Service Scheduling for General Packet Radio Service Classes

Transcription

1 Service Scheduing for Genera Packet Radio Service Casses Qixiang Pang, Amir Bigoo, Victor C. M. Leung, Chris Schoefied Department of Eectrica and Computer Engineering, University of British Coumbia, Vancouver, BC, Canada Motoroa Ceuar Infrastructure Group, Richmond, BC, Canada Abstract- Whie the quaity of service (QoS) profies for a number of genera packet radio service (GPRS) casses has been specified [1] by ETSI, how QoS management is provided by means of traffic scheduing, traffic shaping, and connection admission contro, in a GPRS network is an impementation issue that is attracting significant current research interest. This paper presents an evauation of severa traffic scheduing methods, incuding of, Static Priority Scheduing () and Eariest Deadine First () by simuations, with the objective of meeting the deay profies defined for a number of GPRS casses. Traffic sources representative of GPRS appications, incuding emai, feet management and word wide web appications are empoyed. We focus on the forward ink which represents the botteneck of a typica GPRS data connection. Resuts show that is abe to meet the deay requirements at a much higher channe utiization compared to the other aternatives. I. INTRODUCTION Genera Packet Radio Services (GPRS), as a major component of GSM Phase 2+ [1, 2], aims at providing pubic packet-switched data services over the existing GSM network infrastructure. In order to meet the different requirements of a wide variety of user appications, a number of Quaity of Service (QoS) profies have been specified. There is significant current interest in the deveopment of QoS management functions for GPRS networks that meet the requirements of these profies whie maintaining a high bandwidth efficiency. This is especiay true since the radio spectrum is a imited and vauabe resource. QoS management invoves the characterization, shaping/poicing and scheduing of user traffic, and admission contro of users (Fig.1). In this paper, we focus on traffic scheduing methods for GPRS. Though there are a ot of scheduing methods or service discipines [3-13] that have been studied, e.g. First-In- First-Out (), Static Priority Scheduing (), Virtua Cock, Weighted Fair Queuing (WFQ), Sef-Cocked Fair Queuing, Start-Time Fair Queuing (STFQ), Worst-case Fair Weighted Fair Queuing (WF 2 Q), Eariest Deadine First (), Deay Eariest-Due-Date (Deay-EDD), Jitter Eariest- Due-Date (Jitter-EDD), Stop-and-Go, Weighted Round- Robin (WRR), Deficit Round Robin (DRR), Hierarchica Round Robin (HRR), Rate-Controed Static Priority (RCSP), and Leave-in-time etc, we are not aware of any extensive evauation of traffic scheduing reative to GPRS requirements (predictive and best effort) in the iterature so far. In this paper, the resuts of the foowing scheduing methods are considered. First In First Out () Static Priority Scheduing () Eariest Deadine First () The performance of these scheduing methods are compared and commented. New Traffic Type Traffic Source MS Traffic Contract Traffic Characterization Traffic Conditioning (Shaping Poicing) BSS QoS Management of GPRS SGSN GPRS networks GGSN Admission Contro Traffic Scheduing PDNs or other networks Fig. 1. QoS management of GPRS networks. This paper is organized as foows. Section II describes the QoS profie of GPRS. The descriptions of the scheduing methods are given in Section III. Section IV incudes the traffic source modes, simuation scenarios and simuation resuts, and finay concusions are given in Section V. II. QOS PROFILE OF GPRS There are five aspects in the QoS profie for GPRS, precedence, reiabiity, mean throughput, peak throughput, and deay casses [2]. There are four deay casses in the GPRS QoS profie: casses 1, 2 and 3 offer what we ca predictive services and require QoS management, whie cass 4 provides a best-effort service. Tabe I gives the deay requirements for packets containing 128 and 1024 octets of information. To determine the deay requirements for different packet engths in each predictive service cass, we interpoate between the two packet engths given in the specification. The deay requirement r(c,) for a packet with any size and deay cass c as given in (1) /$10 (c) 1998 IEEE

2 TABLE I DELAY CLASSES AND REQUIREMENTS FOR DIFFERENT PACKET SIZES Deay Cass Packet Size 128 octets 1024 octets Mean 95 % Mean 95 % 1.(Predictive) (Predictive) (Predictive) (Best Effort) Unspecified c = 1; c = 1; > c = 2; 128 r ( c, ) = 128 (1) c = 2; > c = 3; c = 3; > where c stands for deay cass, stands for the size of the packet. A quantifiabe performance measurement with respect to the different service casses and packet engths is needed to faciitate comparisons between different scheduing methods. We deveop the method of normaizing the measured deays to eiminate packet engths as a factor in the comparisons. Let pk denote the packet arrived to the destination, cass(pk) denote the cass of the packet, and pk_en(pk) denote the packet ength of the packet. The normaized deay for pk is given by Experienced rea deay of pk Required deay of pk The required deay of pk is cacuated by the function r ( cass( pk), pk _ en( pk) ) indicated by (1). The mean normaized deay is given by N i=1 Normaizeddeay of pk i (3) N Using the measurement of normaized deay, it is more convenient to evauate the the deay performance of the queuing system with variabe packet size and different deay casses. If the mean normaized deay is beow 1, we can make concusion that the deay performance requirement is met. (2) Deay Requirement sec required deay pk Size rea deay required deay Fig. 2. Normaized deay cacuation. A. First In First Out () III. SCHEDULING METHODS bytes is the simpest scheduing and queuing method. First arrived packet is served first. Two buffers are used, one for predictive services(cass 1, 2 and 3), the other for best-effort service(cass 4). We aways use a separate buffer for besteffort servces in a of the scheduing methods, which receives service ony if the buffers for the predictive casses are empty. B. Static Priority Scheduing () With, each deay service cass has its own buffer and is assigned a fixed (static) service priority: highest for cass 1 and owest for cass 4 (best-effort). When the next downink time sot is avaiabe, a cass i buffer wi receive service ony if a cass j (j < i) buffers are empty. C. Eariest Deadine First () With Eariest Deadine First () or Eariest Due Date (EDD) method, each arrived packet has its own deadine (or due-date). The packets are served in the order of their deadines. Assume the arriva time of a packet is a, and the ength of the packet is. Its deay cass is c(1 c 3), the time-sot capabiity of its destination is s, and rate denote the data rate of one time-sot. Then we have dead _ ine = a + r(c, )- (4) s rate where, the function r(c,) denotes the deay requirement of the packet with deay cass c and ength cacuated by (1). The mechanism needs to sort the packet queue using at east O(ogN) insertion operation for each arrived packet. This may affect its appication due to impementation difficuty /$10 (c) 1998 IEEE

3 A. Traffic Sources IV. PERFORMANCE EVALUATIONS To generate the traffic presented to the simuator (by the sources), we used three types of traffic sources and their combinations incuding emai, feet management and WWW appications. FUNET mode was used for Emai and MOBITEX mode for feet management traffic generation [14]. For generating a reaistic WWW traffic, which takes into account the sef-simiar nature of the traffic, we used avaiabe traces. 1) FUNET mode This mode is based on statistics coected on emai usage from the Finish University and Research Network (FUNET). TABLE II shows the percentage of messages shorter than the indicated ength. Percentage TABLE II DISTRIBUTION OF FUNET MODEL Message ength (Kbytes) < <1 < <2 <3 <5 <10 The trace is downoaded from the web site where stores the socaed Internet Traffic Archive. "The Internet Traffic Archive is a moderated repository to support widespread access to traces of Internet network traffic, sponsored by ACM SIGCOMM. The traces can be used to study network dynamics, usage characteristics, and growth patterns, as we as providing the grist for trace-driven simuations. The archive is aso open to programs for reducing raw trace data to more manageabe forms, for generating synthetic traces, and for anayzing traces". Fig. 3 shows the data rate of the trace we used. The WWW document size distribution in the trace is given in TABLE III. Rate(10 5 bps) TABLE III WWW DOCUMENT SIZE DISTRIBUTION Document size(bits) Number >10,000, >1,000, >100, >10, The probabiity distribution function f(x) for this mode can be approximated by the Cauchy (0.8,1) distribution. f ( x) = Cauchy(0.8,1) = π 1 ( 1 + ( x 0.8) 2 ) The packet inter-arriva time distribution for FUNET mode is exponentia. 2) Feet management mode This mode is based on statistics coected from a feet management appication using the Mobitex wireess packet data network in Sweden. The average message ength on the upink is 30 bytes and 115 bytes on the downink. upink: 30+uniform(-15,15) bytes, downink: 115+uniform(-57,57) bytes. The packet inter-arriva time distribution for Mobitex mode is assumed exponentia as we. 3) Rea WWW browsing trace (5) Time(10 4 sec) Fig.3. Data rate of WWW browsing service (accumuating interva=60 secs). B. Simuation Scenarios In our simuations, we consider appying different service casses to each traffic source type. We aso evauate the effects of varying the number of GSM time sots, n, avaiabe for GPRS between 1, 3 and 7, with a capacity of 13.4 Kbps (CS2) for each time sot. Service scheduing is performed in the LLC ayer based on LLC frames. The number of time sots assigned to a frame is determined by the mobie station s time sot capabiity, number of RLC bocks in the frame and the number of sots eft. The portion of a type of service (emai, feet management or WWW) in the tota traffic is aso changeabe. The combination of a the different parameters, generated tota of 18 simuation scenarios, which were used to evauate and compare the performance of the scheduing methods. In TABLE IV we just give two simuation scenarios /$10 (c) 1998 IEEE

4 I II Services TABLE IV TWO SIMULATION SCENARIOS System Sots Deay Cass Sots Cap. Portion Emai /3 Feet Man /3 WWW /3 Emai /2 Feet Man /4 WWW /4 C. Simuation Resuts In this part, the simuation resuts of the scheduing methods are given. Ony two scenarios are shown in Fig.4 and Fig.5, respectivey. From the simuation resuts, we determine that athough and are very simpe to impement, they do not meet the deay requirements under a high degree of ink utiization. In contrast, by scheduing packets according to their deadines, can achieve up to 90% ink utiization whie meeting the deay requirements. The main reason is that some of the traffic sources are very bursty, and generate packets which sizes vary over a arge range, from severa hundreds bits, e.g., in feet management case, to miions of bytes, e.g., for WWW browsing. It means some of the services may amost at the same time generate tons of traffic and pump them into the wireess networks with very imited capacity. The high variabiity of packet arriva rates and packet sizes reduces the effectiveness of and even for traffic within the same service cass, because short packets are often impeded by ong packets. Note that the service discipine for is essentiay for packets within the same service cass. Many short packets waiting a ong time behind a ong packet resuts in significant increases in the overa mean deay statistics. From our observations, packet ength variation is the main reason why and do not meet the deay requirements under high ink utiization eves. Based on the simuations, we concude that to achieve a good overa performance, packet engths must be considered in scheduing, as in, by servicing short packets ahead of ong packets. The simuation resuts aso show that if a simper scheduing method such as is chosen to reduce the impementation and processing compexity in the scheduer, traffic shaping/poicing may be needed to reduce the burstiness of the traffic sources. This requires additiona processing at the edges of the GPRS network. V. CONCLUSIONS In this paper, we studied the scheduing methods for GPRS. First, we proposed a QoS management framework for GPRS, the packet switched technoogy over GSM. Then we introduced the deay requirement definitions for GPRS in the QoS profie and the normaized deay measurement that more convenient for performance evauations. Through simuations, we evauated the three scheduing methods for GPRS, without any traffic shaping/poicing poicy. The simuation resuts show that the bursty nature of data sources can degrade the performance for and, in the absence of traffic shaping/poicing, whie can provide good performance. However, performance compexity remains an issue for. Other scheduing methods, when combined with traffic shaping/poicing poicies, are expected to provide a more robust performance. This is the topic of our ongoing research. REFERENCES [1] G. Brasche and B. Wake, Concepts, services, and protocos of the new GSM phase 2+ genera packet radio service, IEEE Commun. Magazine, vo. 35, no. 8, pp , Aug [2] ETSI, GSM V6., Digita ceuar teecommunications system (phase 2+); genera packet radio service (GPRS); service description; stage 2, Mar [3] C.Li, R.Bettati, and W.Zhao, Static priority scheduing for ATM networks, Proc. 18th IEEE Rea-time Systems Symposium, pp , [4] T.Hou and C.Chen, Jitter-EDD impementation for transporting MPEG-2 video streams on ATM network, Proc.12th Internationa Conf. on Information Networks, pp , Jan [5] H.Zhang, Service discipines for guaranteed performance service in packet-switching networks, Proceedings of the IEEE, vo.83, no.10, pp , Oct [6] P.Goya, H.M.Vin and H.Cheng, Start-time fair queuing: a scheduing agorithm for integrated services packet switching networks, Proc. of SIGCOMM'96, [7] N.R.Figueira and J.Pasquae, Leave-in-time: a new service discipine for rea-time communications in a packet-switching network, Proc. of SIGCOMM'95, pp , [8] D.D.Cark, S.Shenker and L.Zhang, Supporting reatime appications in an integrated services packet network: architecture and mechanism, Proc. ACM SIGCOMM'92, pp.14-26, [9] L.Zhang, Virtua cock: a new traffic contro agorithm for packet switching networks, Proc. ACM SIGGCOM'90, pp.19-29, Sep [10] J.C.R.Bennert and H.Zhang, WF 2 Q: worst-case fair weighted fair queueing, INFOCOM'96, /$10 (c) 1998 IEEE

5 [11] S.Goestani, A sef-cocked fair queueing scheme for broadband appications, Proc. IEEE INFOCOM'94, pp , Jun [12] H.Zhang and D.Ferrari, Rate-controed static priority queueing, Proc. IEEE INFOCOM'93, pp , Apr [13] C.Liu and J.Layand, Scheduing agorithms for mutiprogramming in a hard rea-time environment, J.ACM, vo.20, pp.46-61, Jan [14] Traffic reference modes for GPRS, GPRS-TG Meeting no 2, Stockhom June, normaized deay of Emai ink utiization normaized deay of Emai normaized deay of Feet Management ink utiization ink utiization normaized deay of Feet Management normaized deay of WWW ink utiization ink utiization Fig. 5. Simuation resut II. normaized deay of WWW ink utiization Fig. 4. Simuation resut I /$10 (c) 1998 IEEE