, Downlink-firs Scheduling of Real-ime Voice raffic in IEEE 802.11 Wireless LANs Dong W. Jeong and Chae Y. Lee Deparmen of Indusrial Engineering, KAIS, 373-1 Kusung Dong, aejon, Korea el : +82-42-869-2916 FAX : +82-42-869-3110 Email : {wander, cylee}@mail.kais.ac.kr Absrac he IEEE 802.11 MAC (Media Access Conrol Proocol suppors wo modes of operaion, a random access mode for nonreal-ime daa applicaions processed by Disribued Coordinaed Funcion (DCF, and a polling mode for real-ime applicaions served by Poin Coordinaed Funcion (PCF. I is known ha he sandard IEEE 802.11 is insufficien o serve real-ime raffic. o provide Qualiy of Service (QoS of real-ime raffic, we propose he Downlink-firs scheduling wih Earlies Due Dae (EDD in Conenion Free Period (CFP wih suiable admission conrol. he capaciy and deadline violaion probabiliy of he proposed sysem is analyzed and compared o he sandard pair sysem of downlink and uplink. Analyical and simulaion resuls show ha he proposed scheme is remarkably efficien in view of he deadline violaion probabiliy. Keywords: WLAN, MAC, PCF, Downlink-firs scheduling, Deadline violaion probabiliy 1. Inroducion Wireless local area neworks (LAN have been growing in populariy, and many producs of wireless LAN have been commercially available. Wih hese backgrounds, he IEEE 802.11 commiee has developed a wireless LAN sandard o saisfy he needs of wireless access. he scope of he sandard is MAC (Media Access Conrol and physical layers. he firs sandard allows daa raes of up o 2Mbps in he 2.4GHz band. hen, he IEEE 802.11a and IEEE 802.11b commiees have developed wireless sandards for higher daa raes of up o 54Mbps in 5GHz band and 11Mbps in he 2.4GHz band, respecively. Furhermore, he IEEE 802.11e commiee is currenly
working o enhance he 802.11 MAC o expand suppor for applicaion wih QoS requiremens (Srinivas Kandala, 2002. ask group E of he IEEE 802.11 working group are currenly working on an exension o he IEEE 802.11 sandard called IEEE 802.11e. he goal of his exension is o enhance he access mechanisms ha can provide service differeniaion. All he deails have no ye been finalized, bu a new access mechanism called Enhanced DCF (EDCF, which is an exension of he basic DCF mechanism, and Hybrid Coordinaion Funcion (HCF have been seleced. Saions, which operae under he 802.11e is called QoS saions (QSAs. A QoS saion, which works as he cenralized conroller for all oher saions wihin he same Basic Service Uni (BSS, is called he Hybrid Coordinaor (HC. he HC will ypically reside wihin an 802.11e access poin (AP. A presen, he IEEE 802.11 sandard MAC proocol suppors wo kinds of access mehods: DCF and PCF. he DCF is designed for asynchronous daa ransmission by using CSMA/CA (Carrier Sense Muliple Access wih Collision Avoidance and mus be implemened in all saions. On he order hand, he PCF is inended for ransmission of real-ime raffic as well as ha of asynchronous daa raffic. his access mehod is opional and is based on polling conrolled by he AP. When boh DCF and PCF are used, he IEEE 802.11 sandard MAC is a hybrid proocol of random access and polling. In his case, a wireless channel is divided ino superframe consising of a CFP for he PCF and CP for he DCF. he performance of he DCF (H.S. Chhaya and Gupa, 1997 and he combined performance of he DCF and PCF (B.P. Crow, 1997 were evaluaed. Wih regard o he PCF, several raffic scheduling schemes o provide QoS were proposed, including Defici Round Robin (M. Shreedhar, 1996 and Disribued Defici Round Robin (R. Ranasinghe, 2001. However, i is hard o saisfy QoS requiremen wih simple round-robin scheme or fair queuing scheduling algorihm, because real-ime raffic generally requess o keep end-o-end delay bound. I is reasonable o assume ha real-ime raffic connecions are esablished wih saions in differen BSSs or DS (Disribued Sysem because he size of BSS is relaively small. In his paper, we focus on he real-ime voice raffic in PCF and propose Downlink-firs scheme in which all downlink raffics are processed earlier han he uplink raffics in CFP. he capaciy and he deadline violaion probabiliy are analyzed using order saisics and simple queuing model. Comparison of he performance of he proposed scheme and ha of he sandard is discussed. I is shown ha he proposed scheme is effecive in providing QoS of voice raffic. 2. Poin Coordinaor Funcion (PCF in IEEE 802.11 Sandard he PCF mode provides conenion-free frame ransfer and he ime period in which he LAN is operaed in he PCF mode is known as he CFP. he AP performs he funcion of he poin coordinaor by gaining conrol of he medium a he beginning of he CFP afer sensing he medium o be idle for PIFS period. During he CFP, CF_Pollable saions are polled by he AP. On receiving he poll he saion ransmis is daa afer a Shor Inerframe Space ( inerval.
Figure 1. Example of PCF ransfer in sandard sysem. he AP iniiaes he CFP by ransmiing a Beacon frame. If he raffic during he CFP is ligh and/or he AP has compleed polling all he saions on he polling lis, i ends he CFP by ransmiing a CF_End frame. A he nominal sar of he CFP, he poin coordinaor (PC senses he medium. If he medium remains idle for a PIFS inerval, he PC ransmis a beacon frame o iniiae he CFP. he PC sars CF ransmission a he inerval by sending a CF_Pollable, Daa, or Daa+CF-Poll frame. If a CF-aware saion receives a CF-Poll frame from he PC, he saion can respond o he PC afer a idle period, wih a CF_ or a Daa+CF_ frame. If he PC receives a Daa+CF_ frame from a saion, he PC can send a Daa+CF_+CF_Poll frame o a differen saion, where he CF_ porion of he frame is used o acknowledge receip of he previous daa frame. he abiliy o combine polling and acknowledgemen frames wih daa frames, ransmied beween saions and he PC, was designed o improve efficiency. If he PC ransmis a CF_Poll frame and he desinaion saion does no have a daa frame o ransmi, he saion sends Null Funcion frame back o he PC. Figure 1 illusraes he ransmission of frames beween he PC and a saion, and vice versa. If he PC fails o receive an for a ransmied daa frame, he PC wais a PIFS inerval and coninues ransmiing o he nex saion in he polling lis. 3. Downlink-firs Scheduling wih EDD and Admission Conrol In IEEE 802.11 sandard, he real-ime raffic is served by PCF and he downlink and uplink ransmission is performed as a pair for each connecion. In oher words, he down/up ransmission of a connecion is performed afer he paired ransmission of he previous connecion as shown in Figure 1. his may cause he serious downlink delay problem of he real-ime raffic in he wireless LAN. o improve he delay problem, he downlinkfirs ransmission is proposed. Afer he beacon frame in he CFP, he downlink ransmission of each connecion is performed firs. hen he uplink raffic is processed as shown in Figure 2. he EDD rule is applied o he downlink raffics o improve he delay problem. In he downlink service, he raffic direced o a saion is acknowledged from he saion by he indicaion afer a. When all of he downlink raffic ha belongs o a polling lis is served, hen he uplink raffic is served. For uplink raffic, he AP polls a saion using Poll or Poll+. hen he polled saion may send a daa frame o is desinaion. he uplink daa frame o he AP is hen acknowledged by he nex Poll+ frame ransmied afer one inerval.
PIFS Figure 2. Example of PCF ransfer in proposed sysem. Now, admission conrol is necessary o balance he real-ime and nonreal-ime raffic in wireless LAN. If excess real-ime raffic is admied, he hroughpu of each nonreal-ime saion is diminished. Also he ransmission delay of real-ime raffic is expeced. he objecive of admission conrol is o mainain a suiable number of real-ime downlink raffic such ha he deadline violaion probabiliy saisfies a cerain limi and o guaranee minimum hroughpu bound for nonreal-ime saions. In wireless LAN, he PC moniors he sae of sysem coninuously and checks he QoS requiremen of ongoing connecions. hus, he admission conrol algorihm applied o he coordinaion funcion will successfully enhance he QoS of real-ime and nonreal-ime raffics. For he admission conrol in he Wireless LAN, we consider he number of real-ime saions ha can be served in he CFP. o obain he maximum number of saions ha can be served in a CFP, he following noaions are employed wih regard o he ime inervals given in Figure 2. B : ransmission ime of beacon _ : CF_END frame CF Poll END inerval : downlink or uplink real-ime raffic frame wihou piggybacking : frame ransmission ime : ransmission ime of poll frame PA : ransmission ime of poll frame piggybacking CFP hen by leing _ is given by Max CFP _ Max = B N max be he maximum number of real ime raffics during he maximum duraion of CFP, + (2 + (2 PA + 4 Poll + 4 ( N max 1 CF _ END he hird erm in he equaion is for he firs scheduled downlink and uplink raffic and he forh erm is for oher raffics. hus, he max capaciy N max of real-ime raffic is obained as N max = CFP _ Max 2 ( B CF _ END PA + 4
Since we have he maximum real ime capaciy in a CFP, he following relaionship holds among raffics generaed N G and raffics ransferred N o he nex CFP ha exceed he capaciy. G N NG + N S Nmax if NG + N S > = 0 oherwise + 1 ( Nmax N is he raffics generaed during 1 superframe and N is he raffics ransferred from 1 o whose delay bound is no violaed a he sar of period. raffics whose delay bound is violaed is discarded. By giving prioriy o he ransferred raffics ha are wihin he delay bound he admission can be conrolled wih he raffic generaed N G. ha is, raffics generaed a superframe 1 are acceped as far as hey saisfy he following limi. N G N max N S S 4. Analysis of Sysem Capaciy and Deadline Violaion Probabiliy he proposed Downlink-firs wih EDD and he sandard paired sysem is compared in erms of capaciy and deadline violaion. 4.1. Capaciy Analysis o compare wo sysems, i is assumed ha all saions ha are acive and belong o he polling lis have he raffics o ransmi and receive. Wih regard o he sandard sysem given in Figure 1, he following noaions are addiionally employed. DAP : ransmission ime of downlink frame piggybacking and Poll UA : ransmission ime of uplink frame piggybacking DP : ransmission ime of downlink frame piggybacking Poll 1 Le CFP and CFP 2 be respecively he duraion of CFP a Downlink-firs sysem and sandard sysem. he difference of CFP in wo sysems is due o he ransmission ime of he uplink and downlink frames. Due o he piggybacked frame and Poll frame, CFP in sandard sysem is less han ha in he Downlink-firs sysem. By leing he number of real-ime raffics be N, he wo CFPs are given as follows. 1 CFP 2 CFP = = B B + (2 + (2 DP UA PA + 4 poll + 4 ( N 1 DAP UA CF _ END + ( + 2 + ( + 2 ( N 1 CF _ END By assuming =, =, and =, we 1 2 DP poll UA havecfp CFP = 2N. Noe ha he duraion o process a connecion ha consiss of uplink and downlink raffic is 2 + 2 DAP PA in he sandard sysem. hus, when he saved ime N PA 2 exceeds he duraion, one more connecion can be served in he sandard sysem. Considering he maximum daa rae of 11Mbps of he IEEE 802.11b sandard and 300-oce voice frame, we have = 218µ sec. By applying = 10µ sec, one more connecion can be processed when he number of real ime connecions N 25. Now,
from _ Max = ms, he maximum number of real ime connecions in he CFP is compued as 55 in he CFP 28 sandard and as 53 in he Downlink-firs sysem respecively. herefore, i can be concluded ha even if he sandard sysem has piggybacking efficiency of polling message, he capaciy difference for he real ime connecion is negligible. 4.2. Analysis of he Deadline Violaion Probabiliy For he analysis of he deadline violaion probabiliy he downlink raffics are generaed following he Poisson process wih he arrival rae λ. Each frame generaed is assumed o have uniformly disribued remaining due over [ Due min, Duemax ] a he poin coordinaor. Noe ha a frame wih is remaining due less han he lengh of a superframe may probably be discarded a he PC. We hus assume ha he Due min is equal o he lengh of a superframe. Le X i, i = 1, 2,..., NG be he random variable of remaining due of he i -h generaed frame o be scheduled a he PC. Noe ha Xi is i.i.d. uniform random variable over [ Due min, Duemax ]. Since he raffics are scheduled by EDD, le X ( j, j = 1, 2,..., NG be he j h smalles remaining due of he X 1, X 2,..., X G. ha is, X ( 1, X (2,..., given by X ( N G are he order saisics corresponding o X 1, X 2,..., X NG. hen he densiy funcion of X ( j is NG! ( j 1 ( NG j f( j ( x( j = [ F( x( j ] [1 F( x( j ] f ( x( j ( j 1!( NG j! Since we assume he Due min is equal o a superframe lengh, he deadline of a real-ime raffic may be violaed when he raffic is ransferred o he nex CFP. Le k, k = N G N +1,..., N G be he index of ransferred raffic, hen X (k is he random variable of he remaining due of he ransferred raffic. Accordingly, he raffic ransferred from previous superframe has he remaining due, Y (l given by Y( l = X Duemin, l = k N G + N. hus, he deadline violaion probabiliy of he l -h raffic is represened as P ( l ( ( l > Y( l, where (l is he scheduled ime of raffic l a he ransferred superframe. When he deadline of a frame is violaed, he raffic is discarded, and he following raffics are served. hus, he scheduled ime (l may be differen from he iniial schedule (l which is he scheduled ime for he l -h raffic before he frame is discarded due o deadline violaion. herefore, P ( l ( ( l > Y( l is represened as he condiional probabiliy. As an example, consider wo ransferred raffics o be served, (l is given by = ( 1 (1 ( 1 : if he firs frame is discarded ( 2 = ( 2 : else he deadline violaion probabiliy P > Y is given by P > Y = P( > ( 1 ( (1 (1 (1 Y(1 ( l ( ( l ( l P > Y = P( > Y > Y P( > Y + P( > Y < Y P( < ( 2 ( (2 (2 (1 (2 (1 (1 (1 (1 (2 (2 (1 (1 (1 Y(1 = P > Y, > Y + P( > Y, < ( ( 1 (2 (1 (1 (2 (2 (1 Y(1 By assuming each real-ime raffic has he same frame lengh, he iniial schedule (l by he EDD rule is
deermined as follows. Iniial schedule (l in he sandard sysem B, if l = 1 = + ( + 2, if l = 2 (l ( 1 DP UA + ( DAP UA + 2 ( l 2, if l 3 ( 2 Iniial schedule (l in he Downlink-firs sysem (l = + if l = 1 B + ( + 2 ( l 1, if l 2 ( 1 o obain he deadline violaion probabiliy he following probabiliy needs o be compued. P ( l ( ( l > Y ( l = P ( ( l > X Due min = ( l + Duemin 0 f ( x dx = ( l + Duemin 0 NG! ( k 1!( N G F( x k! ( k 1 (1 F( x ( NG k Now, from he deadline violaion probabiliy he expeced number of discarded frame in a superframe can be obained by NG P ( k > Y( k k= NG N + 1 (. Figure 3 shows he expeced number of discarded frames for each pair of (N G, N by he sandard sysem and he proposed Downlink-firs sysem. For fair comparison he EDD rule is also applied o he sandard sysem. From he figure i is clear ha more frames are discarded as he number of ransferred raffic N increases. Beer performance by he proposed Downlink-firs is illusraed compared o he sandard. f ( x dx 5. Simulaion resuls of he Real-ime raffic Scheduling he sysem parameers for simulaion are repored in able 1 as specified in he IEEE 802.11b sandard. o simplify he simulaion he propagaion delay, ransmission errors are no considered. ( N G, N Figure 3. Expeced number of discarded frames due o deadline violaion
able 1. Defaul aribue value from IEEE 802.11b sandard Aribue Symbol Value Channel rae CR 11Mbps frame size CF-End frame size Poll frame size Slo ime ime PIFS ime DIFS ime CR 14 oces _ End CR 20 oces CF Poll CR 20 oces S 20 µ s 10 µ s PIFS 30 µ s DIFS 50 µ s he superframe lengh is assumed o be 30 ms wih Duemax = 40ms and Duemin = 30ms. Main characerisics of he real-ime raffic are aken from G.723.1 proocol (D. Minoli e al, 1998. A each saion realime frames are generaed by following he Poisson process wih he arrival rae λ = 0.6 ~ 1.0/30msec. Figure 4 shows he expeced number of discarded frames in a superframe. he number of acive real-ime saions are given by N = 18, 19, 20 wih he sysem capaciy N max = 15. 120,000 superframes ha corresponds o 60 minues are simulaed boh for he sandard and he proposed Downlink-firs sysems. he EDD rule is also applied o he wo sysems. he increase of he number of discarded frames is far degraded by he proposed mehod ha assigns he downlink raffic in fron of he uplink in a superframe. he figure also shows ha he Figure 4. Performance of he Downlink-firs vs. Sandard
proposed admission sraegy effecively conrols new connecions. he expeced number of discarded frames by he oal saions converges o a limi even if he raffic arrival rae is increased. Per saion frame discarded rae is less han 1% wih he proposed sysem. he blocking probabiliies in he wo sysems are compared in Figure 5. he figure shows he performance wih N = 20 real-ime saions when sysem capaciy is fixed o N 15. he blocking probabiliy is max = obained by checking he number of raffics blocked by admission conrol among 120,000 superframes. From he figure i is clear ha no raffics are blocked when he generaed raffic capaciy N max. As he raffic exceeds he capaciy par of i is blocked by he admission conrol. Higher blocking λ N is less han he sysem probabiliy by he Downlink-firs well explains he reduced number of discarded frames as shown is Figure 4. Figure 5. Blocking probabiliies 6. Conclusion A Downlink-firs scheduling is proposed o reduce he delay of he real-ime raffic in he WLAN. he uplink raffics are scheduled afer he downlink in order of polling lis. Admission conrol algorihm is also suggesed such ha i saisfies boh he deadline violaion probabiliy for he real-ime connecions and he hroughpu for he nonreal-ime saions. he accepable number of downlink real-ime raffic is conrolled by he number of frames ransferred from he previous superframe and he maximum number of frames ha can be processed a a superframe. he proposed Downlink-firs wih EDD is compared o he sandard sysem by analyzing he sysem capaciy and he deadline violaion probabiliy. he analysis proves ha he proposed Downlink-firs wih EDD ouperforms he sandard. he number of discarded frames ha violae he deadline is dramaically reduced compared o he sandard sysem where he uplink and downlink ransmission is paired for each connecion. he same resul is
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