IEEE Distributed Coordination Function(DCF): Analysis and Enhancement *

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1 IEEE 802. Dstrbuted Coordnaton Functon(DCF): Analyss and Enhanceent * Hatao Wu, Shduan Cheng, Yong Peng, Keng Long, Jan Ma 2 Natonal Key Lab of Swtchng Technology and Telecouncaton Networks, P.O.Box 206, Bejng Unversty of Posts & Telecouncatons, Bejng 00876, P.R.Chna 2 Noka Chna R&D Center, Noka House, No., He Png L Dong Je, Bejng, 0003, P.R.Chna Abstract- Beng a art of IEEE roject 802, the 802. Medu Access Control(MAC) s used to suort asynchronous and te bounded delvery of rado data ackets. It s roosed that a Dstrbuted Coordnaton Functon(DCF), whch uses Carrer Sense Multle Access wth Collson Avodance (CSMA/CA) and bnary slotted exonental backoff, be the bass of the IEEE 802. WLAN MAC rotocols. Ths aer rooses a throughut enhanceent echans for DCF by adjustng the Contenton Wndow(CW) resettng schee. Moreover, an analytcal odel based on Markov chan s ntroduced to coute the enhanced throughut of 802. DCF. The accuracy of the odel and the enhanceent of the roosed schee are verfed by elaborate sulatons. Key Words: IEEE 802., CSMA/CA, DCF, analyss I. INTRODUCTION Recently, there s an ncreasng need towards ortable and oble couters or workstatons wth the develoent wreless acket coutng technology. Wreless networks need to rovde councatons between oble ternals, oreover, hgh seed access to backbone networks needs to be rovded too. Wreless Local Area Networks(WLANs)[-4], whch rovdes hgher flexblty and convenence than ther wred counter art, are beng develoed to rovde hgh bandwdth access for users n a lted geograhcal area. IEEE Project 802 recoends an nternatonal standard 802.[-2] for WLANs. The standards ncludes detaled secfcatons both for Medu Access Control(MAC) and Physcal Layer(PHY). In WLANs, the hyscal eda, whch s shared by all statons and has lted connecton range, has sgnfcant dfferences when coared to wred eda. The IEEE 802. standards nclude Dstrbuted Coordnaton Functon(DCF) and otonal Pont Coordnaton Functon(PCF). In 802., the DCF s the fundaental access ethod used to suort asynchronous data transfer on a best effort bass. As secfed n the standards, the DCF ust be suorted by all the statons n a basc servce set(bss). The DCF s based on Carrer Sense Multle Access wth Collson Avodance (CSMA/CA). CSMA/CD s not used because a staton s unable to lsten to the channel whle transttng. In 802. CS s erfored both at hyscal layer, whch s also referred to as hyscal carrer sensng, and at the MAC layer, whch s known as vrtual carrer sensng. The PCF n the 802. s a ollng-based rotocol, whch s desgned to suort collson *Ths work s suorted by Noka-BUPT roject free and real te servces. There are two technques used for acket transttng n DCF. The default one s a two-way handshakng echans, also known as basc access ethod. A ostve MAC acknowledgeent(ack) s transtted by the destnaton staton to sgnal the successful acket transsson. The other otonal one s a four-way handshakng echans, whch uses request-to-send/clear-to-send(rts/cts) technque to reserve the channel before data transsson. Ths technque has been ntroduced to reduce the erforance degradaton due to hdden ternal. However, the drawback of RTS/CTS echans s ncreased overhead for short data fraes. In DCF, a bnary slotted exonental backoff s used wth CSMA/CA. Whenever a backoff occurs, the backoff te s set fro a unfor dstrbuton over the nterval [0,CW], whle the Contenton Wndow(CW) wll be doubled for a retry and reset for a new acket. Ths aer rooses a lttle bt conservatve way by resettng the CW to rove the throughut erforance of DCF. To analyze resettng CW, the odelng of 802. has been exaned. Paer [5] gves the theoretcal throughut lt of 802. based on a -ersstent varant. However, t doesn t take the effect of the Contenton Wndow(CW) and bnary slotted exonental back-off rocedure used by DCF nto consderaton. Unlke those ones, Paer [6,7] use Markov rocess to analyze the saturated throughut of The Markov odel n aer[7] can be regarded as an extenson of the odel n aer[6] and t takes the retranssson lt of MAC fraes nto account. Ths aer analyzes the effect of roosed CW resettng schee by a new Markov chan odel, whch s valdated by elaborate sulatons. The aer s organzed as followng. Secton II brefly descrbes the DCF of IEEE 802. MAC rotocols. In secton III, the new CW resettng schee s ntroduced and an analytcal odel s roosed. Secton IV valdates the accuracy of ths odel by sulatons, and the odel s used for throughut erforance analyss. Fnally, secton V concludes the aer. II. DISTRIBUTED COORDINATION FUNCTION IN 802. The basc servce set(bss) s the fundaental buldng block of IEEE 802. archtecture. The geograhcal area covered by the BSS s known as the basc servce area(bsa), whch s slar to a cell n a cellular network /02/$7.00 (C) 2002 IEEE

2 suorts both the ad hoc network and nfrastructure network archtecture. Ths aer only gves a bref ntroducton of 802. DCF, the readers are referred to [,2] for detaled nforaton about It conssts of both a basc access ethod and an otonal RTS/CTS access ethod. A. The basc access ethod In 802., rorty access to the wreless edu s controlled by the use of nter-frae sace(ifs) te between the transsson of fraes. Totally three IFS ntervals have been secfed by 802. standard: short IFS(SIFS), ont coordnaton functon IFS(PIFS), and DCF-IFS(DIFS). The SIFS s the sallest and the DIFS s the largest. The staton ay roceed wth ts transsson f the edu s sensed to be dle for an nterval larger than the Dstrbuted Inter Frae Sace(DIFS). If the edu s busy, the staton defers untl after a DIFS s detected and then generate a rando back-off erod before transttng. The back-off ter counter s decreased as long as the channel s sensed dle, frozen when the channel s sensed busy, and resued when the channel s sensed dle agan for ore than a DIFS. A staton can ntate a transsson when the backoff ter reaches zero. The back-off te s unforly chosen n the range (0, w-). Also (w-) s known as Contenton Wndow(CW), whch s an nteger wth the range deterned by the PHY characterstcs CW n and CW ax. After each unsuccessful transsson, w s doubled, u to value 2 W, where W equals to (CW n +) and 2 W equals to (CW ax +). Uon receved a acket correctly, the destnaton staton wats for a SIFS nterval edately followng the receton of the data frae and transts a MAC ACK back to the source staton, ndcatng that the data frae has been receved correctly. In case the source staton does not receve an ACK, the data frae s assued to be lost and the source staton schedules retranssson wth the CW doubled. When the data frae s transtted, all the other statons hearng the data frae adjust ther Network Allocaton Vector(NAV), whch s used for vrtual CS at the MAC layer, based on the duraton feld value n the data frae, whch ncludes the SIFS and the ACK followng the data frae. B. The RTS/CTS access ethod In 802.[], DCF also rovdes an otonal way of transttng data fraes that nvolve transsson of secal short RTS and CTS fraes ror to the transsson of actual data frae. When the destnaton receves the RTS frae, t wll transt a CTS frae after SIFS nterval edately followng the receton of the RTS frae. The source staton s allowed to transt ts acket only f t receves the CTS correctly. If a collson occurs wth RTS fraes, less bandwdth s wasted when coared wth the stuaton where larger data fraes are n collson. III. DCF PERFORMANCE ENHANCEMENT AND ANALYSIS In ths aer, we focus on the saturated throughut, whch s also exaned n aer [6,7]. It s a fundaental erforance fgure defned as the throughut lt reached by the syste as the offered load ncreases, and t reresents the load that the syste can carry n stable condtons. A. New backoff schee to reset the CW In 802. DCF, a bnary slotted exonental backoff s used wth CSMA/CA. Whenever a backoff occurs; the backoff te s set fro a unfor dstrbuton over the nterval [0, w-]. After each unsuccessful transsson, w s doubled, u to a axu value 2 W, where W equals to (CW n +) and 2 W equals to (CW ax +). Once the CW reaches CW ax, t wll rean at the value of CW ax untl t s reset. The CW wll be reset to CW n after every successful attet of transsson of a data frae or a RTS frae, or where a retry counter reaches ts lt, whose value s dfferent for data and RTS frae n 802.,.e., 5 and 7[2]. Snce a staton use CW to control the backoff wndow of acket transsson, the otal settng of CW wll affect the erforance of DCF. The backoff rocedure of CW can be consdered as a rogress to robe the otal value of CW, however, the resettng schee n 802. breaks ths rogress, whch wll degrades the erforance of DCF. Then the queston s how to control the backoff counter, or CW otally or near otally. Ths aer rooses a sle change for 802. n order to ake the backoff counter oscllate around the otal value wthout colex calculaton and run-te estaton as n aer [5]. The way s conservatve than that used n It can be descrbed as followng: )Whenever the retry counter reaches the lt, the CW s ket and not reset; 2)After a successful transsson, w s set to the value ax[w/2, CW n +]; 3)Whenever a transsson for fals, w s set to the value n[2w, CW ax +]. It wll be referred as new backoff schee through out ths aer. Another contrbuton of ths aer s the analytcal evaluaton of the saturated throughut of the new backoff schee, n the assuton of deal channel condtons. Ths aer does not choose the way n aer [5], whch uses a - ersstent varant that does not cature the effect of bnary exonental settng of CW. Ths aer uses Markov chan to analyze the effect of new backoff schee. A new Markov chan s roosed, whch dfferentates t fro The analyss ncludes two arts: )Wth a Markov chan, the behavor of a staton s exaned, whch we use to get the statonary robablty τ that the staton transt a acket; 2)The throughut of both basc and RTS/CTS access ethods s exaned. B. Markov Chan Model We use the sae assuton n aer [6,7] for the analyss. The contendng statons s suosed to a fxed nuber, n. Let b(t) be the stochastc rocess reresentng the back-off wndow sze for a gven staton at slot te t. Note the slot te s referred to as the constant value σ and the varable te nterval between two consecutve backoff te counter decreents[6]. As n aer [6,7], the key aroxaton n /02/$7.00 (C) 2002 IEEE

3 b 2( 2 x)( x) + + W( (2 x) )( x) + ( 2 x)( x ) = 2( 2 x)( x) W x x + x x + W x x x ' + + ' ' + ' ( (2 ) )( ) ( 2 )( ) 2 ( 2 )( ) ' > ' (9) ths odel s that the robablty that a transtted acket colldes s ndeendent on the state s(t) of the staton. Thus, the b-densonal rocess {s(t), b(t)} of our roosed schee s a dscrete-te Markov chan, whch s shown n Fg.. - 0, 0,2 -,0,,2 -,0,,2 0,W 0-2 0,W 0 - -,0 -, -,2 -,W ,W - -,W -2,W -,W -2,W - Fg. Markov chan odel of new back-off wndow schee Ths aer use all the araeters assgned for Drect Sequence Sread Sectru(DSSS) PHY n 802.; for other PHY layers, the analyss rocess s slar. In DSSS, CW n and CW ax equal to 3 and 023. Therefore, we have W = 2 W ' () ' W = 2 W > ' where W=(CW n +), and 2 ' W = ( CWax + ), so for DSSS, we have ' = 5. Here we use to reresent axu backoff stage. As secfed n 802.[], ths value could be larger than, whle the CW wll be hold after that, whch s shown s equaton (). In fact, here also eans the axu retranssson count, whch s dfferent for data frae and RTS frae,.e., 5 and 7. In ths aer, the backoff schee to reset the CW s dfferent fro that n 802. DCF, so the Markov chan n ths aer and that n aer [7] are dfferent. In ths Markov chan, the only non-null one-ste transton robabltes are Pk {, k, + } = k [0, W 2] [0, ] P {, k,0} = ( )/ W k [0, W 0 ] [, ] (2) Pk {,,0} = / W k [0, W ] [, ] Pk {,,0} = W / k [0, W ] P{0, k 0, 0} = ( ) / W0 k [0, W0 ] These transton robabltes account, resectvely, for: )the decreents of the backoff ter; 2)after a successful We adot the sae short notaton used n aer [0]: P{, k 0, k 0 }=P{s(t+) =, b(t+) = k s(t) = 0, b(t) = k 0 } transsson at backoff stage, the backoff ter of the new acket starts wth a backoff stage (-); 3)an unsuccessful transsson akes the backoff stages ncrease; 4)at the the axu backoff stage, the CW wll kee the sae value f transsson s unsuccessful; 5)at backoff stage 0, t wll stay on the sae stage f the transsson s successful. Let b,k be the statonary dstrbuton of the Markov chan. Frst note that b,0 = ( ) b 0 < (3),0 we have b,0 = xb 0 (4) where x= /( ) (5) Snce the chan s regular, so for each k (0, W ), we have ( )( b,0 + b ) = 0 W k (6) bk, = b,0 + ( ) b 0 < < +,0 W b (,0 + b,0) = Wth (4)(5) and transtons n the chan, equaton (6) can be slfed as W k b = b k,,0 W 0 (7) Therefore, by usng the noralzaton condton for statonary dstrbuton, we have W W W k W + (8) = bk, = b,0 = b,0 = 0 k= 0 = 0 k= 0 W = 0 2 Usng equaton ()(4)(5)(8), we have equaton (9), whch s shown at the to of ths age. Now the robablty τ that a staton transts n a randoly chosen slot te can be exressed as, x τ = = + b,0 b = 0 x (0) where b can be obtaned fro equaton (9) and x s fro equaton (5). In the statonary state, a staton transts a acket wth robablty τ, so we have τ Therefore, equatons (5)(9)(0) and () reresent a nonlnear syste n the two unknowns τ and, whch can be solved by nuercal results. Note that we ust have = ( ) n () (0,) and τ (0,) Snce the Markov chan transtons n Fg. are dfferent fro those n aer [7], the results obtaned for b s dfferent fro that n aer [7], so do τ and. In fact, for the sae other araeters, the value of τ and are saller than those n aer [7] because of the new backoff schee used. C. Throughut Analyss Let P tr be the robablty that there s at least one transsson n the consdered slot te. And let P s be the /02/$7.00 (C) 2002 IEEE

4 robablty that a transsson s successful, gven the robablty P tr. So we have P ( ) n tr = τ (2) n n nτ( τ) nτ( τ) (3) Ps = = n Ptr ( τ ) Now we are able to exress the noralzed syste throughut S as the rato, E[Payload Inforaton n a slot te] S = E[Length of a slot te] (4) PP s tre[ P] = ( P ) σ + PPT + ( P) PT tr s tr s s tr c where we use the sae sybols as those n aer [6]. Here, T s and T c are the average te the channel s sensed busy because of a successful transsson or a collson. The E[P] s the average acket length and σ s the duraton of an ety slot te. Let acket header be H = PHY hdr + MAC hdr and let roagaton be δ. Then we ust have the followng exresson. bas Ts = H + E[ P] + SIFS + δ + ACK + DIFS+ δ (5) bas Tc = H + E[ P*] + SIFS + δ + ACK + DIFS + δ where bas eans basc access ethod and E[P*] s the average length of the longest acket ayload nvolved n a collson. In all our cases, all the ackets have the sae fxed sze, therefore, we have E[P]=E[P*]=P. For the RTS/CTS access ethod, assung that all the staton use the RTS/CTS for the data frae for slcty, then we have rts Ts = RTS + SIFS+ δ + CTS+ SIFS+ δ + H (6) + E[ P] + SIFS + δ + ACK + DIFS + δ rts Tc = RTS + SIFS+ δ + CTS+ DIFS+ δ where rts eans RTS/CTS access ethod. Note that here we suose collson only occurs between RTS fraes. IV. MODEL VALIDATION Ths aer uses the well-known sulaton tool NS-2 [8] fro Lawrence Berkeley Natonal Laboratory. To valdate our odel, we also coare the results of the new backoff schee wth those of 802. n aer [7]. Also, ths aer assues each staton has enough data to send to obtan the saturated throughut erforance of the new backoff schee. All the araeters used n analytcal odel and our sulatons follow the secfcatons n standard[] for DSSS, and are the sae as those n [7]. Note that we assue the alcaton data ayload s 000bytes and IP header and UDP header are 20 and 8 bytes, so acket ayload at MAC layer s 028bytes. Note our MAC Markov odel equatons are ndeendent of the PHY araeters, so t does not atter when choosng araeters for dfferent PHY layers. A. Sulaton results for basc access ethod Frst we see the results of basc access ethod, whch s shown n Fg.2. Here we use 802. to reresent the odel n aer [7] for 802. standards and New backoff to reresent the odel n ths aer for new CW settng schee. For a fxed nuber of statons, we run 0 sulatons wth dfferent rando seed. Each sybol + or reresents a sulaton result, for 802. and new CW settng schee resectvely. Note for soe sulaton seres, soe sybols are suerosed because those results are dentcal or very close to each other. Fro the fgure we are able to see that the analytcal odel of ths aer s accurate and catures the trend exactly. The odel n aer [7] also accurate to trace the results of 802. because t follows the standards closely both n the Markov chan transtons and n the throughut analyss. Our analyss closely odels 802. and new backoff schee esecally when the nuber of statons s over dozens, whch follows our assuton that the Markov chan s fored,.e., that the robablty that a transtted acket colldes s ndeendent of the state s(t) of the staton s accurate when the nuber of statons s large. We also see that the analyss for the new CW settng a lttle bt overestates the sulaton results. Ths s because n sulatons, there exsts the transsson of routng ackets, whch are send by broadcast and CW wll be reset at the sae te, therefore, t wll break the CW resettng schee roosed n ths aer when the nuber of statons s very hgh. Also we can see that the CW resettng schee n ths aer could enhance the erforance of 802. DCF sgnfcant, whch s roved by both analyss and sulaton results. Snce the schee s rather sle and effectve, and can be regarded as an oton for 802., t can rove the stablty of syste esecally when the nuber of statons s hgh. Saturaton Throughut Nuber of Statons 802. sulaton 802. calculaton New backoff sulaton New backoff calculaton Fg.2 Analyss versus sulatons: basc access ethod B. Sulaton results for RTS/CTS access ethod The results coarson of RTS/CTS access ethod s slar to that of basc ethod. Note that n Fg.3, the vertcal axs scale s dfferent to that n Fg.2, so the reader ay feel /02/$7.00 (C) 2002 IEEE

5 that the analyss s uch hgher than sulatons results. Another reason for the analyss overestates the sulaton results s that n our sulatons, the routng ackets n broadcast ay cause collson wth other routng ackets or RTS frae, therefore, the sulaton erforance s degraded. Fro ths fgure, we are able to conclude that RTS/CTS access ethod s useful to coensate the erforance degradaton due to collson, whose robablty ncreases wth the nuber of statons. Note that we can get these results because n ths aer the acket ayload length, 028bytes, s large enough to coensate the overheads of RTS/CTS. Saturaton Throughut sulaton 802. calculaton New backoff sulaton New backoff calculaton Nuber of Statons Fg.3 Analyss versus sulatons: RTS/CTS access ethod Saturaton Throughut = New backoff New Backoff =3 =5 =7 =7 = Nuber of Statons Fg.4 Effect of for basc access ethod C. Perforance Analyss Ths aer uses the roosed Markov odel to analyze the throughut erforance wth dfferent araeters. As we know fro equaton (9), the value of, and W wll affect the results we acheve. Snce the value of and W has been assgned value related to PHY n the standards, we wll exane the effect of n ths aer. The effect of s shown s Fg.4 and Fg.5, for basc and RTS/CTS access ethod resectvely. Fro these fgures, we can see the effectveness of our new backoff schee. Also, we fnd that can affect the erforance greatly. Note n the aer [2], there s =5, whch s for DSSS PHY. Fro the analyss we know that hgher brngs hgher stablty and ncreased delay for retransssons. Fro Fg.4 and Fg.5 we conclude that the value of 5 and 7 for basc and RTS/CTS access ethod s a good tradeoff. Saturaton Throughut New backoff New Backoff =3 =3 =5 =7 =5 = Nuber of Statons Fg.5 Effect of for RTS/CTS access ethod V. CONCLUSIONS Ths aer rooses a sle and effectve Contenton Wndow(CW) resettng schee to enhance the erforance of IEEE 802. DCF. A new and sle analytcal odel based on Markov chan s ntroduced to coute the throughut of the roosed schee. Ths odel can be used n coutaton both for the basc access ethod and the RTS/CTS access ethod. Coarsons wth sulaton results as well as the odel resented n aer [7] show that ths odel s accurate n redctng the erforance and rove the effectveness of the new backoff schee. REFERENCES [] IEEE standard for Wreless LAN Medu Access Control (MAC) and Physcal Layer (PHY) secfcatons, ISO/IEC 8802-:999(E), Aug. 999 [2] B.P. Crow, J.G. K, IEEE 802. Wreless Local Area Networks, IEEE Councatons agazne, Set. 997 [3] G. Anastas, L.Lenzn QoS rovded by IEEE 802. wreless LAN to advanced data alcatons: a sulaton analyss , Wreless Networks 6 (2000) [4] G.Banch. L.Fratta, M.Olver. Perforance Evaluaton and Enhanceent of the CSMA/CA MAC Protocol for 802. Wreless LANs. Proc. PIMRC , Oct.996 [5] F.Cal, M.Cont, E.Gregor. Dynac Tunng of the IEEE 802. Protocol to Acheve a Theoretcal Throughut Lt, IEEE/ACM Trans. On Networkng, V8, N6, Dec [6] G.Banch. Perforance Analyss of the IEEE 802. Dstrbuted Coordnaton Functon. IEEE Journal on Selected Area n Co. V8, N3, March 2000 [7] Hatao Wu, Yong Peng, Keng Long, Shduan Cheng, A sle odel of IEEE 802. Wreless LAN, Proceedngs of ICII 200,.54-59, Oct. 200 [8] NS, URL htt://www-ash.cs.berkeley.edu/ns /02/$7.00 (C) 2002 IEEE