswitching.TheoptimisticallynamedHighSpeedCircuitSwitchedData(HSCSD)systemusedmultipleGSMchannelsandwasratedat28.8to64kilobitspersecond,thoughitrarelyachievedevenafractionofthatspeed.Inthe1990s,HSCSDwasreplacedwiththeGeneralPacketRadioSystem(GPRS)standard,thefirstpacket-switchedtechnologyforGSM.
MobilePhoneHandheldHardwareHardwareRickRogersJohnLombardoO'ReillyMedia,Inc.O'ReillyMediaAndroidApplicationDevelopment,1stEdition
A.3.ImprovedDigitalCellular(2.5G)
Inthelate1990s,operatorscouldseethatdemandforvoicephoneswassaturating.Theycouldforeseethedaywheneverybodywhowantedamobilephonewouldhaveone.Atthesametime,theInternetwasbecomingubiquitous,anduserswerestartingtodemandbetterdataaccessfromtheirmobilephones.Operatorslookedforwaystoexpandthedatacapacityoftheirmobilenetworkswhiletakingadvantageoftheirexistinginfrastructureinvestments.GSMoperatorsexpandedtheirGSM/GPRSnetworkstoanewstandardcalledEnhancedDataforGSMEvolution(EDGE), whichfurtherimprovedavailabledataratesandmadeefficientuseofGSMequipmenttheoperatorsalreadyhadinstalled.CDMAoperatorscapitalizedonsimilarimprovementsinthatdomain,withstandardssuchasCDMA20001X.Thetheoreticaldatarateswerenowinthehundredsofkilobitspersecond,thoughtheactualdatarateswerestillmuchlower.PhonesrunningAndroidcanbeexpectedtohaveatleast2.5Gdataconnectivity.
Asecondwaveofdataaccessimprovement(sometimesreferredtoas2.75G)furtherimproveddatarates,implementedbyHighSpeedPacketAccess(HSPA)forGSMandEV-DO(EVolutionDataOptimized,orsometimestranslatedasEVolutionDataOnly)forCDMA.Theoreticaldatarateswerenowinthemultimegabit-per-secondrange,andmostAndroidphonescanbeexpectedtohavethesetechnologies,ifnot3G.
MobilePhoneHandheldHardwareHardwareRickRogersJohnLombardoO'ReillyMedia,Inc.O'ReillyMediaAndroidApplicationDevelopment,1stEdition
A.4.TheRiseof3G
Alsointhe1990s, theEuropeantelecomcommunitystarteddefiningthenextgenerationofmobiletechnology,firstthroughETSIandthenthroughaneworganizationcalled3rdGenerationPartnershipProgram(3GPP).Thestandarddevelopedby3GPPiscalledUniversalMobileTelecommunicationsStandard(UMTS),and thoughbasedfundamentallyonWidebandCDMA(WCDMA)technology, wascarefullydesignedtoallowbothGSMandCDMAoperatorstoevolvetheirnetworksefficientlyfromtheirinstalledinfrastructuretothenewstandard.Thiswouldallowoperatorsaroundtheworldtoconvergetoanewcommonstandardfor3G.[2]
[2] ExceptforoperatorsinthePeople'sRepublicofChina,wherethegovernmentmandateditsownversionofUMTS,calledTimeDivision-SynchronousCodeDivisionMultipleAccess(TD-SCDMA).TD-SCDMAusesTDMAaswellasCDMAtoprovidesomeuniqueadvantagesfordatatraffic.ItalsoavoidstheneedforPRChandsetmakerstopayroyaltiesformostWCDMAintellectualproperty.
Intheearly2000s,operatorsspenthugesumsofmoneytopurchasespectrumfor3Gwirelessnetworks.3Gnetworksarenowbeingdeployedworldwide,andoverthenextfewyears,newsmartphones(includingAndroid-basedphones)willallincorporate3Gtechnologies.
MobilePhoneHandheldHardwareHardwareRickRogersJohnLombardoO'ReillyMedia,Inc.O'ReillyMediaAndroidApplicationDevelopment,1stEdition
A.5.TheFuture:
4G
Sowhat'snext?
Thestandardsbodies arebackatworkdefiningthefourthgenerationofwirelessnetworkprotocols,sometimestermed LTE(forLongTermEvolution).TheapparentwinnerisagroupofprotocolscalledOrthogonalFrequencyDivisionMultiplexing(OFDM),orsometimesOFDMA(the"A"isforAccess).Theseprotocolsuseradiofrequencysubcarrierstofurtherimprovethedataratesachievableforwirelessdevices.SimilarprotocolsareusedintheWiMAXstandards(thehigherbandwidth,longer-rangefollow-ontoWiFi),butitisnotclearhowWiMAXandLTEwillrelatetooneanother.
Justaswith3G,aroundofspectrumauctionsisstartingtotakeplacefor4G,andoperatorsarealreadyinvestinglargesumsofmoneyintogettingreadyfor4Gservices.SufficetosaythatyourapplicationsbuiltforAndroidwillsomedayencounterphonesrunning4Gprotocols,andwillbeabletotakeadvantageofthehigherdataratesandlowerlatenciesthatwillcomewiththeseprotocols.
Towrapup, FigureA-1 showstheevolutionofprotocolsdiscussedinthischapterinrelationtothedecadeinwhichtheywerefirst deployedandtheeffectivebandwidththeyachieve.
FigureA-1.Mobileprotocols,bandwidth,anddatesofdeployment
MobilePhoneHandheldHardwareHardwareRickRogersJohnLombardoO'ReillyMedia,Inc.O'ReillyMediaAndroidApplicationDevelopment,1stEdition
Appendix.Colophon
Theanimalonthecoverof AndroidApplicationDevelopment isanEasternquoll(Dasyurus viverrinus),anendangeredmarsupialotherwiseknownastheEasternnativecat.Easternquollsgrowt
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