Applications and Challenges of the e EDCA Mechanism: An Experimental Study

Transcription

1 Applcatons and Challenges of the 80.e EDCA Mechansm: An Expermental Study Albert Banchs, Arturo Azcorra, Carlos García, and Rubén Cuevas, Unversty Carlos III of Madrd Abstract In ths artcle we conduct an expermental study of the enhanced dstrbuted channel access mechansm of the upcomng 80.e standard. The man focus of the study s on two applcatons of EDCA: traffc engneerng and servce guarantees. Wth traffc engneerng we am at dstrbutng the bandwdth n the WLAN accordng to a gven throughput allocaton crteron. Wth servce guarantees the objectve s to ensure that the performance metrcs (throughput and delay) experenced by a staton meet a gven qualty crteron. We buld a testbed wth wreless cards that support a substantal subset of the EDCA functonalty and analyze performance aganst dfferent sets of requrements. Expermental results show that EDCA can effectvely be used for traffc engneerng purposes. The goal of provdng servce guarantees wth EDCA s shown to be more challengng; we dentfy future research drectons that need to be addressed n order to acheve ths goal. I n recent years, much nterest has been devoted to the desgn of wreless local area networks (WLANs) wth qualty of servce (QoS) support. The Enhancements Task Group (TGe) was formed under the IEEE 80. project to recommend an nternatonal WLAN standard wth QoS. Ths standard s called 80.e and s beng bult as an extenson of the basc WLAN 80. standard. Whle the standardzaton process of 80.e s stll an ongong effort, the man features of the upcomng standard have already been agreed upon and are unlkely to change. These features are descrbed n the latest verson of the 80.e standard draft []. The standard draft defnes two dfferent access mechansms: the enhanced dstrbuted channel access (EDCA) and hybrd coordnaton functon (HCF) controlled channel access (HCCA). EDCA s a dstrbuted scheme that extends the dstrbuted coordnaton functon (DCF) of 80., whle HCCA s centralzed. Recently, some commercal WLAN products that mplement (to a certan extent) the EDCA mechansm have become avalable. The product roadmap for HCCA s less clear. The focus of the present artcle s on an expermental analyss of the EDCA mechansm wth a real-lfe testbed. To date, a consderable amount of work has been addressed to analyze the performance of ths mechansm (e.g., []); however, to the authors knowledge, all prevous work n the lterature was based on smulatons and/or analyss; none nvestgates the performance of EDCA expermentally. It s well known from The work descrbed n ths artcle s based on results of IST FP6 Integrated Project DAIDALOS. DAIDALOS receves research fundng from the European Communty s Sxth Framework Program. Apart from ths, the European Commsson has no responsblty for the content of ths artcle. expermental studes wth DCF [3, ] that WLANs suffer from a number of nondeal effects that have nonneglgble mpact on performance and are not accounted for n analytcal and smulaton studes. Therefore, experences wth real-lfe experments are needed n order to complement prevous theoretcal studes of EDCA and assess ts real performance. The methodology used n ths artcle to perform the expermental analyss of EDCA s the followng. We consder two wdely accepted applcatons of a QoS archtecture for WLAN, traffc engneerng and servce guarantees, and evaluate how well the requrements of these applcatons are satsfed by our EDCA testbed. The artcle s structured as follows. We frst revew the EDCA mechansm of the 80.e standard draft. Then we descrbe our testbed setup and dscuss some ntal expermental results that gve an understandng of the varous effects observed n the testbed. Next, we take up the ssue of traffc engneerng wth EDCA and study the performance of two dfferent traffc engneerng crtera (symmetrc traffc allocaton and olympc servce model). We then address the provsonng of servce guarantees wth EDCA and outlne some possble future drectons of research n ths context. Fnally, we summarze the results of our work. 80.e EDCA We now brefly summarze the EDCA mechansm as defned n the latest verson of the 80.e standard draft. For a complete descrpton of the EDCA and HCCA mechansms of 80.e, the reader s referred to the standard draft []. EDCA controls the access to the wreless channel on the bass of the channel access functons (CAFs). A staton may run up to four CAFs, wth the frames generated by the staton mapped to one of these CAFs. Each CAF executes an ndependent backoff process to determne the tme of transms /05/$ IEEE

2 son of ts frames. The backoff process s regulated by four confgurable parameters: CW mn, CW max, AIFS, and TXOP lmt. A CAF wth a new frame to transmt montors the channel actvty. If the channel s dle for a perod of tme equal to ts arbtraton nterframe space (AIFS ) parameter, the CAF transmts. Otherwse, f the channel s sensed busy (ether mmedately or durng the AIFS perod), the CAF contnues to montor the channel untl t s measured dle for an AIFS tme. At ths pont, the CAF starts the backoff process by ntalzng ts backoff tme counter to a random value unformly dstrbuted n the range (0, CW ), where CW s the contenton wndow of CAF and depends on the number of faled transmssons. At the frst transmsson attempt, CW s set equal to the mnmum contenton wndow parameter (CW mn ). As long as the channel s sensed dle the backoff tme counter s decremented once every slot tme, where the slot tme duraton (hereafter denoted σ) s a constant defned by the physcal layer. When a transmsson s detected on the channel, the backoff tme counter s frozen, and reactvated agan when the channel s sensed dle for a certan perod. Ths perod s equal to AIFS f the transmsson s receved wth a correct CRC, AND EIFS DIFS + AIFS otherwse, where the extended nterframe space (EIFS) and DCF nterframe space (DIFS) are physcal layer constants. As soon as the backoff tme counter reaches zero, the CAF transmts ts frame n the next slot tme. A collson occurs when two or more CAFs start transmsson smultaneously. In the case of a sngle staton runnng more than one CAF, f the backoff tme counter of two or more CAFs of the staton reach zero at the same tme, a scheduler nsde the staton avods an nternal collson by grantng the channel access to the hghest-prorty CAF. An acknowledgment (ACK) frame s used to notfy the transmttng CAF that the frame has been successfully receved. The ACK s transmtted at the end of the frame after a perod of tme equal to the physcal layer constant short nterframe space (SIFS). If the ACK s not receved wthn a specfed ACK tmeout, the CAF assumes that the transmtted frame was not receved successfully and schedules a retransmsson by reenterng the backoff process. After each unsuccessful transmsson, CW s doubled, up to a maxmum value gven by the CW max parameter. After a successful transmsson, the CAF s allowed to transmt several consecutve frames, the only restrcton beng that t cannot occupy the channel for a perod of tme longer than the transmsson opportunty lmt parameter (TXOP lmt ). In WLANs, the erroneous recepton of a frame may be caused by ether: A collson (.e., two statons transmt smultaneously and thus nterfere wth each other) The nherent nose and nterference of the rado lnk The 80. a/b/g standards tackle the latter by offerng varous modulaton schemes (wth dfferent bt rates and robustness to errors) and usng the one that adapts best to the characterstcs of the rado channel. Specfcally, 80.b (whch s the focus of our experments) reduces the channel bt rate from Mb/s (the nomnal value) to ether 5.5,, or Mb/s dependng on the number of detected errors. The frst cause of erroneous recepton, a collson between two frames, does not always produce an error. On some occasons, the frame receved wth the strongest sgnal survves the collson and s All statons check every receved frame (regardless of ts destnaton) for errors, usng the cyclc redundancy check (CRC) feld of the frame. captured by the recevng staton. Ths s the so-called capture effect. Expermental Setup Although the 80.e standard has not yet been fnalzed, there already are some commercal wreless cards that partally support EDCA. Our experments are conducted wth 80.b wreless cards based on the Atheros AR5 chpsets. The MADWIFI drver provdes full support for wreless adapters usng Atheros chpsets on Lnux platforms. Atheros based cards mplement a substantal subset of EDCA, but also suffer from some restrctons. Out of the parameters defned n the standard draft (CW mn, CW max, AIFS and TXOP lmt), only the frst 3 are supported. In addton, only one CAF per staton s mplemented, nstead of the defned n the standard draft. We beleve, however, that these restrctons do not represent a major lmtaton for our work. Indeed, the multple CAFs per staton feature s not needed for the applcatons we target, whle the TXOP lmt provdes a level of dfferentaton smlar to the CW mn [5]. The confguraton of the CW mn, CW max and AIFS parameters s performed wth some functons that the MADWIFI drver provdes for ths purpose. Specfcally, these functons allow confgurng each wreless card wth three nteger values,, j and k, wth whch the CW mn, CWmax and AIFS parameters of the wreless card are set as follows: CW mn = CW default mn, () CW max = CW default max j, and () AIFS = DIFS + kσ, (3) where CW default mn = 3 and CW default max = 0 correspond to the default values of the DCF mechansm of the 80.b standard. Note that the above restrcts the possble CW mn and CW max values to the ntegers power of. To perform our experments we bult a testbed composed of four desktop PCs under Lnux equpped wth Atherosbased wreless cards. One PC was confgured as the access pont (AP) and the others as statons under the nfrastructure mode; n the rest of the artcle we refer to them as statons,, and 3. Unless otherwse stated, experments were performed wth the four PCs located n the same room at a dstance rangng from to m from each other. Except for the voce applcaton used n the experment of Fg. 6, traffc was generated wth the perf 3 tool sendng (TCP or UDP) 000- byte packets. The expermental results gven are the average of fve runs of 60 s duraton each. In most cases, the dfference between the maxmum and mnmum results of the fve runs falls below 5 percent. In order to gan understandng of the nondealtes of our testbed, we conducted a frst set of experments. Frst, by saturatng the wreless channel wth only one staton sendng UDP traffc, we determned that the throughput obtaned s ndependent of the staton s locaton. Ths shows that wthn the range of dstances n our testbed all PCs work at the nomnal bt rate of 80.b ( Mb/s), and error rates are neglgble. By saturatng the channel wth two statons located at the Multband Atheros Drver for WFI (MADWIFI) conssts of the followng modules: hal, f_ath, and ath_pc. Our work was performed wth hal v. 0.8., f_ath v , and ath_pc v New versons of these modules wth extended functonalty are currently beng developed. See

3 r /r (CW) n Fgure. CW mn and AIFS parameters. same (about m) and dfferent ( and m) dstances from the AP, respectvely, and observng that the AP only detected errors n the former case, we found that the capture effect occurs wthn the range of dstances of our testbed. Furthermore, by comparng the results of the latter experment aganst the analytcal model of [6] for the capture effect, we concluded that ths s the man cause of the devatons observed n our expermental results. EDCA for Traffc Engneerng A r /r (CW) 50 r /r (AIFS) CW mn /CW mn One of the possble applcatons of 80.e EDCA s traffc engneerng. Wth traffc engneerng we am at (by settng approprately the EDCA parameters of the statons) dstrbutng the throughput of the WLAN accordng to a traffc engneerng crteron. We now study the effectveness of EDCA for ths purpose, takng as an example two dfferent traffc engneerng crtera: the symmetrc traffc allocaton and the olympc servce model. We frst propose some confguraton gudelnes of EDCA for traffc engneerng. Next, we assess how well EDCA confgured wth these gudelnes meets the performance goals of the two consdered crtera under UDP traffc. Then we take one of the two crtera, the symmetrc traffc allocaton, and study ts performance under TCP traffc. Fnally, we study, also wth the symmetrc traffc allocaton crteron, the mpact of heterogeneous rado condtons on traffc engneerng. Parameter Confguraton for Traffc Engneerng Most traffc engneerng crtera (n partcular the ones we use as examples here) are based on weghts. Wth these crtera, dfferent statons get assgned dfferent weghts; the goal s to provde them wth a throughput proportonal to the weght. In order to assess whch of the EDCA parameters (CW and AIFS) satsfes the above goal better, we conducted the followng experment. We saturated two statons wth UDP traffc and studed the rato of throughputs (r /r ) when we ncreased, respectvely, the CW mn and AIFS of the second staton (CW mn and AIFS = DIFS + A σ), whle leavng all the other parameters at ther default value. The results, depcted n Fg., show a lnear relatonshp between r /r and CW mn /CW mn, and an exponental one between r /r and AIFS, whch matches the analyss and smulatons of []. From the above we conclude that the CW mn parameter s the most approprate for provdng a staton wth throughput proportonal to ts weght. Specfcally, results show that the throughput obtaned s approxmately nversely proportonal r /r (AIFS) to the CW mn. Followng ths, we confgured the parameters of staton accordng to the followng formulae: AIFS = DIFS, () CW mn CW max = CW default mn round_power(w max /w ), and (5) = CW default max round_power(w max /w ), (6) where w s the weght assgned to staton, w max s the hghest weght n the WLAN, and round_power(x) s the nteger power of closest to x: log ( x), log ( x) x round _ power( x) = log ( x) x. (7) log ( x), otherwse Note that wth the above confguraton the staton wth the hghest weght n the WLAN uses the default CW values (.e., CW default mn and CW default max ), whle the remanng statons use hgher values. Ths ensures that effcency s not degraded due to low CW values yeldng an unacceptably hgh collson rate. In addton, the number of steps between CW mn and CW max s kept equal to the default value for all statons; ths s done n order to preserve the good features of the exponental backoff ncrease algorthm. Fnally, the round_power functon s ntroduced because of the restrcton mposed by the wreless cards that CWs have to be a power of. Symmetrc Traffc Allocaton wth UDP Traffc Throughput n DCF s dstrbuted farly among all contendng statons, n the sense that all statons wth traffc to send are allocated the same throughput. Whle at frst glance ths crteron may seem reasonable, a closer look at the throughput dstrbuton of a WLAN workng n the nfrastructure mode reveals that, as ponted out n [3], ths dstrbuton may be undesrable from a traffc engneerng vewpont. In the nfrastructure mode, statons only send and receve packets from the AP. Therefore, the relevant traffc engneerng parameters n ths case are the rate at whch the statons can send packets (hereafter called the upstream throughput) and the rate at whch the statons can receve (the downstream throughput). Wth the far dstrbuton of DCF, the AP, whch behaves n the same way as any other staton as far as the contenton algorthm s concerned, s allocated the same throughput for sendng packets as any other staton. As a result, the sum of all downstream throughputs n the WLAN s equal to one sngle upstream throughput. In other words, f there are N statons n the WLAN, each staton s allocated N tmes more throughput for upstream than for downstream traffc. A more reasonable throughput allocaton may be to allocate the same throughputs for upstream and downstream traffc (to whch we refer as symmetrc traffc allocaton). A typcal scenaro n whch ths allocaton would be desrable s a WLAN wth all statons holdng bdrectonal voce over IP (VoIP) conversatons. Note that to acheve ths allocaton we need to provde the AP wth a throughput N tmes larger than any other staton, N beng the number of statons n the WLAN. 5 Whle there are other proposals for confguraton of the EDCA parameters (e.g., [7]), one of the advantages of the confguraton proposed here s ts smplcty. 5 In a dynamc envronment, the number of actve statons n the WLAN can be obtaned, for example, by cachng the MAC address of the statons that have transmtted packets recently; ths functonalty s provded by the Wreless Tools ( Lnux/Tools.html). 5

4 Downstream Upstream Throughput (kb/s) nternal Weghted Far Queung (WFQ) scheduler at the AP n order to dstrbute the downstream throughput among the competng flows proportonal to ther weghts. Fgure 3 llustrates the throughput allocatons resultng from the above confguraton wth the three statons saturatng both the upstream and downstream drectons wth UDP traffc. Results confrm the effectveness of EDCA to mplement the OSM; upstream and downstream throughputs are approxmately proportonal to the weghts. Downstream throughputs are about 30 percent larger than upstream; ths s a consequence of the asymmetry n the capture effect dscussed for the prevous experment as well as the error ntroduced by the roundng operaton of Eq. 7. Staton Staton Staton 3 DCF EDCA n Fgure. Symmetrc traffc allocaton wth UDP traffc. To study the effectveness of EDCA n provdng symmetrc throughput allocaton, we saturated the upstream and downstream drectons of three statons wth UDP traffc and examned the resultng throughput allocaton for DCF and EDCA. Results for DCF were obtaned usng the default confguraton. The EDCA confguraton was derved from the formulae gven n Eqs. 6 wth weght w AP = 3 for the AP and weght w STA = for the statons. Results are llustrated n Fg.. We observe that, as expected, DCF allocates approxmately three tmes more throughput to upstream than to downstream, whle, n contrast, our EDCA confguraton behaves much better accordng to the symmetrc traffc allocaton objectve. It s worthwhle to observe that n both the DCF and EDCA results, the AP receves throughput approxmately 0 percent hgher than expected wth the respectve confguratons. Note that f a staton and the AP collde, the staton s transmsson wll surely result n a falure, as the destnaton of the transmsson (the AP) s not n lsten mode. In contrast, there s some probablty that the AP transmsson results n success as a consequence of the capture effect. Ths asymmetry n the capture effect explans why n all our experments the AP s always favored over the statons. Olympc Servce Model wth UDP Traffc Wth the olympc servce model (OSM) [8] users are classfed n three categores (gold, slver, and bronze), and each category s mapped to a weght (w gold > w slver > w bronze ). Then a gold user s allocated throughput proportonal to w gold, a slver user throughput proportonal to w slver and a bronze user throughput proportonal to w bronze. Ths allocaton would be the desrable one for, say, a gold user holdng a vdeoconference wth hgh-rate vdeo, a slver user holdng a low-rate vdeoconference, and a bronze user holdng an audoconference. To study the effectveness of EDCA to provde the OSM, we took w gold =, w slver =, w bronze =, wth one staton for each category, and amed to provde each staton wth balanced downstream and upstream throughputs, both proportonal to the weght of the staton s category. For ths purpose we confgured EDCA wth the formulae gven n Eqs. 6, each staton wth the weght of ts category and the AP wth a weght equal to w AP = w gold + w slver + w bronze (.e., the sum of the weghts of all statons). In addton, we confgured an Staton Staton Staton 3 Traffc Engneerng wth TCP Traffc Traffc engneerng for TCP traffc becomes more challengng as a consequence of the well-known nteractons wth the congeston control algorthm of TCP and the presence of TCP ACKs 6 [3]. In order to gan nsght nto the mpact of TCP on throughput dstrbuton, we repeated the symmetrc trafc allocaton experement of Fg. wth each staton runnng n longlved TCP flows n both upstream and downstream drectons. Experments were performed wth both the default output queue buffer sze n all PCs (B = 99 packets) and (followng the recommendatons of [3]) an output queue buffer sze of B AP = NB at the AP, N beng the number of statons n the WLAN (n our case N = 3). Fgure shows the rato of upstream and downstream throughputs resultng from the above scenaro for DCF and EDCA and the two buffer confguratons. Accordng to our objectve of a symmetrc traffc allocaton, ths rato should deally be equal to. Results show substantal dfferences wth the allocaton obtaned for UDP traffc n Fg.. For n =, TCP balances the throughput obtaned by all flows ndependently of the EDCA parameters and buffer confguraton. For 6 Note that throughput n EDCA s regulated by controllng the number of tmes a staton accesses the channel, and from ths perspectve, a TPC ACK counts as much as any other packet ndependent of ts sze. Trhoughput (kb/s) Gold Slver 35 5 Bronze Downstream Upstream n Fgure 3. Olympc servce model wth UDP traffc. 55

5 r up /r down DCF DCF, B AP = NB EDCA EDCA, B AP = NB 3 5 n n Fgure. Symmetrc traffc allocaton wth TCP traffc. a larger number of flows, the dstrbuton becomes unbalanced wth DCF, whle wth EDCA (both when B AP = B and B AP = NB) the ratos keep relatvely close to. We conclude that the confguraton of EDCA parameters s also useful wth TCP traffc to keep the throughput dstrbuton close to the desred allocaton. We beleve, however, that more extensve studes are needed to better understand the nteractons between TCP and EDCA. Traffc Engneerng Under Heterogeneous Rado Condtons The fact that throughputs n WLAN depend on rado condtons poses an addtonal challenge to traffc engneerng. Rado condtons may mpact the throughput of a staton n 80.b WLAN n the followng ways: Dfferences n the receve power of two statons may yeld the capture effect. Some degradaton of the rado lnk may yeld occasonal transmsson errors. A severe degradaton of the rado lnk yelds frequent transmsson errors, whch trggers the selecton of a new modulaton scheme more robust to errors and wth a lower transmsson bt rate. In order to llustrate the mpact of rado condtons on throughput, we repeated the symmetrc traffc allocaton experment of Fg. wth two statons sendng UDP traffc upstream, the frst staton placed close ( m) to the AP and the second one placed n the followng locatons: At a dstance from the AP smlar to the frst staton ( m) At a longer dstance ( m) but stll wth a good rado lnk Outsde the room, where the bt rate remans at Mb/s At about 00 m, where the transmsson bt rate decreases to Mb/s Results of the total throughput n the WLAN (r) and the rato of throughputs (r /r ) as a functon of the locaton of the second staton are gven n Fg. 5. We observe that n poston both statons obtan approxmately the same throughput. In poston, the capture effect leads to the frst staton obtanng greater throughput (about 0 percent more) than the second staton. In poston 3 transmsson errors occur, whch slghtly degrade the throughput of the second staton and the total throughput; however, the mpact s almost mperceptble. Fnally, n poston the total throughput s drastcally degraded as a result of the second staton transmttng at a lower bt rate, but the rato of throughputs does not change. We conclude from the results that rado condtons have lttle mpact on the rato of throughputs; ndeed, n our experments the rato s always close to, whch s the deal value accordng to the symmetrc traffc allocaton crteron. Another concluson drawn from the results s that when a staton decreases ts transmsson bt rate (poston ), although the ratos are preserved, the throughputs of all the statons n the WLAN are sharply degraded. Indeed, f a gven staton transmts at a lower bt rate, each tme t accesses the channel t occupes t for a longer perod; therefore, less channel tme s avalable for other statons. Ths behavor s studed n detal n []. EDCA for Servce Guarantees We now address the ssue of provdng servce guarantees wth EDCA and analyze the challenges that stll have to be solved n order to reach ths goal. To support the explanatons provded here, we performed a number of experments (Fg. 6) whose setup s descrbed n the followng paragraphs. In experment, staton and the AP held a bdrectonal voce conversaton usng the GnomeMeetng 7 tool, whch measures the round-trp delay. Statons and 3 sent background data traffc of 000-byte packets at a rate of r background. In order to saturate the channel, statons were manually confgured at a low transmsson bt rate; specfcally, the AP and statons and were confgured at a bt rate of Mb/s, and staton 3 at a bt rate of Mb/s. All statons had the default confguraton (.e. the one of DCF). We measured the voce traffc round-trp delay as a functon of the background statons sendng rate, and consdered (as an example of qualty crteron) that voce performance s acceptable as long as ths delay falls below 50 ms (ths s represented by the horzontal straght lne n Fg. 6). In experment we repeated the above setup but usng EDCA. Followng the results of Fg., whch show that the AIFS parameter s more approprate for provdng strct prortes, we confgured real-tme statons wth the mnmum AIFS allowed by the standard draft (AIFS = DIFS) and background statons wth the maxmum allowed AIFS (AIFS = DIFS In the recommendatons gven by the standard draft, real-tme and background statons are also confgured wth dfferent CW mn and CW max parameters. In ths artcle we restrct our analyss to only one parameter for smplcty. r /r Locaton of the second staton n Fgure 5. Heterogeneous rado condtons. r /r r + r r(mb/s) 56

6 Delay real-tme (ms) Experment Experment Experment 3 Experment r background (kb/s) EDCA analyses are restrcted to average delay []. Even for DCF, analyses of the delay dstrbuton are lmted (e.g., the analyss of [0] s restrcted to saturaton condtons). Algorthms to derve the optmal confguraton. The EDCA confguraton proposed here as well as the one gven by the standard draft recommendatons s derved heurstcally and guarantees no optmzed performance. Instead, t would be desrable to use the optmal EDCA confguraton (.e., the confguraton that, gven a set of requests and requrements, provdes the best possble performance). Exstng algorthms to compute the optmal EDCA confguraton [7] are restrcted to saturaton condtons and throughput, and do not serve the purpose of provdng real-tme applcatons wth servce guarantees. n Fgure 6. Experments wth real-tme traffc. 3σ), and left the other parameters at ther default values. 8 In experments 3 and we repeated experment ntroducng the followng varatons. In experment 3, the bt rate of staton 3 was reduced to Mb/s. In experment, staton was moved about m further from the AP than the other statons, such that the lnk qualty was stll good but not as good as the others. Analyss and Algorthms: Future Drectons The provsonng of servce guarantees wth EDCA requres the development of a theoretcal bass that s not yet complete. Indeed, QoS archtectures for wred lnks such as ntegrated servces (IntServ) and dfferentated servces (DffServ) rely on analyses (e.g., [9]) that allow performance predctons and can be used to guarantee QoS va an admsson control algorthm: f acceptng a new request would degrade the performance of admtted requests below the guaranteed metrcs, the algorthm rejects the new request. The development of these analyses and algorthms for EDCA s one of the research challenges that reman yet to be addressed before ths mechansm can be used for provdng servce guarantees. Let us look at experment of the above set. Gven the condtons of the experment and the requrement that the round-trp delay for the voce applcatons cannot be greater than 50 ms, the maxmum sendng rate that can be admtted for the background statons s 00 kb/s. Ths value s hgher than that obtaned wth DCF (experment ), where the maxmum admssble sendng rate s 00 kb/s. Nonetheless, an algorthm s stll needed n EDCA n order to derve the maxmum admssble sendng rate and perform admsson control accordngly. In the last years consderable effort has been dedcated to analyze the theoretcal performance of EDCA; however, there stll exst a number of gaps that need to be flled before analyss can be used to provde servce guarantees. In the followng we outlne future research drectons to fll these gaps: Analyses wth realstc source models. Present analyses of EDCA are restrcted to the unrealstc case of saturated sources (.e., they assume that all statons always have packets to transmt, e.g., []). Even for DCF (the basc mechansm that EDCA extends), although there are some analyses that consder nonsaturaton condtons, no complete analyses wth realstc source models exst (e.g., [5] s restrcted to Posson arrvals). Analyses of the end-to-end delay dstrbuton. Real-tme applcatons need not only a low average delay but also a low delay for most of ther packets, whch requres the computaton of the end-to-end delay dstrbuton. Exstng Inherent Uncertantes wth Moblty: A New Concept for Admsson Control Most of the analyses referenced above assume that all statons transmt at the nomnal bt rate ( Mb/s) under deal condtons (no transmsson errors and no capture effect). These nondeal aspects, as shown by experments 3 and, strongly mpact performance and therefore must be taken nto account to provde servce guarantees. Although there are some analyses that account for the above aspects (e.g., [] provdes an analyss for heterogeneous transmsson bt rates and errors, and [6] provdes an analyss for the capture effect), these analyses requre the knowledge of some data (e.g., the receve power) that s not always easy to obtan. In fact, the receve power depends not only on the transmt power and dstance, but also on some addtonal effects lke fadng that are not easy to derve. Under statc condtons (e.g., a company network based on WLAN where the statons are n fxed locatons), t may be concevable to obtan, from the locaton of each staton and the measured qualty of the correspondng rado lnk, the data necessary to evaluate the mpact of the above aspects and use ths data n the analyses and algorthms that provde servce guarantees. The goal of provdng servce guarantees, however, becomes more challengng when consderng a moble envronment, n whch rado condtons vary wth tme. The problem n ths case arses from the fact that the condtons that yeld a gven admsson control decson at one pont may not hold after some tme. Ths can be seen n the experments of Fg. 6: under the condtons of experment, background statons wth a sendng rate of 00 kb/s may be admtted. However, f the transmsson bt rate of the second background staton s reduced to Mb/s (experment 3) or the real-tme staton moves a bt further (experment ), the performance goals are no longer met, and the admsson control decson needs to be reevaluated. We conclude that under moblty condtons a new concept for admsson control s requred that constantly montors the channel condtons and reevaluates admsson control decsons. Such an algorthm s a subject of future research. Conclusons In ths artcle we have nvestgated the ablty of the EDCA mechansm to satsfy, n a real envronment, the requrements of two of the applcatons for whch ths protocol was desgned: traffc engneerng and servce guarantees. The nvestgaton was performed based on real-lfe experments that have gven us nsght nto a number of aspects not typcally shown by smulatons and analyses. Results for traffc engneerng show that ths applcaton s 57

7 well supported by EDCA when only UDP traffc s present n the WLAN. For the two traffc engneerng crtera studed (symmetrc traffc allocaton and olympc servce model) the results obtaned match, wth reasonable accuracy, the desred allocatons. Only some small devatons are observed because of the capture effect and the restrcton that CW values must be set to nteger powers of. Results wth TCP are promsng; although the nteracton between the TCP congeston control and the EDCA mechansm causes some devatons from the desred allocatons, the throughput dstrbutons we have obtaned are farly good and much better than the ones obtaned wth DCF. Experments under heterogeneous rado condtons reveal that transmsson errors and the capture effect have a small nfluence, whle the statons bt rates have a much stronger mpact on traffc engneerng. The other applcaton we have analyzed, servce guarantees, s much harder to satsfy. A substantal amount of theoretcal work stll needs to be developed before ths applcaton can be mplemented wth EDCA. Ths theoretcal work ncludes analyses under nonsaturaton condtons, analyses of the dstrbuton of the end-to-end delay, and algorthms to determne the optmal EDCA confguraton. In addton, our experments show that the nherent uncertantes of a moble envronment represent a major challenge for servce guarantees. In order to tackle ths problem, we propose the desgn of a new concept for admsson control that constantly montors the WLAN condtons and reacts to changng condtons. As a fnal concluson, we beleve that the expermental results reported n ths artcle show that the EDCA mechansm has many useful applcatons that would not be possble to satsfy wth the legacy DCF protocol, but there are stll some challenges that need to be addressed before EDCA can meet all the desgn goals for whch t was orgnally conceved. Acknowledgments The authors would lke to thank José Félx Kukelka, Chrstos Doulgers, and anonymous referees for ther nsghtful comments on earler versons of ths artcle. References [] IEEE 80.e/D3.0, Part, Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) specfcatons: Medum Access Control (MAC) Enhancements for Qualty of Servce (QoS), draft supp. to IEEE 80. std., Jan [] Z. Kong et al., Performance Analyss of IEEE 80.e Contenton-Based Channel Access, IEEE JSAC, vol., no. 0, Dec. 00, pp [3] S. Plosof et al., Understandng TCP farness over Wreless LAN, Proc. IEEE INFOCOM 03, Mar. 003, pp [] M. Heusse et al., Performance anomaly of 80.b, Proc. IEEE INFOCOM 03, Mar. 003, pp [5] G. R. Canten et al., Performance Analyss under Fnte Load and Improvements for Multrate 80., to appear, Elsever Comp. Commun. [6] R. Ltjens et al., Performance Analyss of Wreless LANs: An Integrated Packet/Flow Level Approach, Proc. 8th Int l. Teletraffc Cong., Sept. 003, pp [7] A. Banchs, X. Perez, and D. Qao, Provdng Throughput Guarantees n IEEE 80.e Wreless LANs, Proc. 8th Int l. Teletraffc Cong., Sept. 003, pp [8] C. Dovrols and P. Ramanathan, A Case for Relatve Dfferentated Servces and the Proportonal Dfferentaton Model, IEEE Network, vol. 3, no. 5, Sept. 999, pp [9] J. C. R. Bennett et al., Delay Jtter Bounds and Packet Scale Rate Guarantee for Expedted Forwardng, IEEE/ACM Trans. Net., vol. 0, no., Aug. 00, pp [0] A. Banchs, Analyss of the Delay Dstrbuton n 80. DCF: A Step Towards Delay Guarantees n WLANs, LNCS, vol. 366, Sept. 00, pp [] H. Pan, S. Sato, and K. Kobayash, On the Throughput of an IEEE 80.a Wreless LAN System wth Termnals under Heterogeneous Rado Condtons, Proc. 8th Int l. Teletraffc Cong., Sept. 003, pp Bographes ALBERT BANCHS [M] (banchs@t.uc3m.es) receved hs M.Sc. and Ph.D. degrees n telecommuncatons from the Techncal Unversty of Catalona n 997 and 00, respectvely. Hs Ph.D. receved the natonal award for best thess on broadband networks granted by the Professonal Assocaton of Telecommuncaton Engneers. He worked for the Internatonal Computer Scence Insttute n 997, for Telefonca I+D n 998, and for NEC Network Laboratores from 998 to 003. Snce 003 he s wth Unversty Carlos III, Madrd. Hs current research nterests nclude resource allocaton, QoS, and performance evaluaton of wreless and wred networks. ARTURO AZCORRA [SM] (azcorra@t.uc3m.es) receved an M.Sc. degree n telecommuncatons from the Techncal Unversty of Madrd n 986, and a Ph.D. degree from the same unversty n 989. Hs Ph.D. receved the natonal award for best thess, jontly granted by the Professonal Assocaton of Telecommuncaton Engneers and the Natonal Assocaton of Electronc Industres. In 993 he obtaned an M.B.A. from Insttuto de Empresa. He was a lecturer at the Techncal Unversty of Madrd from 987 to 990, when he was promoted to assocate professor. In 998 he joned Unversty Carlos III, Madrd, where he has stayed to the present as a full professor. Snce 998 he s also Deputy Vce- Provost for Academc Infrastructures at Unversty Carlos III. He has been program commttee member of several edtons of IEEE PROMS, IDMS, QofIS, CONEXT, and IEEE INFOCOM. He s general co-char for CONEXT 05 and TPC co-char for INFOCOM 06. Hs lst of papers publshed n natonal and nternatonal journals, books, and conferences s over 00 ttles. Current research projects nclude broadband networks, multcast teleservces, actve networks, and advanced IP networks. CARLOS GARCI A (cgg@t.uc3m.es) receved an M.Sc. degree n telecommuncatons from Unversty Carlos III, Madrd, n 00. Hs M.Sc. thess receved the award for best thess granted by the Nortel Networks Char. He s currently a Ph.D. student and lecturer at Unversty Carlos III. Hs research nterests are QoS provson n G networks and IP over DWDM network deployment. RUBÉN CUEVAS (rcuevas@nv.t.uc3m.es) s pursung hs M.Sc. degree n telecommuncatons at Unversty Carlos III. In 00 he receved a scholarshp from Telefonca I+D to work on VPN applcatons. Snce 00 he s nvolved n the development of a QoS management platform for wreless networks as part of hs M.Sc. thess. 58