论文英文参考无线方面文档格式.docx

论文英文参考无线方面文档格式.docx

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thappen.Telephonecompaniesstillchargedifferentamountsbasedonyourdistancefromtheotherendpointofacall.Itwasn'

texactlyunlikeSirArthur'

sprediction,however.InternationalcallsI'

vemadeinthepastfewyearshavecostmeabsolutelynothingbeyondthefixednetworkconnectionrateIpayforabroadbandconnection.ButtheirendpointshavebeenotherdevicesconnectedtotheInternet,ratherthantheoldtelephonenetwork.Inpractice,mostofthecablebetweenthetwoendpointswillbethesame,irrespectiveofwhichlogicalnetworkplacesthecall.

1.TheAll-IPNetwork

ThewiredinfrastructureintheUK（operatedbyBT）andthenext-generationwirelesstelephonenetworksallhaveonethingincommon:

TheyuseIPinternallyandruntelephonecallsasVoiceoverIP（VoIP）.MycurrentphonesupportsUMTS,andisassigneda10/8IPaddresswheneverIuseit.Thismeansthatit'

sbehindaNetworkAddressTranslation（NAT）andthereforecannotreceiveinboundconnections.

The10/8subnetisthelargestprivateallocation.Asa/8,ithas2^24—alittleunder17million—uniqueaddresses.Thisisn'

taverylargenumberincomparisontothenumberofdevicesconnectedtothemobilephonenetwork,whichiswhymobileoperatorsarelikelytobeamongthefirstmajordeploymentsofIPv6.Withv6,acompany（orevenanindividual）caneasilygeta/64,meaningthatthefirst64bitsidentifythenetworkandthelast64bitsidentifythedevice.Toputthisintoperspective,thecompanyhasenoughaddressspaceforeveryvalidIPv4addresstobeanetworkthesizeoftheInternet,orforeverypersontohavethreebilliondevicesonhisorhernetwork.Moretothepoint,itallowseverydevicetohaveitsownIPaddress,andtheroutingtablestobesufficientlysparsethatroutingischeap.

OneotheradvantagethatIPv6adds,ofparticularrelevanceinthisarena,isMobileIPv6,inwhichadevicecanchangeitslocationinthenetworkandstillberoutable,withoutdroppingexistingconnections.Movingbetweentowersisgenerallyhandledlowerdowntheprotocolstack,butthisnewarrangementallowsaphonetomovebetweencurrently-separatenetworkswhilestillmaintainingconnections,aslongasbothendpointshaveIPv6connectivity.

Anall-IPnetworkemphasizesthedistinctionbetweensellingaccessandsellingservices—adistinctionthatmobilephonecompaniescurrentlytrytoblur.Whenyoumakeatelephonecall,you'

reusingtheirnetwork,andyou'

realsomakinguseofthepeeragreementstheyhavewithotherphonenetworks,whileenjoyingtheuseoftheirroutingsystem.

Mappingfromtelephonenumbersismuchlessimportantthanitusedtobe.Iusedtomaketelephonecallsbyacomplicatedprocess.First,Iwouldlookupanumberinapaperdirectoryoralocalcache—eitherapersonaladdressbookormybrain—andthenenterthisnumberonadial（later,onakeypad）.Bycontrast,nowIjustselecttheperson'

snamefrommyaddressbookandpress"

call."

Arecentstudyclaimedthatpeoples'

memoriesaregettingworsebecausetheycan'

tremembertheirfriends'

phonenumbers.Forme,it'

snotjustthatIdon'

tremembermyfriends'

numbersanymore—Ineverknewthem.Inmanycases,afriendsentmeavCardviaemailorBluetooth,andIneverlookedatthenumberfieldinthefirstplace.Already,theideaofcallingaphone（ratherthanaperson）seemsfaintlyarchaic.

2.ConnectionMobility

Oneofthelesswell-knownofthe802.11familyofstandardsis802.11r.Thepointofthisstandardistomakeswitchingbetweenwirelessaccesspointsfast.Therangeofan802.11accesspointistypicallyonlyafewroomsifinside,oraboutahundredmeterswhenoutside.Apersonwalkingaroundmaywanttojumpbetweenahalf-dozenaccesspointsinafewminutes.The802.11rstandardprovidesamechanismfordoingjustthat.Ifyou'

veeversatroughlyanequaldistancebetweentwoaccesspoints,youmayhavenoticedthatyourconnectionsperiodicallypausebrieflyasyournetworkingstackdecidestoswitchbetweentheaccesspoints.The802.11rstandardimprovesthispausetime,droppingtheswitchingtimedowntowellunderasecond.Thischangemakes802.11feasibleformobiletelephoneuse.Inanareawithaloadof.11r-supportingaccesspoints,youcanwalkaroundwhiletalkingonthetelephone,andnotnoticewhenyourcalljumpsbetweenthosepoints—justasyoudon'

tnoticenowwhenyourmobilephonejumpsbetweentowers.（Well,intheoryyoudon'

tnotice;

inpractice,youprobablydo）.

Thiskindofhandoffworkswellifalloftheaccesspointsareonthesamenetworksegment.However,imaginewalkingdownastreetfullofopenaccesspointsconfiguredinamesh（802.11s）.Whenyoustartatoneend,youmakeaphonecallandit'

sroutedthroughthefirsthouse'

snetworkconnection.Asyouwalkdownthestreet,yourcallisseamlesslyhandedoffbetweenaccesspointsuntilyougettothefarendofyourtravels.Atthispoint,yourcall'

spacketsarebeingrelayedviaadozenaccesspointstothefarend,whichisn'

tveryefficient.Ideally,beforethispointyouwouldusethenewaccesspoint'

suplink.Unfortunately,eachaccesspointhasadifferentroutablesubnetaddress,soifyouswitchtothenewaccesspoint'

suplinkyou'

llsuddenlyswitchIPaddresses,whichwillbreakyourconnection.

ThisiswhereMobileIPv6comesin.IPv4alsohadamobilevariant,whichreliedontriangularrouting—insimpleterms,theoldnetworkactedasarelayforpackets,whichaddedlatencyeverytimeyoumoved.WithIPv6,IPSecallowstheroutingtablestobeupdatedsecurely.Currently,theswitchingtimeisaroundasecond,whichisn'

tquitefastenoughforVoIPtraffic,butthistimeisexpectedtoimprove.

WithMobileIPv6,theboundariesbetweennetworksbecomealotmorefluid.Ifyou'

resittinginacoffeeshopwithfreeWiFi,youmaywanttousethecoffeeshop'

snetwork,ratherthanpayingforuseofyourcellularprovider'

snetwork.Ifyouwalkinwhilemakingacall,youwouldwanttoswitchautomaticallytousingthecoffeeshop'

sfreeWiFiratherthanthecellularnetwork,whichisamorescarceresourceandlikelytobemoreexpensive.

3.SellingAccess

Mobilephonecompaniesareverygoodatchargingdifferentamountsfordifferenttypesofdata.Theirentirebusinessmodelisbasedaroundthissetup.Ifyou'

repaying10¢

pertextmessage,thisarrangementworksouttoaround$750/MB.Alotofpeoplearewillingtopaythatmuchfortextmessages,butatthisrateloadingtheInformITfrontpagewouldcostover$500,andIdoubtmanypeoplewouldbewillingtopaythatmuchforwebbrowsing.

Youcanrunaninstantmessagingclientonyourphone,andevenaprotocolasbloatedasXMPPworksouttobeafractionofthecostpermessageofSMS.Unfortunately,youcan'

tuseittocommunicatewithpeoplewhodon'

thaveanIMclientontheirphone,whichreducesthevalue.You'

repayingnotjustforthebandwidthforanIM,butalsoforusingthenetwork'

sservers.

Telephonecallsarealsochargedatadifferentratefromthatofotherdata.Thisdesignmakesmoresense,however,sincetelephonecallshavequitedifferentrequirementsthanthoseofwebbrowsingoremail.GSMandrelatedprotocolsusearound12Kb/sforvoicecalls.Atthisspeed,youonlyusearound5MBforanhour-longcall—hardlyanythingonamodernnetwork.

Bandwidthisn'

teverythingtoavoicecall,however.Ifthelatencyismorethanaround200ms,you'

relikelytonotice.Moreimportanteventhanlatencyisjitter.Ifthelatencyincreasesslightly,therewillbeagapintheconversation.Iflatencydecreases,youcanonlycatchupbydroppingpackets,sinceyoucan'

tplaybackaudioatmorethanreal-timespeedwithoutnoticing.（Youcanspeeditupslightly,butnotmuch.）Youcancompensateforjitterbyhavingareceivebufferateachend,butthisapproachaddslatency,whichiswhatyouweretryingtoavoidoverallanyway.

Soyoupayforthreethingsinnetworkaccess:

bandwidth,latency,andjitter.Withbandwidth,youpaymorewhenyouwantmore.Withlatencyandjitter,youpaymorewhenyouwantless.Evenwhenthey'

rejustsellingIPconnectivity,mobilenetworkscanstillofferdifferentpricingtiersfordifferentqualityofservice（QoS）levels.Ifyouwanttomakeaphonecall,youmightselectamoreexpensiveconnectionwithlowerlatencyandjitterlevels,andpayabitmoreforthebandwidth.

4.SoftwareRadio:

AnEngineer'

sDreamandaRegulator'

sNightmare

Oneinterestingdevelopmentinrecentyearsisthesoftwareradio,withprojectslikeGNURadioproducingopensourceimplementations.Ofcourse,aradiocan'

tbeentirelysoftware,butthehardwarerequiredisrelativelysimple—littlemorethananantennaandaDAC/ADCprocess.

Inthepast,itwaseasytoenforceregulatorycomplianceforradiodevices.Youcheckedthatthehardwarewaslimitedtooperatingatcertainfrequenciesandpowerlevels;

ifitwas,thenyouallowedittobemass-produced.

Forsoftwareradio,thepowerlevelsandthefrequencyarecontrolledinsoftware.Thisisproblematicforregulators,especiallywhencombinedwithopensourcedrivers,sinceitmeansthattheendusercaneasilyturnalegaldeviceintoanillegalone.（Ofcourse,thiswasalwayspossiblebyaddingamplifiersandlargerantennae,but