The Design and Evaluation of a Scalable Wireless MAC Protocol

Transcription

1 Th Dsign and Evaluation of a Scalabl Wirlss MAC Protocol Tchnical Rport UIUCDCS-R--793 Chun-chng Chn, Eunsoo So, and Haiyun Luo Dpt. of Computr Scinc Univ. of Illinois at Urbana-Champaign Urbana, IL {chn3,so}@uiuc.du, haiyun@cs.uiuc.du ABSTRACT Contntion-basd MAC protocols such as IEEE 8.11 DCF ar known to prform wll in a ntwork with small numbr of nods. As th numbr of nods incrass, th cost of collisions incrass and dominats. This incrasing collisions quickly drivs vry clint in th ntwork into wastd and unfair idlnss, rsulting in prformanc dgradation. In this papr, w propos SMAC, a scalabl MAC protocol, that prsrvs th advantags of 8.11 whil avoiding its drawbacks. Th dsign of SMAC lvrags th fficint channl utilization of 8.11 DCF at low load and 8.11 PCF at high load. Th SMAC-nabld AP controls th accss of its clints by combining th contntion and schdulbasd accss mchanisms and dtrmining th optimal contntion lvl among th clints. Th SMAC-clint simply indicats its intntion of channl accss. Through thorough analysis and valuation, w show that SMAC prsrvs th channl utilization fficincy of 8.11 DCF, scals up to any numbr of nods, and prsrvs prfct short-trm and long-trm fairnss among th contnding clints. 1. INTRODUCTION As th IEEE 8.11 bcoms th on dominating standard which is widly accptd for th wirlss ntworking tchnology for Local Accss Ntwork (LAN), many of th laptops, PDAs, cll phons, and vn dsktop computrs ar quippd with IEEE 8.11 NICs whn thy ar manufacturd. Bcaus of th suddn prolifration of IEEE 8.11 dvics, th numbr of nods which is covrd by on Accss Point (AP) has also bn incrasd. In a vry populatd ara such as univrsitis, confrnc sitd and downtowns of big citis, it is obsrvd that mor that on hundrd usrs ar connctd to on AP []. Usrs in thos sits usually xprinc vry low throughput or vn frqunt disconnctions. This is bcaus th original IEEE 8.11 DCF (and PCF, as wll) has not bn dsignd with scalability in mind. This problm will b mor svr in th futur bcaus IEEE 8.11 is bing spradd fast and mor dvics such as digital camras [7] or cars which hav not bn considrd to b connctd to th ntworks bgan to b connctd to th ntworks through IEEE Contntion-basd MAC protocols such as IEEE 8.11 DCF ar known to prform wll in a ntwork with small numbr of nods. As th numbr of nods incrass, th cost of collisions incrass and dominats. On th contrary, polling-basd MAC protocols such as IEEE 8.11 PCF work mor fficintly with largr numbr of nods [11]. Howvr, th prformanc of polling-basd MAC protocols can b dgradd if thr ar many idl nods in th ntwork. W xamin th problms of IEEE 8.11 DCF and PCF in sction. Thn w prsnt how to combin th advantags of ths two approachs: a MAC protocol which maintains th aggrgat throughput as th numbr of nods incrass rgardlss of th ratio of th idl nods in sction 3. For furthr optimization, w propos to us th locality of packt transmission. That is, th nod which transmits a packt is likly to hav mor packts in its output quu. This proprty rducs th cost of polling significantly and incras th ovrall prformanc of th proposd MAC protocol. Thr ar svral tchniqus which may mitigat this problm such as spatial rus by controlling th transmission powr [, 8], usag of multipl orthogonal channls [1] and admission control []. Thy can hlp solving th problm to som xtnt by rducing th numbr of concurrnt contnding nods, but thy ar not scalabl by thmslvs, ithr. W talk about ths tchniqus in sction.. THE SCALABILITY OF IEEE 8.11 In this sction, w xamin th IEEE 8.11 MAC protocols and show that thy do not scal wll as th numbr of nods incrass..1 DCF Th Distributd Coordination Function (DCF) [1] is th mandatory MAC protocol of IEEE It is basd on carrir sns multipl accss with collision avoidanc (CSMA/CA), which uss random backoff to avoid collisions. Figur 1 dpicts how 8.11 DCF works. Whn a nod has a packt to snd, it first snss th carrir. If th channl is idl, it first waits for. Thn it chooss a random backoff tim within th contntion window (CW) and waits for th backoff tim. If th channl is idl for th backoff tim, it snds th packt, waits for and listns to th channl to rciv ACK (figur 1(a)). If th channl bcoms busy during th first tim, th nod waits until th channl bcoms idl and r-starts to wait for (figur 1(b). If th channl bcoms busy during th backoff 1

2 Backoff Data ACK Channl Busy (a) Succssful transmission Backoff Data ACK (b) Mdium busy during Tim Tim. PCF Th othr optional MAC protocol which IEEE 8.11 dfins is th Point Coordination Function (PCF) [1] which can b usd in conjunction with DCF. Contrary to DCF, PCF is a cntralizd solution. PIFS D1 U1 D PIFS D3 U3 CF_nd Backoff Rsidual Backoff Data ACK Tim Channl Busy (c) Mdium busy during backoff Backoff Data Collision (d) Collision CW is doubld Figur 1: IEEE 8.11 DCF tim, it frzs th backoff timr immdiatly and it waits until th channl bcom idl. Thn it first waits for and waits for th rsidual backoff timr (figur 1(c)). If th nod dos not rciv any ACK aftr snding a packt or thr is a collision during th rcption of th ACK, it doubls th contntion window (figur 1(d)). This mchanism is calld binary backoff. A valid valu of CW is in th rang of [CW min, CW max]. Whn a nod succssfully transmits a packt and rcivs an ACK, it rsts its CW to CW min not to wast tim with xcssiv backoff. Whn a nod fails a packt transmission, it doubls CW (with uppr limit of CW max) to rduc collisions. If a nod fails packt transmissions for svn tims srially, it rsts its CW to CW min to nt th long-trm unfairnss. Givn th numbr of contnding nods, th optimal CW can b calculatd [3]. Not that th ncssary information is not th numbr of nods but th numbr of contnding nods which hav nonmpty output buffrs. Howvr, it is impossibl to know th numbr of contndrs, so DCF rsts/doubls th CW whn it succds/fails a packt transmission instad of using a constant CW. On qustion which ariss hr is whthr th numbr two (which is usd for binary backoff) is optimal or not. Th xponntial cofficint (two in binary backoff) dtrmins how fast th CW rachs a good valu which rflcts th currnt congstion lvl. In that sns, if th ntwork has a lot of contnding nods, it is bttr to us larg xponntial cofficint such as 3 or than. Contrarily, if thr ar a small numbr of contnding nods in th ntwork, smallr xponntial cofficint such as 1.3 or 1.7 might b bttr for th prformanc. If th xponntial cofficint is too small, it inducs many collisions. If th xponntial cofficint is too larg, channl is undr-utilizd. As th numbr of nods incrass, th optimal CW also incrass. Thrfor DCF which uss constant xponntial cofficint suffrs from svr collisions as th numbr of nods incrass. Tim Tim Figur : IEEE 8.11 PCF Figur shows how PCF works. Whn it starts, AP first waits for PIFS and snds a downstram data packt (D1) to th first nod (n 1) in its ntwork. If n 1 has an upstram packt, it snds th packt (U1) to th AP aftr. Thn th AP movs on to th nxt nod n and snds anothr downstram packt (U). If th polld nod (n ) dos not hav an upstram packt, it just dos not do anything. Thn AP dtcts it aftr PIFS, and continus polling othr nods immdiatly. Whn AP finishs polling, it snds a CF nd packt as an nd markr. Not that thr is no backoff until CF nd bcaus AP controls th transmission schdul of all nods. If all nods ar backloggd, PCF is scalabl unlik DCF whos frquncy of collisions incras as th numbr of nods incras. Howvr in rality, a ntwork has non-ngligibl portion of idl nods. If it is th cas, AP wasts som tim which is proportional to th numbr of idl nods for polling thm, and it maks th ovrall prformanc not scalabl. 3. DESIGN OF THE SCALABLE MAC 3.1 Scalabl MAC In SMAC, th clints in a singl BSS ar catgorizd into two groups S and C. All clints in groups S must hav frams to transmit and thir transmissions ar schduld by th AP. Th clints in group C ar thos who hav no frams impnding and accss th channl through contntion. Roughly spaking, th AP randomly chooss on of th clints in S to b schduld for transmission. Th clints in group C contnd for th channl in btwn th transmissions of th schduld clints. Whn th AP rcivs th DATA from a clint x in group S, it dtrmins who is th nxt schduld transmission clint and whn it should transmit by including nxtst A and nxtcw in th ACK to x. All th othr clints in th BSS must b abl to ovrhar this ACK and dcod th ACK fram. Th clint in S matching nxtsta backs off at nxtcw slots; whil othr clints in S silntly wait for thir turns. A clint is initially in group C. Whn having DATA to transmit, it waits for th ACK containing th nxtsta and nxtcw, backs off randomly btwn zro and nxtcw 1 slots. Sinc th nxt schduld clint backs off at nxtcw slots, it transmits DATA aftr all th clints in C finishs thir contntion. Th contnding clints indicat in th DATA whthr thy hav mor frams to snd. Whn rciving th DATA from a contnding clint x with mor

3 frams to snd, AP adds x to th st of schduld clints if it is not thr and rplis with ACK. Whn rciving th ACK from th AP, th contnding clint with mor frams to snd changs its mmbrship to S and starts waiting for its schduld turn of transmission. If th contntion fails du to collision, contnding clints simply wait for th nxt ACK fram announcing nxtsta and nxtcw. Not that AP has all th information of th associatd clints, it may dpnd on th numbr of clints in S and clints in C to adjust th opning of th contntion priod. Finally, sinc all th clints in S must hav mor frams to snd, th AP rmovs th transmittr x from S if th rcivd DATA indicats no mor frams to snd. Rgardlss of th contnding clints, SMAC is ssntially a schduling-basd MAC protocol to avoid congstion-inducd collision. This rquirs AP and th clints to hav a consistnt viw of th mmbrship. Howvr, sinc wirlss channl rrors could occur in a fairly hight rat, th clints and AP may not always hav th sam viw of th schduld st. For xampl, a clint x may transmit DATA indicating mor frams to snd, AP will add x into S. But th rplid ACK may b lost, rsulting x thinking itslf is still in C. Similarly, a clint x may transmit DATA asking for rmoval from S, AP rmovs x from S. But th rplid ACK may again b lost, rsulting that x thinks itslf is still in C. For th formr cas, x will contnd for th channl during th contntion priod and vntually b addd to S consistntly. For th lattr cas, w adopt a timr similar to 8.11 that is (r)triggrd whn a schduld clint x transmits DATA. Whn th timr xpirs without rciving th ACK, x changs itslf into C and contnd for th channl. This consrvativ approach guarants that clints will nvr think thmslvs in S whil in rality thy ar in C. But th rvrs is not tru...our solution is to... Idally, if all th clints blong to S, SMAC achivs th bst possibl total throughput sinc no tim is spnt on contntion-inducd collision and idlnss. To account for potntial clints joining and laving th schduld st S, w lav som spac in btwn th transmissions of two schduld clints for othr potntial clints to join S. Howvr, too much spac would lad to wastd idl tim slots whil too littl spac lads to havy collision of th contnding clints. In th nxt sction, w prsnt th mchanism to obtain th optimal tim slots to accommodat th contnting clints whil prsrving th fairnss among th clints in th BSS. 3. Optimal Contntion Window Th clints in C and clint in S ar orthogonal to ach othr in that thy do not contnd with ach othr. Nvrthlss, th transmission of a schduld clint srvrs as a flag indicating th start and nd of a contnding poch. Sinc no tim slot is wastd among th clints in S, optimizing th ovrall throughput is quivalnt to optimizing th throughput among th contnding clints. Svral popl hav proposd optimizing th contntion-basd throughput by tuning th contntion window siz [], but thy fall short of short-trm fairnss among th clints. Huss t.al [3] proposd a nw accss mchanism basd on having ach nod convrging to th sam contntion window siz. W adopt a similar approach to [3] but hav th AP calculat th optimal contntion window siz nxtcw basd on th numbr of collisions in th ious contntion priod and announc th rsult to b usd by th clints in C in th nxt contntion priod. Lt P dnot th channl attmpt probability of th contnding nods, P i dnot th idl probability of th contnding nods, P c dnot th collision probability of th contnding nods, and P t dnot th succssful channl transmission probability, N dnot th numbr of contnding nods in C. W know that: P = cw + 1 (1) P t = NP (1 P ) N 1 () P i = (1 P ) N (3) P c = 1 P t P i = 1 NP (1 P ) N 1 (1 P ) N () Th throughput Thr of th contnding nods can b xprssd as: P te[p] Thr = () P tt t + P ct c + P i σ, whr w assum th following variabls ar known: E[P] is th xpctd DATA siz, σ is aslottim, T t and T c ar th avrag transmission duration and avrag collision duration rspctivly. Substituting P t, P c, P i into Eq (), w obtain Thr as a function of th only unknown variabl P. Thr is maximizd whn P is drivd from th following quation: 1 NP opt,whr η = 1 σ T c = 7.17 for = η(1 P opt ) N (), and us Eq(1) to find th optimal contntion window siz nxtcw to b usd for th nxt round. It turns out th N can b Thrfor, by stimating N, w can calculat P opt asily stimatd ( ˆN) from th collision probability Pc and channl attmpt probability P of th ious contntion priod. In short, lt th numbr of collisions in th ious contntion priod b n c. Lt n s b th numbr of succssful transmissions in th ious contntion priod, and cw b th ious optimal contntion window siz usd. W obtain Eq (7): n c cw = 1 ˆNP (1 P ) ˆN 1 (1 P ) ˆN From Eq (7), w plot in Figur 3 th stimatd numbr of contnding stations ( ˆN) basd on th ious channl attmpt probability (P p rv) and th masurd collision probability ( c n ). W also plot th optimal contntion window cw for diffrnt numbrs of contnding stations in Figur. Finally, obtaining ˆN using Eq (7), w can us Eq () to obtain th nxtcw for AP to announc in th nxt contntion priod. Th optimal contntion window basd on P p rv and n c is plottd in Figur. cw Th psudo-cod for th scalabl MAC protocol is shown is Figur and Figur Illustrativ Exampls Suppos thr ar thr clints A, B, and C in th BSS. Clint A has bn in th schduld st of th AP. Clint B and C hav mor frams to transmit, but thy ar not yt addd to th schduld st by th AP. Figur 8 shows on xampl of how B and C ar addd to th schduld st S of th AP. Th prcivd channl channl status at (7) 3

4 Numbr of contnding nods P =. P =. P =. P =.8 Optimal Contntion Window Optomal contntion window 3 1 P =. P =. P =. P = Collision probability 1 3 Numbr of contnding nods Collision probability Figur 3: Numbr of contnding Figur : Optimal contntion window Figur : Optimal contntion window stations for diffrnt channl attmpt for diffrnt numbrs of contnding stationitis and collision for diffrnt channl attmpt probabil- probabilitis and collision probabilitis probabilitis DATA ACK (A) cw= DATA DATA ACK DATA (B, C) (B, C) DATA ACK DATA ACK cw=1 DATA ACK DATA ACK (A) (A) cw=18 (B) (C) (A) cw= Idl slot Idl slots Idl slots Idl slot Idl slots Idl slots Idl slots Figur 9: Fram xchang squnc and dtaild timing of Figur 8 rcvdata(f, x) f: th rcivd DATA fram x: th transmittr of th rcivd fram 1: if x has mor frams to snd thn : insrt x into S if x / S 3: ls : rmov x from S if x S : if x was in S alrady thn : ACK.nxtCW calcoptcw() 7: if S.siz() > thn 8: ACK.nxtSTA randomly pick y S 9: ls 1: ACK.nxtSTA CONTEND 11: ls 1: if S.siz() > thn 13: if x is just insrtd to S & S.siz() = 1 thn 1: ACK.nxtSTA x 1: ACK.nxtCW calcoptcw() 1: ls 17: ACK.nxtSTA WAIT 18: ls 19: ACK.nxtSTA CONTEND : ACK.nxtCW calcoptcw() 1: snd(ack) Figur : SMAC for AP th AP can b classifid into (1) succssful transmission, () collision, and (3) channl idl. Sinc schduld nods do not contnd with contnding nods, only th contnding nods may rsult in collision. Thus A s transmission is always succssful. Suppos th initial contntion window is st to in th ACK from AP to A, aftr ovrharing this ACK B and C ach randomly backs off in [, ) aslottim and A backs off at aslottim. With only slots, th probability of collision btwn B and C is %. To illustrat th optimal contntion window calculation, lt s suppos B and C both choos to transmit th data aftr 1 aslottim, rsulting in data collision. Now, th AP masurd that thr is a collision during th contntion priod of lngth slots. Sinc th ious announcd contntion window valu is, th collision probability is P c = 1/ =., ious channl atrcvack(f, a) f: th rcivd ACK fram a: th transmittr of th rcivd fram 1: if a = myap & ContTimr is busy thn : stop ContTimr 3: if iously snt DATA indicats mor frams to snd thn : whichst SCHEDULING : ls : whichst CONTENDING 7: if whichst = SCHEDULING thn 8: if I am f.nxtsta thn 9: backoff at f.nxtcw 1: ls 11: if f.nxtsta WAIT thn 1: backoff randomly in [, f.nxtcw) ovrharack(f, a) f: ovrhard ACK fram a: th transmittr of th ovrhard fram 13: if a = myap & ContTimr is busy thn 1: stop ContTimr 1: if a = myap & f.nxtsta WAIT thn 1: if snd timr is busy thn 17: stop snd timr 18: if whichst = SCHEDULING thn 19: whichst = CONTENDING : if whichst = CONTENDING thn 1: if I hav frams to snd thn : if backoff timr is busy thn 3: stop backoff timr : backoff randomly in [, f.nxtcw) : ls : if I am f.nxtsta thn 7: backoff at f.nxtcw Figur 7: SMAC for Clint tmpt probability P = /(+1) = /3. Substituting P c and P into Eq (7), w obtain ˆN to b, which maps to as th optimal contntion window. AP thn announcs this valu in th ACK to clint A. Again A will back off at aslottim, B and C back off in [, ). Although th probability for B and C s transmission to collid again drops to 1/ =., lt s suppos B and C collid again. Following th sam procss, w obtain th optimal contntion window siz to b 3. In fact, if B and C kps colliding with

5 A B C Transmissions for A, B, and C Collision B B A C A C A B C A slots 3 slots Figur 8: Prcivd channl status at th AP for on schduld clint A and two contnding clints B and C ach, th optimal contntion window convrgs at around 8, shown in Figur 1. Not that th optimal contntion window is obtaind by masuring th collision probability at th AP, contnding nods do not nd to prform th xpnsiv binary xponntial backoff which oftn lads to wastd idl channl. Furthrmor, it adjusts th contntion window fastr than th traditional binary xponntial backoff whn thr is a suddn congstion and convrgs to a much smallr valu whn continuous collisions happn. Optomal contntion window Numbr of continuous collisions Figur 1: Optimal contntion window volution whn B and C in Figur 8 kps colliding. PERFORMANCE EVALUATION W implmnt SMAC in ns- simulator vrsion.9. For th currnt ns implmntation, nods rciv packts only whn th RSS from th sndr is gratr than th rciving thrshold, and th impact of any signal with RSS lss than th carrir sns thrshold is compltly ignord - no mattr how many thos signals ar. This is obviously an ovr-simplification of th rality. W rplac this part of 8.11 functions with th ons dvlopd in [], so that all signals from th simulation ar takn into account at rcivr, and th combind signal to intrfrnc-nois (SINR) ratio is usd to dtrmin if an incoming signal can intrfr or b rcivd/capturd. W us Two-Ray Ground radio propagation modl, and th transmitting powr is st so that th communication rang is 11m, and th carrir sns thrshold is st so that th dfault intrfrnc rang is m. W us Mbps basic rat and 11Mbps data rat basd on IEEE 8.11b. Each simulation runs uplink cbr flows with fram siz 1 byts for sconds. For ach random xprimnt, w run th simulaiton for tims and obtain th avrag valus unlss othrwis statd..1 Various Numbrs of Clints W first valuat th prformanc of SMAC in a singl cll by randomly placing varid numbrs of clints in a 8m by 8m ara with saturatd offrd load. Th AP is placd in th cntr of th ara. Figur 11 shows th total throughput for various numbrs of clints in a cll. SMAC clarly scals whn th numbr of clints incrass and kps th total throughput lvling at.mbps. On th othr hand, th total throughput of 8.11 dcrass mor than % from 1 clint to 18 clints in th cll. Total throughput (Mbps)... Scalabl MAC Numbr of clints Figur 11: Systm throughput for varid numbr of clints in a cll W furthr plot th throughput distribution of th 8, 3, and 18 clints contnding in a BSS for 8.11 in Figur 1 and for SMAC in Figur 13. Although 8.11 hav bn thought to achivd long-trm fairnss among th clints, it dos not possss this proprty vn in this fairly long priod of tim ( sc). Th rasons ar twofold. First, whn th numbr of nods is small, thr ar fw collisions among th clints. As th numbr of nods incrass, howvr, th collidd clints in th ious contntion simply doubls its contntion window and compts with othr clints again with smallr channl attmpt probability, rsulting in short-trm unfairnss. Scond, whn captur ffct coms into play, th nods that ar closr to th AP bnfit from its strongr SINR ratio whil th nods that ar farthr from th AP suffrs from th wakr SINR. Whn a collision happns, AP could captur th fram from th closr clint and disrgard th fram from th clints farthr away, rsulting in th intra-bss unfairnss. In fact, th mor clints, th mor svr th unfairnss will b as shown in Figur 1. SMAC (Figur 13), on th othr hand, rlis on th schdul at th AP, and is abl to kp all th flows sharing fair amount of wirlss rsourc (max diffrnc is within % of th highst throughput).. Various Offr Loads It is wll known that 8.11 provids good prformanc whn th channl is undr-utilizd. Howvr, th prformanc dgrads whn th availabl wirlss rsourc bcoms mor and mor scarc. W compar th total throughput of 8.11 and SMAC undr various lvls of offr loads.

6 nods nods nods Normalizd throughput (Mbps) Figur 1: Throughput distribution for 8.11 for 8, 3, and 18 clints in a cll Total throughput (Mbps). x Scalabl MAC Offr load (Mbps) Figur 1: Avrag total throughput of 8 nods for varid offr load Scalabl MAC 18 nods Scalabl MAC 3 nods Scalabl MAC 8 nods channl rror happns, 8.11 clints simply doubls it contntion window siz bfor contnding for th channl again. This is opposit to what it should hav don - contnd for th channl mor aggrsivly. SMAC-nabld AP, on th othr hand, quickly dtcts this rror and rassign th optimal contntion window and nxt clint to transmit without incurring any unncssary wastd tim slots Normalizd throughput (Mbps) Figur 13: Throughput distribution for 8.11 for 8, 3, and 18 clints in a cll Total throughput (Mbps) 3 1 Scalabl MAC 8.11 W randomly plac 8 clints in a 8 by 8 ara with clints bing applid offr load from. Mbps to 1. Mbps. With 8 clints th saturating point for 8.11 is around.8 Mbps as shown in Figur 1. SMAC is abl to prform as fficint as 8.11 whn th offr load is low (offr load lss than.8 Mbps). Whn th offr load is high, it avoids th drawback of contntion and achivs % mor fficint channl utilization..3 Channl Error Effct In our ious simulations, a nod fails to rciv a fram only du to collisions. In rality, failur rcption could also du to bad channl quality. W simulat th wirlss channl for diffrnt bit rror rat (BER) and compar th prformanc of SMAC and Figur 1 shows th total throughput of 8.11 and SMAC for 3 clints in a BSS. Not that 8.11 dos not distinguish failur transmission that is du to collision and bad channl quality. Whn bit rror rat Figur 1: Avrag total throughput of 3 nods for varid bit rror rat (BER). RELATED WORK W rviw blow th approachs to mak MAC protocols to handl a lot of nods mor fficintly: spatial rus [, 8], multipl channls [1], DCF nhancmnts [3, 9], nods grouping [11] and admission control []. Just lik th cllular ntworks, a larg ara can b dividd into many sub-aras and an AP can b assignd to ach subara. Th lss powr th APs (and th nods, of cours) us, th smallr th clls ar. Instad of placing a small numbr of APs, using many APs can rduc th numbr of nods

7 which is covrd by an AP. To avoid intr-cll intrfrnc and hiddn trminal problm, it is dsirabl for two adjacnt clls to us diffrnt orthogonal channls, but it may not always b possibl du to th limitation of th numbr of orthogonal channls. Using multipl channls, th numbr of contnding nods can also b rducd [1]. If an AP has k intrfacs ach of which uss on of th k orthogonal channls and th nods ar uniformly assignd to on of th k channls, th numbr of contndrs is rducd to 1 of th numbr of all nods. k Howvr, whil this can mitigat th problm to som xtnt, it cannot b an ultimat solution bcaus k is limitd. In 8.11b, only thr channls (1, and 11) ar orthogonal. Many nhancmnts to DCF hav bn proposd [3, 9]. [3] shows that, if th numbr of contnding nods is known, th optimal CW can b calculatd and it maks th sum of th collision tim and idl tim almost constant rgardlss of th numbr of nods. This mans th idal DCF is scalabl. Bcaus it is impossibl for th nods to gt th numbr of contndrs, thy us som approximation to mak th protocol not scalabl. [9] rducs CW into half aftr a succssful transmission instad of rstting to CW min and nts rapid changs of CW valu. It improvs th prformanc of DCF, but dos not fiv a solution to th scalability problm. To rduc th numbr of nods which contnd at th sam tim, TMAC [11] has bn proposd. In TMAC, all nods ar dividd into g disjoint tokn-groups. Th AP assigns a tokn to on tokn group to mak th nods in a spcific tokn group to us th channl for som tim. This is don in a coars-tim scal. If th tokn is assignd to on tokn group, th nods in th tokn group contnd using a DCF-lik contntion-basd protocol. It also allows batch transmission in which nods can snd multipl packts aftr on succss in contntion and block ACK which acknowldgs th rcption of multipl packts using only on ACK packt. If too many nods ar associatd to an AP, th ntwork connctivity of almost all nods bcoms unstabl and scarc. This phnomnon is vry common in larg wirlss ntworks. IQU [] uss admission control to solv this problm. Evry nw nod is quud at th AP bfor association. If th currnt channl is highly crowdd, th AP dos not allow th association of a nw nod. Instad, th AP quus th nw nod. Evry associatd nod has a finit tim slot during which it can accss th ntwork. Aftr th timout, it is put to th back of th quu and th connctivity is suspndd. If th channl has som room for nw nods, th AP associats th top nod in th quu to th ntwork and nabls th connctivity. On problm of this approach is that it changs th ntwork srvic modl. Bcaus of th quu and timout, th ntwork connctivity is priodic and it maks th nods to do all ntwork-rlatd jobs whn thy ar connctd. Dfinitly this approach causs problms whn usd with th uppr-layr protocols such as TCP which ar dsignd without considration of this ntwork accss mod.. CONCLUSION In this papr, w propos SMAC, a scalabl MAC protocol, that prsrvs th advantags of 8.11 whil avoiding its drawbacks. Th dsign of SMAC lvrags th fficint channl utilization of 8.11 DCF at low load and 8.11 PCF at high load. Th SMAC-nabld AP controls th accss of its clints by combining th contntion and schdulbasd accss mchanisms and dtrmining th optimal contntion lvl among th clints. Th SMAC-clint simply indicats its intntion of channl accss. Through thorough analysis and valuation, w hav shown that SMAC prsrvs th channl utilization fficincy of 8.11 DCF, scals up to any numbr of nods, and prsrvs prfct short-trm and long-trm fairnss among th contnding clints. 7. REFERENCES [1] P. Bahl, R. Chandra, and J. Dunagan. Ssch: slottd sdd channl hopping for capacity improvmnt in i 8.11 ad-hoc wirlss ntworks. In MobiCom : Procdings of th 1th annual intrnational confrnc on Mobil computing and ntworking, pags 1 3, Nw York, NY, USA,. ACM Prss. [] F. Calí, M. Conti, and E. Grgori. Dynamic tuning of th IEEE 8.11 protocol to achiv a thortical throughput limit. IEEE/ACM Transactions on Ntworking, 8(), Dcmbr. [3] M. Huss, F. Roussau, R. Guillir, and A. Duda. Idl sns: An optimal accss mthod for high throughput and fairnss in rat divrs wirlss lans. In Procdings of ACM SIGCOMM,. [] C. Hu and J. C. Hou. A ractiv channl modl for xpditing wirlss ntwork simulation. In ACM SIGMETRICS Postr,. [] A. P. Jardosh, K. Mittal, K. N. Ramachandran, E. M. Blding, and K. C. Almroth. IQU: practical quu-basd usr association managmnt for WLANs. In MobiCom : Procdings of th 1th annual intrnational confrnc on Mobil computing and ntworking, pags 18 19, Nw York, NY, USA,. ACM Prss. [] T.-S. Kim, H. Lim, and J. C. Hou. Improving Spatial Rus through Tuning Transmit Powr, Carrir Sns Thrshold, and Data Rat in Multihop Wirlss Ntworks. In MobiCom : Procdings of th 1th annual intrnational confrnc on Mobil computing and ntworking, Nw York, NY, USA,. ACM Prss. [7] Kodak asyshar-on zoom digital camra manual. [8] J. Monks, V. Bharghavan, and W. mi W. Hwu. A powr controlld multipl accss protocol for wirlss packt ntworks. In Procdings of IEEE Confrnc on Computr Communications (INFOCOM), pags 19 8, 1. [9] Q. Ni, I. Aad, C. Barakat, and T. Turltti. Modling and analysis of slow cw dcras i 8.11 wlan. In Prsonal, Indoor and Mobil Radio Communications, 3. PIMRC 3. 1th IEEE Procdings on, 3. [1] J. Schillr. Mobil communications. Addison-Wsly Longman Publishing Co., Inc., Boston, MA, USA, scond dition, 3. [11] Y. Yuan and W. Arbaugh. Towards Scalabl MAC Dsign for High-Spd Wirlss LANs. Tchnical rport, Dpartmnt of Computr Scinc, Univrsity of Maryland, Collg Park,. 7