Performance Analysis and Comparison of QoS Provisioning Mechanisms for CBR Traffic in Noisy IEEE e WLANs Environments

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1 Tamkang Journal of Scence and Engneerng, Vol. 12, No. 2, pp (2008) 143 Performance Analyss and Comparson of QoS Provsonng Mechansms for CBR Traffc n Nosy IEEE e WLANs Envronments Der-Junn Deng* and Yu-Fan Yeh Department of Computer Scence and Informaton Engneerng, Natonal Changhua Unversty of Educaton, Tachung, Tawan, R.O.C. Abstract In order to reach better Qualty-of-Servce (QoS) requrements for multmeda applcaton, the e task group was formed and has proposed EDCA (Enhanced Dstrbuted Channel Access) for contenton perod and HCCA (HCF Controlled Channel Access) for contenton-free perod n the HCF (Hybrd Coordnaton Functon) to enhance the orgnal IEEE Medum Access Control (MAC) protocol. However, the problem of choosng the rght set of MAC parameters and strct QoS mechansm to provde guaranteed QoS n IEEE e Wreless Local Area Networks (WLANs) remans unsolved. In ths paper, a pragmatc Call Admsson Control (CAC) scheme wth a novel pollng based uplnk schedulng polcy for CBR traffc n IEEE e WLANs s proposed. The proposed CAC scheme computes the expected delay varaton of every sesson upon arrval of new connecton for admsson decson. In addton, the proposed packet based delay constraned schedulng polcy can derve suffcent condtons such that all accepted sources satsfy ther tme constrants to provde determnstc QoS guarantees. In addton to theoretcal analyss, smulatons are conducted to evaluate the performance of the proposed scheme. As t turns out, our desgn works very well n provdng performance mprovement n the nosy IEEE e WLANs envronment. Key Words: Delay Varaton, EDCA, HCCA, CBR, IEEE e 1. Introducton In WLANs, the MAC protocol s the key component that provdes the effcency n sharng the common rado channel whle satsfyng the QoS requrements for realtme traffc. However, frames n Dstrbuted Coordnaton Functon (DCF), the basc access method n the IEEE MAC layer protocol [1], do not have prortes, and there s no other mechansm to enforce a guaranteed access delay bound. As a result, real-tme applcatons such as voce transmssons may suffer from unacceptable delay wth ths protocol. The alternate access mode of the IEEE MAC layer protocol, Pont Coordnaton Functon (PCF), offers a packet-swtched *Correspondng author. E-mal: connecton-orented servce, whch s well suted for real-tme traffc. However, n order to poll the statons an Access Pont (AP) must mantan a pollng lst, whch s mplementaton dependent. What ths means s that endto-end QoS requrements stll cannot be satsfed n ths scheme snce t does not nclude any access control polcy. Snce the demand for the use of packet-swtched technques for transferrng delay-senstve data n wreless envronments s nevtable for multmeda applcatons, the IEEE workng group s currently workng on a new standard called e [2] to enhance the orgnal MAC sublayer to support applcatons wth QoS requrements. In order to reach better QoS requrements for real-tme applcaton, the e task group has proposed EDCA for contenton perod and HCCA for contenton-free perod n the HCF to enhance

2 144 Der-Junn Deng and Yu-Fan Yeh the orgnal IEEE MAC protocol. However, the problem of choosng the rght set of MAC parameters and strct QoS mechansm to provde guaranteed QoS n IEEE e Wreless LANs remans unsolved. Although n the lteratures there have been adequate excellent dscussons on the ssue of QoS and delay analyss n IEEE WLANs [36], none of the above studes proposed a complete soluton and performance evaluaton to fulfll a strct QoS guarantees for multmeda traffc n IEEE e WLANs. In paper [7], the authors proposed an adaptve EDCF scheme. One problem of the basc EDCA ad-hoc mode s that the sze of contenton wndow and backoff functon of each queue s statc and does not take nto account dynamcty of wreless channel condtons. In AEDCF, relatve prortes are provsoned by adjustng the sze of the contenton wndow of each traffc class takng nto account both applcaton requrements and network condtons. After each successful transmsson, AEDCF does not reset the contenton wndow sze. In paper [8], the authors ntroduced a per-flow dfferentaton scheme. All packets are put n the same queue, ndependent of ther prortes. However, ths scheme ntroduces mutual nterferences between prortes: when the AP serves a low prorty of slow flow, the global speed and effcency of AP depends on the occupaton tme of the slow flow. If most of the tme the flow occupes the AP, even f there are other hgh-prorty fast flows, hence, the AP has to be slow, and servce dfferentaton gets lower. Based on the above observatons, the need for provdng Qualty of Servce (QoS) for real-tme applcatons n e networks has been drvng research actvtes and standardzaton efforts for some tme. However, current research results offer some mechansms to provde basc levels of QoS dfferentaton to aggregate flows manly n the form of prorty servces. In addton, to the best of our knowledge, very lttle work has been dedcated to provdng per-sesson/per-packet QoS guarantees n WLANs, a necessary feature for most multmeda applcatons. In ths paper, a pragmatc CAC scheme wth a novel pollng based uplnk schedulng polcy for CBR traffc n IEEE e WLANs s proposed. Under such a scheme, the CBR sesson was characterzed by ts traffc rate and tolerable jtter. The proposed transmt-permsson polcy for HCCA access method can derve suffcent condtons such that all the CBR sources satsfy ther tme constrants to provde determnstc QoS guarantees. In addton to theoretcal analyss, smulatons are conducted to evaluate the performance of proposed scheme for CBR traffc. We also compared the proposed scheme wth the other two QoS provsonng mechansms wth respect to blockng rate, average access delay, and achevable throughput under dfferent offered traffc load. As t turns out, our desgn ndeed provdes good performance mprovements over the IEEE e standard. The remander of ths paper s organzed as follows. Secton II descrbes the proposed scheme n detal. Smulaton and expermental results are reported n Secton III. Secton IV concludes ths paper. 2. The Proposed Scheme In what follows, we present the proposed scheme n detal. Our method nvolves two basc components: (1) a call admsson control scheme for access decson; (2) a packet based delay constraned up-lnk schedulng polcy. 2.1 Call Admsson Control for Access Decson Upon arrval of a new user, the call admsson control unt decdes to grant or deny admsson permt. The decson s made based on the current nformaton of the system and the analytcal model for estmaton of expected delay varaton. If admsson of the new user does not degrade the achevable performance below ther requrement, the new user can be admtted nto the system. The CBR traffc s characterzed by two parameters (r, ), where r s the rate (number of packets per second) of the source and s the maxmum tolerable jtter (packet delay varaton) for ths sesson. Assume there are n CBR sources (ndexed by =1,2,,n). We denote (r, )as the traffc parameters of the th CBR source. In the followng theorem, we provde suffcent condtons for all the CBR packets to satsfy ther maxmum jtter constrants. Theorem: Defne * to be the parameter calculated by CAC unt. The CAC unt uses ths parameter to decde whether the AP accepts a CBR connecton or not. Let * = t PIFS + 1 rk t p + tp k r, =1,2,,n and t PIFS be the duraton 1 of PCF nterframe space, and t p be the tme to transmt a

3 QoS Mechansms for CBR Traffc n WLANs 145 packet. If *<1/r and for all =1,2,,n, then all the packets generated by the th CBR source meet ther jtter constrants. Proof: Suppose that the frst token generated from the th CBR source has a maxmum watng tme. We want to prove that for 1 n. Consderng the frst CBR source,.e., = 1, ts maxmum watng tme s for the channel to be cleared snce we assume a nonpreemptve prorty. The channel wll be cleared wthn a packet transmsson tme t p. Thus, 1 t PIFS + t p = * 1. Snce * 1 1 r and * (by our 1 1 assumpton), we have 1 1 1, whch establshes the nducton bass. Suppose our nducton hypotheses holds up to the (-1) st CBR sources,.e., j for 1 j 1. Now we consder the th CBR source. Let the arrval nstant of a pollng token as tme 0. Suppose *. Then t means that up to tme * the channel must be servng all the CBR sources from 1 to 1. Snce the total amount of packets that can be served wthn (0, * ) for these 1 1 * voce sources s at most tpifs rk k1. Addng t p for nonpreemptve prorty, hence, the total amount of tme to serve these packets s bounded above by t PIFS + 1 ( * r 1 ) t k1 k p, and snce we assume * 1,we r 1 1 * have tpifs ( rk 1) t p < t PIFS + ( rk ) r k1 k 1 1 t p = *. Thus, the channel cannot be busy all the tme n (0, * ) and ths contradcts our assumpton. Ths shows * Hence,. Based on the prncple of nducton, the statement of the theorem follows. Q.E.D. Snce the proposed packet based delay constraned schedulng polcy can derve suffcent condtons such that all accepted sources satsfy ther tme constrants to provde determnstc QoS guarantees, all admtted CBR sources are starvaton-free. However, as the numbers of CBR connectons ncreases, they tend to grab the channel. Hence, from the performance vewpont, t s equally mportant to guarantee a mnmum bandwdth for data traffc n order to mantan a reasonable bandwdth usage. To acheve ths goal, we can leave a mnmum bandwdth for data traffc smply by changng the value of t p (the total tme of transmts a CBR packet n a contenton free perod) to keep data traffc from starvaton. Fnally, we stll need to analyze the complexty of the proposed theorem. In the worst case, the call admsson control unt makes n-1 comparsons before makng the decson, and each comparson take n steps calculatons. Hence, ts complexty s O(n 2 ). 2.2 Packet Transmt Permsson Polcy In what follows, we propose a delay constraned schedulng scheme for HCCA access method n contenton-free perod to support CBR sessons. In one Basc Servce Set (BSS) of IEEE e nfrastructure network archtecture, the AP mplements a token bucket for each CBR source. In token buckets for CBR sources, the one wth the smallest tolerable jtter constrant has the hghest prorty among all CBR sources. For each CBR source, ts pollng token s generated every 1/r second n the AP. In order to gan control of the medum, the AP performs the functon of the pont coordnator by transmttng a beacon frame at the begnnng of the CFP after sensng the medum to be dle for a PIFS perod. Once the AP has the control of the medum, t performs the followng algorthm. Functon Packet_Transmt Polcy repeat scan the token buffer of CBR sources f a token found n CBR source then Transmts a CFP_Start Beacon after a PIFS perod remove one token from ths token bucket poll correspondng CBR source and transmt one packet f not the last packet then generate next token after (1/r)-tp sec else remove ths token bucket else scan the next token bucket untl no token bucket found n token buffer Transmts a CFP_End Beacon end. As the pseudo code llustrates, AP scans the token buffers of CBR sources accordng to the preset prorty order. If a token s found, t removes one from ths token buffer and polls ths CBR termnal. On recevng a poll

4 146 Der-Junn Deng and Yu-Fan Yeh the Staton transmts ts packet, and the AP wll generate the next token for ths CBR after 1 t r p second f t s not the last packet. When an end-of-fle sgnal s receved, the BS wll remove ths token bucket from token buffer. If there s no token found n all token buffers, the AP wll not know whch, f any, of the statons have real-tme packets to transmt, then, t can end the CFP by transmttng a CF-End frame, and, for assurng the tme constrant of admtted real-tme traffc, the AP shall announce the begnnng of the next CFP nterval by observng the token buffer constantly. 3. Performance Evaluaton Our smulaton model s bult usng the network smulator NS2 [9]. The model represents a BSS n the IEEE b standard WLANs wth all statons n the BSS (Basc Servce Set) capable of drectly communcatng wth the remanng partes. Fgure 1 shows an overvew of the smulated system topology. In our smulatons envronments, the ns wreless lnk s extended to generate an error probablty by employs the Glbert-Ellot (GE) error model [10] to characterze fadng n the communcaton channel. The rate at whch errors occur n the GE model s dependent on the channel condton. As llustrated n Fgure 2, n the good state, G, losses occur wth a low probablty, P G, whereas n the bad state, B, the channel operates n a fadng condton and the loss probablty, P B, s correspondngly hgher. Steady state probabltes of beng n states G and B are gven by G = P BG =(P GB + P BG ) and B = P GB =(P GB + P BG ), respectvely. The average packet loss rate produced by the GE channel s P = P GG + P BB. To focus on the access control ssue and to reduce the complexty of the smulaton, what follows are the basc assumptons n our smulaton envronment. Frst, the hdden termnal and the exposed termnal problems are not addressed n the smulaton model. Second, no statons operate n the power-savng mode. Thrd, no nterference s consdered from nearby BSSs. Fourth, transmsson errors are generated accordng to the Gaussan channel assumpton. Fnally, the transmtted propagaton delay s 1 s. The evaluaton s made wth respect to the frame blockng rate, average access delay, and Fgure 2. State transton dagram for the Glbert-Ellott model. Fgure 1. Smulaton topology.

5 QoS Mechansms for CBR Traffc n WLANs 147 achevable throughput under dfferent offered traffc load. We also compared the proposed scheme wth the other two QoS provsonng mechansms [7, 8]. CBR applcatons are used as traffc generators. The default values used n the smulaton are lsted n Table 1. The values for the smulaton parameters are chosen carefully n order to closely reflect the realstc scenaros as well as to make the smulaton feasble and reasonable. All the smulatons are conducted on FreeBSD 6.0 on a Xeon 3.4 GHz Server wth 2 GB memory. The verson of NS2 s ns-2.29, and each smulaton run lasts for 100 smulaton seconds. Smulaton results are shown below n the form of plots. Fgure 3 depcts the frame blockng rate as the traffc load ncreases. Snce we allow the admtted CBR sources to use bandwdth exclusvely wth prorty over other traffc, the blockng rate of the proposed scheme wll be kept lower than the legacy IEEE e protocol, besdes, the proposed scheme outperforms the other two schemes n most cases. Fgure 4 compares the average access delays of CBR traffc from the proposed scheme wth the legacy IEEE e protocol and the other two schemes. If the request s successfully receved and scheduled by the AP, the CBR connecton may transmt ts packet by usng HCCA access mode n contenton free perod when t s polled by the AP. Otherwse, t can stll transmt ts packet by usng EDCA access mode n contenton perod. We can see that although there s not much dfference n the values of the performance measures when load s lght, however, the proposed scheme provdes sgnfcantly better performance at heavy load. Fgure 5 depcts the achevable throughput as the traffc load ncrease n nosy envronments. As shown n the fgures, the throughput mprovement can be as much as about 20% n congested wreless envronments. It reveals that our proposed scheme could reduce the droppng probablty wthout sacrfcng the overall system performance. Fgure 6 show the BER performance and the traffc load versus the achevable throughput for the proposed scheme. As shown n the fgure, t can be seen that the throughput of our proposed scheme s not affected by Fgure 3. Blockng rate versus traffc load n nosy envronments. Table 1. Default attrbute values used n the smulaton Attrbute Value Meanng & Explanaton R DATA 11 Mb/s Maxmum data rate (WaveLAN DSSS) T slot 20 us Tme needed for each tme slot T SIFS 10 us Duraton of short nterframe space (SIFS) T DIFS 50 us Duraton of DCF nterframe space (DIFS) L DATA 1000 bytes Mean payload sze L ACK 112bts Ackframesze L H _ DATA 224 bts MAC overhead T P 25 us Duraton of a PLCP preamble T H 04 us Duraton of a PLCP header 01 us Propagaton Delay W 31 slots Mnmum contenton wndow sze m 5 Maxmum backoff stages d 250 meters Smulaton topology 250 m 250 m r 10~100 Kb/s CBR set rate (corresponds to nterval of 3.75 ms) L 210 bytes CBR packet sze

6 148 Der-Junn Deng and Yu-Fan Yeh Fgure 6. Throughput versus channel BER and traffc load. Fgure 4. Average access delay versus traffc load n nosy envronments. Fgure 7. Farness ndex versus channel BER and traffc load. Fgure 5. Achevable throughput versus traffc load n nosy envronments. channel BER between 10-8 to Besdes, the throughput of our proposed scheme changes lttle as the traffc load vares. Ths ndcates that the proposed scheme s qute resstant towards nfluences form wreless envronments. In Fgure 7, we nvestgate and analyze the farness of the proposed scheme. We use the farness ndex defned by Jan [11] to evaluate how far t s. The farness ndex s defned as where n s the number of connectons, and T s the throughput of connecton. From Cauchy-Schwartz nequalty, we obtan Farness Index 1, and the equalty holds f and only f all T are equal. As shown n the fgure, as the number of connectons and channel BER ncreases, the dfference of throughput also ncreases. 4. Concluson Currently, many wreless applcatons and devces are emergng, and ths trend s expected to contnue n the foreseeable future. Many problems reman further ntensve nvestgaton and need to be solved. In ths paper we have propose a pragmatc CAC scheme wth a novel pollng based uplnk schedulng polcy for CBR traffc n

7 QoS Mechansms for CBR Traffc n WLANs 149 IEEE e WLANs, offerng easly mplemented and yet flexble crtera for traffc prortzaton n a wreless envronment. The schedulng algorthm for CBR traffc s performed at each AP n a dstrbuted manner. Through extensve smulatons, we have demonstrated a satsfactory performance of our proposed scheme n a quanttatve way. It shows that the proposed scheme has proven t satsfactory superorty n most cases. References [1] Wreless Medum Access Control and Physcal Layer WG, IEEE Draft Standard P802.11, Wreless LAN, IEEE Stds. Dept, D3, Jan. (1996). [2] IEEE Draft Std e/D8.0, Medum Access Control (MAC) Qualty of Servce (QoS) Enhancements, Feb. (2004). [3] Wu, H. K. and Chuang, P. H., Dynamc QoS Allocaton for Multmeda Ad Hoc Wreless Networks, Moble Networks and Applcatons., Vol. 6, pp (2001). [4] Sobrnho, J. L. and Krshnakumar, A. S., Qualty-of- Servce n Ad Hoc Carrer Sense Multple Access Wreless Networks, IEEE Journal on Selected Areas of Communcatons., Vol. 17, pp (1999). [5] Yen, J. Y. and Chen, C. H., Support of Multmeda Servces wth the IEEE MAC Protocol, Proc. ICC., Vol. 1, pp (2002). [6] Coutras, C., Gupta, S. and Shroff, N. B., Schedulng of Real-Tme Traffc n IEEE Wreless LANs, Wreless Networks., Vol. 6, pp (2000). [7] Romdhan, L., N, Q. and Turlett, T., Adaptve EDCF: Enhanced Servce Dfferentaton for IEEE Wreless Ad Hoc Networks, Proc. WCNC., Vol. 2, pp (2003). [8] Aad, I. and Castellucca, C., Remarks on Per-Flow Dfferentaton n IEEE , Proc. EW., pp. 16 (2002). [9] The Network Smulator, ns-2, [10] Glbert, E. N., Capacty of a Burst-Nose Channel, Bell System Techncal., Vol. 69, pp (1960). [11] Jan, R., Durres, A. and Babc, G., Throughput Farness Index: An Explanaton, Proc. ATM Forum/ , Feb. (1999). Manuscrpt Receved: Nov. 21, 2007 Accepted: Jun. 27, 2008

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