and assigned priority levels in accordance with the QoS requirements of their applications.

Transcription

1 Effect of Priority Class Ratios on the Novel Delay Weighted Priority Scheduling Algorithm Vasco Quintyne *, Adrian Als Deartment of Comuter Science, Physics and Mathematics University of the West Indies Cave Hill, Barbados * vasco.quintyne@mycavehill.uwi.edu, adrian.als@cavehill.uwi.edu ABSTRACT In this aer, the novel Delay Weighted Priority Scheduling (DWPS) algorithm roosed for the management of cellular network's channel access will be resented. Whereas the simle round-robin algorithm, like that used in the oular Point Coordinated Function (PCF), fails to consider the delay imlications on stations with different riority levels, in the DWPS algorithm, mobile stations are queued and assigned riority levels in accordance with the quality of service (QoS) requirements of their alications. Results will show that the DWPS outerforms the simle round-robin algorithm for cell sizes of u to 25 stations. Keywords: Delay Tolerance Threshold, Delay Weighted Priority Scheduling, Incidence of QoS Degradation, QoS Tolerance Threshold and Ready Ratio 1. INTRODUCTION Attemts at devising fair channel access schemes for cellular environments have traditionally focused on ensuring guaranteed access to all mobile stations within each cell or ensuring that the available bandwidth is adequately allocated amongst the active stations. Non-contentious scheduling algorithms, articularly the round-robin scheme used within the Point Coordinated Function (PCF) [1-7], have roven to give guaranteed channel access. However, the demand for the highest quality of service (QoS) levels by endusers not only requires guaranteed channel access and adequate bandwidth to be taken into account but also the delay tolerance of the alications being executed on the mobile stations. The differences in the various delay tolerance levels [1] resent in a multialication environment is significant enough to validate the use of riority classes. In the roosed Delay Weighted Priority Scheduling (DWPS) algorithm, mobile stations are queued and assigned riority levels in accordance with the QoS requirements of their alications. Note that the QoS exerienced by a station is artially determined by the queue delay, which is the time a station sends waiting in the queue before it is granted access to the channel. Note that if the scheduling algorithm does not emloy rioritisation then the combination of long queue lengths and a high QoS requirement would rove detrimental to stations using delay sensitive alications. Conversely, when rioritization is introduced, the QoS may also be adversely affected due to channel access starvation caused by the referential treatment of higher riority stations. The treatment of the DWPS algorithm in this aer is aimed at finding the right balance between these extremes. In section 2, the literature review focuses on the PCF scheduling algorithm. Additionally, the motivation for a new aroach to scheduling channel access in cellular networks is resented as a recursor to the novel DWPS algorithm. The DWPS algorithm is then resented in section 3. Afterwards, the methodology along with the assumtions and arameters that were used in the simulations are resented in section 4. The simulation results are then rovided in section 5. Finally, the key findings are discussed in section LITERATURE REVIEW To rovide a non-contentious facility for channel sharing the PCF, which emloys a round-robin technique to administer channel access, was incororated into the IEEE MAC rotocol [2;4]. More secifically, the IEEE MAC rotocol, is comrised of the Distributed Coordinated Function (DCF) [2;8] and the PCF algorithms. Together they incororate the five (5) timing intervals shown in Table 1[5].

2 Tyes of Intervals Duration (µs) Short Interframe Sace (SIFS) Slot time Priority Interframe Sace (PIFS) Distributed Interframe Sace (DIFS) Extended Interframe Sace (EIFS) Variable but > DIFS Table 1: IEEE MAC Protocol Timing Intervals The reader is asked to refer to [2;8] for more information on the DCF algorithm. The PCF rotocol, illustrated in Figure 1, is described below: 1) The base station (or access oint, in other wireless networks) olls the first mobile station on its list. Note that the olling message may also iggyback additional data for the olled mobile station. 2) The olling station then resonds by sending its data, otherwise only an acknowledgment (ACK) frame is sent before the SIFS exires. 3) The base station then olls the next station in accordance with one of the following scenarios: i) after it receives an ACK from the olled station or ii) after it receives any data the olled station has to send or iii) after the SIFS exires and before the PIFS has assed. 4) Reeat from ste 1 until the olling list is emty or the non-contentious eriod of the channel access scheduling has ended. The PCF rotocol is inefficient as it olls stations that do not require the immediate use of the channel. Moreover, as it does not incororate any enhancing riority-based schemes, high-riority traffic may be adversely affected. As a result, mobile stations using delay sensitive alications may suffer a significant decrease in the Quality of Service (QoS). 3. THE VEL DWPS ALGORITHM The DWPS algorithm is being roosed as an alternative to the PCF for roviding fair channel access to mobile stations within a network. The mechanism by which this is achieved is based on the use of: 1. the alication riority levels listed in Table 2, 2. first-in-first-out (FIFO) queues and 3. a ready ratio (RR), defined as the ratio between a station s queue delay and the alication s delay tolerance threshold (DTT). Priority Level Media Tye 1 Voice Examle Mobile VoIP DTT ms Priority Status Very High 2 Video Video ms High MS5 1. Base Station olls MS1 3 Transaction Services (Low Priority) Small Messaging Service (SMS) s Medium 2. MS1 resonds with an ACK within the SIFS MS4 MS1 3(i). Base Station olls MS4. MS4 resonds with ACK within the SIFS 3. Base Station olls MS2 BS 3(ii). Base Station olls MS3. MS3 resonds with a data acket within the SIFS KEY MS Mobile Station BS Base Station ACK Acknowledgement SIFS Short Interframe Sace PIFS Priority Interframe Sace Figure 1 Oeration of PCF MS3 MS2 3(iii). MS2 fails to resond within the PIFS 4 Bulk Data Transfer File Transfer s Table 2: Alication Priority Levels Low Note that the Delay Tolerance Threshold (DTT) is the alication s absolute delay tolerance threshold, which when surassed results in alication function failure. However, the QoS of the alications noticeably degrade before delay reaches the DTT value. For examle, the QoS of voice alications degrades after 1ms but can still oerate u to ms [1]. For this reason the QoS Delay Threshold (QTT) was devised. This is the referred uer delay limit of an alication. The QTT is used to calculate

3 the RR, which reresents the degree of channel access urgency required by a mobile station. The RR is based on an oerating system scheduling algorithm called Highest Resonse Ratio Next (HRRN) [9-11]. The modified equation used for the DWPS is defined as: RR ( QTT W ) Px q Px = (1) QTT + QTT Poll chosen mobile station Allow mobile station to transmit 1 acket START Add incoming station to olling list Is channel free? where: W q = time sent waiting to use the channel QTT Px = QTT of a station with riority level x QTT = QTT of a station with riority level 1 Udate all queued stations delay counters Free the channel Does the mobile station need more channel access? In the DWPS algorithm, each station wishing to access the shared transmission media is assigned a riority level ranging from 1 (highest) to 4 (lowest) in accordance with the tye of alication being used on the mobile station. The riority levels are used to determine which queue the wireless stations will be inserted into and thus by extension, the associated QoS exerienced by the alication. Mobile stations monitor a control channel to determine when they have been assigned access to the communication channel. In the initial instance the stations are assigned to the channel on a first-come first-served basis. The remaining mobile stations, if any, are then laced into a channel access riority queue as illustrated by the flowchart in Figure 2. The DWPS algorithm accommodates the use of both riority and multile queues. The algorithm was used to test the effect of riority ratios on the delay exerienced by stations within the cell. The methodology emloyed to investigate this is described in the next section. Remove mobile station from the olling list Are all queues emty? Is alication used Voice? Place mobile station in the riority 2 FIFO queue () Place mobile station in the riority 1 FIFO queue () Choose mobile station based on lowest RR value Place mobile station in the riority 4 FIFO queue () Is it the queue s turn to use the channel? Is current mobile station delay sensitive? Is alication used Bulk Data Transfer? Place mobile station in the riority 3 FIFO queue () Are 2 or more RR values equal? Choose mobile station based on highest riority STOP Figure 2: DWPS Algorithm Flowchart

4 4. METHODOLOGY IQD is the number of IQDs for the riority The DWPS algorithm was imlemented in Java mobile stations and the simulating environment was based on IQD is the average IQD for the riority the following simlifying assertions aimed at reducing rocessing time: mobile stations 1. Each mobile station is associated n is the number of riority stations with a single riority level. 2. Communication only occurs with 5. RESULTS stations in an adjacent cell. 3. The number of mobile stations within Figures 3, 4 and 5 show the results of the cell is limited to 15, or 25. simulating the DWPS algorithm when the total 4. All riority level 3 mobile stations number of stations in the cell is 15, and 25, send 2 ackets inclusive of the resectively. In each grah, a lot of the ratio control acket. All other stations send between high riority ( and ) and low 61 ackets in total. riority ( and ) stations against the average 5. All stations initially have data to send IQD for each riority level is resented. Note when olled by the base station (BS). that the queue length for each riority level is 6. When a olled station has finished its directly roortional to the total number of communication a control acket is mobile stations ( n T ) within the cell. transmitted to the base station and the mobile station is removed from the olling list. Note that the simulation is not real-time and measurements such as transmission time and roagation delay are calculated. The major simulation arameters used in these calculations are listed in Table 3 below. Attribute Cell radius Measurement 1 m Link Caacity 2Mbs Transmission Seed ms -1 MS-to-BS distance m Table 3: Simulation Parameters Calculation of the Average IDQ in the DWPS Algorithm An incidence of QoS degradation (IQD) occurs when a articular mobile station s delay surasses its QTT value. The average IQD for each riority class is calculated by Eq. (2). IQD = n x= 1 IQD n, for 1 4 (2) Incidences of QoS Degradation Incidence of QoS Degradation (IDQ) Figure 3: Effect of Priority Ratios on the DWPS Algorithm in a cell containing 15 mobile stations Figure 4: Effect of Priority Ratios on the DWPS Algorithm in a cell containing mobile stations where: reresents the riority level

5 Incidence of QoS Degradation (IDQ) Figure 5: Effect of Priority Ratios on the DWPS Algorithm in a cell containing 25 mobile stations Observe that in Figure 3, there is a negligible increase of.2 in the average IDQ value for the riority 2 mobile stations when the ratio is 8:2. This average value increases raidly to ~58 which is slightly above the average IQD for the riority 1 mobile stations. This trend, which is also resent in figures 4 and 5, is due to the favoured treatment of the riority 1 mobile stations over the riority 2 mobile stations by the DWPS algorithm. Note that the stee increases in the average IQD are an indicator that the algorithm is failing and that the end used may notice degradation in the QoS. In figures 4 and 5 these increases occur near the 6:4 ratios. This lower ratio, comared to the one in figure 3, indicates that as the queue length increases the ability of the DWPS algorithm to successfully manage large numbers of high riority stations decreases. The almost identical grahs roduced in figures 4 and 5 suggests that when the cell has stations or more the DWPS algorithm reaches its otimum oerating caacity at a high riority to low riority ratio of 4:6. However, this still remains manageable with low IQD values occurring at a ratio of 6:4. Figure 6 and 7 show the results of simulating the Round-Robin algorithm using the same conditions as those set out in the DWPS simulation. In articular, the behaviour for riority 1 and 2 mobile stations when the total number of stations in the cell is equal to 15 is resented in figure 6. Indeed the results highlight that the algorithm begins to fail after the ratio exceeds 1:1; thus end users may exerience degraded QoS levels. In figure 7 all the delay sensitive queues maintain an almost constant average IQD of. This indicates that when the number of stations reaches, the round-robin algorithm is unable to oerate efficiently with regard to the delay sensitive mobile stations. Consequently, the results for a cell containing 25 mobile stations are not resented. Average Incidence of QoS Degradation (IQD) Figure 6: Use of Priority Ratios on the Round-Robin Algorithm in a cell containing 15 mobile stations Incidence of QoS Degradation (IQD) Figure 7: Use of Priority Ratios on the Round-Robin Algorithm in a cell containing mobile stations Note that all the grahs roduced for both the DWPS and round-robin algorithms show an IQD value of zero for the delay tolerant riority 3 and 4 alications. Thus the mobile stations using these alications can oerate within their QoS limits for both algorithms as the riority ratios has no affect on these mobile stations when n T CONCLUSION Results indicate that the ratio of high riority to low riority stations influences the oeration of the DWPS algorithm. Indeed the success in the DWPS algorithm can be seen when comaring figures 3 and 6 with resect to the delay in the onset of higher IQD values. Additionally, in comaring figures 4 and 7 the DWPS

6 categorically outerforms the round robin for all ratios. Otimum ratios for the DWPS algorithm are below 4:6, when the cell contains stations or more. For cells containing 15 stations or lower the otimum ratio can increase to at least 7:3. In essence, to work at otimum levels DWPS requires the ratio of high riority to low riority stations to decrease as the queue sizes increase. These riority ratios could be used to regulate cell admission schemes where QoS is a major concern. Results show that the DWPS algorithm can be used in reference to the round-robin algorithm as it erforms better in each of the cases examined. However, an investigation of the DWPS algorithm when the assumtions are relaxed will be essential to further ascertain its viability in the field. Note that in the future, more research will be done to include other established noncontentious algorithms such as RETHER [2;6]. REFERENCES [1] A. Tee and J. Cleveland, "A new otion roosed for 82. requirements on latency and acket error rates", IEEE, 4. Available at tt:// /Contribs/C t [2] S. Sharma, "Analysis of 82.11b MAC: A QoS, Fairness, and Performance Persective", 3. Available at htt:// [3] B. Bensaou, Y. Wang, and C. C. Ko, "Fair medium access in 82_11 based wireless ad-hoc networks", IEEE/ACM MobiHoc Worksho,, Protocol Design and Performance", IEEE International Conference on Communications, 2, [6] T. Chiueh, "RETHER: A Software-Only Real-Time Ethernet for PLC Networks", Proceedings of the Worksho on Embedded Systems on Worksho on Embedded Systems, 1999,. 6. [7] L. Chandran-Wadia, "Throughut Performance of the Distributed and Point Coordination Functions of an IEEE Wireless LAN", S. Mahajan and S. Iyer, Eds., Proceedings of the Worksho on Embedded Systems on Worksho on Embedded Systems, 2, [8] L. Zheng, A. Dadej, and S. Gordon, "Fairness of IEEE Distributed Coordination Function for Multimedia Alications", 3. Available at htt:// eng3-fairness.df [9] C. Ritcjie, Oerating Systems incororating UNIX & Windows, 3 ed. New York: Continuum, [] W. Stallings, Oerating Systems, 2 ed. Uer Saddle River, New Jersey: Prentice Hall, [11] W. Stallings, Oerating Systems: Internals and Design Princiles, 6 ed. Uer Saddle River, New Jersey: Pearson Prentice Hall, 8, [4] B. P. Crow, I. Widjaja, J. G. Kim, and P. T. Sakai, "IEEE Wireless Local Area Networks", IEEE Communications Magazine, Vol. 35, No. 9, 1997, [5] K. K. Leung, B. McNair, L. J. Cimini, and J. H. Winters, "Outdoor IEEE Cellular Networks: MAC