Achevable Bandwdth Estmaton for Statons n Mult-Rate IEEE 802. WLAN Cells Eduard Garca, Davd Vamonte, Rafael Vdal and Josep Paradells Wreless Networks Group - echncal Unversty of Catalona (UPC) {eduardg, vamonte, rvdal, teljpa}@entel.upc.edu Abstract hs paper analyzes the effect of mult-rate transmssons n a CSMA wreless LAN envronment. Observatons n a real testbed showed that bandwdth resources (n Bytes/s) are shared farly among all statons even though transmssons carred out at lower rates capture the medum for longer perods, whch drastcally reduces the overall throughput. he ntrnsc concept of farness n a CSMA scheme wth multple rates s quantfed by means of a new formulaton whch s valdated through smulatons and practcal measurements. he algorthm presented provdes the maxmum achevable bandwdth that can be offered to a gven IEEE 802. staton. Havng ths nformaton has evdent applcatons n realtme multmeda transmssons over WLANs. he algorthm was also run n commercal APs as a proof of concept, after analyzng ts mplementaton ssues. he model presented n secton 4 of ths paper has been revsed as a result of a posteror analyss. Please refer to () for an updated and extended verson of secton 4 only. (): http://hdl.handle.net/27/2045. Introducton he present popularty of WLANs, especally those based on IEEE 802. standards, requres specal attenton wth the am of explanng all observable phenomena whch affect ther effcency and ther provson of qualty. IEEE 802. standards defne several sets of modulatons and codng rates for the dfferent physcal layers. Each dfferent scheme provdes a dfferent transmsson rate, but the hgher the chosen rate, the worse t performs n presence of nose and nterference. As the qualty of the sgnal gets worse, the physcal rate must be lowered n order to acheve an acceptable packet error rato (PER). Furthermore, CSMAtype access s used: a staton wshng to transmt frst probes the medum and transmts only f the medum s sensed dle, ensurng long-term channel access farness among all actve statons. In a mult-rate envronment, ths concept of farness nvolves a consderable loss of effcency for the whole cell, snce slow statons wll capture the common medum for longer perods. In ths way all statons obtan an equal bandwdth (expressed n Bytes/s). In consequence farness s acheved at the cost of penalzng hgher rate statons, whch leads to a loss of effcency. In other words, on the one hand far access n a mult-rate CSMA/CA network s translated nto an equtable bandwdth allocaton (n Bytes/s), but on the other hand t entals an unfar sharng of tme, whch reduces the throughput that fast statons can acheve. he problem was frst descrbed as a performance anomaly n []. hese statements are verfed n the example of fg. : connected to one IEEE 802.b AP, two hosts (A and B) are n saturaton state as they sent 500-byte UDP datagrams. Dfferent rates were used n B (represented on the x-axs n Fg. ) for a fxed rate n A. No matter what rate they use, the ndvdual throughput s almost the same for both statons. As we decreased the rate of one of the statons, the overall throughput decreased sgnfcantly. In ths paper we dscuss the effect of a mult-rate envronment n WLANs and provde a formulaton that allows an accurate estmaton of the achevable throughput, motvated by ts applcaton n multmeda transmssons over WLANs. Most of the related prevous work n the feld was focused on fndng the maxmum capacty for real-tme flows based on saturaton condtons and wthout consderng background traffc [2]. In [3] a specal nterest was devoted to the effect of mult-rate statons. We provde an analytcal model that s valdated through practcal measurements and smulatons. From ths formulaton we derved an algorthm that was developed to run n commercal IEEE 802. Access Ponts. he algorthm performs an estmaton of the maxmum bandwdth that the Access Pont (AP) can offer to each of ts assocated statons. he results of the mechansm we present can also be used to compute a load metrc based on tme share, whch descrbes the actual load of a cell more precsely than the number of actve users or carred traffc. he use of our analytcal method to provde the capacty that a network s able to offer to a gven staton avods the undesred effects of exstng nvasve mechansms (e.g. see [4]), whch consst of njectng real data over the ar nterface, thus consumng valuable resources and possbly producng a negatve mpact on ongong real-tme transmssons [5][6].
hroughput Mbps 3,5 3 2,5 2,5 0,5 0 5,5 2 PHY rate (Mbps) of contendant B Our contrbuton s thus the desgn and mplementaton of a mechansm that accurately performs an estmaton of the cell throughput and the achevable ndvdual bandwdth for a set of statons transmttng wthn a WLAN cell, usng potentally dfferent modulatons due to varyng wreless envronments. Beng able to perform accurate throughput estmatons s of captal mportance n several scenaros, and n partcular n the case of real-tme multmeda applcatons. hs estmaton can be used as a bass to perform a number of decsons, such as: - Audo / Vdeo CODEC selecton and parameterzaton for multmeda sessons. - Admsson Control and Polcng based on network condtons. he rest of the paper s organzed as follows: Frst, secton 2 presents the motvaton of the proposed mechansm through a dscusson of ts applcatons. Secton 3 brefly ntroduces the basc concepts of IEEE 802. standards. he formulaton and the algorthm used to compute the avalable bandwdth s detaled through Secton 4. In Secton 5, mplementaton detals are dscussed. Secton 6 presents the evaluaton results, and fnally our concludng remarks are gven n Secton 7. 2. Applcablty statements 2.. Multmeda CODEC adaptaton A ( M bps) B (A= M bps) A (5.5 M bps) B (A=5.5 M bps) A (2 M bps) B (A=2 M bps) A ( M bps) B (A= M bps) Fg.. Average throughput of statons A and B for dfferent 802.b physcal rates A frst scope where ths knd of mechansm can be used relates to the ablty to adapt audo/vdeo CODEC settngs to the actual network condtons. For example, a staton under bad rado coverage condtons wshng to use a hgh qualty vdeo stream may actually affect performance of all users connected to the same AP (possbly wth better rado condtons). An AP mplementng the proposed algorthm would be able to perodcally provde nformaton about network condtons to all statons, so that they are able to perform the best decson at any gven tme (e.g.: CODEC selecton). In [7] authors propose an archtecture where the CODEC of VoIP calls n a mult-rate WLAN s selected accordng to both MAC layer nformaton and RCP (Real- me Control Protocol) statstcs. In a smlar way and also n the context of IEEE 802. WLANs, n [8] frame sze and rate of an MPEG-2 stream are vared dependng on detected channel condtons. Both solutons propose small ncremental adaptatons based on measurements carred out n the termnal. But, as we wll explan n the next sectons, the actual capacty for a gven staton also depends on the channel condtons seen from neghborng statons. hs nformaton would allow streams to be adapted rapdly wth the optmal parameters. Alternatvely, the AP could provde network nformaton to a network element operatng at the applcaton layer, so that t s able to control the CODEC settngs of endponts wshng to ntate multmeda sessons. hs model s smlar to the approach followed by 3GPP for IP Multmeda sessons based on the Sesson Intaton Protocol (SIP) [9]. SIP proxes may modfy meda stream offers so that they adapt to operator polces and capabltes. By connectng the nformaton provded by the AP (runnng the proposed algorthm) to the polcy functons mplemented by the 3GPP IP Multmeda Subsystem (IMS) a decson can be made at any gven tme to determne the optmum set of encodng settngs to be used, based on properly calculated throughput. 2.2. Admsson Control adaptaton In addton to CODEC selecton, the proposed algorthm can be used to enhance the Admsson Control strategy mplemented by the AP. Effectvely, the AP may use the proposed algorthm to estmate the effect that a new user would cause n the overall cell throughput. Consder for example the case n whch several multmeda sessons are runnng smoothly under a gven AP, and suddenly a new node tres to connect usng the lowest transmsson rate. As we have explaned above, the staton wth lower modulaton severely affects the overall cell throughput. Dfferent AP capacty estmatons were used n [0] and [] as a metrc for admsson control n WLANs. Beng able to perform proper Admsson Control mechansms s of nterest n certan cases, and of crucal mportance n others. Consder for example the case of a GSM user that roams nto a WLAN cell and wshes to make use of the Generc Access Network (GAN) functonalty adopted by 3GPP. hs servce makes use of the WLAN nfrastructure to replcate the GSM user experence, thus allevatng operators from havng to deploy costly ndoor cellular coverage n complex envronments. In ths case, the GSM user expects the same qualty of experence (QoE) from the GSM servce, regardless of t beng supported by a pure GSM nfrastructure or from a GAN-WLAN AP. In such
case, the access network should support the followng Admsson Control capabltes: - When a GSM user wshes to connect to ts servce provder usng a dual GSM-GAN devce over the WLAN nfrastructure, the AP must be able to provde reasonable certanty that the GSM call wll be successful. Otherwse, the user should be rejected n case t s stll feasble to keep the call under pure cellular coverage (ths s the case f WLAN and GSM coverage overlap). - When a new data staton wshes to attach to the AP, the network must be able to ensure that already ongong GSM/GAN sessons wll not experence any degradaton due to the upcomng user. Otherwse, the new request may be rejected. 3. IEEE 802. protocols Snce the defnton of the frst IEEE 802. standard for WLANs [2], several varants have appeared that ncrease the bt rate. However, the MAC workng procedure has remaned the same. he IEEE 802. MAC procedure provdes two operatng modes: Dstrbuted Coordnaton Functon (DCF) and Pont Coordnaton Functon (PCF). he DCF uses the contenton MAC algorthm CSMA/CA, whereas the PCF offers contenton free access. he two modes are used alternately n tme. he DCF works as follows: before ntatng a transmsson, a staton senses the channel to determne whether t s busy. If the medum s sensed dle durng a perod of tme called Dstrbuted Interframe Space (DIFS), the staton s allowed to transmt. If the medum s sensed busy, the transmsson s delayed untl the channel s dle agan. A slotted bnary exponental backoff nterval (BO) s unformly chosen n [0, CW-], where CW s the contenton wndow. he backoff tmer s decreased as long as the channel s sensed dle, stopped when a transmsson s n progress, and reactvated when the channel s sensed dle agan for more than DIFS. When the backoff tmer expres, the staton starts transmttng. After each successfully receved data frame, the recever wats for a Short Interframe Space (SIFS) perod and transmts an acknowledgment frame (ACK). he value of CW s set to ts mnmum value, CW mn, n the frst transmsson attempt, and ncreases nteger powers of two at each retransmsson, up to a pre-determned value CW max. he protocol descrbed above s called basc or two-way handshakng mechansm. In addton, the specfcaton also contans a four-way frame exchange protocol called RS/CS mechansm, whch works as follows: a staton gans channel access through the contenton process descrbed prevously, and sends a specal frame called Request to Send (RS), nstead of the actual data frame. In response to that, the recever sends a Clear to Send (CS) frame after a SIFS nterval. Subsequently, the requestng staton s allowed to start the data frame transmsson after a SIFS perod. he man objectve of RS/CS handshake s the resoluton of the hdden termnal problem. he mechansm s also employed to mnmze the lost perods caused by collsons the RS frame s much shorter than data fames. See fg. 2 for the complete message exchange sequence for basc access and RS/CS. Fnally, the IEEE 802. DCF MAC protocol supports two knds of Basc Servce Set (BSS): the ndependent BSS, known as ad-hoc networks, whch have no connecton to wred networks, and the nfrastructure BSS, whch contans an AP connected to the wred network. he nfrastructure BSS can be used to provde wreless access to IP networks and servces n a smlar way to that of cellular systems. he PCF mode s only allowed n the nfrastructure BSS. In ths case, the AP polls ts assocated statons one after another by sendng pollng messages. Moreover, f the access pont has data ready to send to a staton beng polled, t can be ncluded n the pollng message. If the polled staton has data to send to the AP, t s transmtted n the response message after a SIFS perod. Moreover, as of late 2005, the IEEE 802. task group E released a new standard [3] that defnes a set of Qualty of Servce enhancements for the IEEE WLANs; t defnes procedures for managng network QoS usng classes of servce. he extensons ntroduced consder the two access mechansms: DCF and PCF. he new MAC protocol s called Hybrd Coordnaton Functon (HCF). It combnes a contenton channel access mechansm, the Enhanced Dstrbuted Channel Access (EDCA), and a pollng-based channel access, the HCF Controlled Channel Access (HCCA). EDCA s desgned to manage 8 dfferent traffc prortes. Packets belongng to the dfferent traffc prortes are mapped nto 4 access categores (ACs); each of them represents a dfferent prorty level. he HCCA (whch s not mandatory) works lke the PCF wth traffc classes. HCCA s the most complex coordnaton functon. Wth HCCA, QoS can be parameterzed wth great precson. However, IEEE 802.e enabled devces usually mplement EDCA only, whch s not ntended to offer any form of QoS guarantee to the users n terms of bandwdth allocaton, bounded delay, etc. Nonetheless, ths servce prortzaton s translated nto an ncrement of the probablty to wn the contenton for the access to the common channel. 4. Estmaton for a new staton Basc RS/CS Fg. 2: Message sequence for basc and RS/CS access Dervng the throughput that s currently devoted to a staton wth ongong data transmssons can be acheved by means of straghtforward measurements. On the other hand,
predctng the throughput that a new staton wll get before t actually starts transmttng, or the maxmum bandwdth that can be allocated to a staton f t ncreases ts offered traffc n a mult-rate cell requres a deeper study. In ths secton we frst analyze an deal scenaro n the smplest case: all statons are n saturaton. Dspensng wth the assumpton of saturaton, we then study a more general scenaro where statons can have dfferent bandwdth requrements. hs addtonal complexty brngs the need to develop an algorthm to derve the achevable throughput. Fnally, the algorthm s adapted to the partculartes of IEEE 802. WLANs. 4.. he Saturaton case Let N be a set of n nodes attached to the base staton of a cell. All statons from N share bandwdth resources by means of a MAC protocol whch allows the assgnment of a specfc prorty p to the -th staton. Every staton transmts at rate r. We defne the tme perod cycle as the average tme between two consecutve transmssons of the staton wth lowest prorty, takng nto account the fact that there are frames watng to be sent n all the statons queues at any tme (.e. n saturaton): cycle = n j= p j r j Consderng that the packet sze dstrbuton functon s the same for all statons after a long perod, we used n () normalzed length frames to smplfy subsequent formulaton. Each node adds an ndvdual load L defned as the porton of cycle that s used by node : p L = r cycle hus, t s trval to obtan the ndvdual throughput S and the total throughput S as: p S = r L = S = n = cycle S = n cycle = 4.2. Non-Saturated Statons p In the prevous sub-secton we have studed how the bandwdth of a cell s shared among mult-rate statons, whch are n saturaton state. In a more general scenaro, n whch the statons may have dfferent bandwdth requrements wthout reachng saturaton, IEEE 802. MAC keeps assurng farness n terms of access probablty. All () (2) (3) (4) statons contrbutng wth a load equal to or smaller than the bandwdth that should be allocated under saturaton condtons wll be able to carry all ther offered traffc. Saturated statons, n addton to ther correspondng bandwdth, wll farly share the excess tme that s not used by the non-saturated nodes. Based on these consderatons, we propose a new algorthm for the load calculaton n a mult-rate 802. WLAN cell, takng nto account the traffc offered by each staton (O ) and the values of cycle, L and S computed n saturaton condtons as depcted n formulatons (), (2) and (3). he algorthm not only provdes each staton s load; t also allows bandwdth predcton for greedy applcatons (e.g. FP, e-mal, etc.), whch try to get all the bandwdth avalable to them. hs defnton s also appled to UDP applcatons that requre more bandwdth than that already allocated to them. We defne the parameter δ as the porton of the maxmum throughput achevable by staton (f n saturaton: S ) that s actually used: δ = O / S. he proposed algorthm s as follows: Algorthm : AAC Algorthm cycle cycle OrderIncr(N, δ ) for all N do f δ L O /r exc (-δ )/ r cycle cycle / r for all j N and δ j > do L j L j (+ exc / cycle ) S j L j r j δ j O j /S j end for end f end for Statons are frst ordered on the bass of the ncreasng value of parameter δ. Statons wth δ use fewer (or equal) resources than those that would be allocated n saturaton condtons and ther offered traffc wll therefore be carred. he porton of tme not used by non-saturated statons ( exc ) wll be dvded farly between the remanng statons, accordng to a new tme cycle n whch all the statons that have already been served are not counted. Note that greedy applcatons are modeled wth δ j > regardless of S j. 4.3. he Case of IEEE 802. WLANs One partcular case occurs as a result of not usng prortes,.e. the prorty p = /n N. In ths case, as we can see n (3) for saturaton condtons, all statons wll obtan an equal throughput, ndependently of ther own rate r, and S = S/n N. hs s the behavor that can be observed n an IEEE 802. WLAN (QoS opton not mplemented), snce long-term channel access probablty s
guaranteed to be equal for all statons due to CSMA/CA. In order to know the actual bandwdth avalable to applcatons we must take Layer 2 overheads nto account. Effcency s also affected by the number of competng statons and the channel qualty. We must expand expresson () orgnally based on normalzed length frame transmssons, to nclude overheads and the effect of channel errors and collsons for every node (usng basc access): t = DIFS + ( RE ) + ( ) + SIFS + n cycle = t = BO data Note that t s defned for a basc CSMA/CA access; f RS/CS handshake s used, 2 SIFS, RS and CS must be added to t. he duraton of the data frame s data. BO represents the average tme a staton wats for the backoff tmer to expre before attemptng to transmt. BO depends on the number of prevous transmsson attempts. he average value of the backoff nterval after j consecutve transmssons s gven by: BO 2 ( j) = j ( CWmn + ) 2 CWmax slot 2 slot j 6 ACK 0 j 6 As prevously done n [4], we consder that the number of packet retransmssons necessary to successfully transmt a sngle packet s a geometrcally dstrbuted random varable. If P f () s defned as the probablty that an s frame has to be retransmtted (ncludng both the effect of collsons and channel errors), the average number of transmssons for a frame s: Pf ( ) RE = + P ( ) f data can be further decomposed nto : data ( ) = preamble 8 ( H + MAc + MSDU ) r where r s the physcal btrate at whch node sends data frames wth a payload of MSDU Bytes. he rest of the values vary dependng on the standard and the modulaton. Standard values for these parameters are summarzed n table I. We defne the overhead produced n a node s layers and 2 as OH = r t /(8 MSDU ). For the computaton of the actual O (offered traffc) we must take nto account ths overhead and the possble retransmssons knowng the traffc offered by upper layers (O app ): case of DSSS-CCK modulatons, for more detals on OFDM see [5]. (5) (6) (7) (8) (9) OH O = O P ( ) f app ( ) Applyng these changes to the formulaton and algorthm presented n 4, we can do capacty estmatons from real measurements. he AP s the only node that s able to compute n real tme the avalable bandwdth for a gven user snce t mantans statstcs from whch all requred parameters can be derved, ncludng the physcal rate used by each staton, MSDU sze and P f. Summarzng, we have presented a new algorthm that provdes an estmaton of the throughput that the statons attached to a gven AP can obtan. Our formulaton and algorthm s easly applcable n EDCA enabled WLANs, selectng the proper values for p (out of the scope of ths work). he core of the complexty for runnng ths algorthm s dscussed n the next secton. 5. Implementaton Issues A key ssue to solve when t comes tme to mplement the proposed algorthm les n the acquston of the requred parameters. Most of them can be easly derved from statstcs provded drectly by drver/frmware functons. Nevertheless, obtanng P f s not trval snce the actual number of frames sent (and lost) by ts assocated statons s not known by the AP wthout any knd of nformaton exchange. But the AP knows the SNR at whch t receves frames from all ts clent nodes. From SNR values and the modulaton m used by the staton for ts transmssons, the AP s able to derve the BER (Bt Error Rato) for a certan clent, whch can be obtaned theoretcally usng the formulas gven n [6]. However, we have used tabulated values from the emprcal curves provded wth the Intersl Prsm HFA3863 transcever data sheet [7]. hus we avod unnecessary complex calculatons n the code. As n [8] [9], we use the followng approxmaton to compute PER (Packet Error Rato), knowng BER m and the length of the packet (L bts): m m L PER L = ( BER ) (0) () Note that we have omtted the effect of losng an ACK able I: IEEE 802. PHY Parameters DSSS - CCK OFDM r (Mbps) 2 5.5 6 2 24 54 ACK (μs) 304 248 22 202 44 32 28 24 SIFS (μs) 0 9 DIFS (μs) 50 34 slot (μs) 20 9 preamble (μs) 92 20 CW mn 3 5 CW max 023 023 H MAC (Bytes) 28 28
frame snce ts probablty s neglgble due to ts short sze and the fact that t s usually transmtted at the slowest (.e. more relable) btrate. On the other hand, the probablty that a packet sees a collson (P col ) s the same for all statons (ncludng the AP). Knowng ts own P f (# of packets not acknowledged / # of packets sent), the AP can do an estmaton of P col and an estmaton of any P f () followng: P ( ) = PER + P f col PER P col 5.. Implementaton detals he algorthm s fast and ts mplementaton requres lttle resources. herefore t s sutable for runnng on devces wth lmted features, such as any commercal AP. In order to prove the vablty of our proposal, we mplemented the algorthm n a commercal AP runnng a Lnux-based OS: the 4G Systems AccessCube 2. he algorthm presented n secton 4.2 wth the modfcatons mentoned n 4.3 was programmed n standard C for Lnux and cross-compled to run on the AccessCube s MIPS archtecture. Some of ths AP s features nclude: 400MHz MIPS processor 32MB flash 64MB RAM 2 Prsm 2.5 based WLAN IEEE 802.b nterfaces Lnux-based OS: NyLon (kernel 2.4.27) he AccessCube s OS ncludes the HostAP 3 drver for the Intersl Prsm 2.5 based 802.b devces on board. hs drver provdes a helpful collecton of statstcs whch are accessble to applcatons va the proc flesystem. For each assocated staton, the drver regularly updates nformaton ncludng the number of packets and the number of Bytes sent/receved by that staton, the number of packets sent at a gven rate, and the SNR of the last receved packet. he drver also provdes nformaton about the performance of the AP tself, e.g. packet dscards, retransmsson attempts, etc. All these parameters allow an applcaton to fll n the varables needed to process the algorthm n a tmely manner, so the effects of varyng flows and channel condtons can be taken nto account. he resources employed by a sngle run of the algorthm are nsgnfcant n most of the cases. hs way, the performance of the AP s not affected by the extra load even though the AP s servng a large amount of clents, snce most of the AP functonalty reles on the wreless card frmware, whch does not share resources wth user space applcatons. In the extreme that the AP has to nspect statstcs of 50 assocated statons, the AccessCube s CPU 2 http://www.meshcube.org 3 http://hostap.eptest.f (2) tme usage needed to compute the capacty avalable for a sngle staton ranges from 20 to 30 ms and the memory usage reported by the OS s below.%. 6. Evaluaton In order to valdate our mechansm, frst of all, several Opnet smulatons were run usng the 802.g standard set of parameters. Wthout losng generalty, the smulated scenaro conssted of a sngle IEEE 802.g cell wth an AP and three statons (A, B and C): A used a physcal rate of 48Mbps to carry the requrements of a greedy applcaton (FP), B offered traffc descrbed by a 7Mbps UDP CBR source whle transmttng at 24Mbps, and C ncreased ts traffc demands lnearly from 0 to 5Mbps usng a physcal rate of 2Mbps. Fgure 3 shows the carred throughput measured at the applcaton layer for all three statons. Sold lnes are drawn for values obtaned by smulaton and the dotted lnes are the values provded by our algorthm. In saturaton condtons and followng (3), the throughput that any of these three statons wll obtan s about 5Mbps, and ths s the value to whch each staton converges, as seen n fgure 3. As C ncreases ts offered traffc, not only t s clear that less capacty s avalable for greedy statons, but also snce C s the slowest staton, the global throughput s decreased as well. By applyng the same concept to 802.b, as shown n Fg., we can compare our results wth practcal measurements. For example, f A transmts at a rate of 5.5Mbps, B transmts at Mbps and P f (A) = 0.03, P f (B) = 0.04 (from measurements at the AP, as explaned n 5), followng (5) and (6), one cycle s cycle = 6.94ms. he ndvdual loads are L A = 0.9 and L B = 0.8,.e. 8% of the tme, the channel s captured by staton B. However, both statons are gettng smlar bandwdth, S A = 79 kbps and S B = 7 kbps. hese numbers are close to the values shown n Fg. (S A = 725±5 kbps; S B = 698±44 kbps). Note that the dfference between the two statons bandwdth s due to the exstence of packet losses: f P f (A) = P f (B) = 0, both statons would get an dentcal bandwdth. he cell s fully loaded (L = 00%), but note that a load metrc based on throughput measurements wll provde an erroneous noton of the actual hroughput (kbps) 8000 6000 4000 2000 0000 8000 6000 4000 2000 0 0 500 000 500 2000 2500 3000 3500 4000 4500 5000 raffc traffc Load load offered by staton C (kbps) C (kbps) A Sm. A Analt. B Sm. B Analt. C Sm. C Analt. Fg. 3. hroughput of A (48M), B (24M) and C (2M) for dfferent traffc demands n C: A n saturaton and 7Mbps CBR n B.
load snce the carred throughput measured (.43Mbps) seems a low load n comparson to the 6.50Mbps that can be acheved n an IEEE 802.b WLAN. 6.. Real-me estmatons A PC A real testbed was also set up n order to evaluate the fdelty of the measurements wth the algorthm runnng on a commercal IEEE 802.b AP (as detaled n 5.). he testbed s completed wth three statons. Before detalng the confguraton of our testbed, t s worth havng a look at the next fgure: fg. 4, obtaned analytcally wth our model, provdes a helpful mage llustratng how the avalable bandwdth per staton vares dependng on the modulatons used n a cell wth three transmtters n saturaton, and sendng maxmum sze frames;.e. the curves n fg. 4 are an upper bound for saturaton. Observe that the presence of a sngle node usng the slowest modulaton lowers drastcally the global throughput, ndependently of the physcal rates used by the rest of the statons (see dotted surface n fg. 4 s nearly flat). Once these upper bounds are known, we can choose the approprate offered traffc and physcal rates for our scenaro so that the undesred effects of mult-rate contenders are clearly vsble. Back to the testbed descrpton (see fg. 5); there are three statons A, B and C, assocated wth the same IEEE 802.b AP. All three statons have dfferent traffc profles: A s the source of an MPEG-2 vdeo stream. he MPEG-2 vdeo codec can be formatted as constant length packets of 88 bytes (ths s called ransport Stream) that can buld payloads of k x 88 bytes. he transport of S packets over IP/UDP/RP usually ncludes 7 S packets = 36+40 bytes, n order to approach the Ethernet MU and maxmze effcency. A s packets are spaced out so that a 2Mbps CBR stream s obtaned. In B, a greedy applcaton s always tryng to send as many 500 byte UDP packets as possble, whle C follows a bursty pattern: the average tme between consecutve bursts s 20s, the average duraton of a burst s 8s; bursts consst of 000 byte UDP packets n such a way that SA C reaches Kbps x SA Modulaton SA B SA A: Mbps Modulaton SA C SA A: Mbps Fg. 4. Maxmum achevable throughput for 3 statons n saturaton dfferent modulatons (, 2, 5.5 and 4=Mbps) AP Fg. 5. estbed for real tme: AP runs algorthm and serves three SAs. Connected to PC and PC2 wth 00M Ethernet saturaton. he testbed topology s shown n fgure 5. he stream orgnated n A s sent to PC whle PC2 s the destnaton of data flows orgnated n B and C. PC and PC2 are drectly connected to the AP by means of a 00Mbps Ethernet segment, so we can guarantee that the bottleneck resdes n the ar nterface. A sends frames at Mbps, C uses Mbps and B decreases ts btrate one level (modulaton) every 20s, startng wth Mbps. For ths experment, the AP runs the algorthm once per second to provde an estmaton of the maxmum capacty avalable for staton A. Measurements are shown n fg. 6: durng the frst 0s staton A has to compete wth only one element, staton B, whch s sendng frames at Mbps. he measured capacty s greater than A s requrements so A can therefore carry all ts offered traffc. From 0 to 20s, a burst of C s packets makes the capacty measurements for A fall below 2Mbps, whch corresponds to the real throughput obtaned by A s flow. Durng the perod 20-28s, B sends frames at 5.5Mbps, and the capacty for A measured at the AP s agan greater than. As t can be seen n the fgure, the subsequent measurements are representatve of the actual throughput obtaned n A s transmssons. hs experment has shown that the mplementaton n a commercal AP of the algorthm presented n prevous sectons s able to detect sgnfcant capacty fluctuatons that affect a gven staton. We argue that ths real-tme knowledge n the AP wll outperform any end-to-end estmaton, n vew of the fact that the AP s usually the system s bottleneck and s thus the best place to measure the actual capacty; but we leave a detaled comparson for a future work. Note that the algorthm s slghtly overestmatng the actual capacty; the values provded are, n average, less than a 0% above the actual throughput measured at the same tme nstant, whle the error measured n the prevous smulatons were smaller than 5%. 7. Conclusons and future work B PC2 C We examned the effect of mult-rate transmssons n an IEEE 802. WLAN cell. We verfed that a far bandwdth dstrbuton s provded by CSMA/CA at the cost of
penalzng statons that use hgher physcal rates. hese observatons led us to develop a mechansm for computng throughput n a mult-rate 802. WLAN, whch was valdated aganst smulatons and practcal measurements. We also stated that the computaton n terms of tme provdes an evaluaton whch s more representatve of the actual cell load than the use of traffc (n Bytes/s). he avalablty of ths metrc has evdent applcatons n the transmsson of multmeda streams. hese applcatons were wdely dscussed n the paper. As a proof of concept, an mplementaton of the studed algorthm was developed to run n Lnux-based commercal APs, showng the accuracy that can be provded n a real scenaro. Once the algorthm has been valdated, n the near future ths testbed wll be extended to evaluate the benefts of applyng our metrc n the scenaros dscussed: admsson control and dynamc CODEC adaptaton. Observe that the consderatons taken n the ntal applcablty statements may requre some extensons to the basc AC capabltes of deployed WLAN AP s. In partcular, AC n a WLAN-GAN or WLAN-IMS envronment may be shared across a set of network elements, ncludng the AP tself, Polcy Decson and Polcy Enforcement Ponts, Applcaton Layer nodes (e.g.: SIP proxes) and operator polces. Alternatvely, f a staton wshes to perform proper CODEC selecton based on estmated throughput, a mechansm to transmt such nformaton from the AP towards the staton should be developed (at present, only the AP s able to mplement the algorthm, whle the statons reman unaware of ts results). Acknowledgment hs research work was funded by the ERDF, the Spansh Government through EC2006-04504, and the 2CA foundaton. References [] M. Heusse, F. Rousseau, G. Berger-Sabbatel, and A. Duda, Performance anomaly of 802.b, n Proceedngs of the 22nd IEEE INFOCOM'03, vol. 2, pp. 836-843, March 2003. [2] S. 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