Voice capacity of IEEE b WLANs

Transcription

1 Voce capacty of IEEE 82.b WLANs D. S. Amanatads, V. Vtsas, A. Mantsars 2, I. Mavrds 2, P. Chatzmsos and A.C. Boucouvalas 3 Abstract-There s a tremendous growth n the deployment and usage of Wreless Local Area Networks (WLANs n recent years. There s also an ncreasng nterest n supportng voce applcatons over WLANs. In ths paper we use analytcal models presented n the lterature for saturaton throughput, average packet delay and packet drop probablty and smulatve results for jtter to evaluate voce traffc over WLANs. We propose a methodology that determnes the voce capacty of WLANs and we examne the effect of packetzaton nterval and transmsson rate on voce capacty usng G.7 codec. Analytcal results for voce capacty match smulatve results presented n the lterature usng extensve smulatons. I. INTRODUCTION Over the past few years the popularty of Wreless LANs has grown sgnfcantly. WLANs provde prncpal value n envronments wth a large and dynamc set of end users allowng easy access to the network. The man reasons for the success of WLANs are the smplcty of setup, hgh data rates and shrnkng equpment costs. Advances n technology are enablng the delvery of laptop computers wth enhanced and ntegrated WLAN capabltes. Recently, there has been growng nterest n usng WLANs to support voce communcatons, especally n an offce envronment. Thus, employees can be engaged n voce communcatons whle stll beng moble. Furthermore, as WLAN applcatons share a common transmsson medum, voce applcatons must compete wth other voce applcatons as well as wth data applcatons for bandwdth. Therefore, voce qualty and capacty can be sgnfcantly affected by the smultaneous transmsson of data and other voce traffc n WLANs. IEEE has developed the 82. [] protocol famly for WLANs. IEEE 82.b [2] s the most popular and operates n the 2.4 GHz Industral/Scentfc/Medcal (ISM band wth data rates up to Mb/s. IEEE 82.a [3] operates at hgher data rates (up to 54 Mb/s n the less crowded 5 GHz ISM band. The IEEE 82. standard defnes two modes of Medum Access Control (MAC operaton: a mandatory Dstrbuted Coordnaton Functon (DCF mode and an optonal Pont Coordnaton Functon (PCF mode. DCF allows for hghly dstrbuted medum Department of Informaton Technology Technologcal Educatonal Insttuton, Thessalonk, Greece, {dma, vtsas, pers}@t.tethe.gr 2 Department of Appled Informatcs Unversty of Macedona, Thessalonk, Greece, {mants, mavrds}@uom.gr 3 School Desgn, Engneerng & Computng Bournemouth Unversty Fern Barrow, Poole, UK tboucouv@bournemouth.ac.uk access usng Carrer Sense Multple Access wth Collson Avodance (CSMA/CA technques. PCF uses a pollng mechansm controlled by a pont coordnator operatng at the Access Pont (AP of each Basc Servce Set (BSS. The PCF mode was desgned for real-tme servces such as voce. Many manufacturers are choosng not to mplement PCF, clamng that t nhbts nteroperablty wth other APs [4] and does not, n fact, always allocate bandwdth better than DCF [5]. Furthermore, authors n [4], clam that PCF provdes margnal mprovement n performance when compared wth DCF for real-tme servces such as voce. In addton to the MAC modes, the standard also specfes two network confguratons called the Infrastructure and Ad Hoc. The Infrastructure mode requres one node to coordnate all communcatons and s based on PCF, whle the Ad Hoc mode provdes drect communcaton between peers and s based on DCF. Evaluatng voce traffc under DCF Ad Hoc confguraton s the subject of our nterest. As defned n the lterature, voce capacty s the maxmum number of voce sessons a WLAN can support smultaneously at specfc qualty lmtatons [6]. Expermental and analytcal results for voce qualty usng DCF n Infrastructure confguraton have frst been reported n [7]. Authors n [7] proposed an approxmate analytcal model whch notably llustrates the mportance of choosng a voce packet sze that s as large as possble. The voce capacty of a sngle AP s quantfed n [6] whch also examnes the effect of codec and packetzaton ntervals on voce capacty for IEEE 82.b networks. In [8] the speech capacty s evaluated usng the same conversatonal model as n [6], comparng analyss and smulaton for 82.a, 82.b and 82.g. It was concluded n [8] that channel capacty strongly depends on the channel bandwdth, voce codec packetzaton nterval and the data traffc n the system. Authors n [9] proposed solutons to performance problems usng a multplexmultcast scheme for overcomng the large overhead effect of VoIP over WLAN. In [] the co-exstence of data and voce s beng nvestgated. An analyss of the performance s provded for WLANs whch use TCP/IP for data and UDP/IP protocol for the voce packets.

2 A mathematcal analyss based on a Markov chan model has been developed n [] for a WLAN operatng n saturaton condtons,.e., the transmsson queue of each staton s assumed to be always nonempty. The analyss calculates the saturaton throughput whch s defned as the maxmum load that the system can carry n stable condtons [] and evaluates the performance for dfferent scenaros and parameter values for unlmted packet retres. The throughput model s extended n [2] to take nto account packet droppng when the packet retransmsson lmt s reached. In [3] the same model s expanded to calculate average packet delay, packet drop probablty and the packet drop tme. Research work presented n the lterature [6], [7], [8] calculates the voce capacty usng extensve smulatons. As analytcal models are much easer to use, ths work calculates the voce capacty of a WLAN based on the mathematcal model of [2], whch calculates the maxmum WLAN throughput performance assumng saturaton condtons. An ad hoc network scenaro consstng of a fnte number of statons s assumed. We also assume deal physcal channel condtons, (no transmsson errors, no hdden statons and that all statons transmt packets of constant length. Analytcal models presented n [3] that calculate average packet delay and packet drop probablty are utlzed to examne whether the voce qualty requrements are met for all consdered scenaros. Our analyss dentfes that, n ad hoc WLAN voce communcatons, the average packet delay should not exceed the packetzaton nterval,.e. a voce packet should be successfully receved before a new voce packet s generated. Ths paper s organzed as follows: Secton II descrbes the DCF method for channel access and the backoff procedure. Secton III revews the mathematcal model that calculates throughput, average delay and drop probablty. Secton IV descrbes the consdered network scenaro, the voce qualty metrcs and the methodology utlzed to calculate voce capacty. Analytcal results for voce capacty and comparson wth results presented n the lterature are placed n Secton V. Fnally, secton VI presents the conclusons. II. DISTRIBUTED COORDINATION FUNCTION (DCF DCF s based on CSMA/CA technques and adopts a slotted Bnary Exponental Backoff (BEB scheme to reduce collsons due to statons transmttng smultaneously. Each node wth a packet to transmt frst senses the medum to ascertan whether t s n use. If the medum s sensed to be dle for a tme nterval greater than the Dstrbuted Inter-Frame Space (DIFS, the staton proceeds wth the packet transmsson. If the medum s sensed busy, the staton defers transmsson and ntalzes ts random backoff nterval. Ths backoff tmer s decremented when the medum s dle and s frozen when the medum s sensed busy. After a busy perod, the backoff resumes only after the medum has been dle for longer than DIFS. A staton transmts ts packet when ts backoff counter reaches zero. After the successful recepton of a packet, the destnaton staton reples wth an mmedate postve acknowledgment (ACK after a tme nterval equal to Short Inter-Frame Space (SIFS. Snce SIFS s shorter than DIFS, the staton sendng the ACK transmts before statons attemptng to send new packets and hence takes prorty. If the source staton does not receve an ACK packet wthn a specfed ACK tmeout nterval, the data packet s assumed to have been lost and a retransmsson s scheduled. Each staton holds a retry counter that s ncreased by one each tme a data packet s transmtted unsuccessfully. If the counter reaches the retransmsson lmt m the packet s dscarded. The random number of empty slots for the backoff tmer s chosen n the nterval [,W -], where s the backoff stage, [,m] and W s the current contenton wndow (CW sze. The value of CW depends on the number of faled transmssons of a packet. At the frst transmsson attempt, CW s set equal to W whch s the mnmum contenton wndow sze CW mn =W=W. After each retransmsson due to a collson, CW s doubled up to a maxmum value W m' =CW max =2 m' CW mn where W m' s the largest contenton wndow sze. Once the CW reaches CW max, t wll reman at ths value untl t s reset. We have: W = 2 W m ( m W 2 = W > m Here m represents the maxmum backoff stage whle m' represents the number of dfferent wndow szes. The model consders that m can have a value larger or smaller than m'. The 82.b standard [2] specfes that m = 6 and m' = 5. III. MATHEMATICAL MODEL Ths study assumes deal channel condtons (no transmsson errors or hdden statons, the contentng statons are of fxed number n and each staton has always a packet avalable for transmsson of the same fxed sze. Let b(t and s(t be the stochastc processes representng the backoff tme counter and the backoff stage (, m respectvely for a gven staton at tme t. We utlze the same dscrete-tme Markov chan wth [2][3] n order to model the b-dmensonal process {s(t, b(t}. The key approxmaton n ths model s that each packet colldes wth constant and ndependent probablty p regardless of the number of retransmssons suffered. Let b, k = lm P{ s( t =, b( t = k} be the statonary dstrbuton t of the Markov chan, where [,m], k [,W -]. The probablty τ that a staton transmts a packet n a randomly chosen slot tme can be expressed as [2]: 2 2p W (2p + ( 2p τ = 2 2p + W (2p + ( 2p + W 2 p 2p m, m The probablty p that a transmtted packet encounters a collson s gven by: n p = ( τ (3 Equatons (2 and (3 represent a non-lnear system wth two unknown τ and p, whch can be solved usng numercal methods and has a unque soluton [2]. +, m> (2

3 A. Saturaton Throughput Let P tr be the probablty wth that at least one staton (out of n transmts n a consdered slot tme: P = ( τ tr The probablty P s that an occurrng packet transmsson s successful s gven by the probablty that exactly one staton transmts and the remanng n- statons defer transmsson, condtoned on the fact that at least one staton transmts: n n τ ( τ Ps = (5 n ( τ Consderng that a random slot s empty wth probablty( P tr, contans a successful transmsson wth probablty Ptr P and a collson wth probablty S Ptr PS, the saturaton throughput S s gven by: Ptr PS l S = (6 ( Ptr σ + Ptr PS TS + Ptr ( PS TC where l s the length of the transmtted packet, σ the duraton of an empty slot tme, T s and T c are the average tmes that the medum s sensed busy due a successful transmsson or a collson respectvely. The values of T s and T c depend on the channel access mechansm and n the case of basc access: bas TS = D IF S + H + l + SIF S + A C K bas TC = D IF S + H + l + SIF S + A C K where H = MAC hdr + PHY hdr s the packet header. B. Packet drop probablty The packet drop probablty s defned, as the probablty that a packet s dropped when the retry lmt s reached and t s equal to: P = drop p (7 snce a packet s dropped f t encounters collsons. C. Average packet delay The delay D for a successfully transmtted packet s defned to be the tme nterval from the tme the packet s at the head of ts MAC queue ready to be transmtted, untl an acknowledgement for ths packet s receved. If a packet s dropped because t has reached the specfed retry lmt, the delay tme for ths packet wll not be ncluded nto the calculaton of the average delay. The average packet delay E[D], s gven by: E D = E X E slot (8 [ ] [ ] [ ] where E[slot] s the denomnator of (6 and E[X] s the average number of slot tmes requred for successfully transmttng a packet and s gven by: n (4 + = m m + W ( p p E [ X ] m + (9 = 2 p where (- p s the probablty that the packet s not dropped and (p - p /(- p s the probablty that a packet that s not dropped reaches the stage. IV. NETWORK SYSTEM MODEL A. Descrpton We consder an 82.b DCF system n ad hoc mode. We assume full duplex VoIP calls between the statons. We also assume that: the physcal channel condtons are deal, (no transmsson errors, and no hdden statons and we gnore all propagaton delays. the source always has a packet to send, ether a voce packet or a data packet of the same sze, all statons are wthn range of each other and hence acheve the maxmum data rate. Voce s typcally sampled at a fxed rate, and collected bts are assembled nto packets at fxed ntervals. The voce codec used s the G.7 codec whch generates voce packets at a Constant Bt Rate (CBR of 64 kbps and assemble packets every, 2, 3 or 4 ms, resultng n packets wth payloads of 8, 6, 24 and 32 bytes, respectvely. Each call results n two Real-tme Transport Protocol (RTP streams. Each packet s attached a 68-byte packet header (2 bytes RTP, 8 bytes UDP, 2 bytes IP, and 28 bytes MAC layer header. Each MAC frame has a physcal layer header (preamble of 24 bytes, transmtted always at Mbps. The MAC layer acknowledgement (ACK s 4 bytes long and s transmtted at varable rate. The tme requred to transmt the packet payload depends on the sze of the packet, and thus on the packetzaton nterval of the voce codec. For example f voce s generated at 64 kbps and packetzed every ms, each packet payload s 64 bts and takes 58 μs to transmt at Mbps rate. Whle the headers (RTP, UDP, IP, MAC are 544 bts and ACK s 2 bts, the addtonal tme needed to transmt them at Mbps s almost 6 μs. In addton, we need 6 μs for DIFS+SIFS and 92 μs for transmttng the physcal packet headers and 92 μs for the physcal packet headers of the MAC ACK packet. It s easly seen that for 8 bytes (58 μs of payload, the overhead takes about ten tmes the tme needed to transmt ths packet at Mbps. Thus, when voce s packetzed less frequently, the result s proportonately larger payloads and more tme to transmt, but the number of packets requred to convey a fxed payload scales nversely. Therefore much of the packet overhead also scales nversely wth respect to the payload when packetzaton ntervals are ncreased, and the medum s used more effcently. B. Qualty metrcs The human ear s senstve to voce tme delays larger than 2 ms [6, ]. Thus, f a voce packet s delayed more 2 ms, t should be consdered as a lost packet. In addton, a packet s dropped when the retransmsson lmt s reached. Caputo n [4] defnes the packet drop rate lmt, the delay lmt and the jtter (n lterature t s defned also as delay varaton whch are tabulated n table I for good voce qualty.

4 TABLE I REQUIREMENTS FOR GOOD VOICE QUALITY Requrement Delay Jtter Lmt 5 ms 75 ms Drop rate 3 % In our model we take n account these qualty metrcs as these values are commonly accepted by many vendors and researchers. C. Methodology In ths paper, we try to calculate the maxmum number of VoIP clents whch can be supported wth good voce qualty n IEEE WLANs. As defned n the lterature, capacty for voce traffc s the maxmum number of voce sessons than can be supported smultaneously at specfc qualty lmtatons [6]. We use the mathematcal model presented n secton III to calculate average packet delay and packet drop probablty and examne whether the voce qualty requrements presented n secton IV-B are met. We calculate voce capacty assumng that the average packet delay should not exceed the threshold of packetzaton nterval. In other words, a voce packet should be successfully receved before a new packet s generated.e. f the packetzaton nterval s 3 ms, the maxmum number of statons that have average packet delay less than 3 ms s the voce capacty of the consdered scenaro. Ths number depends on the maxmum throughput the channel can acheve, whch s a functon of the packet sze. Other factors affectng the channel throughput nclude the packet overheads and the transmsson rate. V. RESULTS Unless otherwse specfed, the values reported n the followng fgures have been obtaned usng the system parameters n table II and are based on the Drect Spread Sequence Spectrum (DSSS physcal layer used n 82.b standard. TABLE II IMPORTANT PARAMETERS IN 82.B Parameter Value Slot 2 μs SIFS μs DIFS 5 μs PHY layer header 92 bts MAC layer header 224 bts ACK packet 2 bts + PHY layer header CW mn 32 Number of CW szes, 5 Retry lmt 6 Packet payload Depends on codec Channel bt rate, 2, 5.5, Mbps Fg. plots the average packet delay aganst the number of statons for dfferent packetzaton ntervals usng G.7. In the fgure, the horzontal lnes are the lmts for packets assembled every, 2, 3 and 4 ms. The pont where the packet delay for a specfc packetzaton nterval crosses the horzontal lne of ths packetzaton nterval ndcates the voce capacty of the WLAN f the G.7 codec mplements the specfc packetzaton nterval. Voce capacty values are tabulated n Table III. TABLE III MAXIMUM NUMBER OF VOICE STATIONS AND VOICE SESSIONS Packetzaton Interval Number of statons Voce Capacty (number of sessons ms ms ms ms 4 2 The number of sessons s rounded to an even value ndcatng that when the number of statons s an odd number, the last sesson s assumed as ncomplete. The voce capacty results of Table III match the expermental results presented n [7] ndcatng the correctness of the analytcal methodology presented n ths work. Packet delay (ms Packetzaton nterval ms Packetzaton nterval 3 ms Packetzaton nterval 2 ms Packetzaton nterval 4 ms Fgure Average packet delay aganst number of statons at Mbps Jtter (ms % Jtter, ms Jtter, 2 ms Jtter, 3 ms Jtter, 4 ms Packet drop probablty, ms Packet drop probablty, 2 ms Packet drop probablty, 3 ms Packet drop probablty, 4 ms Fgure 2. Packet drop probablty and jtter versus number of statons at Mbps In Fg. 2 we plot jtter and drop probablty for the same packetzaton ntervals as n prevous fgure. Fgure 3% 2% % Packet drop probablty

5 ndcates that packet drop probablty s ndependent of the packetzaton nterval. Ths can easly be explaned by consderng equaton (7, whch ndcates that p drop depends only on m and p; p s calculated by solvng the non-lnear system of (2 and (3, thus p depends on W, m' and m. Fgure ndcates that the packet drop rate qualty lmt of 3% s never reached n the consdered confguraton and thus, packet droppng does not affect the voce capacty of WLANs n ths knd of setup. Fgure 2 also plots smulatve results for jtter as no analytcal model for jtter s known to us n the lterature for the consdered confguraton. The jtter qualty lmt of 75 ms (Table I s exceeded for hgh n and the average delay qualty lmt s reached before the jtter qualty lmt for the packetzaton ntervals 2 ms and 3 ms. The jtter results for the 4 ms packetzaton nterval ndcate that jtter exceeds the qualty lmt of 75 ms for a WLAN consstng of 34 or more transmttng statons. Thus, jtter lmts the voce capacty to 7 sessons, a value lower than the voce capacty consderng the average packet delay (fg., Table III. Packet delay (ms Transmsson rate Mbps Transmsson rate 5.5 Mbps Transmsson rate 2 Mbps Transmsson rate Mbps Fgure 3 Packet delay for dfferent transmsson rates wth l=92 bts Fgure 3 examnes the effect of transmsson rate on voce capacty by plottng the average packet delay for all transmsson rates of the 82.b for a packet payload of 24 bytes (92 bts when G.7 s used for packetzaton every 3 ms. Our proposed threshold of 3 ms crosses the lnes representng the transmsson rates at dfferent number of statons ndcatng the voce capacty value at specfc transmsson rate. At Mbps there are only 8 statons, at 2 Mbps 3 statons, at 5.5 Mbps 25 statons and at Mbps 33 statons. These results ndcate that when transmsson rate ncreases, voce capacty ncreases. When ths packetzaton nterval s used, as n most commercal mplementatons of IP phones, the voce capacty s reduced from 6 at Mbps to only 4 at Mbps. As t s shown n Fgure 2 the packet drop probablty and jtter lmts are never exceeded for ths packetzaton nterval. The domnant factor s the packet delay lmt whch restrcts the voce capacty for the specfed packetzaton nterval. VI. CONCLUSIONS In ths paper, we present a methodology that calculates the voce capacty of ad hoc WLANs based on mathematcal models presented n the lterature. We present the requrements for good voce qualty n terms of delay, jtter and drop probablty. We propose that the average packet delay should not exceed the packetzaton nterval ndcatng the voce capacty for the packetzaton nterval of the codec used. The calculated results match results presented n the lterature usng smulatons. We have also shown that the transmsson rate has a sgnfcant effect on voce capacty. The number of users ncreases when the transmsson rate ncreases. ACKNOWLEDGMENT Ths work s funded by the Greek Mnstry of Educaton (25% and European Unon (75% under the EPEAEK II program Archmedes. REFERENCES [] IEEE standard for Wreless LAN Medum Access Control (MAC and Physcal Layer (PHY specfcatons, ISO/IEC 882- :999(E, Aug [2] Wreless LAN Medum Access Control (MAC and Physcal Layer (PHY Specfcaton: Hgh-Speed Physcal Layer Extenson n the 2.4 GHz Band, IEEE 82.b Sept [3] Wreless LAN Medum Access Control (MAC and Physcal Layer (PHY Specfcaton: Hgh-Speed Physcal Layer n the 5 GHz Band, IEEE 82.a, Sept [4] Koepsel A., Ebert J.-P., and Wolsz A., A Performance Comparson of Pont and Dstrbuted Coordnaton Functon of an IEEE 82. WLAN n the Presence of Real-Tme Requrements, n Proc. of the 7thInternatonal. Workshop on Moble Multmeda Communcatons (MoMuC2, Tokyo, October, 2. [5] Veeraraghavan M., Cocker N. and Moors T., Support of Voce Servces n IEEE 82. 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