Performance Evaluation of Implementing Calls Prioritization with Different Queuing Disciplines in Mobile Wireless Networks

Transcription

1 Journal of Compuer Science 2 (5): , 2006 ISSN Science Publicaions Performance Evaluaion of Implemening Calls Prioriizaion wih Differen Queuing Disciplines in Mobile Wireless Neworks Salman A. AlQahani and Nasser-Eddine Rikli Deparmen of Compuer Engineering, King Fahd Universiy of Peroleum & Minerals, Dhahran 31261, Saudi Arabia Absrac: I is known ha efficien call handling mechanisms can grealy improve cellular nework performance. One way o improve sysem performance of cellular nework is o use efficien handover schemes when users changes beween cells. This paper focuses on he performance evaluaion of originaing and handover calls prioriizaion using differen queue size and discipline. Boh calls will be queued unil hey reach a cerain hreshold, or a channel becomes available. Higher prioriy will be given saically or dynamically o he handover calls based on differen crieria. Tow differen queuing policy known as minimum waiing ime firs ou (MWFO) and firs in firs ou (FIFO) are compared. In addiion, he effec queue size on calls prioriizaion and sysem performance is sudied. In his proposed scheme, we aim o decrease he probabiliies of blocking and forced erminaion and increase he oal carried raffic while improving he service qualiy. Key words: Call prioriizaion, mobile wireless, performance evaluaion, qualiy of service, queuing discipline. INTRODUCTION I is known ha efficien call handling mechanisms can grealy improve cellular nework performance. One way o improve sysem performance of cellular nework is o use efficien handover schemes when users changes beween cells. This paper focuses on he performance evaluaion of originaing and handover calls prioriizaion using differen queue size and discipline. Boh calls will be queued unil hey reach a cerain hreshold, or a channel becomes available. Higher prioriy will be given saically o he handover calls or dynamically based on some crieria. Tow differen queuing policy known as minimum waiing ime firs ou (MWFO) and firs in firs ou (FIFO) are compared. In his scheme, we aim o decrease he probabiliies of blocking and forced erminaion and increase he oal carried raffic while improving he service qualiy. In [1-6] differen calls prioriizaion schemes are sudied, bu he effec of queue size and queuing discipline on he call prioriizaion and sysem performance, which are he main focus of our sudy, were no sudied. The handover process usually consiss of wo phases. One is he handover iniiaion phase and anoher one is he handover execuion phase. In he handover iniiaion phase, he QoS level is moniored in order o decide when o rigger he handover. In he handover execuion phase, allocaion of new resources o he handover call is performed. I should be noed ha he focus of his paper is pu on he handover execuion phase, and we assume ha he handover reques deecion and iniiaion procedures are perfec (i.e., all valid requess are deeced and no invalid requess acivae he handover procedure). Due o he increased raffic in cellular mobile neworks, he availabiliy of service wihin he supply area depends more and more on availabiliy on free channels and hus on he proper raffic configuraion of he sysem. From he raffic poin of view, he QoS is deermined by he probabiliy of he wo evens, which occur due o he occupancy of all available channels. These evens are he probabiliy of call blocking and he probabiliy of forced erminaion. There is rade off beween hese wo performance measures and he configuraion parameers. The performance parameer measures esimaed by his sudy are he probabiliy of call blocking, he probabiliy of forced erminaion and he raio of carried raffic o he oal offered load. Sysem descripion: We consider a cellular mobile nework where here are R cells in he nework. Each base saion has is zone where is radio waves can be received. The zone of each base saion is called a cell. We divide he cell ino overlapping areas and nonoverlapping area. The area, which is covered by more han one cell, is called handover area. For simpliciy, we assume ha here are no areas where hree or more cells overlap. Here, a service area is assumed o be of homogeneous opology (Fig. 1). We ake ou a cell from i, which is called marked cell (Fig. 2). There is a base saion and channels in a cell. When a moving user holding a channel approaches from a neighboring cell oward he marked cell and he received signal srengh goes below he handover hreshold of he neighboring cell, a handover reques is generaed in he marked cell. All he handover and he new calls were assumed o be independen of each oher. We se a finie queue for Corresponding Auhor: Salman AlQahani, Deparmen of Compuer Engineering King Fahd Universiy of Peroleum & Minerals, Mail Box 5065, Dhahran 31261, Saudi Arabia Tel.: , Mobile:

2 each call ype. As shown in Fig. 2, in he base saion here are wo separae queues Q h and Q n wih capaciies K, and L for handover requess and new call requess, respecively. We also assume ha he channel number needed for each call classes (bandwidh) are S h and S n for handover requess and new call requess, respecively. Queuing boh new calls and handover sreams is very likely o have meri of improving he perceived service qualiy. Firsly, fewer handovers will be erminaed in he middle of a call due o queuing. Secondly, he oal carried raffic will be increased, since he new calls are no blocked when no channels are available for hem, bu simply delayed for small period of ime and will ulimaely go hrough he sysem, conribuing herefore o he carried raffic. Cell There are hree characerisics of a raffic model: 1) Call arrival process,2) Call holding ime, and 3) Cell Residence (dwell) ime. A lo of research has been done o define he mos suiable raffic model for wireless neworks. For he call arrival process, we can safely assume ha he call arrival is a Possion process. For he call holding ime and he cell residence ime, wo main approaches can be found in he lieraures. One way is o model he call holding ime and he cell residence ime as general independen idenically disribued (iid) wih nonlaice disribuion using several disribuions, namely: gamma, hyper-exponenial, lognormal, hyper- Erlang, Weibul, or deerminisic. This approach has been inensively sudied by [8,9]. The second approach is based on he user s mobiliy, he shape and size of he cell, and exponenial disribuion o deermine he disribuion of he cell residence ime and he channel holding ime. This mehod has been exensively used by [1-9]. A lo of researches use he exponenial assumpions o obain analyical soluions for cellular sysems. Handoff Handoff area Call arrival raes: New call requess and handover requess arrive a every cell according o Poisson processes wih raes λ h and λ n for handover reques and new call reques respecively. All call generaion processes are assumed muually independen [1-9]. Mobile Terminals Base Saion Fig. 1: Service area Sysem raffic model: When calls arrive o he sysem (Fg. 2), hey can be eiher: 1) Blocked, if on is arrival i finds is queue full 2) Dropped, if waiing ime of he call in he queue exceeds he maximum ime-ou before geing he service (energy drops below he minimum), 3) Served by he sysem and compleed wihin he cell if call holding ime is less han cell residence ime oherwise call is lef for he adjacen cell. Call holding imes ( c ): The call holding ime of a call is he ime duraion beween he beginning of a call and he compleion of a call. I is also assumed ha he call holding ime is exponenially disribued wih probabiliy densiy funcion f ( ) = e and has c c µ µ mean µ. The call holding ime can span several cells before he call is erminaed. Call holding imes are muually independen irrespecive of areas in which hey are generaed and base saions a which hey are served. Call holding imes are independen of call generaion [1-9]. Each call class can have is own call holding ime wih mean µ h and µ n for handover reques and new call reques respecively. Handover Calls Originaing Calls Call Classifier λ h B locked λ n B locked Tim eo u Tim eo u Call Secheduling Fig. 2: Sysem model C η µ σ Residence imes ( r ): If a mobile is given a channel in a cell, and he mobile remains in he cell s coverage area for a period of ime r, which is called he residence ime. I is assumed o be exponenially disribued wih probabiliy densiy funcion f r ( r ) = ηe η and has mean η. The relaion beween he call holding ime and residence ime are shown previously in Fig. 3. If he holding ime of a call served a a base saion ends before he residence ime are over, hen he call leaves he sysem (ends). If he residence ime is over before he holding ime ends, hen he call ry o move o a neighboring cell. Call residence imes are independen of call generaion process and call holding imes [1-9]. 467

3 Channel holding ime ( h ): If he mobile is given a channel, his channel would be released eiher by he compleion of he call in he cell or by a handover process o a neighboring cell. In his way, he channel occupancy ime is he smaller of he call holding ime and he cell residence ime (because of he memoryless characerisic of he exponenial disribuion). Since he call holding ime and cell residence ime are assumed o be exponenially disribued wih means µ and η respecively, and using he memoryless propery of he exponenial PDF, he channel holding ime, which is equal o min ( c, r ), is exponenially disribued. Since he call holding ime and cell residence ime are exponenially disribued and are independen of each oher as assumed before, ( ) hen ( µ ) x η η f x µ µ + η = µ ( ) h e e + η e e = µ + η e. For more deail [6]. Then he channel holding ime being he minimum of he call holding ime and cell residence ime, is also exponenially disribued wih a mean equal o σ where σ = ( η + µ ). Call Arrival c Call Compleion f ( ) = γ e γ and mean γ. Since for handover q q requess, queued handover requess are dropped as he mobile moves ou of he handover area before he handover is compleed, he ime-ou period will equals o q. For simpliciy, we assume ha each new call reques has an average ime-ou idenical o he queuing ime of a handover reques wih differen mean. Performance measures: The performance parameer measures esimaed by his simulaion are he probabiliy of call blocking, he probabiliy of forced erminaion and he raio of carried raffic o he oal offered load. These parameers will be used in he presenaion of numerical resuls and are defined as follows: Blocking probabiliy ( P b ): is he probabiliy ha a new call aemp does no ge service. The blocking probabiliy of a new call is he sum of wo erms (he blocking probabiliy of new call and he ime-ou probabiliy). No. of New Call blocked + No.of New Call Timed ou = P b No. of New Call Arrived Ener Cell 0 r, 0 r 1 Ener Cell 1, k Ener Ener Cell Cell 2 K-1 r, Ener Cell K r, k Fig. 3: Call holding ime and cell residence ime Time Queuing ime( q ): Each handover call arriving o he cell and having o wai in he queue has a paience ime (saying power ime). If he virual waiing ime (i.e., he ime ha he handover call would have o wai unil service) exceeds he paience ime, he handover depars from he sysem and ges los by impaience. The paience ime of a handover call is assumed o be exponenially disribued wih mean γ. There are a lo of papers dealing wih impaience phenomena [1-9]. Therefore, he ime-ou period deermines he maximum ime a call says in he queue before deparing from he sysem. We assume ha he handover requess and new calls requess are subjec o ime-ou wih mean γ h and γ n respecively. For all calls, his ime ou period is assumed o be exponenially disribued. When a mobile is moving ou of a cell (moves across he handover area), is RSS decreases unil i becomes unaccepable for he communicaion. The average ime handover requess will say in queue (wihin he handover area), before i is forced o erminae, is q (his is also referred as degradaion inerval). This ime is exponenially disribued wih densiy funcion The probabiliy of forced erminaion of handover ( P f ): The probabiliy ha he mobile experiences an unsuccessful handover aemp. The blocking probabiliy of a handover call is he sum of wo erms ( he blocking probabiliy of handover and he ime-ou probabiliy). No.of Handoff Callblocked + No of Handoff CallTimed ou = P f No. of Handoff CallArrived Grade of service (GoS): This is a cos funcion ha penalizes he fac ha handover forced erminaion probabiliy is much more annoying han new call blocking and i is used as a reference of he grade of service offered by he sysem. The expression associaed wih his variable is: GoS = P b + 10P f Offered load: new calls and handover calls ha arrived λ = λ h + λ n Toal carried raffic: is he amoun of raffic admied o he cellular nework as opposed o he offered load. In general, he carried raffic is less han he offered load because of he blocking of calls and handover forced erminaion probabiliy. The percenage of he offered load ha is carried is cerainly desired o be as high as possible. This percenage decreases wih he 468

4 increase of offered load and probabiliy of call blocking and forced erminaion. P f b Toal Carried Traffic = ((1 ) λh + (1 P ) λ n) Offered TrafficUilizaio n = ToalCarriedTraffic OfferedTraffic In general, allowing each call ype o wai for service will increase he sysem performance and herefore decrease he blocking probabiliy more han he sysem wihou using hese queues. In addiion, he queuing of new calls increases he sysem uilizaion a he expense of sligh increase in he forced erminaion of handover calls. NUMERICAL RESULTS Effecs of queuing on he sysem performance: In his secion, we es he effec of increasing he queue size of each call ype on he sysem performance. Firsly, we will sudy he performance of wo-queue sysem (queues are used for all call ypes) as shown in Fig Secondly, sysems wih one queue and wo queues will be compared. In all cases, blocking probabiliies, probabiliy of forced erminaion and offered raffic uilizaion versus queue size will be compared. We consider he parameers shown in Table Fig. 3: P f versus queue size Table 1: Sandard sysem parameers used o es queue usage Call ypes Arrival Rae (call/sec) c (sec) r (sec) q (sec) Queue size handover call Variable new calls Variable Number of channels =50 and handover requess are 25% of new calls. Figure 3 and 5 show he impac of increasing he queue size of all ypes of call simulaneously on he oal sysem performance. As he queue size of each call increases, more raffic of each ype eners he nework. Therefore, he sysem uilizaion will increase as shown in Fig. 9. Figure 9 shows he over all sysem offered raffic uilizaion. The impac of queue size on he sysem uilizaion will no have any effec afer a cerain queue level in general ( in his case level 17 (calls)) since no one of any call ype will have more arrival raffic can ener he nework (Fig. 3 and 5). The forced erminaion and blocking probabiliies of each call ype remains subsanially consan when he queue size of each call ype exceeds a consan value due o he limiaion of queuing ime of each call ype in is queue. This is explained as follows. When he queue size of handover call increases, he carried raffic will increase. This coninues unil he queue size exceeds a size of 5 (calls). Afer ha, he queuing delay will exceed he maximum waiing ime of handover call and he carried raffic will no increase. The same idea will apply for new calls (17 calls). So ha he oal offered raffic uilizaion of he sysems will increase as he queue size increases, unil he queue size equals o 17 (calls). Afer ha, any increase in any queue size will no have any impac on he sysem performance (imeou) (blocking) Fig. 4: Time ou and blocking probabiliy of handover requess Pb Pb-w ih-q Fig. 5: P b versus queue size Figure 6 shows P f as arrived reques increases, wih and wihou using queues. As we allow he handover reques o be queued, P f will improve. Bu his will has small effec on he new call P b. As we allow handover reques o be queued, he blocking probabiliy will increase, bu

5 his can be solved by queuing he new calls hemselves. Figure 8 shows he blocking probabiliy of new calls versus he arrival raes using and wihou using he queues. Wihou allowing he new calls o be queued and allowing handover reques o be queued he blocking probabiliy of new calls will increase, bu when we allow he new calls o be queued he blocking probabiliy of new calls will increase Qh=5 Qh=0 Fig. 6: P fh wih and wihou queuing versus arrived requess Pb Qh=5,Qn=0 4.5 Qh=5,Qn=10 Qh=0,Qn=0 Fig. 7: P b wih and wihou queuing versus arrival raes Offerd Traffic Ulilizaion Ui1.Q Ui2.Q Fig. 8: Carried raffic uilizaion versus queue size Figure 9 and 10 show P f and P b versus queue size a differen handover reques queuing imes. As queuing ime of handover increase he P f will decrease, bu his will be a he expense of small increase on he blocking probabiliy of new calls as shown in Fig (q=5) (q=10) (q=15) Fig. 9: P f versus queue size a differen queuing ime Pb Pb(q=5) Pb(q=10) Pb(q=15) Fig. 10: P b versus queue size a differen handover reques queuing ime Sysem performance wih differen queuing policy: For he FIFO scheme he calls are insered in heir queue based on he call arrival imes, hen he call ha arrive a firs will be served firs wihin is queue. For he Minimum Waiing ime firs ou (MWFO), he calls are insered based on he call dropping imes, hen he call in he head-of-line of he waiing queue (wih minimum drop ime) is served firs wihin is waiing queue. In his secion, we sudied he performance of he sysem when FIFO and MWFO are used a wo differen cases. In he firs case he handover calls have he same average speed (one mean of waiing ime). The second case, he handovers call are divided in wo pars, slow handover calls and fas handover calls and each par have a differen mean of waiing ime. In he firs case, we use he parameers in Table 1 wih K=5, L=10. Figure 11 shows he handover calls forced erminaion probabiliy using FIFO and MBPS policy. From he Fig. 11, i indicaes ha he MWFO has is roughly he same as he FIFO and he difference beween he wo schemes is negligible. 470

6 1.5 -FIFO MWFO Fig. 11: P fh versus arrival rae using FIFO and MWFO wih one mean of waiing ime In he second case, we assume ha he handover calls have differen speeds. Tha is he mobile user can be slow or fas. In his case, we assume ha he handover calls have wo differen mean of waiing ime. The parameers used are he same as in previous case excep ha he second mean of waiing ime is equal o halve of firs waiing ime. Figure 12 and 13 show forced erminaion probabiliy of handover calls using FIFO and MWFO policy. From he Fig. 13, i indicaes ha he MWFO has a less forced erminaion probabiliy han FIFO. Hence, he oal carried raffic will increase as he handover calls forced erminaion probabiliy decreases. As expeced, as he difference beween he call waiing ime become bigger he difference beween he wo schemes become clear. In case 2, wo mean of waiing ime for each call ype one of hem is half of oher he waiing ime (he mobile user classified as slow and fas), and as Figures indicae, he sysem performance of he MWFO is beer han FIFO scheme. Therefore, we can say ha, he performance of MWFO will be beer in case of large variaion beween he waiing imes of handovers. Finally, in case1 and case2 he difference beween MWFO and FIFO is negligible a high call arrival raes. -FIFO -MWFO Fig. 12: P f versus arrival rae using FIFO and MWFO wih differen mean of waiing imes (Timeou) (Timeou)-FIFO (imeou)-mwfo Fig. 13: Time ou probabiliy of handover versus arrival rae using FIFO and MWFO wih differen mean of waiing imes CONCLUSION In his paper, a prioriized handover and admission conrol scheme in cellular mobile wireless neworks has been sudied. Finie buffers (queues) are used for boh new calls and handover calls. We have focused on making blocking probabiliy of new calls and he probabiliies of forced erminaion of handover calls smaller han no queuing sysem by allowing new calls and handover calls o wai for services (using queue). According o he simulaion resuls, as shown in all figures of he resul secion, we can make he blocking probabiliies of each call ype smaller han no queuing sysem and his increases he oal carried raffics of he sysem. However, by making he blocking probabiliies of handover calls small (as we increase he queue size of each handover calls), he ime-ou probabiliies of handover calls become large. We hink ha making he blocking probabiliy of handover calls small should be given prioriy over making he ime-ou probabiliies of handover calls small so ha he avoidance of blocking should be paid more aenion han ha of ime-ou. Then, he probabiliy ha handover calls are blocked as soon as hey move o new cell becomes smaller. In addiion, he idea of queuing policy was presened and he FIFO and MWFO policy were discussed and compared in wo cases. The firs case is when he handover calls have one mean of waiing ime and he second case is when he handover calls have more han one mean of waiing ime (varying waiing ime). The effec of MWFO will be in he cell wih varying waiing ime. The resuls we gain from our simulaion indicae ha MWFO has beer impac on he oal sysem performance han FIFO policy in second case. REFERENCES 1. Wang, J., Q.-A. Zeng and D.P. Agrawal, Performance analysis of a preempive and prioriy reservaion handoff scheme for inegraed servicebased wireless mobile neworks. IEEE Trans. Mobile Compuing, 2:

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