Modeling Throughput and Delay in Infrastructure Mode Networks with QoS Support from the Point Coordination Function

Transcription

1 Proceeding of the World Congre on Engineering and Computer Science 2012 Vol II, October 24-26, 2012, San Francico, USA Modeling Throughput and Delay in Infratructure Mode Network with QoS Support from the Point Coordination Function Mahaweta Sarkar, Chritopher P. Paolini, and Santoh Nagaraj Abtract Thi paper analye a hybrid medium acce control (MAC) cheme that alternate between contention and polling period in an infratructure mode wirele local area network (WLAN). The cheme can be thought of a a tandard channel acce cheme operating both in the Ditributed Coordination Function (DCF) and Point Coordination Function (PCF) mode for enhanced Quality of Service (QoS) performance. An analytical model ha been propoed by the author for uch a hybrid MAC, and dimenionle expreion depicting throughput and delay have been derived by the author. Graph diplaying throughput and delay characteritic of the hybrid MAC a a function of packet colliion probability and high priority tation count are preented. The author' dimenionle expreion characterizing throughput and delay a a function of the contention free period ratio i ignificant becaue the model can be ued in infratructure mode wirele networking by an acce point to dynamically adjut the contention free period ratio to an optimal value that minimize delay while maximizing throughput, baed on current high priority tation count. Index Term WLAN; QoS; Throughput Analyi; Delay Analyi, Infratructure Mode; Hybrid MAC Model; Point Coordination Function; Ditributed Coordination Function T I. INTODUCTION raffic generated in any data network ytem i neither uniform nor homogeneou. Performance enitive traffic uch a voice and video application require tringent delay contraint while data packet of a file tranfer application, for example, can operate over a much broader delay and throughput requirement. In order to provide differentiated ervice to different categorie of traffic, the IEEE e MAC tandard [1] ha the proviion of traffic claification and prioritization. The tandard claifie network traffic into four different priority level or acce categorie (AC). Node maintain eparate queue for each AC and packet at the head-of-line () of each queue contend for channel acce uing AC-pecific parameter. Manucript received July 24, 2012; revied Augut 10, M. Sarkar i with San Diego State Univerity, San Diego, CA USA (phone: ; fax: ; markar2@mail.du.edu). C. P. Paolini i with San Diego State Univerity, San Diego, CA USA ( paolini@engineering.du.edu). S. Nagaraj i with San Diego State Univerity, San Diego, CA USA ( nagaraj@mail.du.edu). On the other hand, legacy [2] ha the proviion for node to be operating in two different mode (a) DCF or Ditributed Coordination Function and (b) PCF or Point Coordination Function. While DCF i baed on the contention baed CSMA/CA [3] mode of channel acce, PCF i baed on the polling mechanim. Limited QoS upport in the legacy tandard i available through the ue of the PCF. Previou work on analyzing throughput and delay in wirele network ha focued on modeling performance apect of the IEEE tandard [3,4,5,6,7,8,9] while conidering the ditributed coordination function (DCF) only. Other publication that analyze e have provided an overview of the QoS enhancement [10,11], or an analyi of performance for particular application [12], but have not attempted to model throughput and delay in analytical form. Thi paper take a firt tep toward finding analytical expreion for modeling throughput and delay characteritic of a MAC protocol that mimic the IEEE e in every eential repect. We firt propoe a implified model of the IEEE e MAC. Thi model can be thought of a a hybrid MAC model which operate in both the contention and contention free phae alternately akin to a legacy MAC protocol with both it DCF and PCF mode enabled. The ret of the paper i organized a follow: Section III decribe our e-like Hybrid MAC model before enumerating the detail of the ytem model in Section IV. Section V preent analytical expreion for dimenionle throughput and delay for our Hybrid MAC. Section VI preent graph to viualize delay and throughput a a function of colliion probability. The paper i concluded in Section VII. II. NOMENCLATUE MAC media acce control i a ublayer of the data link layer that provide channel acce control facilitie to allow everal tation to have multiple acce to a wirele network. LP low priority packet belonging to background or bet effort protocol uch a FTP and SMTP. HP high priority packet belonging to video or voice AC protocol uch a TP, TSP, and VoIP. acce category i a traffic priority claification. In thi paper we ue two AC: HP and LP. DCF ditributed coordination function i a fundamental acce method that ue the CSMA/CA (Carrier Sene Multiple Acce with Colliion Avoidance)

2 Proceeding of the World Congre on Engineering and Computer Science 2012 Vol II, October 24-26, 2012, San Francico, USA cheme to ene a channel and wait until the channel i free before tranmitting. If the channel i buy, the DCF will backoff a number of lot time before attempting to tranmit again. PCF point coordination function i a channel acce method that i centrally controlled by a point coordinator implemented in an acce point. The point coordinator maintain a lit of tation eligible for polling and poll tation in a round-robin manner to give each tation guaranteed channel acce. contention period i a reoccurring time in which tation compete for channel acce uing the CSMA/CA cheme. Station ene a channel and wait until the channel i free before tranmitting. If the channel i buy, the tation will backoff for a random period before reattempting channel acce. contention free period i a reoccurring time in which tation are polled by an acce point' point coordinator and provided guaranteed channel acce for a pecified time. Station with delay enitive traffic (voice and video) can determine an upper bound on packet latency becaue tation are guaranteed a tranmiion opportunity after receiving a poll from a point coordinator. head-of-line, the front of a FIFO queue where packet are equeued for tranmiion. AIFS arbitration inter frame pacing i a variable period of time a tation mut wait before tranmitting packet for each acce category (LP or HP). To provide HP packet higher priority to the channel, the AIFS period for HP claified packet i horter than the AIFS period for LP claified packet. STA tation i a mobile node other than an acce point. α value between 0 and 1 that identifie the ratio of the time pent in the to the total time panned by a uperframe. σ lot time i a unit of time equal to the um of the xtx turnaround time (time for a tation to witch from receive to tranmit mode), the channel ening time, the channel propagation delay, and the MAC proceing time. III. SIMPLIFIED IEEE E HYBID MAC MODEL The IEEE e MAC tandard provide ditributed ervice differentiation or QoS by employing a priority ytem. Network traffic i claified into four different priority level or AC. Node maintain eparate queue for each AC and packet at the head-of-line () of each queue contend for channel acce uing AC-pecific parameter, namely cutomized back-off and channel ening duration [3]. Such a mechanim facilitate differentiated QoS where high priority, performance enitive traffic, uch a voice and video application, will experience le delay and greater throughput, compared to low priority traffic (e.g., FTP and SMTP tranfer). In thi paper, we analyze the e MAC protocol and propoe a Hybrid-MAC model that reemble the e MAC in mot eential repect. Our MAC model provide u with an abtraction of the eential feature of e MAC, while avoiding complex detail. We believe that the inight obtained by uing our model are applicable to the e cenario and can inform future tandard evolution. In our ytem, application are claified into two priority level or AC and each node maintain two queue, namely a high priority traffic (HP) queue and a low priority traffic (LP) queue. Our model can be generalized to incorporate more AC. Traffic aigned to the HP category i delay enitive wherea LP traffic i delay tolerant. The network can operate in both contention and contention-free phae and thee phae alternate periodically. During the contention phae, a node with packet to tranmit will contend for channel acce uing the tandard CSMA/CA algorithm [6]. QoS differentiation i enforced by allowing packet in the HP queue preferential channel acce by enabling the interface to ene the channel prior to data tranmiion for a horter period of time (AIFS) and alo to back-off for a horter duration, when faced with a colliion or buy channel ignal, than the packet mut wait in the LP queue. Thi mechanim i imilar to the IEEE e preferential channel acce cheme. We aume that node are tranmitting to an acce point (AP) that can invoke the contention-free period by iuing a poll requet to one or more node. Thee polled node can then tranmit without any contention during the contention-free period. Thu, our protocol i very imilar to the e Hybrid Coordination Function (HCF), with the contention period correponding to e random acce or enhanced ditributed coordination function (EDCF) functionality and the contention-free period correponding to the e polled acce or HCCA functionality. A diagram howing an example cenario involving communication between an AP and one node i illutrated in Figure 1. IV. SYSTEM MODEL Our elected ytem model i a Baic Service Set (BSS) of N low priority and M high priority traffic flow. We aume that each flow i generated by a node which we refer to a a STA (tation) a done in the tandard. During the, each STA ue the baic acce mechanim only. That i, no STA i aumed to be hidden from another STA and the TS/CTS mechanim i not employed. During the, the M high priority traffic STA are placed in a circular queue and are polled equentially by the PCF a dicued hortly below. In our imulation, each STA i aumed to have a ingle IEEE b tranceiver with an omni-directional antenna. The PCF implement two period of channel acce in a duration of time referred to a the uperframe : (i) a contention free period () and (ii) a contention period (). The proportion of time allocated to each period within a uperframe i not defined by the tandard; however, the length of time allocated to the mut be at leat long enough to accommodate the tranmiion of one MAC Service Data Unit (MSDU) with a maximum frame length of 2304 byte. The period of a uperframe i delimited by a beacon frame tranmiion. The beacon i tranmitted by the deignated acce point (AP) within a Baic Service Set (BSS) and carrie with it protocol related parameter that are ued by STA to ynchronize local timer and learn when the following beacon frame will be tranmitted.

3 Proceeding of the World Congre on Engineering and Computer Science 2012 Vol II, October 24-26, 2012, San Francico, USA Synchronized data exchange within the i accomplihed by polling STA. The polling proce i coordinated by the PCF implementation within an AP. When the begin, the AP wait a brief duration of time known a a hort interframe pace (SIFS) which erve a a delay between beacon, data, acknowledgement, and end frame that are tranmitted during the. The value of SIFS varie by the particular tandard implemented by a tranceiver. For a, b, and g, the value are 16, 10, and 10 μ, repectively. After waiting an initial SIFS time period, the AP commence with polling by tranmitting a Data/CF-Poll frame to the firt STA in a polling lit. Data/CF-Poll frame erve a dual purpoe by piggybacking data carried by the AP which, in an infratructure mode network, i attached to a wired network via a wired Ethernet interface. The Data/CF-Poll frame poll the receiving STA while imultaneouly carrying higher layer datagram originating from another STA within a BSS or a device external to a BSS via a wired LAN. The colliion avoidance (CA) mechanim of CSMA/CA cannot guarantee colliion will not occur. A colliion can occur, for example, if two STA compute exactly the ame backoff time after detecting a channel idle for a DCF interframe pace duration (DIFS) and then tranmit a MPDU when the backoff timer mature. To determine if a tranmiion reulted in a colliion, each data frame (MPDU) mut be acknowledged through the tranmiion of an ACK frame ent by the STA receiving a data frame. If a ending STA doe not receive a correponding ACK after waiting a SIFS period, the ending STA conclude a colliion occurred and will repeat the tranmiion. DIFS value for a, b, and g are 34, 50, and either 28 or 50 μ, depending on lot time, repectively. In IEEE g, the lot time can be either 9 μ if no legacy b STA are preent in the BSS, or 20 μ if the BSS ha a mix of b and g STA. DIFS i a function of SIFS and i computed according to DIFS SIFS 2 (1) where σ i the lot time defined to be twice the maximum propagation time τ. The lot time i therefore an amount of time a STA wait to determine if another STA ha acceed Fig. 1. Example cenario involving communication between the point coordinator and one tation during the contention free period. the channel at the tart of the previou lot. Slot time value for a and b are 9 and 20 μ, repectively, for a PHY that ue a Direct Sequence Spread Spectrum (DSSS) modulation technique and 50 μ for a PHY that ue a Frequency Hopping Spread Spectrum (FHSS) tranmiion method. Acknowledgement frame may alo piggyback data originating from a receiving STA and intended for another STA in the BSS or an external device. If the point coordinator fail to receive a repone from a polled STA within a PCF interframe pace (PIFS) period of time, the PCF will move on and poll the next STA in it polling lit. PIFS i alo a function of SIFS and i computed according to PIFS SIFS (2) and thu the PIFS value for a, b, and g are 25, 30, and either 19 or 30 μ, repectively. The PIFS duration alo erve a a gap between the and. From (1) and (2) we have the following inequality SIFS PIFS DIFS (3) which prevent the PCF from tranmitting a poll frame in between a Data/CF-Poll and Data/CF-ACK tranaction. The point coordinator ubytem reiding in an AP will continue to poll STA in it polling lit until the duration expire, at which time a pecial CF-End frame i tranmitted by the PCF to mark the end of the. V. MODELING THOUGHPUT Our analytical model for overall ytem throughput i a dimenionle multivariable function S of N, M, p, and α, S S( N, M, p, ) (4) where p i the probability of a ucceful frame tranmiion and α i a value between 0 and 1 that identifie the ratio of the time pent in the to the total time panned by a uperframe which form a repeating interval of contention and contention free time period, A α tend toward 0, the BSS revert to a contention only baed environment where the point coordinator i not ued to poll STA. With a non-zero α, dimenionle throughput S become a weighted um of time pent in the and the, (5) S( N, M, p, ) (1 ) S S (6) We define S and S a dimenionle throughput for each repective period, S S I U B U (7) (8) B The definition of S from [1] i given by equation (7) where U i the average duration of time ueful data i received by a STA during the, I i the average duration of time the channel remain idle during the, and B i the average duration of time the channel i buy tranmitting data and the overhead bit incurred by the data, and time taken handling colliion. Equation (7) i then a dimenionle quantity between 0 and 1 that repreent throughput efficiency a the ratio of time the channel i ued for ending ueful data to total time during the contention period. S i imilar to S, but doe not include the idle term in the denominator ince it i aumed the channel i never idle during the. The definition of U I, and, B are adopted from [1], with the light modification that the total STA count N in [1] ha been replaced by (N+M), that i N M Tp U (9) N M 1 p 1 1 p

4 Proceeding of the World Congre on Engineering and Computer Science 2012 Vol II, October 24-26, 2012, San Francico, USA (10) 1 1 p I N M B T 1 p N M (11) where T i the time pent ening the channel during a ucceful frame tranmiion and T i the time pent tranmitting ueful data in the. Subtituting (9), (10), and (11) into (7), we obtain S N N M Tp1 p T T 1 p M1 NM (12) The expreion for T i given by H P ACK T DIFS SIFS 2 (13) Our derivation of S proceed in a imilar way. Let q repreent the probability a STA ha a non-null data frame to tranmit during the. U i the average time pent during the to tranmit ueful data. By ueful data we mean data bit and not bit belonging to beacon, pure ACK, or CF-End frame. If we denote P a the number of data bit tranmitted during the, then P U (14) where i the fixed tranceiver data rate. In our imulation we ue the b maximum data rate of 11 Mbit/. To derive an expreion for B, the time the channel i buy in the during a ucceful polling tranaction, we need to account for all the individual frame tranmiion hown in Figure 1, B CF Beacon PIFS 2SIFS H P oll ( NM) q 2 H P CFACK N M SIFS ( NM) H P CFNull 1q (15) where CF Beacon, CF Poll, CF ACK, and CFNull are the length of the beacon, Data/CF-Poll, Data/CF-ACK, and CF-NULL frame, repectively. CF-NULL frame are tranmitted by a polled STA if the STA doe not have any pending data to end. τ i the propagation delay of the wirele LAN and H i the length of the header and frame check equence (FCS) of an frame. In our imulation we aume each frame ha a 30 byte header and a 4 byte FCS and thu H=34. VI. MODELING DELAY Our analytical model for overall ytem delay i a dimenionle multivariable function D of N, M, p, and α, Oberve D D( N, M, p, ) (16) 0 1 (17) D actual where D ideal i the theoretical minimum delay a STA can experience in a uperframe while D actual i the true delay experienced. If we define D uch that D D 1 D ideal actual (18) Then D 0 a the actual delay approache the ideal, and D 1 a actual delay diverge from the ideal. We firt conider delay incurred by the DCF. A decribed in [3], ideal delay in the can be expreed a the um of ideal head-of-line () delay and ideal queuing delay, Queuing D D D (19) ideal ideal ideal where repreent the minimum time required in the to tranmit an frame uccefully, upon the firt attempt, and i equal to T. Ideal queuing delay i given by the Pollaczek-Khinchine formula [3,13] Queuing cv (20) that decribe the mean time a frame wait in queue to be erviced by the MAC, where the queue i modeled a a M/G/1 queue (a ingle erver with frame arrival having a Poion ditribution and ervice time having a general ditribution). Total actual delay D actual i modeled in [3] a the um of (21) and an expreion for the expected value of delay which take into account backoff delay, CWmin E Dactual T P P 1 21P 121P P P Pr max 1 11P 1 P T P (21) where β i the average phyical time between two decrement of the backoff counter, CW min i the minimum contention window ize, P 1 p MN1 i the probability a STA frame tranmiion i ucceful, and r max i the maximum number of retranmiion permitted. In our imulation, CW min i et to 2 4 and CW max i et to 2 10 which are the value ued by a PHY that employ a Frequencyhopping pread pectrum (FHSS) method of tranmitting radio ignal. In addition, r max i defined a r log CW CW (22) max 2 max min i (21) without any backoff delay, D T (23) ideal

5 Proceeding of the World Congre on Engineering and Computer Science 2012 Vol II, October 24-26, 2012, San Francico, USA Conidering now the PCF, each STA ha an opportunity to tranmit when polled while the i in progre. If the maximum predetermined duration of the in a given uperframe expire before every STA ha been polled, STA that were not given an opportunity are more likely to be polled in the following a the PC ue a circular queue to chedule tation polling. Let Ψ repreent the expected value of the ize of a frame tranmitted by a polled STA during the and ψ repreent the ize of the body of data within thi frame, 34 E[ ] (24) Auming the length of data in frame tranmitted during the i uniformly ditributed, the total time for one i given by T, CF N M PC Beacon STA T (25) CFEnd 2 N M 1SIFS N M In our imulation, CF Beacon and CF End are et to 180 and 20 byte, repectively. Let D repreent the average time a frame mut wait at the head-of-line once the begin, CFBeacon N M PC STA D 1 2 N M N M 1 SIFS 2 From (19), (20), (23), and (26) we now have 2 T 1 cv 2 1 CFBeacon N M PC STA 1 2 N M N M 1 SIFS 2 (26) (27) Accounting for backoff delay, (27) i modified to give D actual, CWmin Dactual T P 2 1 P 1 21P 121P 1 P Pr max 1 11P 1cv P 1 P T P (28) CFBeacon N M PC STA 1 2 N M N M 1 SIFS 2 A plot of our analytical expreion for dimenionle throughput and normalized delay baed on our derivation i hown in Figure 2 and 3. Parameter value ued in both plot are given in Network Simulation Parameter1. TABLE I. NETWOK SIMULATION PAAMETES1 Data ate () 2 Mbp Frame Data Uniformly ditributed in range [0,2312] B Size PHY Header 24 Byte Size MAC Header 34 Byte Slot Time σ 50 μ SIFS 28 μ Prop. Delay τ 10 μ CW min, CW max 2 4, 2 10 Beacon Size 90 Byte Packet interarrival 2*10-8 rate (λ) Figure 2 and 3 how a urface plot that quantifie the relationhip between colliion probability, number of HP uer, and the effect thee parameter have on ytem delay and throughput, repectively. In Figure 2 we ee that, a the number of HP tation increae, a aturation condition at normalized delay D = 1 i attained with lower value of colliion probability p. Colliion probability p i defined a the probability a given frame tranmiion attempt i unucceful due to a colliion occurring in the. Looking at Figure 2, one can ee that for a mall number of HP tation, the directional derivative dd/dp i le than it i for a large number of HP tation. Becaue the rate of change in delay increae fater with repect to tation count a colliion probability increae, a aturation condition will arie ooner in a BSS with many high priority traffic tation if tation begin to experience a greater number of colliion in the contention period. Similarly, in Figure 3 we ee how mall change in colliion probability can greatly affect throughput a the HP tation count increae. We alo ee the appearance of an optimal throughput contour along the maxima of the urface S. Fig. 2. Normalized delay urface plot D = D(HP, p).

6 Proceeding of the World Congre on Engineering and Computer Science 2012 Vol II, October 24-26, 2012, San Francico, USA Fig. 3. Dimenionle throughput urface plot S = S(HP, p). VII. CONCLUSION In thi paper, the author have preented analytical expreion to model the throughput and delay of a hybrid MAC cheme akin to IEEE e. The author have extended the work of Khalaf and ubin in [3] and have made the following new contribution: the development a dimenionle expreion for throughput a a function of the ratio α for two acce categorie (HP and LP), the development a dimenionle expreion for delay a a function of α for two acce categorie, expreing dimenionle delay a a function that range from 0 to 1, baed on ideal and actual delay time, expreion for ideal and actual delay time, and a code to viualize throughput and delay a a function of HP STA count and colliion probability for elected value of α. The author conider thee contribution are ignificant becaue the concept and expreion can be ued in an optimization framework by an acce point to dynamically adjut α, in real-time, a HP STA aociate and diociate from the acce point. For example, an acce point could repeatedly etimate the colliion probability baed on pat datagram retranmiion tatitic and elect a value for α that minimize delay and maximize throughput baed on current HP STA count. The author have hown the value of α ha a ignificant effect on ytem performance with repect to throughput and delay. An increaing number of HP uer create higher contention in the phae leading to longer backoff time and thereby a drop in throughput and an increae in delay. A future direction will be to introduce a numerical Min-Max optimization trategy to find the optimal value of α that minimize delay while maximizing throughput, and to imulate the ue of an optimal α in a dicrete event network imulator uch a n-3, OPNET Modeler, or QualNet. [2] "IEEE Standard for Information Technology-Telecommunication and Information Exchange between Sytem-Local and Metropolitan Area Network-Specific equirement - Part 11: Wirele LAN Medium Acce Control (MAC) and Phyical Layer (PHY) Specification," IEEE, IEEE Std , June 12, [3]. Khalaf and I. ubin, "Throughput and Delay Analyi in Single Hop and Multihop IEEE Network," in 3rd International Conference on Broadband Communication, Network and Sytem, 2006 (BOADNETS 2006), San Joe, CA, 2006, pp [4] M. M. Carvalho and J. J. Garcia-Luna-Aceve, "Delay Analyi of IEEE in Single-Hop Network," in Proceeding of the 11th IEEE International Conference on Network Protocol (ICNP), Wahington, DC, November 04-07, 2003, p [5] G. Bianchi, "Performance Analyi of the IEEE Ditributed Coordination Function," IEEE Journal on Selected Area in Communication, vol. 18, no. 3, pp , March [6] O. Tickoo and B. Sikdar, "Modeling Queueing and Channel Acce Delay in Unaturated IEEE andom Acce MAC Baed Wirele Network," IEEE/ACM Tran. Netw., vol. 16, no. 4, pp , Augut [7] T. Suzuki and S. Taaka, "Performance Evaluation of Integrated Video and Data Tranmiion with the IEEE Standard MAC Protocol," in Global Telecommunication Conference (GLOBECOM '99), vol. 1B, io de Janeireo, Brazil, 1999, pp [8] O. Tickoo and B. Sikdar, "Queueing Analyi and Delay Mitigation in IEEE andom Acce MAC Baed Wirele Network," in Proceeding of IEEE INFOCOM, Hong Kong, P.. China, 2004, pp [9] L. Yun, L. Ke-Ping, Z. Wei-Liang, and W. Chong-Gang, "Analyzing the Channel Acce Delay of IEEE DCF," in IEEE Global Telecommunication Conference (GLOBECOM '05), vol. 5, St. Loui, MO, 2005, pp [10] S. Mangold, S. Choi, G. Hiertz, O. Klein, and B. Walke, "Analyi of IEEE e for QoS Support in Wirele LAN," IEEE Wirele Communication, vol. 10, no. 6, pp , December [11] A. Lindgren, A. Almquit, and O. Schelén, "Quality of Service Scheme for IEEE Wirele LAN: An Evaluation," Mob. Netw. Appl., vol. 8, no. 3, pp , June [12] A. Köpel, J.-P. Ebert, and A. Woliz, "A Performance Comparion of Point and Ditributed Coordination Function of an IEEE WLAN in the Preence of eal-time equirement," Waeda, Tokyo, Japan, October 23-26, [13] I. Mitrani, "Modelling of Computer and Communication Sytem," Cambridge Computer Science Text ed.: Cambridge Univerity Pre, EFEENCES [1] "Medium Acce Control (MAC) Quality of Service (QoS) Enhancement," IEEE e Working Group, New York, P802.11e/D13.0, January 2005.