HIERARCHICAL SCHEDULING WITH ADAPTIVE WEIGHTS FOR W-ATM *

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1 Copyrght Notce c 1999 IEEE. Personal use of ths materal s permtted. However, permsson to reprnt/republsh ths materal for advertsng or promotonal purposes or for creatng new collectve wors for resale or redstrbuton to servers or lsts, or to reuse any copyrghted component of ths wor n other wors must be obtaned from the IEEE. Ths materal s presented to ensure tmely dssemnaton of scholarly and techncal wor. Copyrght and all rghts theren are retaned by authors or by other copyrght holders. All persons copyng ths nformaton are expected to adhere to the terms and constrants nvoed by each author s copyrght. In most cases, these wors may not be reposted wthout the explct permsson of the copyrght holder. Insttute of Communcaton Networs and Computer Engneerng Unversty of Stuttgart Pfaffenwaldrng 47, D Stuttgart, Germany Phone: , Fax: Emal: mal@r.un-stuttgart.de,

2 HIERARCHICAL SCHEDULING WITH ADAPTIVE WEIGHTS FOR W-ATM * Hu Huang, and Danny H. K. Tsang Dept. of Electrcal & Electronc Engneerng The Hong Kong Unversty of Scence & Technology Clear Water Bay, Kowloon, Hong Kong Rolf Sgle Unversty of Stuttgart, Insttute of Communcaton Networs & Computer Engneerng, Pfaffenwaldrng 47, D-7056 Stuttgart, Germany Damler-Benz AG, Communcaton Technology, Ulm, Germany Paul. J. Kuehn Unversty of Stuttgart, Insttute of Communcaton Networs & Computer Engneerng, Pfaffenwaldrng 47, D-7056 Stuttgart, Germany Abstract: Medum access control (MAC) protocol s one of the ey components for provdng qualty of servce (QoS) n wreless ATM (W-ATM) networs. In ths paper, we propose a herarchcal schedulng scheme coupled wth far queueng algorthms wth adaptve weghts. Ths scheme s ntended to be applcable to a TDMA/TDD based MAC protocol. Specfcally, the performance of the far-queueng algorthm usng fxed weghts and adaptve weghts are nvestgated. Smulaton results show that the proposed herarchcal far-queueng schedulng wth adaptve weghts (HAW) can yeld a lower cell transfer delay and hgher channel utlzaton whle mantanng farness among multple users. I. Introducton Wreless ATM (W-ATM) has been proposed as a potental soluton for supportng future wreless multmeda communcatons [1]. One of the most challengng and mportant ssues n W-ATM s the medum access control (MAC) protocol, whch has a sgnfcant mpact on the performance of W-ATM networs [, 3]. The W-ATM MAC protocol needs to support dfferent ATM servces (e.g., CBR, VBR, ABR, and UBR) wth dfferentated QoS (qualty of servce) requrements, such as cell loss rato, maxmum cell transfer delay, and cell delay varaton, etc., over wreless channels whle achevng a hgh channel utlzaton. To realze ths goal, the MAC protocol must support the solaton of dfferent traffc flows such that multple users wll apprecate farness n sharng the avalable resources of W-ATM networs [4]. In ths paper, we consder a centralzed W-ATM archtecture where the base-staton (BS) acts as a central controller whch s responsble for the traffc management of the remote moble termnals (MTs). Moreover, a TDMA/TDD (Tme Dvson Multple Access/Tme Dvson Duplex) access scheme s employed n order to acheve a hgher multplexng gan, especally under asymmetrc traffc condtons,.e. dfferent upln and downln bandwdth [5, 6]. Due to the requrement of an accurate control of the remote moble termnals, the base-staton and the moble termnals must exchange connecton status nformaton. As these updates consume valuable bandwdth,.e. overhead, whch could otherwse be used for data transmsson, ther frequency should be ept as small as possble. In order to mnmze the overhead, Sgle et al proposed a herarchcal schedulng scheme coupled wth far-queueng algorthms [4]. In ths paper we further extend the algorthm by the use of adaptve weghts. Numercal results ndcate that the herarchcal far-queueng schedulng wth adaptve weghts can yeld a lower cell transfer delay and a hgher channel utlzaton whle mantanng farness among multple users. The remander of ths paper s organzed as follows. In Secton, we ntroduce a general framewor of the MAC protocol n W-ATM networs. In partcular, the detal of the TDMA/TDD frame structure s descrbed. Secton 3 presents the proposed herarchcal far-queueng algorthm. Smulaton results and performance comparson are gven n Secton 4. Fnally, the conclusons are drawn n Secton 5. II. Medum Access Control n Wreless ATM The man tas of an advanced MAC protocol s to control the allocaton of rado resources n a far and effcent manner. To acheve ths goal, a centralzed access archtecture s employed, for whch the most mportant technques * Ths wor s supported n part by the DAAD/RGC Jont Research Scheme under the proect no. G-HK96/97.EG03

3 beng consdered are TDMA schemes wth dynamc slot assgnment. Ths mechansm allows a flexble accommodaton of bandwdth by allocatng more or less tme slots dependng on current traffc condtons. TDMA can operate n ether TDD or FDD (Frequency Dvson Duplex) mode. Snce TDD does not need separate upln and downln bands but operates wth a sngle carrer, a hgher multplexng gan can be acheved whch leads to a hgh channel utlzaton especally n the case of asymmetrc traffc from upln and downln. In other words, the TDMA/TDD MAC protocol s able to effcently and dynamcally allocate the avalable bandwdth to connectons based on ther current need and traffc load through reservaton and/or allocaton cycles [3, 5, 6]. Tx-Rx turn-around tme Downln perod Upln perod An example of a TDMA/TDD frame structure s shown n Fg. 1. It s noted that, n the TDMA/TDD transmsson mode, tme frames may be of varable or fxed length. A frame s ntated by the BS by sendng a frame header pacet, whch ncludes nformaton about the allocaton of the tme slots of ths frame. The allocaton of dataslots s then undertaen by the BS based on the used schedulng algorthm. The frame header s followed by a downln (from the BS to the MTs) traffc perod (dataslots 1 ), an upln traffc perod (from the MTs to the BS), and a request perod (mnslots). Status nformaton from the MTs can ether be pggybaced on the dataslots or sent n one of the mnslots. The access to these slots may ether be based on random access and/or pollng. Dependng on the schedulng algorthm adopted, the BS and the MTs exchange varous amount of control nformaton. If all connectons are served on a per connecton bass by the base staton scheduler, the MTs must send an update pacet every tme when the status of a sngle connecton changes. In order to reduce the MAC overhead, the base staton may handle all connectons of one MT as one flow, whch s called herarchcal schedulng [4]. In ths case, an addtonal scheduler nsde the MT s necessary. A detaled descrpton of ths mechansm wll be gven n the next secton. Another mportant aspect of the MAC protocol s the farness guarantee whch can be acheved by the use of far-queueng schedulng. Note that far-queueng s an mportant mean to provde QoS guarantees. As a consequence, herarchcal far-queueng schedulng, f properly desgned, should have a combned nature of reducng the MAC overhead as well as mantanng farness. In [4], herarchcal far-queueng schedulng s mplemented n order to reduce the bandwdth consumed for nformaton exchangng between the basestaton and the mobles. However, snce fxed weghts equal to the sum of all the weghts of sessons n the MT are assgned to a MT, the mprovement n the channel throughput s often sacrfced for farness. Therefore, we propose a herarchcal far-queueng schedulng scheme whch employs adaptve weghts. In ths scheme, an effcent statstcal multplexng at the wreless access part can be acheved whlst farness can be mantaned among multple users. A. Far-Queueng Schedulng Algorthm Frame header Dataslot Mnslot Fg. 1 TDMA/TDD frame structure. III. Proposed Herarchcal Far-Queueng Scheme As dscussed above the schedulng algorthm s one of the most mportant mechansms n a wreless MAC protocol. In order to provde guaranteed end-to-end delay and farness among multple users wth dfferent traffc flows, farqueueng schedulng s suggested for pacet swtched networs [7, 8]. Far-queueng schedulng s based on the flud flow model of the generalzed processor sharng (GPS) mechansm. In GPS a weght φ s assgned to each sesson and the avalable resources are shared among all baclogged sessons accordng to these weghts. It s noted that varous far-queueng strateges, although they were orgnally developed for fxed pacet swtched networs, also play an mportant role n wreless networs [4, 9] to ensure an effcent and far sharng of the lmted resources. For GPS, farness s defned as follows. For any arbtrary tme nterval [t 1, t ] wth sesson and sesson beng baclogged contnuously, W ( t t W ( t, t ) 1, ) 1 =, (1) φ φ 1 Normally, one dataslot contans one ATM cell and ts cyclc redundancy code (CRC).

4 where W (t 1,t ) s the amount of sesson traffc served n the nterval [t 1, t ]. In other words, the server serves all baclogged sessons smultaneously, n proporton to ther servce weghts. The weghts are assgned durng connecton setup dependng on the source parameters of the connectons and ther QoS requrements. As the flud flow model of GPS can not drectly be appled to pacet swtched networs, pacet-by-pacet GPS (PGPS, also called weghted far queueng) s proposed n [10]. In PGPS, the noton of vrtual tme s ntroduced for schedulng purposes. The vrtual tme s used to emulate the GPS model wthn the pacet envronment. Pacets are served n ncreasng order of ther servce tags whch are assgned to each pacet upon ts arrval at the queue. Specfcally, the servce tags can be calculated by the followng equaton: L 1 F = + max( F, v( a )) () φ where F = servce tag of the -th pacet from sesson, L = length of the -th pacet from sesson, a = arrval tme of the -th pacet from sesson, v (t) = vrtual tme functon, φ = weght of connecton. Snce the vrtual tme functon of PGPS requres frequent recalculaton, a modfed algorthm called self-cloced far queueng (SCFQ) [11], was proposed. In SCFQ, the v( t) n Eqn. () s defned as the servce tag of the pacet n servce at tme t. Throughout, SCFQ wll be chosen as the bass for the development of our proposed effectve herarchcal far-queung scheme. B. Herarchcal Far-Queueng Schedulng wth Adaptve Weghts As mentoned above, herarchcal far-queueng schedulng wth proper desgn can potentally mnmze the MAC overhead and guarantee reasonable farness. A model of herarchcal schedulng s depcted n Fg.. D server Scheduler n BS φ 1 φ n Scheduler Scheduler r B1 r Bm r r 1, K 1 1,1 Sessons r n,1 Sessons r nk, n Sessons n BS Fg. Herarchcal far queueng schedulng strategy. In ths approach, a two-level far queueng schedulng s mplemented n the BS and MTs. We assume that the termnal MT has K sessons wth queue lengths q,1, q,,, q, and servce weghts r,1, r,,,. Lewse, n BS, the downln traffc may have B 1, B,, B m sessons wth servce weghts r B1, r B,, r Bm. The BS scheduler serves MTs based on ther weghts φ, =1,,, n. In the second level, the MT scheduler decdes whch connecton nsde the staton wll be served on the bass of the weghts r,. We consder the traffc n moble termnals. Let W t 1, t ) K denote the amount of traffc served by MT n the nterval [t 1, t ]. That s r, K MT (

5 W MT K ( t, t ) = W ( t, t ), (3) 1, = 1, q, 0 where W t, ) represents the amount of sesson traffc served n the nterval [t 1, t ] n MT., ( 1 t In [4], the weghts φ are assgned once durng connecton setup (we refer to ths mechansm as herarchcal schedulng wth fxed weghts, HFW). If not all sessons of one MT are baclogged, these sessons receve more servce compared to the case when all connectons are served ndvdually. Ths may lead to a dscrmnaton of other sessons. Therefore, we propose to adapt the weght φ of a MT dependng on the set of baclogged sessons. That s, K r, = 1, q, 0 1 φ =. (4) In order to realze ths scheme, the MT announces φ together wth the aggregated queue length n ts data and request slots. It s worth notng that herarchcal far-queueng operates n a frame-based manner. In practcal cases such as wth the TDMA/TDD access scheme, schedulng method may have an mpact on farness guarantee. Recall that the queue state and the current weght of the MTs can only be updated by the scheduler n the BS at the begnnng of next frame. On the other hand, n the MTs, the scheduler wors based on the nstantaneous queue nformaton. As a result, when some queues n MTs change ther state from empty to non-empty or vce versa, the real schedule weght of a MT could be smaller or larger than the weght assgned to the MT by the base staton at the begnnng of the frame. In order to mnmze the mpact of ths effect, the weght of each MT should be updated qucly. Therefore, the frame length must be bounded. As a consequence, the sessons n MTs wll be served n terms of ther servce weghts adaptvely whle mantanng reasonable farness. In the followng secton, smulaton results are presented whch wll demonstrate that herarchcal schedulng wth adaptve weghts has a better property of farness than that wth fxed weghts. III. Smulaton Results In computer smulatons, we evaluated the delay characterstcs and the farness property of the proposed herarchcal far queueng wth adaptve weghts (HAW), and compared t to the non-herarchcal (NH) case and the herarchcal schedulng wth fxed weghts (HFW). In partcular, we assume an error free channel wth a capacty of 10 4 cells/sec and a varable frame length wth maxmum number of 0 dataslots ncludng the frame header and mnslots. Moreover, the transcever turn around tme s gnored and the guard tme and physcal layer preamble are ncluded n the wreless header. The parameters for the TDMA/TDD frame structure are shown n Table 1. Table 1. Parameters of TDMA/TDD frame structure Defnton value Channel rate 10 4 cells/sec Frame length 0 dataslots Mnslot length 1/4 dataslot Frame header dataslots wreless header for upln traffc 1/4 dataslot We consder a scenaro wth four MTs and three bdrectonal connectons between each MT and the BS, two rtvbr (real-tme varable bt rate) and one UBR (unspecfed bt rate) connectons whch are modeled as on-off and Posson sources, respectvely. The traffc parameters and the correspondng weghts are shown n Table. It s noted that the bursty traffc has a hgher weght due to ts tghter delay requrement. Table. Traffc Parameters Type Average rate Pea rate Mean Burst Sze Delay Lmtaton Weght (cells/s) (cells/s) (cells) (ms) On-Off Posson Fg. 3 shows the complementary delay dstrbuton functon of the upln and downln. It s observed that the HAW schedulng outperforms the NH for both upln and downln traffc. Although the HFW performs best for the upln traffc, t has a very poor performance whch s even worse than that of NH for downln traffc. Ths s because n HFW the weght of each moble s equal to the sum of weghts of all connectons n the moble, regardless of whether the queues for the connectons are empty or not. Hence, the upln traffc can get more resources than the downln

6 traffc when some queues of the MT are not baclogged. Ths unfar bandwdth allocaton could lead to an even worse delay performance of downln traffc when the traffc load of downln traffc s much hgher than the load of upln traffc as n most clent-server applcatons. non herarchcal herarchcal wth adaptve weghts herarchcal wth fxed weghts non herarchcal herarchcal wth adaptve weghts herarchcal wth fxed weghts (a) Upln (b) Downln Fg. 3 Upln and downln delay dstrbuton of bursty traffc. The delay performance for Posson sources s shown n Fg. 4. It s noted that the HFW, although t has better delay performance for upln traffc compared to HAW, suffers from a sgnfcantly ncreased delay for downln traffc. In contrast, our proposed HAW can always balance well the delay performance between upln and downln non herarchcal herarchcal wth adaptve weghts herarchcal wth fxed weghts non herarchcal herarchcal wth adaptve weghts herarchcal wth fxed weghts (a) Upln (b) Downln Fg. 4 Upln and downln delay dstrbuton of Posson sources. The numercal results for evaluatng the farness of HAW and HFW are gven below. In smulatons, we use the same settng as before (.e., each MT has rt-vbr and 1 Posson sources) and compare the Posson sources only. We consder two sessons of Posson sources,.e. sessons,, wth weghts r, r, respectvely. Moreover, sessons and are from upln and downln traffc, respectvely. Let W = the maxmum value of W ( t t W ( t, t ) 1, ) 1, where r r W (t 1, t ) and W (t 1, t ) are the amount of traffc served for the baclogged sessons,. For dfferent t ( t = t - t 1 ) whch s the tme perod that sessons and are contnuously baclogged, W s measured and plotted n Fg. 5. W ( t t W ( t, t ) 1, ) 1 Clearly, for any value of t, HAW eeps the maxmum value of qute small. But, wth HFW, r r

7 the farness becomes ncreasngly worse as t gets larger. Hence, we conclude by notng that the farness can be better sustaned by the use of the proposed HAW schedulng algorthm. 8 7 herarchcal wth adaptve weghts herarchcal wth fxed weghts 6 5 W (cells) t (slots) Fg. 5 Measured farness performance of HAW and HFW. IV. Conclusons Ths paper proposes a W-ATM MAC protocol whch s based on a herarchcal far-queueng schedulng. The applcaton of our weght adaptaton mechansm n a centralzed TDMA/TDD access archtecture s demonstrated and ts performance s evaluated. Smulaton and numercal results ndcate that the proposed protocol can yeld lower cell delay and hgher channel utlzaton whle mantanng farness compared to the protocol usng fxed weghts. In further study, channel errors wll be consdered n the protocol by reservng bandwdth for retransmsson purposes. References [1] D. Raychaudhur and N. D. Wlson, ATM-Based Transport Archtecture for Multservces Wreless Personal Communcaton Networs, IEEE J-SAC, Dec. 1994, pp [] C. S. Chang and K. C. Cheng, Guaranteed Qualty-of-Servce Wreless Access to ATM Networs, IEEE J-SAC, Jan. 1997, pp [3] J. Sanchez et al, A Survey of MAC Protocol Proposed for Wreless ATM, IEEE Networ, Nov. 1997, pp [4] R. Sgle and T. Renger, Herarchcal Schedulng Strateges for Wreless ATM MAC Protocols, Proc. of Personal Wreless Communcatons 98, Toye, Jappan, Aprl [5] D. Raychaudhur et al, WATMnet: A Prototype Wreless ATM system for Multmeda Personal Communcaton, IEEE J-SAC, Jan. 1997, PP [6] N. Passas et al, Qualty-of-Servce-Orented Medum Access Control for Wreless ATM Networs, IEEE Communcatons Magazne, Nov. 1997, pp [7] L. Gn and R. Guern, Bandwdth Management and Congeston Control Framewor of the Broadband Networ Archtecture, Computer Networs and ISDN Systems, vol. 6 (1), 1993, pp [8] Jon C. R. and Hu Zhang, Herarchcal Pacet Far Queueng Algorthms, IEEE/ACM Transactons on Networng, Vol. 5, Oct. 1997, pp [9] R. Kautz and A. Leon-Garca, A Dstrbuted Self-Cloced Far Queueng Archtecture for Wreless ATM, Proc. of PIMRC 97, pp [10]A. K. Pareh and R. G. Gallager, A Generalzed Processor Sharng Approach to Flow Control n Integrated Servces Networs - The Sngle Node Case, IEEE INFOCOM 9, pp [11]S. J. Golestan, A Self-Cloced Far Queueng Scheme for Broadband Applcatons, IEEE INFOCOM 94, pp