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1、Now that the 802.11g standard has been finalized, comparisons with the other standards in the 802.11 family are inevitable. One conclusion that is frequently drawn is that 802.11g offers similar speeds to 802.11a. After all, both products are advertised as having a data rate of 54 Mbps. This article

2、 develops a simple model for the maximum TCP throughput of 802.11 networks so that a comparison can move beyond a simple comparison of nominal bit rates. According to the model, 802.11g is significantly faster than 802.11b. In a network consisting only of 802.11g clients, it is even slightly faster

3、than 802.11a. However, protection mechanisms added to 802.11g to ensure backwards compatibility with legacy 802.11b clients can cut the throughput by 50 percent or more. TechhnollogyyTrannsacctioons perr seeconndMbpss off TCCP ppaylloadd thhrouughpputTrannsacctioonall sppeedd reelattivee too 8002.11

4、1b11b, 111 Mbbps4795.61.011a, 544 Mbbps2,333627.334.911g, 544 Mbbps/no prootecctioon2,333627.334.911g, 544 Mbbps/CTSS-too-seelf prootecctioon1,111313.002.311g, 544 Mbbps/RTSS/CTTS pprottecttionn7508.81.6The Transactional ModelThe basic transactional model assumed by this article is a maximum-length

5、frame containing a single TCP segment followed by a TCP acknowledgement. To cope with the inherent unreliability of radio waves, the 802.11 MAC requires positive acknowledgement of every transmission. Each TCP packet must therefore be wrapped up in a frame exchange. The complete transaction consists

6、 of the two 802.11 data frames carrying TCP embedded in their respective exchanges: The TCPP daata seggmennt: Disttribbuteed IInteerfrramee Sppacee (DDIFSS): thiis iinteerfrramee sppacee inndiccatees tthatt ann exxchaangee haas ccomppletted, annd iit iis ssafee too acccesss tthe meddiumm aggainn. Th

7、e datta fframme cconttainningg thhe TTCP seggmennt. A Shhortt Innterrfraame Spaace (SIIFS), wwhicch iis aa smmalll gaap bbetwweenn thhe ddataa frramee annd iits ackknowwleddgemmentt. The 8022.111 ACCK fframme. The TCPP ACCK:DIFSS The datta fframme cconttainningg thhe TTCP ACKK SIFSS 802.11 ACKK frra

8、mee For thiis mmodeel, I wwilll asssumme tthatt thhe hhostt opperaatinng ssysttem limmitss thhe ooutggoinng fframme ssizee too 1,5000 byytess. 8802.11 perrmitts mmuchh laargeer fframme ssizees, butt thhis fleexibbiliity hass noot ttradditiionaallyy beeen useed bby cclieent prooduccts. Moost acccesss

9、 poointts cconnnectt too exxisttingg neetwoorkss wiith Ethhernnet, annd ttherrefoore limmit thee paaylooad sizze tto tthe maxximuum EEtheerneet ppaylloadd siize. (IIn ffactt, tthiss siimplle ppreccauttionn iss reequiiredd too obbtaiin WWi-FFi ccerttifiicattionn.) EncaapsuulattionnIn aaddiitioon tto

10、tthe payyloaad ddataa, ttherre aare 36 addditiionaal bbytees oof ddataa adddedd inn thhe eencaapsuulattionn prroceess. Thhe 8802.11 MACC heeadeer aaddss 288 byytess off daata forr vaarioous conntrool aand mannageemennt ffuncctioons, errrorr deetecctioon, andd adddreessiing. A furrtheer eeighht bbyte

11、es aare addded by thee SNNAP enccapssulaatioon hheadder to ideentiify thee neetwoork layyer prootoccol. (FFor furrtheer eencaapsuulattionn deetaiils, seee FFiguuress 3-9 aand 3-113 oof 8802.11 Wirreleess Nettworrks.) TThe tottal sizze oof tthe MACC paaylooad forr thhe TTCP datta ssegmmentt iss 1,536

12、6 byytess (112,2288 bitts). Foor tthe TCPP ACCK, thee 400 byytess off TCCP/IIP hheadder infformmatiion alsso ggetss a furrtheer 336 bbytees oof SSNAPP heeadeer ffor a ttotaal oof 776 bbytees (6088 biits).ThrooughhputtThiss arrticcle meaasurres thee thhrouughpput of thee neetwoork at thee TCCP ppayll

13、oadd laayerr. EEachh trranssacttionn deelivverss onne ddataa frramee. TTherre aare 1,5500 byttes of Ethhernnet payyloaad, butt 400 byytess muust be subbtraacteed ffor heaaderrs. By adddingg upp thhe ttotaal ttimee reequiiredd foor eeachh coompoonennt oof tthe traansaactiion, a traansaactiion ratte c

14、can be derriveed. Forr thhe tthrooughhputt, ssimpply mulltipply by 1,4460 byttes.Weakknesssess off thhe MModeelThe traansaactiionaal mmodeel iis qquitte ssimpple, annd nnegllectts sseveerall immporrtannt eeffeectss. FFirsst oof aall, itt asssummes a ssteaady strreamm off weell-orddereed fframmes wit

15、th nno cconttenttionn foor tthe meddiumm. 8802.11 impplemmentts ccolllisiion avooidaancee annd eexpoonenntiaal bbackkofff, sso iin rreallityy, tthe timme bbetwweenn frramee exxchaangees wwilll bee loongeer tthann onne DDIFSS. EExpoonenntiaal bbackkofff inn thhe ppressencce oof cconttenttionn wiill f

16、urrtheer ddecrreasse tthrooughhputt. MMy iinittiall baack-of-thee ennvellopee esstimmatee iss thhat conntenntioon ffor thee meediuum wwoulld rreduuce thee maaximmum thrrougghpuut ffiguuress abbovee byy 255% tto 550%, deepenndinng oon tthe exaact asssumpptioons madde. Furttherrmorre, theere is alwway

17、ss thhe pposssibiilitty oof ssimuultaaneoous traansmmisssionn. 8802.11 willl oordeer llayeer ttwo fraamess annd pprevventt coolliisioons, buut hhighher-layyer paccketts mmay be traansmmittted simmulttaneeoussly. 8002.111 ttreaats TCPP seegmeentss annd aacknnowlledggemeentss ass unnrellateed hhighher

18、-layyer fraamess, sso aa ceertaain nummberr off coolliisioons aree exxpecctedd. (Onee ennginneerr haas ttoldd mee thhat up to 10% reetraansmmisssionns sshouuld be exppectted forr TCCP/IIP oon 8802.11 duee too thhis efffectt.) Furttherrmorre, thee moodell iggnorres thee soophiistiicattionn inn thhe T

19、TCP ackknowwleddgemmentt moodell. TTo aavoiid cconsstraainiing thrrougghpuut, TCPP usses sllidiing winndowws andd alllowws mmulttiplle ooutsstanndinng fframmes to be traansmmittted befforee accknoowleedgeemennt. In praactiice, TCCP aacknnowlledggemeentss caan aapplly tto mmulttiplle ssegmmentts, so

20、thiis mmodeel ooverrstaatess thhe iimpaact of higgherr-laayerr prrotoocoll accknoowleedgeemennts. The Basseliine: 8002.111b at 11 MbppsThe basseliine speeed commes froom 8802.11bb. IIt iis nnot as fasst aas tthe newwer speecifficaatioons, buut iitss whhat mosst oof tthe 8022.111 caardss ouut iin tth

21、e worrld do. Ill do thee caalcuulattionn foor 8802.11bb fiirstt, aand theen ccompparee evveryythiing elsse tto tthe ressultt foor 8802.11bb. Firsst ooff, thhe bbasiic ttimiing nummberrs ffor 8022.111b: SIFSS = 10 s Slott tiime = 220 s DIFSS = 2 xx Sllot timme + SIIFS = 550 s 802.11bb reequiiress thh

22、at a ppreaamblle bbe ppreppendded to eveery fraame befforee itt iss trranssmitttedd too thhe aair. Thhat preeambble mayy bee eiitheer tthe traadittionnal loong prreammblee, wwhicch rrequuirees 1192 s forr trranssmisssioon, or it mayy bee ann opptioonall sshorrt preeambble thaat rrequuirees oonlyy 96

23、6 ss. (Forr a diaagraam oof tthe phyysiccal layyer heaaderrs iin 8802.11bb, ssee Figguree 100-266 inn 8002.111 WWireelesss NNetwworkks.) Suuppoort of thee loong preeambble is manndattoryy, aand is thee deefauult setttinng oon mmostt deevicces. Inn thhe iinteeresst oof rreduucinng tthe nummberr off c

24、aalcuulattionns, thiis aartiiclee wiill perrforrm ccalcculaatioons usiing onlly tthe lonng ppreaamblle. 802.11bb ruunniing at thee maax sspeeed ddiviidess daata up intto 88-biit ssymbbolss. TTherre aare 1,5536 8-bbit bloockss inn thhe TTCP datta ssegmmentt, aand 76 8-bbit bloockss inn thhe TTCP ackk

25、nowwleddgemmentt. TThe ACKK dooes nott haave SNAAP hheadderss, aand is onlly 114 bbytees llongg. Encoodinng tthe MACC frramees iis eeasyy. 8802.11bb diividdes up thee MAAC fframme iintoo a serriess off 8-bitt ssymbbolss, andd thhen traansmmitss 1.3755 miilliion symmbolls pper seccondd. SSo aadd up t

26、hee inndivviduual commponnentts oof tthe traansaactiion to gett thhe ttotaal dduraatioon: TCP dattaTCP ACKKDIFSS50 s50 s802.11 Datta192 s + 115366/1.3755 Mssps= 1992 s + 1,1188 ss= 1,3100 ss 192 s + 776/11.3775 MMspss= 1992 s + 566 ss= 2448 s SIFSS10 s10 s802.11 ACKK192 s + 114/11.3775 MMspss= 1992

27、s + 111 ss= 2003 s = 2003 s Framme eexchhangge ttotaal1,5773 s511 sTrannsacctioon TTotaal2,0884 sEachh trranssacttionn reequiiress 2,0844 ss. AAt tthatt duurattionn, 4479 excchanngess caan ccompplette pper seccondd. WWithh a TCPP paaylooad of 1,4460 byttes perr exxchaangee, tthe thrrougghpuut iis 55

28、.7 Mbpps. 802.11aa802.11aa iss faasteer tthann 8002.111b forr twwo rreassonss: ttimiing rellatiionsshipps bbetwweenn frramees iin tthe excchanngess arre ttighhterr, aand thee enncoddingg ussed by 8022.111a ddoess noot rrequuiree suuch lonng ppreaamblles forr syynchhronnizaatioon. Heree arre tthe bas

29、sic timmingg nuumbeers forr 8002.111a:SIFSS = 16 sSlott tiime = 99 ss DIFSS = 2 xx Sllot timme + SIIFS = 334 s Likee 8002.111b, 8002.111a divvidees ddataa upp innto a sseriies of symmbolls ffor traansmmisssionn. HHoweeverr, tthe enccodiing useed bby 8802.11aa usses mucch llargger symmbolls. At 54 Mb

30、pps, eacch ssymbbol enccodees 2216 bitts(每每個符號號的時間間為4s). (Forr a fulll llisttingg off enncoddingg bllockk siizess, ssee Tabble 11-3 iin 8022.111 Wiirellesss Neetwoorkss.) Thee OFFDM enccodiing useed bby 8802.11aa addds sixx biits forr enncoddingg puurpoosess too thhe eend of thee frramee, sso oour m

31、axximuum-llenggth fraame of 1,5536 byttes beccomees aa seeriees oof 112,2288 datta bbitss pllus thee siix ttaill biits. Thhe ttotaal bbit strringg off 122,2994 bbitss caan bbe eencoodedd inn 577 syymbools. Thhe 6608 bitt(400+366=766 Byyte) TCCP AACK pluus ttaill biits reqquirres thrree symmbolls, an

32、dd thhe 8802.11 ACKK allso reqquirres jusst oone symmboll. Eachh frramee iss prrepaaredd foor ttrannsmiissiion in thee aiir wwithh a 20s hheadder to synnchrroniize thee reeceiiverr. FFolllowiing thee 200 ss heeadeer iis aa seeriees oof ssymbbolss, eeachh reequiirinng 44s forr trranssmisssioon. TCP d

33、attaTCP ACKKDIFSS34 s34 s802.11 Datta20 s + 577 * 4 s/ssymbbol= 200 ss + 2288 ss= 2448 s20 s + 3 * 44 ss/syymbool= 200 + 12 s= 322 ssSIFSS16 s16 s802.11 ACKK20 s + 1 * 44 ss/syymbool= 200 ss + 4 s= 244 ss= 244 ssFramme eexchhangge ttotaal322 s106 sTrannsacctioon TTotaal428 sAt 4428 s perr trranssact

34、tionn, iit iis pposssiblle tto ccompplette 22,3336 eexchhangges perr seeconnd. Thaat ccorrresppondds tto aa thhrouughpput of 27.3 MMbpss.（注注：4228uss*23336=1s）802.11ggPerfformmingg thhe ssamee caalcuulattionns ffor 8022.111g iis aa biit ttriccky. 8002.111g opeerattes in thee saame freequeencyy baand

35、as 8022.111b, andd iss reequiiredd too reemaiin bbackkwarrds-commpattiblle. Thee enncoddingg ussed by 8022.111g wwilll noot bbe rrecoogniizedd byy 8002.111b staatioons, soo pprottecttionn mmechhaniismss arre ddefiinedd too liimitt thhe ccrosss-ttalkk inn miixedd b/g eenviironnmennts. Esssenntiaallyy

36、, tthe prootecctioon mmechhaniismss reequiire thaat 8802.11gg sttatiionss opperaatinng aat hhighh raatess prre-rreseervee thhe rradiio mmediium by usiing sloowerr, 8802.11bb-coompaatibble resservvatiion mecchannismms. 802.11gg SIIFS = 110 s802.11gg shhortt sllot timme = 9 s (8002.111g-onlly mmodee w

37、iith no leggacyy sttatiionss) 802.11gg loong sloot ttimee = 20 s (miixedd moode reqquirres sloow sslott tiime) 802.11gg usses manny oof tthe samme ttimiing parrameeterrs aas 8802.11aa. HHoweeverr, iit iis ssadddledd wiith bacckwaardss coompaatibbiliity reqquirremeentss. IIt iinheeritts tthe shoort 1

38、0 s SIFFS ttimee frrom 8022.111b, butt thhe hhighh-raate coddingg inn 8002.111g neeeds addditiionaal ttimee. TTherrefoore, 8002.111g addds aa 6 s siignaal eexteensiion tiime at thee ennd oof eeverry fframme. 802.11gg-onnly BSSSWhenn noo 8002.111b staatioons aree prreseent, noo prroteectiion is reqqu

39、irred. Thhis sittuattionn iss exxtreemelly uunliikelly, givven thee huuge insstallledd baase of 8022.111b ccardds. As aalwaays, sttartt wiith thee baasicc tiiminng pparaametterss:802.11gg SIIFS = 110 s 802.11gg faast sloot ttimee = 9 s (cann onnly be useed wwhenn noo 8002.111b staatioons aree prrese

40、ent) 802.11gg DIIFS = 22 x Sloot ttimee + SIFFS = 288 ss The 8022.111g EERP-OFDDM PPHY is neaarlyy iddentticaal tto tthe 8022.111a PPHY, exxceppt tthatt itt opperaatess inn a diffferrentt frrequuenccy bbandd annd uusess a shoorteer SSIFSS tiime. Phhysiicall laayerr heeadeers aree iddentticaal, as is

41、 thee coodinng. Theerefforee, tthe callcullatiion forr thhe ttimee reequiiredd too trranssmitt a fraame is neaarlyy iddentticaal, witth oonlyy miinorr chhangges to thee innterrfraame spaace timmes. TCP dattaTCP ACKKDIFSS28 s28 s802.11 Datta20 s + 577 * 4 s/ssymbbol +6 s= 200 ss + 2288 ss= 2554ss20 s

42、 + 3 * 44 ss/syymbool + 6 s= 200 + 12 s= 388 ssSIFSS10 s10 s802.11 ACKK20 s + 1 * 44 ss/syymbool + 6 s= 200 ss + 4 s + 6 s= 300s= 300 ssFramme eexchhangge ttotaal322 s106 sTrannsacctioon TTotaal428 sThe traansaactiion lenngthh off 8002.111g is ideentiicall too 8002.111a. Thhe iinteerfrramee sppacee

43、iss sllighhtlyy shhortter, buut tthiss iss offfseet eexacctlyy byy thhe ssignnal exttenssionn fiieldd.Prottecttionn 1: CTTS-tto-sselffOncee thhe ffirsst 8802.11bb sttatiion asssociiatees wwithh ann 8002.111g acccesss poointt, hhoweeverr, pprottecttionn iss reequiiredd. TThe minnimaal pprottecttionn

44、coonteempllateed bby tthe staandaard is thaat 8802.11gg sttatiionss wiill prootecct tthe fasst 8802.11gg frramee exxchaangee wiith a ssloww Cllearr Too Seend (CTTS) fraame thaat llockks oout othher staatioons acccesss too thhe mmediium. Prroteectiion draamatticaallyy reeducces thee maaximmum theeore

45、eticcal thrrougghpuut bbecaausee thhe aaddiitioonall CTTS ttrannsmiissiion is reqquirred witth iits lonng 8802.11bb heeadeers. Longger intterfframme sspaccingg iss reequiiredd whhen leggacyy clliennts aree coonneecteed aand prootecctioon iis eengaagedd. TThe shoort sloot ttimee iss onnly avaailaable

46、e whhen no 8022.111b sstattionns aare preesennt. Oncce ttheyy arre ppressentt, tthe fraame spaacinng rreveertss too thhe 8802.11bb sttanddardd: SIFSS = 10 s Slott tiime = 220 s DIFSS = 2 xx Sllot timme + SIIFS = 550 s A CTTS fframme iis 114 bbytees. It willl bbe ttrannsmiitteed aat tthe higghesst rr

47、atee unnderrstoood by alll sttatiionss atttacchedd too thhe aacceess poiint, whhichh wiill be at mosst 111 MMbpss. TThe CTSS frramee wiill be enccodeed qquiccklyy, bbut onlly aafteer iits lonng hheadder. Foolloowinng tthe CTSS wiill be thee Daata-ACKK seequeencee att thhe hhighh 8002.111g speeedss.

48、TCP dattaTCP ACKKDIFSS50 s50 sCTS192 s + 114/11.3775 MMspss= 1992 s + 111 ss= 2003 s= 2003 sSIFSS10 s10 s802.11 Datta20 s + 577 * 4 s/ssymbbol + 66 ss= 200 ss + 2288 ss +66 ss= 2554 s20 s + 3 * 44 ss/syymbool + 6 s= 200 + 12 s + 66s= 388sSIFSS10 s10 s802.11 ACKK20 s + 1 * 44 ss/syymbool +6 s= 200 ss

49、 + 4 s + 6 s= 300 ss= 300 ssFramme eexchhangge ttotaal557 s341sTrannsacctioon TTotaal898sThe tottal traansaactiionaal ttimee iss ovver twiice as lonng bbecaausee off thhe pprottecttionn meechaanissm. At 8988s perr trranssacttionn, oonlyy 1,1133 trranssacttionns ccan commpleete perr seeconnd, andd th

50、he tthrooughhputt drropss drramaaticcallly tto 113.00 Mbbps. Prottecttionn 2: RTTS-CCTSUsinng oonlyy a CTSS frramee too reeserrve thee meediuum iis tthe minnimuum rrequuireemennt, butt itt maay ffaill inn soome casses wheere theere aree soo-caalleed hidddenn noodess tthatt doo noot ssee thee CTTS. T

51、o fullly resservve tthe meddiumm, tthe iniitiaal eedittionn off thhe 8802.11 staandaard inccludded a ttwo-fraame excchannge thaat wwoulld ffullly aannoouncce tthe imppenddingg trranssmisssioon ccompposeed oof aa Reequeest To Sennd (RTSS) fframme ffollloweed bby tthe CTSS frramee. AAlthhouggh tthe st

52、aandaard reqquirres onlly aa CTTS-tto-sselff, uusinng tthe fulll RRTS/CTSS wiill bettterr prroteect thee innnerr exxchaangee frrom intterffereencee. TThe finnal callcullatiion is quiite simmilaar tto tthe preevioous onee, wwithh thhe aaddiitioon oof tthe RTSS frramee att thhe sstarrt:TCP dattaTCP AC

53、KKDIFSS50 s50 sRTS192 s + 220/11.3775 MMspss= 1992 + 155 ss= 2007 s= 2007 sSIFSS10 s10 sCTS192 s + 114/11.3775 MMspss= 1992 s + 111 ss= 2003 s= 2003 sSIFSS10 s10 s802.11 Datta20 s + 577 * 4 s/ssymbbol + 66 ss= 200 ss + 2288 ss + 6 s= 2554 s20 s + 3 * 44 ss/syymbool + 6 s= 200 + 12 s + 66 ss= 388 ssS

54、IFSS10 s10 s802.11 ACKK20 s + 1 * 44 ss/syymbool + 6 s= 200 ss + 4 s + 6 s= 300 ss= 300 ssFramme eexchhangge ttotaal774 s558 sThee tootall trranssacttionnal timme iis eevenn loongeer bbecaausee a morre rrobuust (annd hhencce, timme-cconssumiing) prroteectiion mecchannismm iss ussed. A fulll RRTS/CTS

55、S exxchaangee maakess thhe TTCP ACKK reequiire morre ttimee thhan a sstraaighhtfoorwaard 11 Mbpps ttrannsmiissiion usiing oldder 8022.111b eencoodinng. Theerfoore, a traansaactiion wouuld connsisst oof tthe TCPP daata prootecctedd byy RTTS/CCTS at 7744 ss foolloowedd byy a TCPP ACCK aat tthe 5111 ss prreviioussly callcullateed ffor 8022.111b. Thee tootall trranssacttionn tiime is 1,2285 s perr trranssacttionn, sso oonlyy 7778 ttrannsacctioons cann coomplletee peer ssecoond, annd tthe thrrougghpuut ddropps bbackk innto