Achieving Fairness in Wireless LANs by Enhanced DCF *

Transcription

1 U.C. ECECS Techical Report August 2005 Achievig Fairess i Wireless LANs by Ehaced 802. DCF * by Nagesh S. Nadiraju, Hrishikesh Gossai, Dave Cavalcati, Kaushik R. Chowdhury, Dharma P. Agrawal Ceter for Distributed ad Mobile Computig, Dept. of ECECS, Uiversity of Ciciati - Ciciati,OH 4522 (adirs, cavalcdt, kaushir, dpa)@ececs.uc.edu Mesh Research ad Developmet Group Motorola, Ic., Maitlad, Florida Hrishikesh.Gossai@motorola.com

2 Nagesh S. Nadiraju et al. of 6 Abstract Over the past few years, Wireless Local Area Networks (WLANs) have gaied a icreased attetio ad a large umber of WLANs are beig deployed i uiversities, compaies, airports etc. Majority of the IEEE 802. based WLANs employ Distributed Coordiatio Fuctio (DCF) i Wireless Access Poits (AP) to arbitrate the wireless chael amog Wireless Statios (STAs). However, DCF poses serious ufairess problem betwee uplik ad dowlik flows. To overcome this ufairess problem, we propose a simple ehacemet to the IEEE 802. DCF which provides priority to the AP ad thus eables it to acquire a larger share of the chael whe required. We have demostrated the ufairess problem through systematic measuremets i a experimetal test bed of WLAN usig the legacy 802. DCF. We also developed aalytical models to calculate the throughput of AP ad the STAs ad verify these results through thorough simulatios i s-2. We observe that our simulatio results fid i good agreemet with our aalytical models. Results show that our proposed ehacemet achieves a fair distributio of badwidth ad improves the throughput (by early 300%) ad reduces the delay (by early 60%) for the dowlik flows as compared to the DCF, without severely affectig the performace of uplik flows. Keywords: Fairess, MAC protocols, Performace Evaluatio, Test beds, WLANs.

3 Nagesh S. Nadiraju et al. 2 of 6. INTRODUCTION I the last decade, Wireless LANs experieced a proliferatig growth due to their flexibility ad ubiquitous ature. I particular, WLAN hotspots are typically foud i uiversities, compaies, airports, shoppig malls, etc. With the icreasig iterest i the itegratio of various wireless etworks (4G etworks), WLANs are gaiig more popularity tha ever. The capacity of WLANs has rapidly icreased from 2Mbps to 54Mbps ad proposals (IEEE 802.) to achieve early 00Mbps are also uderway. May efforts are also beig made to make QoS provisios for real time traffic (IEEE 802.e). Most of the curret WLAN implemetatios are based o the IEEE 802. [] stadard, which supports two basic mechaisms for chael arbitratio: Distributed Co-ordiatio Fuctio (DCF) ad Poit Coordiatio Fuctio (PCF). The implemetatio of DCF i IEEE 802. compliat devices is madatory while provisio of PCF is optioal. DCF is based o the traditioal CSMA/CA paradigm ad provides equal chael access privileges to all participatig Wireless Statios (STAs). I cotrast, PCF is a cetralized schedulig algorithm. It requires a poit coordiator (PC) at the AP to cotrol the chael access. The default schedulig algorithm of IEEE 802. PCF is a roud robi scheme ad may ot always be ideal. Due to the iheret complexity ivolved with the deploymet of PCF [2], most of the curret implemetatios of IEEE 802., eve i hot spot scearios, use DCF access mechaism. However, DCF poses serious ufairess problem betwee uplik ad dowlik flows. With DCF, the chael share of the AP would be a fractio of total umber of trasmittig STAs i its service area. All STAs, icludig the AP, have the same chael access privileges. As a result, the share of the chael obtaied by the AP is early equal to the share of ay other STA uder its coverage. This results i ufair sharig of the badwidth amog uplik ad dowlik flows. All the dowlik flows (flows that are destied for wireless statios) have to utilize the AP s chael share while the uplik flows origiatig from differet STAs ejoy a larger share. With the icrease i the umber of STAs uder the AP s coverage, the dowlik flows would suffer from relatively low share of the available badwidth. To better illustrate the ufairess problem, cosider a typical WLAN hotspot sceario as show i figure. A set of wireless users (STAs) are coected to the iteret through a AP. All the traffic to ad from the STAs passes through the AP ad thus the wireless lik becomes the bottleeck for all the traffic. Assume STA-STA5 are receivig UDP traffic from server S (dowlik flows); whereas STA6-STA0 are trasmittig UDP traffic to server S (uplik flows). I this sceario, the AP, ad STA6-STA0 will coted for the wireless chael through the regular IEEE 802. DCF. Recall from previous paragraph DCF gives early equal chael share to all the cotedig odes i a eighborhood. Thus, the chael share obtaied by the AP (ad hece the combied dowlik traffic) would early be equal to the idividual shares of other cotedig STAs (STA6- STA0). I other words, the badwidth available to the idividual flows edig at STA- STA5 would be a fractio of the badwidth available to the idividual uplik flows of STA 6- STA 0. Iterestigly, this sceario is just the reverse of a typical wired access etwork (e. g. DSL.), where badwidth available to dowward flows is always higher tha upward flows. S Primergy AP Primergy STA STA 2 W STA0 STA 9 STA 5 STA 6 Figure : IEEE 802. DCF based WLAN Access Sceario I order to overcome the aforemetioed problem, we propose a simple MAC layer ehacemet to IEEE 802. DCF, called Bidirectioal-DCF (BDCF). We specifically address the uplik/dowlik ufairess by providig the AP with more cotetio free trasmissio opportuities whe high load is experieced. I particular, if a AP s MAC receives a DATA packet, istead of trasmittig a regular MAC layer ACK, it checks the buffer for a outstadig packet to ay of the STAs i

4 Nagesh S. Nadiraju et al. 3 of 6 its Basic Service Set (BSS). If a packet is foud, the it will sed the DATA with a piggybacked ACK after SIFS time, thus elimiatig the eed for a fresh chael cotetio to trasmit this packet. I this way, the AP gets a preferetial treatmet resultig i a relatively higher badwidth share as compared to its STAs. Clearly, this kid of preferetial treatmet for the AP is desirable i hot spots scearios. This is because most of the users accessig iteret i these hotspots use applicatios (e.g. , web browsig, Iteret radio, etc.) that typically geerate large volume of dowlik traffic for a sigle uplik request. It is worthwhile to ote that if the AP does ot have traffic to sed i dowlik directio, BDCF works exactly as DCF. Our specific cotributios through this paper are: Experimetal demostratio of the ufairess problem i a IEEE 802.b based test-bed. A simple ehacemet to DCF for overcomig the ufairess problem i AP based etworks. A aalytical model to evaluate the throughputs of AP ad STAs complemeted by extesive simulatio study. Our aalytical ad simulatio results show that BDCF has a better throughput ad delay performace for dowlik flows ad also has a fair chael sharig i both directios as compared to DCF. I additio, we compare BDCF with DCF+ [5], which uses a similar idea to reduce MAC layer overhead ad icrease the throughput, while it does ot give ay priority to the AP. The outlie for the rest of this paper is as follows. I sectio 2, we briefly describe DCF, PCF, ad EDCA (802.e) for the completeess of the text. We the experimetally demostrate the ufairess problem caused by DCF i WLAN hot spot scearios i sectio 3. We describe our proposed BDCF mechaism i sectio 4. Next, we develop a aalytical model for the uplik ad dowlik throughputs with BDCF i sectio 5. Sectio 6 provides comprehesive simulatio results, comparig BDCF with DCF ad DCF+. I sectio 7, we discuss the related work, ad fially, we coclude the paper i Sectio 8, highlightig some ope problems ad future research directios. 2. BACKGROUND DCF follows the CSMA/CA mechaism for chael access. I this method, ay STA before trasmittig a packet seses the chael. If the chael is sesed busy, the STA waits for the ogoig trasmissio to complete. Otherwise, if the chael is idle for DIFS (Distributed Iter-Frame Space) period ad if the backoff timer is zero, the STA will trasmit. After trasmittig a DATA packet, the STA waits for a ackowledgemet (ACK) from the receiver, which is set after SIFS (Small Iter-Frame Space) period. If the seder does ot receive the ACK, it assumes a collisio has occurred ad schedules a retrasmissio after a radom backoff time. I additio to this basic access scheme, DCF also supports virtual carrier sesig, i a attempt to reserve the chael before every data trasmissio. I this case, the seder trasmits a RTS (Request To Sed) ad the receiver replies with CTS (Clear To Sed) if it is ready to accept the DATA packet. Upo receivig the CTS packet, the seder trasmits the DATA packet ad waits for the ACK. Both RTS ad CTS packets carry a duratio field that idicates the time period for which the chael is reserved for the followig trasmissio. Thus, all the odes overhearig the RTS/CTS hadshake, record this iformatio i the data structure, Network Allocatio Vector (NAV), ad defer their trasmissio for the specified time frame i the NAV. PCF, o the other had, is maily desiged for ifrastructure operatio mode, i.e., for Access Poit (AP) based systems, ad it rus o top of DCF. I PCF, there is a poit coordiator (PC), which is geerally located i the AP, ad cetrally cotrols the chael access. A cotetio free period (CFP) ad a cotetio period (CP) form a super frame, which is periodically iitiated by the PC after each beaco trasmissio. I CFP, the PC polls STAs for collisio free trasmissios accordig to a pollig list. The CFP is followed by a CP i which all statios follow DCF for chael cotetio. I order to efficietly poll the STAs, the AP should be iformed about the curret traffic patter beforehad. With the chagig traffic patters i the cosidered WLAN access sceario, it is ot feasible to provide such iformatio to the AP. Moreover the pollig scheme of PCF geerates a lot of overhead, thereby makig it more iefficiet. Due to this iheret complexity ad limitatios of PCF, it is ot employed for practical deploymet. The IEEE 802. workig group is attemptig to ehace the curret 802. MAC ad make provisios for quality of service (QoS) provisioig. This ehaced MAC (802.e) provides varyig chael access priorities for differet traffic categories (TCs). It specifies eight differet TCs ad QoS-eabled STAs mark their frames accordig to the applicatio service requiremets. EDCA provides service differetiatio amog the various traffic classes while maitaiig backward compatibility with legacy DCF. However it does ot address ode level access priorities ad either limits the traffic geerated by ay STA.

5 Nagesh S. Nadiraju et al. 4 of 6 3. THE DCF UNFARINESS PROBLEM IN WLAN HOT SPOTS I this sectio we illustrate the ufairess amog dowlik/uplik flows whe DCF is employed i a WLAN. We have set up a experimetal test bed to model the typical WLAN hot spot sceario as show i Figure. We cofigured a ifrastructure based IEEE 802.b etwork with a data rate of Mbps ad coected the AP to a desktop PC (with a Itel Petium 4 processor). We start the server S i the same machie, so that the coectio betwee the AP ad the server is ot a bottleeck. Furthermore, the AP uses the hostap driver package [9], ad the STAs use IEEE 802.b USB adapters, Netgear WG3 or Atheros wireless cards. We have systematically examied the TCP ad UDP throughput performace with a symmetric ad a asymmetric traffic cofiguratio. TCP ad UDP traffic is geerated by Iperf v.7 [20] which rus i both the cliet ad server modes. We first cosider a asymmetric traffic sceario where 3 UDP flows origiatig at the differet wireless odes destied to the server (called uplik flows) ad 7 UDP flows from the server towards the wireless odes (called dowlik flows). Each flow geerates traffic at a rate of 3Mbps, which is eough to saturate the wireless lik. We have coducted 0 differet rus for each traffic sceario ad measured the throughputs of idividual flows. The throughputs of flows i the same directio do ot have much variatio. Thus, for the sake of brevity, we oly show the aggregate throughputs of uplik ad dowlik flows for all the four scearios i Figure 2. As ca be see from the plot (i Figure 2 (a)), i the case of asymmetric UDP traffic, the three uplik flows obtai a throughput of Kbps (749.6 Kbps per flow o a average) while the 7 dowlik flows obtai a throughput of oly Kbps ( Kbps per flow o a average). It ca be observed that the throughput of a idividual dowlik flow is early /3 of the throughput achieved by ay uplik flow. Clearly the MAC level fairess achieved by DCF leads to a udesirable situatio. For example, cosider a typical WLAN hotspot sceario where a couple of studets (who are a part of peer-to-peer file sharig etwork) are uploadig sogs/movies through the wireless etwork. Their applicatios (are uplik) cosume fairly large share of the wireless chael ad thus limit the badwidth for the dowlik traffic. Thus other users who are checkig their or usig other predomiatly dowlik traffic based applicatios experiece larger dowload delays ad icreasig frustratio Up Lik Dow Lik Up Lik Dow Lik Throughput (kbps) Asymmetric (3Up-7D) Symmetric (5Up-5D) Throughput (kbps) Asymmetric (3Up-7D) Symmetric (5Up-5D) Figure 2 (a) UDP Traffic Figure 2 (b) TCP Traffic These results motivate the eed for a preferetial treatmet to the AP i order to allot a fair share of badwidth for the dowlik flows. Although with PCF, the chael access for each statio is cotrolled by the AP (through pollig), it is ot flexible. By default the AP polls all the statios i the pollig list i a roud robi fashio. I order to efficietly poll the STAs, the AP should be iformed about the curret traffic patter beforehad. With the chagig traffic patters i the cosidered WLAN access sceario, it is ot feasible to provide such iformatio to the AP. Moreover the pollig scheme of PCF geerates a lot of overhead thus makig it more iefficiet. We propose a simple ehacemet to DCF i the ext sectio, which prioritizes the AP without the requiremet of ay additioal iformatio ad achieves the required fairess amog the uplik ad dowlik flows. 4. BIDIRECTIONAL DISTRIBUTED CO-ORDINATION FUNCTION (BDCF) I order to hadle the ufair badwidth availability to the dowlik traffic i WLAN hot spot scearios, we propose a Bidirectioal DCF (BDCF), which provides preferetial treatmet to the dowlik flows at the AP. For implemetig BDCF, we modified IEEE 802. DCF to support piggybackig of ACK packets i the DATA trasmissio from the AP. Similar to

6 Nagesh S. Nadiraju et al. 5 of 6 IEEE 802. DCF, BDCF supports both basic (DATA-ACK) ad 4-way hadshake (RTS-CTS-DATA-ACK) chael access mechaisms. I the remaider of this sectio we describe the details of BDCF. STA(i) DIFS SIFS RTS DATA AP SIFS SIFS CTS DATA AP NAV(RTS) NAV(DATA ) STA(j) SIFS ACK STA(i) AP STA(j) DIFS DATA SIFS DATA AP NAV(DATA ) SIFS ACK NAV(RTS) NAV(DATA ) DIFS Other (Source Rage) (Destiatio Rage) NAV(CTS) Defer Access Cotetio Widow Backoff Starts Others NAV(DATA ) Defer Access NAV(DATA AP) DIFS Cotetio Widow Backoff Starts Figure 3(a). Timig diagram of BDCF with RTS-CTS hadshake Figure 3(b). Timig diagram of BDCF with basic access 4. BDCF with RTS-CTS hadshake We first start by describig the operatio of BDCF whe RTS-CTS hadshake is eabled. Figure 3(a) shows the timig diagram of the BDCF operatio. Iitially all STAs ad the AP coted for the chael with equal privilege. Durig the cotetio resolutio period, if the AP gets the access to the chael, BDCF works exactly like DCF. The differece comes whe a STA wis the chael cotetio. Figure 3(a) illustrates this case i detail. STA(i) iitially seds a RTS to the AP i order to reserve the chael for its DATA trasmissio. The AP respods this request of STA(i) by sedig a CTS. Upo receptio of the CTS packet, STA(i) seds its DATA packet (DATA i ) to the AP. After receivig DATA i, the AP checks its MAC buffer for ay packet to trasmit. If o packet is foud, the AP simply seds the correspodig ACK to STA(i). However, if the AP has a outstadig packet for ay of its servig STA(j) (where j may be same as i), it trasmits that DATA packet (idicated by DATA AP i Figure 3(a)) with a piggybacked ACK, after a SIFS time period. If STA(i) seses trasmissio from the AP after a SIFS period, it will implicitly recogize the ACK set by the AP. As log as the STAs are withi the coverage regio of the AP, they are able detect the piggybacked ACK (STA(i) was waitig for a ACK for DATA i from AP). Sice all the STAs i the servig BSS should be withi the coverage of AP our assumptio is valid. After a successful receptio of the DATA AP, the destiatio STA(j) will sed back a ACK to the AP i the usual way. However, if the AP does ot receive the iitial data frame (DATA i ) correctly, it does ot sed ay DATA AP or ACK, thus STA(i) becomes aware of its usuccessful trasmissio ad schedules a retrasmissio. It is importat to ote that BDCF allows piggybackig of DATA packets oly at the AP, thus avoidig the chace of formig ay cycles (pheomeo where AP ad ay oe of STAs repeatedly access ad capture the chael). Furthermore, whe the AP trasmits the DATA packet with piggybacked ACK (DATA AP ), it freezes its backoff timer, such that the trasmissio of DATA AP is totally trasparet to the regular chael cotetio at the AP. The chages made at the MAC layer for implemetig BDCF does ot require ay chages to the upper layers ad thus totally trasparet to the upper layers. As we will show i the ext sectio, by usig this simple ehacemet, BDCF ca provide fair access to the wireless chael i both the directios, irrespective of the type of trasport layer (UDP or TCP) uder cosideratio. It should also be oted that with BDCF, we avoid sedig ACK, RTS, ad CTS packets while seakig a DATA AP packet from the AP, thus reducig MAC layer cotrol overhead as compared to DCF. Moreover we also reduce the time wasted i ay chael cotetio ad backoff mechaism. I order to avoid ay egative effect of BDCF o the uplik flows whe there are fewer dowlik flows tha uplik flows, we adopt a dyamic piggybackig strategy. With this strategy the AP records the umber of STAs trasmittig the uplik flows ad the umber of STAs receivig dowlik flows over a time widow. The AP piggybacks a DATA packet oly with a probability equal to the ratio of dowlik ad uplik flows. This way, we esure that the dowlik flows do ot get ay udue advatage ad ifluece fairess of the system. 4.2 BDCF Basic Access mechaism The workig priciple of BDCF i the basic access mechaism (without RTS-CTS hadshake) is similar to its operatio whe RTS-CTS hadshake is eabled. Figure 3(b) illustrates the timig diagram of BDCF for this case. Oce STA(i) has

7 Nagesh S. Nadiraju et al. 6 of 6 captured the chael it seds its DATA packet (DATA ) to the AP. After receivig DATA, the AP checks its MAC buffer for ay outstadig packet. If a packet for ay STA(j) is foud, the AP seds it (DATA AP i Figure 3(b)) after SIFS period, otherwise it simply respods with a ACK packet to STA(i). 5. ANALYTICAL EVALUATION OF BDCF As poited out i sectio 4, BDCF esures that the dowlik flows gets a fair share of the system badwidth. We validate this claim by derivig aalytical expressios for the throughput of the AP (dowlik) ad the remaiig STAs (uplik) whe BDCF is used with the RTS-CTS hadshake mechaism. We assume fixed STAs ad oe AP, each of them havig a packet to trasmit at all times (saturatio traffic coditios with equal umber of uplik ad dowlik flows). We also assume perfect chael coditios ad o hidde termials. We defie the throughput by the followig equatio S = E[payload iformatio trasmitted i a slot time] E[legthof slot time] () ( p)/w0 0,0 ( p)/w0 0, 0,2 0,W0 2 0,W0 p/w i,0 p/wi p/wi i,0 i, i,2 0,Wi 2 0,Wi p/wi+ p/wm m,0 m, m,2 m,wm 2 p/wm p/wm p/wm Figure 4. Biachi s Markov Chai model We first study various evets that ca occur i ay arbitrary slot, the time duratios for the idividual evets, ad fially the total trasmissio time. We follow a approach similar to that used by Biachi [2] i derivig our expressios. It is assumed that each statio icludig the AP trasmits i a radomly chose slot time with probability ad collisios occur with a costat probability p, irrespective of the umber of previous collisios before a successful trasmissio. We cosider the slot time to be of legthσ. Let E p, H ad deote the average packet payload size, the packet header size (calculated as PHY hdr + MAC hdr ) ad the propagatio delay, respectively. E p ad H are measured i time uits. For simplicity we defie, W = CW mi ad m as the maximum backoff slots so that, CW max = 2 m * CW mi.the probability of trasmissio () i a give time slot as derived i [2] is 2( 2 p)( p) τ = m ( 2 p)( W + ) + p( W ( 2 p ( ) ) I what follows, we idetify the various evets that ca occur i a arbitrary time slot uder BDFC operatio ad calculate their probabilities of occurrece. Recall from sectio 4 that i BDCF, the AP trasmits a data packet uder two coditios: by cotetio or by piggybackig. I. Whe AP coteds for the chael I this case, the AP coteds for the chael afresh with all the other odes (this evet is deoted by tr). I this sceario, BDCF works similar to DCF ad collisios ca occur whe two or more RTS packets are trasmitted simultaeously. The probability of a trasmissio by the AP i ay arbitrary slot is give by: P tr(ap) = (2) m,wm

8 Nagesh S. Nadiraju et al. 7 of 6 tr(ap) Oce the AP wis the chael, the time spet for a successful trasmissio of the DATA packet by the AP is calculated as: T s(ap) = O rts + {H + E p + + SIFS + ACK + + DIFS}, (4) Where, O rts = {RTS ++ SIFS + CTS+ + SIFS} is the time required for RTS/CTS exchage. I case of a collisio, whe RTS/CTS scheme is used, the collisio time is give by T c(ap) = RTS + + SIFS + CTS+ + DIFS (5) II. Whe the AP Piggybacks Wheever the AP receives a DATA packet from a STA, it will have the optio of sedig a DATA packet with the piggybacked ACK after SIFS time period. We deote this trasmissio as tr2. With the assumptio of saturatio traffic coditios, the AP always has a DATA packet to sed to ay oe of the STAs. As the chael is reserved for the ACK trasmissio from the AP, this trasmissio (DATA with piggybacked ACK) is guarateed (assumig o hidde termials) ad there is o eed for cotetio. Thus the collisio probability i this case is 0. The trasmissio probability for the AP (P tr2(ap) ) i this case is the same as the probability of successful trasmissio by ay STA i the prior data trasfer stage, which is give by This trasmissio is successful oly if oe of the other STAs trasmit i the same slot. Thus, the probability of success give that the trasmissio has occurred is: τ (-τ ) P s(ap) = = (-τ ) (3) P P tr2(ap) = Probability AP ot trasmittig i the previous trasfer probability that oly oe STA trasmitted - = (-τ ) τ (-τ ) = τ (-τ ). (6) P s2(ap) = (7) The time for which the chael is busy (we have to cosider the time spet by the STAs as well) is give by: T s2 (AP) = O rts + {H + E p + + SIFS + H + E p + + SIFS + ACK + + DIFS} (8) We ow proceed with the calculatios for the STAs. STAs ca oly trasmit by cotedig ad wiig the chael; ad the probability that at least oe STA trasmits is give as P tr (STA) = [- (-τ ) ] (9) The probability of successful trasmissio give that there is trasmissio from a STA is the probability that the AP did ot trasmit ad oly oe STA trasmitted, - (-τ ) τ (-τ ) τ (-τ ) P s (STA) = = (0) (-τ ) P tr ( STA) Each time a STA udergoes a successful trasmissio, the AP piggybacks aother packet immediately ad hece the total time for successful trasmissio accouts for both the packet duratios ad is the same as T s2(ap) T s(sta) = T s2(ap). () Similarly, the time spet i a collisio i this case is same as the case whe the AP coteds for the chael. T c(sta) = T c(ap). (2) After we have examied the various cases of trasmissios ad their correspodig probabilities, we ow calculate the average time spet i each case. The time duratio i which the system is active is give by: 4 T total = = p ( i) T i i Where, p ( ) T = Probability of idle chael Slot duratio

9 Nagesh S. Nadiraju et al. 8 of 6 p ( 2) T = Probability of AP successfully trasmits T s(ap) 2 = P tr(ap) P s(ap) T s(ap) = τ (-τ ) Ts(AP) p ( 3) T = Probability of STA successfully trasmits T s(sta) 3 = c(ap) T AP (E ) = p τ. (4) Similarly, evaluatig the useful trasmissio time for the remaiig STAs we have, T (E ) = τ. (5) Let P tr deote the probability of at least oe trasmissio i a cosidered slot time ad P s deote a successful trasmissio. As the AP ad the STAs coted for the chael with equal privileges, the share of the AP i the system throughput is S /( + ) S /( + ) where S is the system throughput as derived by Biachi [2]: while that of the remaiig STAs is [ ] tr s p S = (9) + = [(-τ ) (-τ ) ] σ = (-τ ) σ = P tr(sta) P s(sta) T s(sta) = τ (-τ ) Ts(STA) It is importat to ote that the time the AP occupies the chael i case II (AP piggybacks) is accouted i the evet of a successful trasmissio by the STAs. p ( 4) T = Probability of collisio T 4 cl(ap) = (- Probability of a sigle or o trasmissio) T cl(ap) + [ ( τ ) ( + ) τ (-τ ) ] T Thus, + + T total = (-τ ) σ + τ (-τ ) T + s(ap) τ (-τ ) T + s(sta) [ ( τ ) ( + ) τ (-τ ) ] T (3) c(ap) For the AP, the time spet i useful trasmissio cosiderig both the cases i which trasmissios occur is give by: P tr(ap) P s( AP) E p + P tr2(ap) P s2(ap) E p = [ τ (-τ ) + τ (-τ ) ] Ep = [ (-τ ) ( + )] Ep P STAs p tr(sta) P s(sta) E p - = (-τ ) E p We fially calculate the AP s throughput usig (), (3) ad (4) as: [ τ (-τ ) ( + )] Ep SAP = (6) TTotal From (), (3) ad (5) the throughput of the STAs is give by: - τ (-τ ) Ep Sode =. (7) TTotal We ext evaluate the throughput for the STA ad AP for 802. DCF based o the model preseted i [2]. The time, T s, for which chael will be busy due to a successful trasmissio is same as T s(ap). Also the collisio time remais costat i both BDCF ad DCF. Thus, T s = T s(ap) ad T c = T c(ap) (8) (- P ) + P P T + ( P )P T tr P P E tr s s s tr c Figure 5(a-c) shows the umerical results of the throughput as a fuctio of umber of statios ad validate our aalysis through simulatios. We cosider Mbps chael ad other parameters are same as described i ext sectio. Figure 5(a) shows the obtaied throughput by AP ad STAs with DCF. Clearly, we ca observe a substatial ufairess betwee the throughputs of AP (dowlik flows) ad STAs (uplik flows). This is because of the CSMA/CA mechaism, which provides equal access privileges to the AP ad STAs. Thus the AP oly gets /(+) of the total available badwidth (with STAs ad a AP), while the STAs obtai a higher share (/(+)). O the other had, with the preferetial treatmet for AP i BDCF the AP achieves fair share of the badwidth (almost equal to that obtaied by the STAs).

10 Nagesh S. Nadiraju et al. 9 of 6 (a) Throughput compariso of AP ad STAs with DCF (b) Throughput compariso of AP with BDCF (c) Throughput compariso of STAs with BDCF Figure 5. Aalytical ad Simulatio throughput comparisos of BDCF 6. SIMULATION RESULTS I this sectio, we compare the performace of BDCF with DCF ad DCF+ [5]. Before we proceed to the aalysis, we briefly describe the workig of DCF+. I DCF+, a STA trasmits a DATA packet after reservig the chael via RTS-CTS hadshake as i DCF. Upo receivig a DATA packet, the destiatio STA checks its buffer for ay outstadig packet destied for the seder. If such a packet is foud, it attempts to reserve the chael for trasmittig this DATA packet. The reservatio is accomplished by modifyig the duratio field i the ACK packet, thus turig it ito a implicit RTS packet. The, the seder may respod with a CTS packet (if it agrees for the followig trasmissio) or discard it treatig as a regular ACK (if it does ot agree for the followig trasmissio). Origially desiged for ad hoc etworks, DCF+ also uses the piggybackig cocept, but applies this idea to every STA, i.e., it does ot give ay priority to the AP. We have implemeted BDCF ad DCF+ i the s-2 simulator (versio 2.26) [6]. For our simulatios, we have used the sceario described i Figure, where a AP is servig 0 statioary STAs. We placed the STAs such that all were i the trasmissio rage of each other, thus avoidig ay hidde termial problem. The badwidth of the wireless chael is set to 2 Mbps ad the AP is coected to a server through a wired lik with badwidth of 00 Mbps ad 2ms of propagatio delay. Other simulatio parameters are summarized i Table I. We have cosidered both symmetric ad asymmetric traffic patters: For symmetric traffic we cosidered 5 uplik ad 5 dowlik flows, deoted by 5Up-5D. I case of asymmetric traffic we agai cosidered two differet cases: 3 uplik flows ad 7 dowlik flows, deoted by 3Up-7D ad 7 uplik flows ad 3 dowlik flows, deoted by 7Up-3D. We assume that o two flows start/ed at the same ode. We have doe a comparative aalysis of the aggregate throughput, average ed to ed delay, ad per stream fairess of IEEE 802. DCF, BDCF ad DCF+ for the aforemetioed traffic patters. All results preseted here are averaged over 0 simulatio rus with differet seed values. Table I: Simulatio Parameters. Parameters Value Number of Nodes 0 Packet Size 000 bytes Simulatio Time 250s Trasmissio Rage 250m Badwidth 2 Mbps Node Placemet Fixed Radio Propagatio Model Two Ray Groud MAC Protocol IEEE 802. Routig Protocol DSDV Trasport/Applicatio Protocol UDP(CBR), TCP(FTP)

11 Nagesh S. Nadiraju et al. 0 of 6 6. Aalysis of UDP traffic I this sectio, we discuss ad aalyze the various performace metrics for UDP traffic. 6.. Aggregate Throughput We first compare the performace of the aggregate uplik/dowlik throughputs achieved by the three schemes (BDCF, DCF ad DCF+). We vary the DATA rate of the CBR traffic ruig over UDP from 00Kbps to 800Kbps ad measure the aggregate throughput achieved by the flows i either directios (uplik ad dowlik). Cosiderig the two differet traffic patters metioed above, Figures 6(a-c) ad 6(d-f) show the aggregate uplik ad dowlik throughputs respectively. We observe a huge differece betwee the uplik ad dowlik throughputs whe DCF/DCF+ is employed. As show i Figure 6(a-c), at low loads all the three protocols have similar performace. This is because the chael is relatively free ad all odes ca trasmit their packets without much cotetio. With DCF/DCF+, as we icrease the traffic load, the aggregate uplik throughput rapidly icreases ad dowlik flows starve. Eve for margial icrease i the traffic load, with IEEE 802. DCF or DCF+, the uplik flows obtai a very high throughput ad completely domiate the access to the chael. Dowlik flows experiece severe cogestio resultig i drastic reductio of the aggregate dowlik throughput. Oe importat thig to ote from Figure 6(d) is that, till 200Kbps load, the throughput of dowlik traffic icreases as there is sufficiet badwidth to accommodate all uplik ad dowlik flows. O the other had, with BDCF the available badwidth is fairly distributed amog uplik ad dowlik traffic. The dowlik flows achieve a fair share of the total badwidth, without affectig the performace of uplik flows. This ca be attributed to the preferetial treatmet for the AP. AP obtais substatial chael share to accommodate all the dowlik flows ad thus limitig the uplik traffic from the odes. I DCF, the throughput of idividual uplik flows is early 4 times the throughput of idividual dowlik flows. However whe BDCF is employed, this sharp differece does ot occur. The throughput of idividual uplik flows is almost equal to ay of the idividual dowlik flows. The aggregate dowlik throughput is improved by early 300% whe compared with DCF/DCF+. Hece, by providig preferetial access to the AP, BDCF esures fair sharig of badwidth amog uplik ad dowlik flows Average ed-to-ed packet delay We ow aalyze the average ed-to-ed delay of the uplik/dowlik traffic. We observe that BDCF sigificatly reduces the average ed-to-ed delay of dowlik traffic without severely affectig the delay of uplik traffic. Figure 7(a-f) shows the average ed-to-ed delay of the data packets. The average ed-to-ed delay for both uplik ad dowlik flows icreases with the icrease i the data rate. With DCF/ DCF+, the AP drops a large umber of packets belogig to the dowlik traffic. The packets that are trasmitted by the AP experiece similar delays as experieced by the packets of uplik flows. Thus the edto-ed delay of dowlik traffic does ot reflect the true problem with DCF. As ca be see from Figure 7(a-c), BDCF slightly icreases the delay for the uplik flows. This is because the AP takes the chael more frequetly ad thus the STAs have to wait for a relatively loger time i BDCF, to trasmit their packets. O the other had, the average ed-to-ed delay for the dowlik traffic is sigificatly reduced (see Figure 6(d-f)). Dowlik flows experiece substatially smaller ed-to-ed delays (by a factor of 2 to 3), sice the AP trasmits the packets headig for the STAs more ofte. Although there is slight icrease i the delay for uplik flows, this icrease is acceptable cosiderig the icreased trasmissio by the AP. Furthermore, these results are ecouragig as most of the traffic is expected to be i dowlik directio ad from the AP (e.g. , web browsig, dowloadig music etc.). The higher average ed-to-ed delay with DCF/DCF+ may result i higher dowload times ad cosequet user dissatisfactio Fairess Idex I this sectio we compare the fairess amog all the flows. We have used the Jai s fairess idex (f) [8] to measure the fairess amog the flows. It is give by: 2 xi i= f = i, 2 * xi i= Where, there are flows i the etwork ad x i is the throughput achieved by flow i. The fairess idex is always positive ad whe it approaches oe, it implies that all the flows are gettig equal share of the available badwidth. Whe the fairess idex drops or has egative slope, the it idicates that the available badwidth is ot fairly shared amog the flows. The fairess

12 Nagesh S. Nadiraju et al. of 6 idexes for asymmetric (3Up-7D) ad symmetric traffic (5Up-5D) cofiguratios are show i Figure 8(a) ad (b), respectively. The fairess idex is ear costat ad is close to oe with BDCF i both traffic cofiguratios. This is because all the flows obtai a fair share of the available badwidth. However, this is ot the case whe legacy DCF ad DCF+ are employed, as the dowstream flows achieve very low throughputs whe compared to the throughputs of uplik flows. (a) 3Up-7D (b) 5Up-5D (c) 3Up-7D (d) 3Up-7D (e) 5Up-5D (f) 5Up-5D Figure 6. Aggregate Throughput for UDP Traffic 6.2. Aalysis of TCP traffic I this sectio, we aalyze the performace of TCP traffic with differet traffic patters. Oce agai we observe similar ufairess amog the uplik/dowlik flows as illustrated i the experimetal results (preseted i sectio 3). As ca be oted from the previous sub-sectio, for UDP traffic flows, BDCF solves the ufairess problems by sigificatly improvig the performace of dowlik flows. I this sectio, we demostrate how BDCF overcomes the ufairess problem for TCP traffic as well. Furthermore we also otice that BDCF also achieves early 5% improvemet i the overall throughput of TCP Aggregate Throughput Figure 9 shows the throughput performace of TCP traffic for the differet traffic patters (asymmetric ad symmetric). From figures 9(a-c), we ca observe the disgraceful performace of the dowlik TCP traffic flows whe legacy 802. DCF is employed. I cotrast, whe BDCF is employed, we observe fair sharig of the available badwidth amog uplik ad dowlik flows for all traffic patters. For istace, with BDCF i the asymmetric traffic (3Up-7D) sceario, the dowlik flows achieve Kbps (20 Kbps per flow); while the uplik flows obtai aroud 389 Kbps (29 Kbps per flow). However, with DCF ad asymmetric traffic (3Up-7D), the dowlik flows get oly a meager 60 Kbps (aroud 87 Kbps per flow), while the aggregate uplik throughput (Figure 9(b)) is as high as 470 Kbps (aroud 57 Kbps per flow). The poor performace of TCP with DCF ca be explaied as follows. TCP geerates bi-directioal traffic. TCP ACKs of the uplik flows ad TCP DATA packets of the dowlik flows compete for the limited chael share of the AP. This icreased dowlik traffic load at the AP leads to overflowig of the lik layer queues at the AP ad resultig i excessive packet drops. TCP cogestio cotrol algorithm further worses the situatio by reducig the cogestio widow of a flow whe it detects a packet drop. If a TCP DATA packet of a dowlik flow is dropped at the AP, timeout occurs at the source ad hece the cogestio widow is decreased, leadig to a lower throughput. However, if a TCP ACK of a uplik

13 Nagesh S. Nadiraju et al. 2 of 6 flow is dropped, due to the cumulative ature of TCP ACKs, evetually aother ACK with a higher sequece umber will reach the uplik flow source ad the TCP cogestio widow will ot be reduced. Thus the dowlik traffic experieces severe cogestio cotrol while the uplik traffic is ot affected at the same rate. Cosequetly the uplik flows ejoy higher et throughputs whe compared to dowlik flows. (a) 3Up-7D (b) 5Up-5D (c) 7Up-3D (d) 3Up-7D (e) 5Up-5D (f) 7Up-3D Figure 7. Average Ed to Ed Delay for UDP Traffic Per Flow Fairess BDCF DCF DCF+ u3-d7 u5-d5 u7-d3 8(a) UDP Traffic (3Up-7D) (b) UDP Traffic(5Up-5D) (c) TCP Traffic Figure 8. Fairess Idex I summary, dowlik flows are ot oly affected by the limited chael availability for the AP, but also due to TCP- DATA packet drops at the AP. We carefully observed the cogestio widow growth of all flows ad otice that the cogestio widow of dowlik flows experiece frequet cut dow due to packet drops at the AP (ad thereby leadig to timeouts). I cotrast, the cogestio widow for all the uplik flows grows cotiuously. Similar observatio is also reported by the authors i [4]. Oce agai BDCF overcomes this problem by givig a preferetial treatmet to the AP. Wheever a packet from the STAs is received by the AP; it ca immediately trasmit a packet (recall from sectio 4) from its buffer without ay eed for chael

14 Nagesh S. Nadiraju et al. 3 of 6 cotetio. I this way, at high traffic loads, the AP ca avoid large queues ad overflows by immediately trasmittig either the TCP-ACKs/TCP-DATA packets. As more ad more dowstream DATA packets are trasmitted the dowlik flows achieve acceptable throughputs without affectig the uplik flows. Throughput (Kbps) Uplik Dowlik Throughput (Kbps) Uplik Dowlik Throughput (Kbps) Uplik Dowlik 0 BDCF DCF DCF+ 0 BDCF DCF DCF+ 0 BDCF DCF DCF+ (a) Asymmetric traffic (U3-D7) (b) Symmetric traffic (U5-D5) (c) Asymmetric traffic (U7-D3) Figure 9. TCP Aggregate Throughput Fairess Idex As show i Figure 8(c), BDCF maitais high fairess idex with TCP traffic as well. It is iterestig to ote that DCF performs better i the case of TCP traffic as compared to the UDP traffic, but it still has a low fairess idex whe compared with BDCF. With BDCF, all flows achieve uiform throughput which is idicated by the high fairess idex. I the case of symmetric traffic (5Up-5D), the fairess idex for DCF is just 0.9 while our scheme still maitais a fairess idex close to (i.e. all flows get equal share of the chael badwidth). DCF+ works better tha DCF for TCP traffic. 7. RELATED WORK Fairess provisioig i wireless etworks has bee a attractive area of research ad has bee explored at various layers. Most of the research addresses the problem of ufairess observed i the upper layers. A large amout of work aalyses the iteractio of TCP with the IEEE MAC layer [0][2][3]. Fair amout of research also reports the ufairess of IEEE 802. i multi-hop wireless etworks. I particular, Xu et al. [7] demostrated the istable behavior of TCP i ad hoc etworks ad the ufairess effect o the selectio of the TCP cogestio widow. The ufairess problem betwee uplik ad dowlik TCP flows i a AP based WLAN was iitially reported by Ramjee et al. [2].They studied the iteractio betwee TCP ad IEEE 802. MAC protocol ad idetified the buffer size at the AP as the cause for ufairess, the they proposed receiver widow size maipulatio i the TCP ACK to gover the access of wireless lik. Bottigliego et al. [3] studied the ufairess due to chael uavailability. They aalyzed the chael coditios betwee the STA ad the AP, choosig the STA with best chael coditio for trasmissio ad compesatig the other STAs later with a burst trasmissio. Yag et al. [6] studied the ufairess betwee a sigle uplik ad dowlik flow i multi-hop etworks ad suggested a rate limitig i the MAC layer o to decrease the ufairess amog the flows. However, their scheme reduces the throughput achieved. The cocept of reverse chael reservatio i WLANs has bee first itroduced i the DCF+ scheme [5]. The goal of DCF+ is to improve the performace of TCP i WLANs (usig DCF) by a implicit reservatio of the chael. However, as our simulatio results have idicated, DCF+ also has the ufairess problem because the implicit reservatio is used by ay STA. Recetly, attempts to improve performace of TCP i multi-hop ad hoc etworks by similar techiques were proposed i [3] ad [5]. I [3], the authors try to reduce itra ad iter flow cotetio. Itra flow cotetio is ecoutered betwee TCP-DATA ad TCP-ACK packets of a sigle TCP flow while Iter-flow cotetio is experieced betwee TCP-DATA packets of differet TCP flows. They propose two schemes Quick exchage, i which the seder reserves the reverse chael for the TCP-ACK packets ad Fast-forward, i which the seder allows the receiver to forward the curretly received packet towards the destiatio. Kuag et al. [5] proposed a multi-chael MAC ad reverse chael reservatio for mitigatig the DATA-DATA ad DATA-ACK cotetios. However they have ot addressed the fairess problem for the ifrastructure

15 Nagesh S. Nadiraju et al. 4 of 6 based etworks. Moreover, it should also be oted that the above schemes are desiged for MANETs. Furthermore, this ability to seak a packet is eabled for ay receivig STA that has a packet to trasmit. Hece, whe applied to AP based WLANs, these schemes ca ot distiguish betwee a AP ad other STAs ad will ot be able provide a preferetial treatmet to AP packets oly. Piggybackig, implicit ACK ad IFS based preferetial access are also preset i PCF. I PCF, the PC waits for a small PIFS time period after the chael is sesed idle before pollig a STA for trasmissio. I this way, it gets guarateed chael access before other STAs. However, PCF is complex to implemet ad it has ot bee used i practice. O the other had, we observe that our very simple ehacemet to the DCF ca provide substatial fairess improvemet. It is also worth to metio the IEEE 802.e stadard amedmet, which is supposed to provide QoS support for WLANs, ad is expected for the ed of this year. I IEEE 802.e, several traffic classes are defied ad the AP ca give differet access priority to differet classes. 802.e icludes more sophisticated facilities to support QoS compared with legacy 802., such as call admissio cotrol ad maximum trasmissio opportuity provisios. Clearly, BDCF ca ot provide QoS support, but its basic priciple ca still be applied i the case where all STAs use the best effort service provided by 802.e. 8. CONCLUSION The log term MAC layer fairess idea of IEEE 802. DCF does ot guaratee trasport layer fairess amog uplik ad dowlik flows. Cosequetly, the dowlik flows suffer from very low throughputs ad higher ed-to-ed delays. O the other had, the uplik flows ejoy relatively higher throughput ad low ed to ed delays. The mai cotributios of our paper are three fold: We experimetally demostrate the existig ufairess problem i typical WLAN hotpots. We propose a simple ehacemet to DCF for overcomig the ufairess problem. We develop aalytical models to evaluate the throughputs of AP ad STAs ad verify these models through extesive simulatio study. Our proposed BDCF protocol eables the AP to access the chael more frequetly by gratig a preferetial treatmet. I additio to this, our protocol also reduces the time wasted i chael cotetio ad backoff mechaism at the MAC layer. BDCF grats fair share of the badwidth to the dowlik flows ad avoids the moopoly of uplik flows. We otice that the aalytical results fid i good agreemet with the simulatio results. Comprehesive simulatios are also coducted o various traffic patters for both TCP ad UDP traffic. It has bee demostrated that BDCF successfully solves the ufairess problem alog with substatial improvemets i the throughputs ad reductio i the ed-to-ed delays of the dowward traffic (aroud 300%). REFERENCES [] IEEE Std IEEE Stadard for Wireless LAN Medium Access Cotrol (MAC) ad Physical Layer (PHY) Specificatio, ISO/IEC 8802-:999 (E), Aug., 999. [2] S. Pilosof, R. Ramjee, D. Raz, Y. Shavitt, ad P. Siha, Uderstadig TCP Fairess over Wireless LAN, IEEE INFOCOM, vol. 2, pp , April [3] M. Bottigliego, C. Casetti, C. F. Chiasserii ad M. Meo, Short-term Fairess for TCP Flows i 802.b WLANs, IEEE INFOCOM [4] I. Aad ad C. Casette, Differetiatig mechaisms i IEEE 802., IEEE INFOCOM 200. [5] H. Wu, Y. Peg, K. Log, S. Cheg, ad J. Ma, Performace of Reliable Trasport Protocol over IEEE 802. Wireless LAN: Aalysis ad Ehacemet, IEEE INFOCOM, vol. 2, pp , Jue [6] UCB/LBNL/VINT Network Simulator (s-2), Available at [7] V. Bharghava et al., "MACAW: A Media Access Protocol for Wireless LANs," Proc. ACM SIGCOMM '94, vol. 24, o. 4, 994, pp [8] R. Jai, The Art of Computer Systems Performace Aalysis, Joh Wiley ad Sos, 99. [9] H. Balakrisha ad V. Padmaabha, How Network Asymmetry affects TCP performace, IEEE commuicatio magazie, 60-67, April 200. [0] S. Xu ad T. Saddawi, Does the IEEE 802. MAC protocol work well i multihop wireless ad hoc etworks?, IEEE commuicatio magazie, 39(6):30-37, Jue 200. [] S. Lu, V. Bharghava, ad R. Srikat, Fair schedulig i wireless packet etworks, ACM Sigcomm 97, Caes, Fracem Sep 997.

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