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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. TechnnoloogyTranssacttionns pper secconddMbps of TCPP paaylooad thrrougghpuutTranssacttionnal speeed rellatiive to 8022.111b11b, 1
4、1 Mbpps4795.61.011a, 54 Mbpps2,336627.34.911g, 54 Mbpps/nno pprottecttionn2,336627.34.911g, 54 Mbpps/CCTS-to-sellf pprottecttionn1,113313.02.311g, 54 Mbpps/RRTS/CTSS prroteectiion7508.81.6The Transactional ModelThe bbasiic ttrannsacctioonall moodell asssummed by thiis aartiiclee iss a maxximuum-llen
5、ggth fraame conntaiininng aa siinglle TTCP seggmennt ffollloweed bby aa TCCP aacknnowlledggemeent. Too coope witth tthe inhhereent unrreliiabiilitty oof rradiio wwavees, thee 8002.111 MMAC reqquirres possitiive ackknowwleddgemmentt off evveryy trranssmisssioon. Eacch TTCP pacckett muust theerefforee
6、 bee wrrappped up in a fframme eexchhangge. Thee coomplletee trranssacttionn coonsiistss off thhe ttwo 8022.111 daata fraamess caarryyingg TCCP eembeeddeed iin ttheiir rresppecttivee exxchaangees: The TTCP datta ssegmmentt: Distrribuutedd Innterrfraame Spaace (DIIFS): tthiss innterrfraame spaace ind
7、dicaatess thhat an excchannge hass coomplleteed, andd itt iss saafe to acccesss thhe mmediium agaain. The ddataa frramee coontaainiing thee TCCP ssegmmentt. A Shoort Intterfframme SSpacce (SIFFS), whhichh iss a smaall gapp beetweeen thee daata fraame andd itts aacknnowlledggemeent. The 8802.11 ACKK
8、frramee. The TTCP ACKK: DIFS The ddataa frramee coontaainiing thee TCCP AACK SIFS 802.111 AACK fraame For tthiss moodell, II wiill asssumee thhat thee hoost opeerattingg syysteem llimiits thee ouutgooingg frramee siize to 1,5500 byttes. 8002.111 ppermmitss muuch larrgerr frramee siizess, bbut thiis
9、fflexxibiilitty hhas nott trradiitioonallly beeen uusedd byy clliennt pprodductts. Mosst aacceess poiintss coonneect to exiistiing nettworrks witth EEtheerneet, andd thhereeforre llimiit tthe payyloaad ssizee too thhe mmaxiimumm Ettherrnett paaylooad sizze. (Inn faact, thhis simmplee prrecaautiion i
10、s reqquirred to obttainn Wii-Fii ceertiificcatiion.) EncappsullatiionIn adddittionn too thhe ppaylloadd daata, thheree arre 336 aaddiitioonall byytess off daata addded in thee enncappsullatiion proocesss. Thee 8002.111 MMAC heaaderr addds 28 byttes of datta ffor varriouus cconttroll annd mmanaagemme
11、ntt fuuncttionns, errror dettecttionn, aand adddresssinng. A ffurttherr eiightt byytess arre aaddeed bby tthe SNAAP eencaapsuulattionn heeadeer tto iidenntiffy tthe nettworrk llayeer pprottocool. (Foor ffurttherr enncappsullatiion dettaills, seee Fiigurres 3-99 annd 33-133 off 8002.111 WWireelesss N
12、Netwworkks.) Thhe ttotaal ssizee off thhe MMAC payyloaad ffor thee TCCP ddataa seegmeent is 1,5536 byttes (122,2888 bbitss). Forr thhe TTCP ACKK, tthe 40 byttes of TCPP/IPP heeadeer iinfoormaatioon aalsoo geets a ffurttherr 366 byytess off SNNAP heaaderr foor aa tootall off 766 byytess (6608 bitts).
13、ThrouughpputThis artticlle mmeassurees tthe thrrougghpuut oof tthe nettworrk aat tthe TCPP paaylooad layyer. Eaach traansaactiion delliveers onee daata fraame. Thheree arre 11,5000 bbytees oof EEtheerneet ppaylloadd, bbut 40 byttes musst bbe ssubttracctedd foor hheadderss. BBy aaddiing up thee toota
14、ll tiime reqquirred forr eaach commponnentt off thhe ttrannsacctioon, a ttrannsacctioon rratee caan bbe dderiivedd. FFor thee thhrouughpput, siimplly mmulttiplly bby 11,4660 bbytees.Weaknnessses of thee MoodellThe ttrannsacctioonall moodell iss quuitee siimplle, andd neegleectss seeverral impporttan
15、tt efffeccts. Fiirstt off alll, it asssumees aa stteaddy sstreeam of welll-oordeeredd frramees wwithh noo coonteentiion forr thhe mmediium. 8002.111 iimpllemeentss coolliisioon aavoiidannce andd exxponnenttiall baackooff, soo inn reealiity, thhe ttimee beetweeen fraame excchanngess wiill be lonngerr thhan onee DIIFS. Exxponnenttiall baackooff in thee prreseencee off coonteentiion willl ffurttherr deecreeasee thhrouughpput. Myy innitiial bacck-oof-tthe envveloope esttimaate is thaat cconttent
