THE IEEE standard for wireless local area networks

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1 IEEE RANSACIONS ON VEHICULAR ECHNOLOGY, VOL. 58, NO. 2, FEBRUARY Determnstc Prorty Channel Access Scheme for QoS Support n IEEE e Wreless LANs Sunmyeng Km, Rongsheng Huang, Student Member, IEEE, and Yuguang Fang, Fellow, IEEE Abstract he enhanced dstrbuted channel access EDCA) of IEEE e has been standardzed to support qualty of servce QoS) n wreless local area networks LANs). he EDCA statstcally supports the QoS by dfferentatng the probablty of channel access among dfferent prorty traffc and does not provde the determnstcally prortzed channel access for hghprorty traffc, such as voce or real-tme vdeo. herefore, lower prorty traffc stll affects the performance of hgher prorty traffc. In ths paper, we propose a smple and effectve scheme called determnstc prorty channel access DPCA) to mprove the QoS performance of the EDCA mechansm. o provde guaranteed channel access to multmeda applcatons, the proposed scheme uses a busy tone to lmt the transmssons of lower prorty traffc when hgher prorty traffc has packets to send. Performance evaluaton s conducted usng both numercal analyss and smulaton and shows that the proposed scheme sgnfcantly outperforms the EDCA n terms of throughput, delay, delay jtter, and packet drop rato under a wde range of contenton level. Index erms Busy tone, enhanced dstrbuted channel access EDCA), IEEE e, medum access control MAC), qualty of servce QoS). I. INRODUCION HE IEEE standard for wreless local area networks WLANs) defnes a medum access control MAC) protocol for sharng the channel among statons [1]. he MAC protocol s desgned wth two methods of communcaton for statons: 1) dstrbuted coordnaton functon DCF) and 2) pont coordnaton functon PCF). he DCF has two data transmsson methods: 1) default basc access and 2) optonal request-to-send RS)/clear-to-send CS) access. he basc access method uses the two-way handshakng DAA-ACK) mechansm. he RS/CS access method uses the four-way handshakng RS-CS-DAA-ACK) mechansm to reserve Manuscrpt receved March 1, 2007; revsed August 22, 2007, March 26, 2008, and March 30, Frst publshed May 14, 2008; current verson publshed February 17, he work of S. Km was supported n part by the Korean Government Mnstry of Educaton and Human Resources Development) under Korea Research Foundaton Grant KRF D he work of R. Huang was supported n part by the U.S. Natonal Scence Foundaton under Grant CNS and Grant CNS he work of Y. Fang was supported n part by the U.S. Natonal Scence Foundaton under Grant CNS and Grant CNS and n part by the Natonal Scence Councl NSC) under the NSC Vstng Professorshp under Contract NSC E and Chunghwa elecom under Contract NBY he revew of ths paper was coordnated by Prof. B. L. S. Km s wth the School of Computer and Software Engneerng, Kumoh Natonal Insttute of echnology, Gum , Korea e-mal: sunmyeng@ kumoh.ac.kr). R. Huang and Y. Fang are wth the Department of Electrcal and Computer Engneerng, Unversty of Florda, Ganesvlle, FL USA e-mal: rshuang@ufl.edu; fang@ece.ufl.edu). Color versons of one or more of the fgures n ths paper are avalable onlne at Dgtal Object Identfer /V the channel before transmttng long data packets. hs technque s ntroduced to avod the hdden termnal problem. he DCF s used to support best-effort data traffc, whereas the PCF supports tme-senstve traffc. he access pont AP) perodcally transmts a beacon frame to statons. Between beacon frames, the channel tme s dvded nto a contenton-free perod CFP) and a contenton perod. o elmnate the channel contenton among statons, the AP grants channel access to a staton by pollng the staton durng the CFP. Statons can only transmt ther packets after beng polled by the AP. he wdespread use of multmeda applcatons requres new features, such as hgh bandwdth and small average delay and delay jtter n WLANs. Unfortunately, the IEEE MAC protocol cannot support qualty-of-servce QoS) requrements [2] [6]. he DCF does not dfferentate between traffc types [7], and a staton mght have to wat for an arbtrarly long tme to send a packet so that multmeda applcatons, such as voce and vdeo, may suffer ntolerable delay [8]. o support multmeda applcatons wth tght QoS requrements n the IEEE MAC protocol, the IEEE e has been standardzed [9]. It ntroduces a new channel access mechansm called the hybrd coordnaton functon HCF), whch combnes functons from the DCF and PCF wth some enhancements. he contenton-based channel access mechansm of the HCF s referred to as the enhanced dstrbuted channel access EDCA). he EDCA supports the QoS requrements by ntroducng four access categores ACs). Each packet arrves at the MAC layer wth prorty from the hgher layer and s mapped to an AC accordng to the prorty. AC 3, AC 2, AC 1, and AC 0 are for voce, vdeo, best-effort data, and background traffc, respectvely. o dfferentate the traffc types, the EDCA uses a set of AC specfc parameters, whch nclude mnmum contenton wndow CWmn[], maxmum contenton wndow CWmax[], and arbtraton nterframe space AIFS) AIF S[] for AC =0,...,3). he AIFS s, at least, dstrbuted nterframe space DIFS) long and s calculated wth the AIFS number AIF SN[]. he duraton of AIF S[] s defned by AIF S[] =SIFS + AIF SN[] aslot me, where SIF S s a short nterframe space IFS), and aslot me s the duraton of a slot tme. For 0 <j 3, the EDCA has CWmn[] CWmn[j], CWmax[] CWmax[j], and AIF SN[] AIF SN[j]. Note that, n the precedng nequaltes, at least one must be not equal to. heedcaassgnsasmallercwmn, CWmax, oraif S to hgher prorty AC to provde the hgher probablty to access the channel. herefore, n the EDCA, the support of QoS can be acheved by dfferentatng the probablty of channel access among dfferent prorty ACs [10], [11]. However, there stll /$ IEEE

2 856 IEEE RANSACIONS ON VEHICULAR ECHNOLOGY, VOL. 58, NO. 2, FEBRUARY 2009 reman problems n the EDCA smlar to the DCF snce t s a contenton-based mechansm [12] [14]. At hgh loads, there are a large number of collsons, even for hgh-prorty ACs, because low-prorty ACs keep attemptng to access the channel and collde wth hgh-prorty ACs. herefore, the EDCA does not ensure the QoS requrements [15]. In ths paper, we propose a novel dstrbuted contentonbased MAC algorthm called determnstc prorty channel access DPCA). he proposed DPCA scheme uses short-duraton busy-tone sgnals.e., pulses of energy) [16] [18] to provde a determnstcally prortzed channel access for hgh-prorty ACs and avod collsons caused by low-prorty ACs. hs paper s organzed as follows: he related work s presented n Secton II. In Secton III, the proposed DPCA scheme s presented n detal. he channel tme rato used by voce and data traffc s analyzed n Secton IV. In Secton V, we dscuss the numercal and smulaton results. Fnally, we draw a concluson n Secton VI. II. RELAED WORK o provde QoS guarantee, many studes dfferentate traffc types by offerng them dfferent QoS parameters or by usng busy tones. Several prorty schemes have been studed n the lterature over the DCF. Kwon et al. proposed QoS support for voce servce over IEEE WLANs by makng a reservaton before the data channel access [3], [4]. Xao proposed a smple prorty scheme for real-tme applcatons by dfferentatng the ntal wndow sze, the wndow-ncreasng factor, and the maxmum backoff stage [2]. Veres proposed a prorty scheme that sets dfferent values of mnmum and maxmum contenton wndows for dfferent traffc types and dfferent levels of servce [19]. he schemes proposed n [2] and [19], as well as the EDCA, statstcally support the QoS requrements by dfferentatng the probablty of channel access among dfferent prorty traffc types [10], [11], [20]. Although the prortzed channel access s provded n a long-term tme scale, t s not guaranteed n a short-term tme scale. he reasons are gven as follows: A staton decreases ts backoff counter by one at each slot when the channel s dle after sensng an dle channel for the DIFS/AIFS perod. he staton transmts ts packet, regardless of ts prorty, when ts backoff counter reaches zero. In addton, a hgh-prorty staton cannot always be assured to have a smaller backoff counter snce the backoff counter s randomly selected based on the unform dstrbuton. herefore, lower prorty statons can transmt ther packets pror to hgher prorty statons, and the prorty of channel access cannot be ensured. As a result, hgher prorty traffc may wat a long tme for the channel contenton. he short-term prortzed access s very mportant for delay- or jtter-senstve traffc, such as voce and vdeo [21]. Herenafter, we call ths undesrable case the short-term prorty problem. Aad and Castellucca assgned dfferent DIFSs to dfferent prorty traffc for servce dfferentaton n the IEEE DCF [7]. o ensure that no staton wth hgher prorty has packets n ts queue when a staton wth lower prorty starts packet transmsson, the DIFS for lower prorty s set as the sum of the DIFS and the maxmum contenton wndow for hgher prorty traffc. hs scheme can prevent the short-term prorty problem prevously mentoned. However, when hghprorty statons do not have packets to transmt, ths scheme results n wastng the channel bandwdth snce low-prorty traffc statons have to wat for a long tme to transmt ther packets due to the long DIFS value. he scheme proposed n [20] makes use of two narrow-band busy-tone sgnals.e., B1 and B2). he channel bandwdth s dvded nto three parts: 1) the B1 channel; 2) the data channel; and 3) the B2 channel. Low-prorty statons determne the presence of hgh-prorty statons by sensng the carrer on the busy-tone channels. When a hgh-prorty staton has a packet to transmt, t wll send a B1 lastng for one slot tme) every M slots durng DIFS and backoff stages before t acqures the channel. M s a constant and should be smaller than the IFS of low-prorty statons so that they are able to sense the busy-tone sgnal before they attempt to acqure the data channel. When hgh-prorty statons wthout packets to transmt and low-prorty statons hear ths B1, they wll send a B2 to avod the hdden termnal problem. All statons wth low-prorty packets that hear ether B1 or B2 wll defer ther transmssons for some duraton. hs scheme wll waste consderable channel bandwdth and energy to send busy-tone sgnals for each packet transmsson. In addton, ths scheme just supports two prorty traffc types. In [10] and [11], the authors proposed a determnstc prorty access scheme to avod the short-term prorty problem. A staton sends a busy tone, nstead of ts backoff procedure, after sensng an dle channel for the AIFS perod. he length of the busy tone s equal to ts backoff counter. When the staton completes the transmsson of the busy tone, t checks the channel status. If the channel s busy, the staton defers the current contenton. Otherwse, the staton transmts ts packet. he packet transmsson delay of ths scheme s larger than that of the EDCA snce ths scheme, unlke the EDCA, prefers the staton wth the largest backoff counter, nstead of the staton wth the smallest backoff counter. hs scheme also consumes energy to transmt longer busy tones. Furthermore, n the hdden termnal envronment, hdden statons cannot sense the other statons busy tones and send ther busy tones so that dfferent prorty statons can operate together. herefore, there exst effects among dfferent prorty statons. In ths paper, we propose a DPCA scheme, whch uses a busy tone to solve the short-term prorty problem. he proposed scheme does not waste the channel bandwdth, even though there are no hgher prorty traffc. It sends a busy tone twce for each packet transmsson so that t can consume much less energy than the schemes n [10], [11], and [20]. In addton, t can have a shorter packet transmsson delay compared to the schemes n [10] and [11]. Furthermore, the proposed scheme elmnates effects among dfferent prorty traffc types n the hdden termnal envronment. III. DPCA SCHEME In ths secton, we descrbe the prorty access scheme that ensures the channel contenton based on traffc prorty. Our

3 KIM et al.: DPCA SCHEME FOR QoS SUPPOR IN IEEE e WIRELESS LANs 857 Fg. 1. Busy-tone tmng dagram for the DPCA scheme. scheme needs mnor modfcatons to the EDCA. However, the basc operaton of the proposed scheme s the same as that n the EDCA. o ensure the QoS requrements, hgh-prorty ACs should not be affected by low-prorty ACs. o do ths, the proposed scheme blocks the transmssons of low-prorty ACs by usng a busy-tone sgnal shorter than aslot me when hgh-prorty ACs have packets to transmt. In other words, lower prorty ACs do not transmt ther packets untl no hgher ACs contend for the channel. In the EDCA, a flow of a gven AC frst senses the wreless channel medum. After sensng the dle duraton of the AIFS perod, the flow wats for random backoff tme before transmttng. However, n the proposed scheme, a flow sends a busy tone after sensng an dle channel for the AIFS aslot me) perod see Fg. 1). On recevng the busy tone from the flow, the AP sends a busy tone at the next tme slot so that every flow n a network can recognze the presence of the flow that sends the busy tone. After recevng the busy tone from the AP, the flow operates lke the EDCA. hus, t decreases ts backoff counter as long as the channel s sensed dle, does not decrease when a transmsson s detected on the channel, and tres to transmt a packet when the backoff counter reaches zero. If the channel s determned to be busy at any tme wthn the AIFS aslot me) perod, then a busy tone and the backoff procedure are suspended. In other words, the flows of lower ACs, whch receve a busy tone from ether flows of hgher AC or the AP, stop ther current channel contenton and wat untl a packet transmsson occurs. As long as at least one flow of hgher AC exsts, all the flows of lower ACs wll sense a busy tone wthn ther AIFS aslot me) perods. he proposed scheme ensures that flows of the hghest AC always access the channel. o let all the flows sense a packet transmsson n the hdden termnal envronment, the AP sends a negatve ACK NACK) packet for the basc access method or a negatve CS NCS) packet for the RS/CS access method after recevng a collded packet. Flows that are not n the transmsson range of a flow that transmts a packet can detect a packet transmsson from the NACK or NCS packets. hs extra sgnalng cost does not decrease the performance of the proposed scheme snce, n the IEEE e standard, flows wat for the duraton Fg. 2. Busy-tone transmsson tme accordng to the packet arrval tme. a) Packet arrval before the LCB. b) Packet arrval after the LCB. of the EIFS DIFS + AIFS) perod to start the backoff operaton when a packet collson occurs, and the EIFS DIFS) value s equal to the tme needed to transmt a NACK or an NCS packet, where the EIFS s an extended IFS. For the operaton of the proposed scheme, let us assume that, unlke the EDCA, a hgher prorty AC always has a smaller AIFSN than a lower prorty AC,.e., AIF SN[] > AIF SN[ +1]+1for 0 2. he busy-tone transmsson tme vares wth the packet arrval tme and channel status. o determne the tme, the proposed scheme uses three parameters: 1) the packet arrval tme PA) at the MAC layer; 2) the last channel busy tme LCB) due to the recent packet transmsson; and 3) the AIFS of the lowest prorty AC LAIFS). he LCB s set to the completon tme of a packet transmsson.e., the end tme of an ACK, a NACK, or an NCS packet). here are two possble cases that determne the transmsson tme of a busy tone see Fg. 2). If a flow receves a busy tone between LCB and PA, t defers ts operaton untl t senses the channel busy by a packet transmsson. Otherwse, t operates as n the followng two cases: Frst, n the case where a flow receves a new packet from the upper layer before the LCB, t sends ts busy tone after sensng an dle channel for the AIFS aslot me) perod [see Fg. 2a)]. Second, when a packet arrves after the LCB, the channel should be determned to be dle untl LCB + LAIFS N + AIFS aslot me) before the busy tone s allowed to transmt [see Fg. 2b)], where N s used to algn the start tme of the AIFS perod wth the ntegral multples of LAIFS and s PA LCB/LAIF S. x rounds to the smallest nteger greater than or equal to x. hs algnment s needed to ensure the channel contenton to be among flows wth the same prorty AC. If a flow sends ts busy tone after sensng the dle duraton of the AIFS aslot me) perod wthout the algnment, t may cause a busy-tone collson wth flows of other prorty ACs. hen, the flows wth the collded busy tone contend for the channel at the same tme so that there exst effects among dfferent prorty ACs. Before the busy-tone transmsson tme expres, f a busy tone s detected from other flows or the AP,

4 858 IEEE RANSACIONS ON VEHICULAR ECHNOLOGY, VOL. 58, NO. 2, FEBRUARY 2009 Fg. 3. Operaton procedure of the DPCA scheme. then a flow should defer ts operaton untl a packet transmsson occurs and operate as the frst case. Fg. 3 shows the operaton procedure of the proposed scheme. In the DPCA scheme, dstngushng a busy tone from a packet transmsson s very mportant to guarantee the proper operaton. o do ths, the duraton of a transmsson s used. he transmsson tme for a packet has a duraton of at least three tme slots, because t ncludes the physcal preamble and header of 20 μs, whch s from able I n Secton V. A busy-tone duraton s smaller than one tme slot. Estmatng the duraton s smple wthout any addtonal overhead or cost, because every staton performs carrer sensng. Each staton, by usng carrer sensng, observes the channel status and measures the duraton of the busy perod. herefore, the proposed scheme can dscrmnate between a busy tone and a packet transmsson when recevng a sgnal. IV. PERFORMANCE ANALYSIS In the proposed scheme, each prorty traffc separately performs ts own packet transmsson operaton and s provded the determnstcally prortzed channel access. As loads of hgher prorty traffc types ncrease, they use the larger amount of channel tme to meet ther QoS requrements and affect the performance of lower prorty traffc types. In ths secton, we analyze the channel tme usage rato, whch s the fracton of tme durng whch the channel s used to transmt each prorty traffc. We assume that all the statons are n the transmsson range of one another so that whenever a staton s transmttng, all the other statons can detect ths transmsson,.e., there s no hdden termnal problem. We denote a transmsson nterval as the tme duraton between two consecutve packet transmssons, whch s made up of four components: 1) AIFS; 2) busy tone from the AP; 3) backoff; and 4) transmsson see Fg. 4). Fg. 4. Structure of the transmsson nterval. We consder a WLAN supportng two types of traffc: 1) voce and 2) data. Voce traffc s assgned a hgher prorty. Suppose there are fxed N v voce flows and N d data flows admtted n the WLAN. For data traffc, each flow always has packets to transmt, and each packet has to wat for random backoff tme before beng transmtted. Data flows share the channel tme unused by voce flows. Under the consdered envronment, a flow, whch has a packet arrval durng the ongong transmsson nterval, does not contend for the channel. For example, n Fg. 4, there are three new packet arrvals of dfferent flows. he frst packet arrves durng the AIFS perod and has to wat untl LCB + LAIFS + AIFS aslot me) [see Fg. 2b)]. It wll receve a busy tone before the tme expres snce there s always at least one flow wth a packet to transmt. For the second packet, t already receved a busy tone from other flows before arrvng. For the thrd packet, t defers ts access operaton snce the channel s sensed busy. herefore, only flows wth a packet arrval before the LCB contend for the channel. he proposed scheme lmts the operaton of flows of lower prorty ACs by busy tones. herefore, only flows wth the same prorty AC contend for the channel durng a transmsson nterval.

5 KIM et al.: DPCA SCHEME FOR QoS SUPPOR IN IEEE e WIRELESS LANs 859 o calculate the transmsson nterval, the AIFS, busy tone tme, and transmsson tme can easly be obtaned, but backoff duraton vares wth tme, because t depends on how many flows are contendng for the channel durng the current ongong transmsson nterval. he number of data flows s always N d because of the saturaton condton. he number of voce flows vares wth tme under the nonsaturated condton. he number of voce flows, whch wll contend for the channel durng the next transmsson nterval, s determned at the end of a packet transmsson, regardless of a successful transmsson or a collson. he number of contendng voce flows s descrbed by a process wth state {0, 1, 2,...,N v }. After each transmsson nterval, a state wll reman n the current state or move to the next state. For example, n Fg. 4, assume that there are n v voce flows wth a packet at the end of the n 1)th transmsson nterval, and three new packets of dfferent voce flows arrve durng the nth nterval. At the end of the nth nterval, the number of contendng flows becomes n v +2 f the nth transmsson s successful) or n v +3f collded). We can express the channel tme usage rato n terms of statonary probablty S and the correspondng duraton of transmsson nterval D at state 0 N v ). As no voce flows contend for the channel, data flows wll occupy the channel. D 0 s the duraton of the transmsson nterval for data flows. he ratos for voce U v and data U d are clearly gven by Nv =1 U v = D Nv =0 S. D 1) U d =1 U v. 2) Our analyss s dvded nto two parts: Frst, we study the behavor of the state model and obtan the statonary probablty and the correspondng duraton of transmsson nterval for voce traffc. hen, we fnd the duraton for data traffc. A. Statonary Probablty and Duraton of ransmsson Interval for Voce raffc We make several assumptons. Frst, voce packets do not accumulate n the transmsson queue, whch means that each voce flow has up to one voce packet. herefore, herenafter, a packet arrval means that a voce flow can start contendng for the channel. Second, we do not consder the retry lmt. hrd, the maxmum contenton wndow s equal to the mnmum contenton wndow for voce traffc. We defne, for convenence, CW = CWmn = CWmax. hese assumptons are reasonable, because of the followng: Voce traffc does not collde wth data traffc so that ts collson probablty remans low n steady state. herefore, a packet can be transmtted wthn the packet nterarrval tme, and a voce flow can mantan the packet drop probablty at a relatvely small value. he IEEE e EDCA standard recommends that the values of CWmn and CWmax for voce traffc are set to acw mn +1)/4 1 and acw mn +1)/2 1, respectvely, where acw mn s the mnmum contenton wndow for data traffc [9]. herefore, the dfference between CWmn and CWmax s so small that ts effect s neglgble. Fg. 5. State transton dagram for voce traffc. In Fg. 5, we use the state transton dagram to model the number of contendng voce flows. Here, we explan the transtons only for state. Any number of voce packets less than or equal to N v can arrve, but only one packet can successfully be transmtted durng a transmsson nterval. herefore, state can enter any state above t and but can only enter state 1 for states lower than. Smlarly, state s entered from any state below t and from state +1. State can also reman n the same state. Packets from a voce flow arrve at constant nterval.he start tme for each voce flow s randomly gven. herefore, we assume that the arrval probablty of a voce packet s D / at state, where D s less than.wealsoassume that packets collde wth one another wth a constant and ndependent probablty Pc, v at state, regardless of the number of retransmssons, and succeed wth probablty Ps, v. he state transton dagram has seven transton probabltes. 1) here are no new packet arrvals at state 0,.e., p, = 1 D ) Nv, =0. 2) here s a successful transmsson and one new packet arrval, or there s a collson and no new packet arrvals,.e., p, = Ps, v Nv 1 ) D ) 1 D ) Nv 1 + Pc, v 1 D ) Nv, 1 N v 1. 3) here s an unsuccessful transmsson at state N v,.e., p, = P v c,, = N v. 4) here s a successful transmsson and no new packet arrvals,.e., p, 1 = Ps, v 1 D ) Nv, 1 N v. 5) here are one or more new packet arrvals at state 0,.e., p,j = Nv j ) ) j D 1 D ) Nv j, =0, 1 j N v.

6 860 IEEE RANSACIONS ON VEHICULAR ECHNOLOGY, VOL. 58, NO. 2, FEBRUARY ) here are j +1)new packet arrvals when a transmsson s successful or j ) new packet arrvals when collded,.e., p,j = Ps, v Nv j +1 + P v c, Nv j ) D ) D ) j +1 1 D ) Nv j 1 ) j 1 D ) Nv j 1 N v 2 +1 j N v 1. 7) here are j ) new packet arrvals when a transmsson s collded,.e., p,j = P v c, D ) j, 1 N v 1 j = N v. After each transmsson, state transts to state j j = : transton probabltes 1 3, j = 1: transton probablty 4, j>: transton probabltes 5 7). We can derve the statonary probablty S that the state s n, whch s a recursve expresson and can be gven n terms of a sngle unknown constant S 0,.e., S = 1 p 2 1, 1 j=0 S 1 p j, 1S j, 0 < N v. p, 1 p, 1 3) S 0 s determned by the normalzaton equaton,.e., N v S =1. 4) =0 Now we calculate the average number of backoff slots at state.letp, l, k) be the probablty that l voce flows choose backoff counter k and the other contendng flows choose the counter larger than k at state. hen, we have p, l, k) = ) l 1 CW +1 ) l ) l CW k. 5) CW +1 From 5), we obtan the probablty p, k) that, at state, the number of backoff slots s k when any transmsson occurs,.e., p, k) = p, l, k). 6) l=1 he average number of backoff slots bo, v s calculated from 6)asfollows: v bo, = CW k=0 p, k) k. 7) he average duraton of transmsson nterval D s gven by D = AIF S[voce]+ v bo, σ + P v s, v s + P v c, v c + B 8) where Ps, v s the probablty of a successful transmsson that exactly one flow transmts and that the remanng flows defer transmssons, and Pc, v s the collson probablty. c v and s v are the average tme ntervals that the channel s sensed busy due to collson and successful transmsson, respectvely. σ s the duraton of a tme slot. B s the tme duraton for a busy tone from the AP.e., = σ). Ps, v and P c, v are gven by P v s, = CW k=0 p, 1,k) 9) P v c, =1 P v s, 10) respectvely. For the basc and RS/CS access methods, c v and s v are gven by { v c,basc = H + L + SIFS + NACK +2δ s,basc v = H + L + SIFS + ACK +2δ 11) { v c,rts/cts = R S + SIFS + NCS +2δ s,rts/cts v = R S + SIFS + CS + SIFS 12) + H + L + SIFS + ACK +4δ where H= PHYhdr+ MAChdr) s the tme to transmt a packet header; δ s the propagaton delay; SIFS s the SIFS tme space; R S, CS, ACK, NACK, and NCS are the tmes to transmt an RS, CS, ACK, NACK, and NCS, respectvely; and L s the packet transmsson tme. B. Duraton of ransmsson Interval for Data raffc In the proposed scheme, data flows contend for the channel when there are no contendng voce flows. Durng the ongong transmsson nterval, new arrvng voce packets have to suspend ther busy-tone transmsson and backoff procedure untl the completon of the current successful transmsson or collson of data flows. herefore, D 0 s the duraton of transmsson nterval only for data traffc. o calculate the duraton, we use the same Markov chan model proposed n [22] and [23]. he analyss of the proposed scheme can be obtaned from the analyss n [22] and [23]. herefore, we omt the dervatons of the equatons that can straghtforwardly be derved along the lnes of [22] and [23]. P tr s the probablty that at least one flow transmts a packet n a gven slot tme, and Ps d s the probablty of a successful transmsson that exactly one flow transmts and that the remanng N d 1 flows defer transmssons. Pc d s the collson probablty. c d and s d are the average tmes that the channel s sensed busy due to collson and successful transmsson, respectvely. τ s the transmsson probablty that a flow transmts a packet n a randomly chosen slot tme [22], [23].

7 KIM et al.: DPCA SCHEME FOR QoS SUPPOR IN IEEE e WIRELESS LANs 861 ABLE I NUMERICAL AND SIMULAION PARAMEERS ABLE II RAFFIC PARAMEERS Fg. 6. Numercal and smulaton results for the channel tme usage rato. P tr, P d s, and P d c are gven by P tr =1 1 τ) N d 13) Ps d = N dτ1 τ) N d 1 = N dτ1 τ) Nd 1 P tr 1 1 τ) N d 14) Pc d =1 Ps d 15) respectvely. he average number of backoff slots per transmsson d bo s gven as follows: bo d = 1 P tr. 16) P tr he average duraton of the transmsson nterval for data traffc D 0 s gven by D 0 =AIF S[data]+ d bo σ + P d s d s + P d c d c +B 17) where d c and d s can be obtaned from 11) and 12). Substtutng 3), 8), and 17) nto 1), we can obtan the channel tme usage rato. V. N UMERICAL AND SIMULAION RESULS In ths secton, we dscuss the numercal and smulaton results of the proposed DPCA scheme. o study the performance of the DPCA scheme and valdate the accuracy of the analytcal model, we have mplemented t wth the NS-2 smulator. We compare them to the results of the IEEE e EDCA and guaranteed prorty and enhanced farness GPEF), whch s proposed n [10] and [11]. System parameters used n the numercal analyss and smulaton are lsted n able I. We smulated an IEEE a network wth transmsson rates of 54 Mb/s for data packets and 6 Mb/s for control packets such as ACK, respectvely. We have three types of traffc: 1) voce; 2) vdeo; and 3) data. he traffc parameters are lsted n able II. A constant bt rate model s used for all three traffc types. For voce traffc, the overhead 40 octets), such as the Fg. 7. Collson probablty when CWmn =7, CWmax =7) and CWmn =7, CWmax =15) for voce traffc. RP/UDP/IP headers, s added so that the sendng rate becomes 96 kb/s at the MAC layer. In the smulaton, we consder the basc access method and only uplnk traffc. In addton, we assume that each staton has a sngle flow of voce, vdeo, or data traffc. Smulatons run for 100 s, and all smulaton results are averaged over ten smulatons. Man performance metrcs of nterest are throughput, collson probablty, average delay, delay jtter, drop probablty, and channel tme usage rato. Delay s the tme elapsed from the moment a packet arrves at the MAC layer queue untl the packet s successfully transmtted to the ntended staton, ncludng the recept of acknowledgement, and delay jtter s the standard devaton of the delay. Drop probablty s the rato between the number of packets dropped due to the retry lmt and the total number of data packets beng exceeded. A. Sngle-Hop opology In ths scenaro, we consder one WLAN where all statons are n the transmsson range of one another. In the smulaton, all three traffc types have the same number of flows. Fg. 6 shows the numercal and smulaton results for channel tme usage ratos. Fg. 7 shows the collson probablty

8 862 IEEE RANSACIONS ON VEHICULAR ECHNOLOGY, VOL. 58, NO. 2, FEBRUARY 2009 Fg. 8. Performance accordng to the number of flows per traffc n a sngle hop. a) hroughput. b) Channel tme usage rato. c) Collson probablty. d) Average delay. e) Delay jtter. f) Packet drop probablty. accordng to the mnmum and maxmum contenton wndows for voce traffc. For voce and data, we use the same values as traffc parameters n able II. However, we change the nterarrval tme of data traffc to a small value for the saturaton condton. In these fgures, Sm7,7) and Sm7,15) mean the smulaton results wth CWmn =7, CWmax =7) and CWmn =7, CWmax =15) for voce traffc, respectvely. Fg. 6 shows a close match between the numercal and smulaton results. here s also no dfference between Sm7,7) and Sm7,15). Sm7,15) makes collson probablty slghtly lower than Sm7,7), as shown n Fg. 7. However, ths may also make the average backoff tme longer. herefore, the channel tme usage rato s almost smlar. From Fg. 6, we also observe that voce traffc s only allowed to occupy a certan channel tme, whereas the remanng channel tme s dedcated to data traffc. Fg. 8a) shows the effect of the number of flows on the throughput, where for data traffc, we see that there s no performance dfference among the DPCA, GPEF, and EDCA schemes at lght loads. However, as the number of flows ncreases, the dfference becomes notceable such that, n the EDCA, the throughput for data sharply goes to zero, whereas, n the DPCA and GPEF, t slowly decreases compared wth the EDCA. As for vdeo traffc, almost the same behavors as data traffc can be observed. When the load becomes hgher, the throughput for the EDCA becomes worse than the other schemes, partcularly over the range where the throughput for data traffc becomes zero. he performance dfference between the DPCA and GPEF becomes markedly notceable at hgh loads. For voce traffc, the throughput of the EDCA soon becomes saturated and decreases as the number of flows becomes larger. hs s because of the fact that, n the EDCA, voce cannot gan exclusve access over vdeo and data, and data and vdeo traffc types stll try to access the channel and collde wth voce traffc. At hgh loads, the throughput of the GPEF becomes rapdly worse. However, the throughput of the DPCA becomes saturated at the pont wth a larger number of flows. Fg. 8b) shows the channel tme usage rato for the DPCA and GPEF schemes. Data and vdeo traffc types have almost the same behavors, as shown n Fg. 8a). he reason for ths s that, to guarantee the QoS requrements for hgher prorty traffc types, the DPCA and GPEF schemes dscrmnate lower traffc and reallocate the amount of saved channel tme to hgher prorty traffc. From ths fgure, we can also observe that the voce flows obtan a sgnfcant porton of the channel tme as the number of voce flows ncreases. At hgh loads, for voce traffc, the GPEF needs more channel tme than the DPCA snce t prefers the staton wth the largest backoff counter. herefore, the channel tme for the vdeo traffc of the GPEF dramatcally decreases. Fg. 8c) shows the results for collson probablty. Herenafter, we omt the results for data traffc, because t requres hgh throughput, but other QoS metrcs are less strngent than voce or vdeo traffc. In the EDCA, collson probablty gets hgher as the number of flows becomes larger snce all prorty traffc types always try to access the channel and cause collsons wth one another. In the DPCA and GPEF, collson probablty slowly ncreases compared wth the EDCA. At hgh loads, most voce flows contend one another to access the channel. herefore, the collson probablty of the DPCA and GPEF ncreases lke the EDCA. However, the GPEF s more steep than the DPCA. he reason s gven as follows: he GPEF prefers the staton wth the largest backoff counter so that more voce packets are queued. herefore, the channel contenton probablty ncreases. Fg. 8d) and e) shows the results for average delay and jtter, respectvely, where we can see that, at lght loads, the

9 KIM et al.: DPCA SCHEME FOR QoS SUPPOR IN IEEE e WIRELESS LANs 863 Fg. 9. hroughput and average delay of voce traffc n the hdden termnal envronment. delay and jtter for the DPCA and GPEF schemes are slghtly worse than those for the EDCA. here are two reasons for the DPCA: Frst, f a packet arrves after the LCB, the flow has to wat for a longer tme than ts AIFS to send a busy tone, as shown n Fg. 2b). Second, when a flow receves a busy tone from other prorty traffc, t defers ts backoff operaton untl sensng a packet transmsson, although the channel s already sensed dle. For the GPEF, t s because ths scheme prefers the flow wth the largest backoff counter. However, the delay and jtter dfferences between three schemes are small, and the QoS requrements can be met at lght loads. As the load ncreases, the DPCA outperforms the EDCA and GPEF for all prorty traffc types, whch s another advantage of our scheme. Fg. 8f) shows the results for drop probablty. It can be observed that there are fewer packet drops n the DPCA. On the contrary, the EDCA and GPEF cause many packet drops due to ts hgh collson probablty, as shown n Fg. 8c), partcularly when there are many flows n the network. B. Hdden ermnal opology o valdate the proposed scheme n the hdden termnal envronment, we smulated a network where the flows of each AC are dvded nto two groups that are not n the transmsson range of one another. In the smulaton, voce traffc has ten flows, and vdeo and data traffc types have the same number of flows. Fg. 9 shows the throughput and average delay of voce traffc accordng to the number of vdeo and data flows. For the EDCA, the throughput and average delay become worse as the number of vdeo and data flows ncreases snce all prorty flows always try to access the channel and make collsons wth one another. For the DPCA and GPEF, the throughput remans the same and always met wth voce traffc s requrement, regardless of the number of vdeo and data flows. he DPCA always has better throughput performance than the GPEF. he average delay of the DPCA s kept low and stable. However, t slghtly ncreases when there exst vdeo and data traffc types. hs s from the fact that, n the proposed scheme, voce flows wth a new packet arrval durng transmsson ntervals of vdeo or data flows must wat untl sensng a packet transmsson. VI. CONCLUSION IEEE e EDCA provdes only statstcally prortzed channel access. herefore, the EDCA does not completely ensure the QoS requrements n practce. In ths paper, we proposed the DPCA to mprove the QoS performance over the EDCA. he proposed DPCA scheme lmts the operatons of lower prorty traffc through a busy tone. A determnstcally prortzed channel access s provded to hgher prorty traffc. 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10 864 IEEE RANSACIONS ON VEHICULAR ECHNOLOGY, VOL. 58, NO. 2, FEBRUARY 2009 [20] X. Yang and N. Vadya, Prorty schedulng n wreless ad hoc networks, Wreless Netw., vol. 12, no. 3, pp , Jun [21] C. E. Koksal, H. Kassab, and H. Balakrshnan, An analyss of short-term farness n wreless meda access protocols, ACM Sgmetrcs, vol. 28, no. 1, pp , Jun [22] G. Banch, Performance analyss of the IEEE dstrbuted coordnaton functon, IEEE J. Sel. Areas Commun., vol. 18,no.3, pp , Mar [23] H. Wu, Y. Peng, K. Long, S. Cheng, and J. Ma, Performance of relable transport protocol over IEEE wreless LAN: Analyss and enhancement, n Proc. IEEE INFOCOM,Jun.2002,vol.2,pp Rongsheng Huang S 07) receved the B.S. and M.S. degrees n electrcal engneerng from X an Jaotong Unversty, X an, Chna, n 1996 and 1999, respectvely. He s currently workng toward the Ph.D. degree wth the Department of Electrcal and Computer Engneerng, Unversty of Florda, Ganesvlle. From 1999 to 2001, he was wth Huawe echnologes Co. Ltd., Shenzhen, Chna, as an R&D Engneer, workng on GPRS and 3G projects. From 2002 to 2005, he was wth UStarcom Reseach Center, Shenzhen, as a Senor Engneer and eam Leader on a 3G project. Hs research nterests nclude meda access control, protocol, and archtecture for wreless networks. Sunmyeng Km receved the B.S., M.S., and Ph.D. degrees n nformaton and communcaton from Ajou Unversty, Suwon, Korea, n 2000, 2002, and 2006, respectvely. From May 2006 to February 2008, he was a Postdoctoral Researcher n electrcal and computer engneerng wth the Unversty of Florda, Ganesvlle. In March 2008, he then joned the School of Computer and Software Engneerng, Kumoh Natonal Insttute of echnology, Gum, Korea, as a Full- me Lecturer. Hs research nterests nclude resource management, wreless LANs and PANs, wreless mesh networks, and qualty-of-servce enhancement. Yuguang Fang S 92 M 93 SM 99 F 08) receved the Ph.D. degree n systems engneerng from Case Western Reserve Unversty, Cleveland, OH, n 1994 and the Ph.D. degree n electrcal engneerng from Boston Unversty, Boston, MA, n From July 1998 to May 2000, he was an Assstant Professor wth the Department of Electrcal and Computer Engneerng, New Jersey Insttute of echnology, Newark. In May 2000, he joned the Department of Electrcal and Computer Engneerng, Unversty of Florda, Ganesvlle, as an Assstant Professor. He was promoted to Assocate Professor n August 2003 and to Professor n August He has authored more than 200 papers n refereed professonal journals and conference proceedngs. Prof. Fang has served on several edtoral boards of techncal journals, ncludng the IEEE RANSACIONS ON COMMUNICAIONS, the IEEE RANSACIONS ON WIRELESS COMMUNICAIONS, the IEEE RANSACIONS ON MOBILE COMPUING, andacm Wreless Networks. He was the recpent of the U.S. Natonal Scence Foundaton Faculty Early Career Award n 2001 and the U.S. Offce of Naval Research Young Investgator Award n 2002.