Cooperatve Load Balancng n IEEE 82.11 Networks wth Cell Breathng Eduard Garca Rafael Vdal Josep Paradells Wreless Networks Group - Techncal Unversty of Catalona (UPC) {eduardg, rvdal, teljpa}@entel.upc.edu; +34 934137218 Abstract IEEE 82.11 WLANs (W-F) are wdely deployed for provdng Internet access n publc spaces, known as Hot Spots. In these scenaros, users tend to be gregarous and essentally statc. Snce assocaton and roamng decsons are made by clent devces followng sgnal strength crterons (.e. a clent staton selects the AP that provdes the strongest sgnal), the users and ther load are unevenly dstrbuted between neghborng APs. In ths paper we propose a dstrbuted algorthm wth whch the APs n an IEEE 82.11 WLAN are able to tune ther cell sze accordng to ther load and also to ther neghbor s load. Ths technque mproves the farness and the performance levels and s known as Cell Breathng. 1. Introducton It could be sad that WLANs based on the IEEE 82.11 set of standards are vctms of ther own success. The great popularty of these networks has led to ts expanson n scenaros for whch they had not been orgnally desgned (e.g. mesh topologes, large scale networks, outdoor lnks, etc.). For example, varous studes have shown (e.g. see [1]) that users tend to be concentrated both temporally and spatally, creatng hghly congested areas known as Hot Spots. Therefore, the load s unevenly dstrbuted across a small number of Access ponts (APs) n the WLAN. Moreover, although moblty s ncreasng as users get nto the habt of usng wreless access, the moblty pattern can stll be consdered quas-statc n the sense that users tend to reman n the same locaton for long perods. Ths stuaton s compounded by the fact that the assocaton wth base statons s determned by the clent devces on the bass of sgnal level measurements, whch means that users are generally assocated wth the closest AP. In other words, although a Hot Spot s served by several APs, most of the users wll be connected through the AP that provdes the strongest sgnal. As an nherent consequence, over-loaded APs offer the users n congested areas a very low QoS whle nearby APs reman under-utlzed. Ths behavor s determned by the roamng process, whch goes as follows [2]. A staton (STA) keeps track of the Beacon frames receved from ts current AP. When the qualty of beacons drops below the cell search threshold (1< CS Th <3dB), the STA ntates an actve scan and sends out Probe Request messages on all the avalable channels. The APs recevng the Request wll send a Probe Response back. When an AP s found whose responses mprove the current AP s Beacons qualty by at least SNR (usually 6 SNR 8 db), the STA ntates a cell swtch. If a better canddate s not found, the STA returns to the current AP s channel and the scan sweep s repeated perodcally. There are dfferent solutons to the unfar stuaton explaned above, the most evdent of whch conssts of usng a dfferent roamng crteron (e.g. AP load), but t requres deep changes n clent devces. We choose to keep any modfcaton transparent to the end user who can be equpped wth off-the-shelf devces. For these reasons, n ths paper we propose a new AP-drven load balancng scheme based on cell breathng, ntended to allevate the congeston n hot spots: congested APs reduce the sze of ther cells; alternatvely, underutlzed APs ncrease ther cells to attract further statons. In our approach, neghborng APs cooperate n order to mprove performance and farness levels. To ths am, APs can make use of the nformaton avalable to clent statons through the mechansms provded by the new IEEE standards: 82.11e, 82.11h and, prncpally, 82.11k. The rest of the paper s structured as follows: Secton 2 dscusses the defnton of load n the partcular case of 82.11 WLANs. Secton 3 provdes an overvew of related work. In Secton 4 we descrbe our algorthm. Secton 5 dscusses some mplementaton ssues. Secton 6 contans an evaluaton of our proposal n comparson wth other schemes; fnally, conclusons are gven n Secton 7. 2. Defnton of load Load balancng n overlappng areas has tradtonally been used n crcut-swtched cellular networks. Snce each user n these types of networks represents an dentcal utlzaton of avalable resources, load
balancng could be appled by usng call level nformaton,.e. load s represented by the number of actve calls served by a BS. Nevertheless, call level nformaton s not suffcent for modelng the actual load that s carred by a BS n current wreless packet networks, gven that users may have dfferent traffc profles. Ths asserton s vald for IEEE 82.11 WLANs. Therefore, a new metrc based on packet level nformaton s requred. However, the number of actve users stll provdes valuable nformaton n networks that use CSMA-based access: more collsons occur as the number of actve users ncreases, whch leads to decreased performance. The number of competng statons can be calculated from any staton by usng the formulaton gven n [3], but ths parameter can only be used to estmate the saturaton throughput of a cell and does not provde nformaton about the actual load. Dfferent load metrcs based on packet level nformaton have been proposed. The authors of [4] used the number of retransmsson attempts needed to successfully transmt a sngle packet, whch can be derved f all hdden pars are known. The same concept was also used n [5] to derve the Gross Load metrc usng a dfferent formulaton. Reference [5] also suggests usng the packet loss estmaton as a new load metrc. Traffc (n bytes/s) was used as a load metrc n [6] and [7]. However, we should bear n mnd that the IEEE 82.11 standards defne several modulatons wth dfferent physcal bt rates (e.g. 1, 2, 5.5 and 11 Mbps for 82.11b); n ths case an AP could be congested when carryng traffc of 1 Mbps f there are assocated statons transmttng at the slowest bt rate. On the other hand, the same AP could also be consdered underutlzed wth a load of 3 Mbps f all of ts clents use faster modulatons. Therefore, carred traffc s not a vald representaton of the load on an AP n a mult-rate scenaro. Instead, n [8] and [9] the measure of busy tme s proposed as the representatve load metrc. More precsely, n [9] the network congeston level s estmated usng channel occupaton tme and by montorng the occupaton of the AP s buffer queue. 2.1 Load nformaton n new IEEE standards The IEEE 82.11k group (Rado Resource Measurement) s currently developng a standard whch s ntended to mprove the provson of data from the physcal and medum access layers by defnng a seres of measurement requests and reports that can be used n the upper layers to carry dfferent rado resource management mechansms. The current draft verson s 9. [1], although the fnal standard s expected to be released soon (at the tme of wrtng). The current IEEE 82.11 standard [11] and the future 11k defne a set of load metrcs that are ether broadcast by APs or measured drectly by clent statons: Channel Load Report: defned as the proporton of the tme durng whch ether the physcal carrer sense, the vrtual carrer sense (Network Allocaton Vector or NAV) ndcate that the channel s busy. Ths measurement s smlar to the CCA report n 82.11h. Beacon Frames: these management frames are extended wth three new elements that provde nformaton about the load of an AP. BSS Average Access Delay: average medum access delay for any transmtted frame measured from the tme the frame s ready for untl the actual frame transmsson start tme. BSS AC Access Delay: n QAPs (QoS enabled APs), average medum access delay for each of the ndcated Access Categores defned by the IEEE 82.11e. BSS Load ncludes the followng felds: Staton Count: the number of statons currently assocated wth the AP. Channel Utlzaton: the percentage of tme that the AP senses the medum s busy. Avalable Admsson Capacty (AAC): the remanng amount of medum tme avalable va explct admsson control. Although channel busy tme provdes a good representaton of the cell load even n a mult-rate scenaro, we consder that t s not a valuable metrc n the presence of greedy applcatons (e.g. FTP). For example, a channel busy tme of 85% can be acheved wth a sngle greedy staton, but also wth 1 users offerng 5kbps each. However, a new staton wll get much more bandwdth f t only has to compete wth one user than f t has to share the medum wth 1 other statons. The advantage of AAC over access delay resdes n ts ablty to antcpate the potental effects on load and throughput of addng a new user. The IEEE 82.11 standard defnes AAC as the remanng amount of medum tme avalable n unts of 32 μs, although t does not specfy how t should be calculated. In [12] we proposed a new load metrc based on a more precse defnton of AAC. We expand the current defnton of Avalable Admsson Capacty to be the proporton of tme a new staton can take up f t s assocated wth the AP at a gven physcal rate. Ths new metrc provdes a vson of cell load that takes nto account the effect of mult-rate statons, the presence of greedy users, the average frame sze, the number of actve users and also transmssons errors and collsons. Any AP can easly derve ts AAC value by nspectng statstcs that are usually provded by the wreless nterface drver. For detals on AAC dervaton and mplementaton ssues see [12].
3. Load Balancng n WLANs Dfferent approaches have been proposed n the lterature that try to change the clent-drven nature of IEEE 82.11 assocaton and roamng decsons. The authors of [6] and [8] propose network-controlled schemes n whch clent statons send the requred nformaton to a central unt, whch also has access to the load nformaton for each cell. The scheme proposed n [6] provdes the best AP for assocaton and the network also suggests roamng to APs located further away f nearby APs are consdered unable to cover the staton s requrements. In order to mplement these solutons t s necessary to modfy the clent devces: frstly, they have to send new management frames before they are actually assocated; secondly, they wll no longer be responsble for assocaton or roamng decsons. The frst ssue can be solved by usng new rado measurements (e.g. IEEE 82.11k). Reference [13] provdes a survey of dfferent load balancng technques and dscusses the applcablty of the new 11k procedures. However, there s no standardzed procedure for solvng the second ssue as yet, but t s expected to be revsed by the IEEE 82.21 group, whch wll provde mechansms ntended to assst handovers, and by IEEE 82.11v, whch wll nclude management capabltes to allow network-drected roamng. It s not vtal to solve the second of these ssues, snce t s also possble to perform mplct admsson control/assocaton management. Ths nvolves actons taken on the network sde that nduce the desred clent behavor and therefore leave the roamng and assocaton decsons to clent statons so that hardware/software modfcatons are not requred. In [9] the APs accept or deny new assocaton requests dependng on the respectve load. When the frst choce s rejected, the statons wll send assocaton requests to the next AP n the sgnal strength-arranged lst, untl they are admtted. The algorthm proposed n [7] s more sophstcated but follows smlar logc. There are three possble AP states: under-loaded (wll accept any request), balanced (wll not accept extra load) and overloaded (wll expel the staton on the assumpton that t wll automatcally request a less loaded AP). Cell breathng s a sde effect n CDMA networks that reduces the cell coverage when more users are supported, but ths could be advantageous n load balancng technques f optmal strateges are appled. Cell breathng technques consst n dynamcally modfyng cell dmensons usually ncreasng or reducng transmtted power. The concept of cell breathng for load balancng n WLANs s explaned n [14]: a hghly congested AP reduces ts coverage radus so that the furthest statons lose connectvty and try to roam to a neghborng AP (less loaded). An underutlzed AP may ncrease ts transmt power n order to expand ts coverage. Consequently, new users wll roam to ths AP and the load on neghborng APs wll decrease. In [15], APs could even buld a custom radaton pattern to balance load, but besdes a very specalzed RF hardware, ths soluton reles on the APs perfect knowledge of ther own coverage and the exact poston of clents, whch s hardly feasble. Reference [16] provdes an n-depth analyss of cell breathng n IEEE 82.11 WLANs and proposes a centralzed soluton based on two dfferent algorthms: one s amed to reduce the load of the most congested AP and the other tres to fnd the mn-max load balanced soluton. However, as stated n [13], the furthest statons may sometmes be expelled arbtrarly as they may contrbute an nsgnfcant load dependng on the applcatons they run. On the other hand, cell breathng provdes a network-nduced assocaton management and therefore t does not generally requre any change on clent devces. 4. Dstrbuted load balancng algorthm wth cell breathng We have to dstngush between two ndependent types of transmtted power management: cell dmenson management (Cell Breathng) and transmtted power control (). As prevously explaned, Cell Breathng tres to mprove load balancng among neghborng APs, whle s amed to reduce power consumpton, nterference and the near-far effect [17]. From the clent staton s pont of vew, the cell dmensons are determned by the energy of receved Beacon frames and Probe Responses. Then, an AP can set ts optmal cell dmenson so that the farthest clent that the AP must serve, receves Beacons wth SNR > CS Th. But, as n [16], the power used to transmt data frames can be hgher so that the user s experence s not degraded. Hence, an optmum algorthm s assumed (e.g. [18][19]) for the exchange of data frames between an AP and ts clents, usng the mnmum power that does not degrade the performance of the communcaton. For ths reason we dstngush between tx range (determned by the maxmum transmsson power allowed) and cell sze (determned by beacons and Probe Responses). In our approach, APs are responsble for computng ther own load and let ther neghbors know about t by ether perodc or trggered updates. Smlar to [7], APs can be n one of the followng three states, accordng to ther load, as compared wth ther neghbors : Far: the AP s load s smlar to the average load n the neghborhood. An AP n ths state wll not take any acton regardless of ts neghbor s behavor. Gull: the AP s load s larger than the average load n the neghborhood. An AP n ths state s wllng to
reduce ts cell and wll try ask ts neghbors for help. Wllng: the AP s load s below the average load n the neghborhood. An AP n ths state s wllng to ncrease ts cell n response to a neghbor s appeal. In order to determne the AP s load we propose the AAC metrc, defned as the capacty avalable for a new staton that uses the fastest modulaton [12]. Logcally, as the congeston ncreases, the APs AAC decreases. Then, we consder that an AP s Gull f AAC < AAC δ, where AAC s the capacty avalable n AP, AAC s the average capacty n s neghborhood and δ a threshold value used to add hysteress, thus mprovng the stablty of the system. Analogously, an AP s Wllng f AAC > AAC +δ. If none of the prevous condtons s met, the AP s n state Far. The value of δ s set dynamcally accordng to AAC / 3. The optmum value was chosen after a prevous smulatonbased study that s not detaled here for the sake of brevty. Two APs are neghbors f there s at least one clent wthn transmsson range of both APs. The behavor of Gull and Wllng APs s detaled n fgure 1. Note that ntal and fnal states are connected a) b) Fg. 1: Behavor of a) a Gull AP, and b) a Wllng AP and that the process can be nterrupted f the state of the AP changes. The frst acton taken by an AP s to arrange ts cell sze accordng to ts state. Far and Gull APs wll run txpowergull() to reduce the sze of ther cells so that the clent recevng the poorest sgnal detects beacons wth SNR > CS Th, or the mnmum transmsson power s reached; Far APs wll not take any further acton. A Wllng AP wll run txpowerwll() ncreasng ts cell sze so that no STA assocated to a neghborng AP roams to t, or the maxmum transmsson power s reached. We defne S as the set of STAs (s ) and A the set of APs (a j ); S as s the subset of the elements of S that contans the STAs assocated to a gven AP a j, and S rg s the lst of STAs wthn a j s range. Both lsts are arranged n decreasng order accordng to the SNR computed from a j s beacons. SNR j, s the SNR of aj s beacons as seen from s, whle SNR s the SNR of the beacons that s receves from ts current AP. Then, for any a j : j S = s S SNR > SNR S j as = { j, mn } { s S SNR > SNR k A} rg j, k, We assume that S as and S rg are always updated thanks to the complete collecton of statstcs provded by an ndependent process (see next secton). A Gull AP wll then select the frst s from ts S as that s able to roam to a Wllng AP. The Gull AP wll send an SOS message to all APs wthn range of s. A Wllng AP recevng a SOS message wll compute the AAC value for that partcular STA and wll forward ths value to the requestng Gull AP. Ths AAC s computed takng nto account the fact that an optmal s used for data exchange. The Gull AP then sends an acknowledgement only to the best AP canddate and adjusts ts cell sze expellng the selected STA (s ). In turn, the adoptve AP adjusts ts cell sze to accommodate s. In order to avod undesred handovers, all APs recevng a SOS message wll ban the announced s (e.g. va ACL) untl the process s complete. 5. Implementaton Issues Although the algorthm presented n ths paper has not been fully mplemented, some of the functonaltes requred have been prevously tested n real testbeds. For example, the sgnalng requred to communcate neghborng APs could be easly carred out by means of a common wred backbone. If there s no such common backbone, APs could stll partcpate n the dstrbuted algorthm usng a wreless dstrbuton system based on mesh concepts as proposed n [2]. As n [16] and [18], one of the requrements for the APs s the possblty to set the transmsson power n a per-packet manner. In ths way, APs can arrange ther
Functon txpowerwll: end = false whle!end do f Ptx + step > PtxMax then end = true for all S S rg do f SNR < CS th AND f SNR j, > SNR + SNR + step then end = true done f!end then Ptx = Ptx + step done Functon txpowergull: end = false MaxSNR = Max(SNR k, k A) whle!end do f Ptx - step < PtxMn then end = true for all S S gs do f SNR < CS th AND f SNR step < MaxSNR + SNR then end = true done f!end then Ptx = Ptx - step done cell sze adjustng the transmtted power for Beacons and Probe Responses and at the same tme runnng an effectve for data. The AAC computaton adds another requrement for the APs: an updated collecton of statstcs s requred at applcaton level n order to allow the AAC estmaton n real tme. It s worth to menton that the extra processng load ntroduced by the AAC computaton s affordable despte the AP s lmted resources, as stated n [12]. Nevertheless, the nformaton needed by the APs to run the algorthm descrbed n the prevous secton represents the man mplementaton ssue. Any AP should know the complete lst of STAs wthn transmsson range and the lst of APs that any of these STAs can reach, ncludng SNR of beacons and potental SNR for data. The new standard [11], whch ncludes the 82.11h amendments, along wth the upcomng 82.11k standard [1] wll ease the acquston of ths nformaton, as detaled next. The potental SNR for data exchange between an AP and all the clent STAs n range can be obtaned by means of an 11h s Request/Report or an 11k s Lnk Measurement Request/Report. These two mechansms are smlar and allow the estmaton of the lnk margns between two statons. The requestng AP announces the transmtted power used to send the request (maxmum allowed transmtted power) and the requested STA responds wth the lnk margn accordng to the SNR of the receved request. The response also ncludes the transmtted power used to send the frame. APs are also able to retreve nformaton about the SNR of receved beacons usng the Beacon Request/Report defned n 82.11k. A STA recevng a Beacon Request wll respond wth a Report contanng statstcs, ncludng SNR, power, channel and BSSID, of receved Beacons and Probe Responses. The AP stll has to know the potental SNR for data between ts n-range STAs and the neghborng APs. Ths could be solved ether addng an extra sgnalng among APs or ndependently, usng the 82.11k frames: Measurement Plot. Smlarly to Beacons, these frames are transmtted pseudoperodcally by APs at a small nterval, but a Measurement Plot s smaller than a Beacon and s transmtted more often than a Beacon. STAs also nclude statstcs of receved Measurement Plots n Beacon Reports, so, f APs send these frames at the maxmum allowed transmsson power, APs could fnally gather all requred nformaton. However, our approach s also feasble wth no 82.11k enabled devces. We have to note that n ths case, many of the parameters can only be approxmated and that t s requred that STAs perform actve scans. In ths way, all APs wthn the STA s range are able to obtan the uplnk margn from Probe Request messages, and thus estmate the downlnk margn assumng that the path s symmetrc and that the power used to send the Probe message s known (max. allowed power). These assumptons also allow APs to estmate the power of receved Beacons (knowng the power of transmtted Beacons and the estmated path loss). Furthermore, more sgnalng s requred to exchange ths nformaton among APs. 6. Performance Evaluaton 6.1 Scenaro The evaluaton process we desgned s based on extensve smulatons n a 38x38 m square ndoor scenaro wth 16 IEEE 82.11b APs evenly dstrbuted. The smulator was developed n C and mplements all the detals of the algorthms descrbed n 4. We ran a large number of ndependent smulatons and obtaned small confdence ntervals, whch are therefore not shown n the fgures. The throughput carred by an AP and the throughput avalable to the STAs s computed accordng to the model presented n [12]. Usng a path loss PL(d) = 4-33 log(d), where d s de dstance between a transmtter and a recever, ptxmax=15dbm and a ptxmn=1dbm (hghest and lowest allowed transmsson power), we assume that wth all APs transmttng at ptxmn, there s no coverage gap n the scenaro, and that transmttng at ptxmax no
Agg regat e throug hpu t (Mb ps) 7 6 5 4 3 2 1 c (%) co-channel nterference s produced (usng a 4-colorng scheme). As stated n [1], users are statc and tend to be spatally concentrated. We smulate these characterstcs by placng users at random, but forcng that a gven percentage of users, c %, are concentrated n a randomly selected area of 1x1 m. Ths ensures that a realstc scenaro s met. The physcal rate used for data transmssons depends on the dstance between an STA and ts selected AP: f d < 46m, rate = 11Mbps; f d < 61m, rate = 5.5Mbps; f d < 75m, rate= 2Mbps; and f d < 92m, rate = 1Mbps. For d 92m SNR mn = 1dB s not met. Fnally CS Th = 2 db and SNR = 7 db. The PER of each STA depends on the SNR and modulaton used for data transmssons, accordng to the performance of an Intersl Prsm HFA3863 [21]. A collson probablty s also provded for each cell, dependng on the number of actve users (see [3]). Our approach (Dstrbuted Cell Breathng ) s compared aganst dfferent mechansms. The centralzed approach () of [16] s used as a reference snce, as we understand, a complete knowledge of the scenaro wll allow better assgnments. Both and use AAC as the load metrc. We call the soluton that mplements solely an optmal for data exchange, but that keeps the sze of the cells fxed. Fnally, the behavour of current IEEE WLANs s also represented n the smulatons. 6.2 Smulaton results The frst concluson derved from the smulatons was that the proposed algorthm runs wthout loops, and converges rapdly n the scenaro depcted n the prevous subsecton. Then we measured the aggregate throughput n dfferent stuatons. Fgure 2 a) and b) are obtaned n saturaton condtons, that s, all STAs have always buffered frames (15 Bytes) ready for transmsson. Fgure 2 a) shows the effects of ncreasng the concentraton (c %) wth a fxed number of users (65), whle b) has a fxed c (55%) and a varyng number of users. It s not a surprse that the soluton presents the best results, snce t always guarantees that all STAs use the best possble rate. In the case where STAs have dfferent traffc profles (packet sze from a) 7 b) 7 c) 6 6 Aggregate throughput (Mbps) 5 4 3 2 1 5 to 1 Bytes and offered load rangng from.2 to 2Mbps), outperforms the other approaches (see Fgure 2 c), but as the number of saturated users ncrease, t becomes slghtly worse. The aggregate throughput has a maxmum wth 3 or 4 STAs per AP and decreases wth more users due to the ncreasng collson probablty. Logcally, as c ncreases (more users use less APs), the aggregate throughput decreases. However, a maxmzed aggregate throughput does not nvolve that the throughput of all STAs s maxmzed. For ths reason we also measured the farness degree among STAs and among APs. Farness s measured usng the known Jan s Index: β s a value between (unfar) and 1 (far): β = n n n r 2 r 2 Aggregate throughput (Mbps) ; β 1 When we measure farness among STAs, r s the traffc carred by STA and n s the number of STAs. When we measure farness among APs, r s the AAC of AP and n s the number of APs. We observed that presents the best farness values n all the cases, regardless of the number of users, c or number of STAs n saturaton (e.g. see fgure 3). Another measure of farness can be provded by measurng r _mn. In ths case, snce s desgned to maxmze AAC mn, ts results are logcally the best (see Fgure 4a). But although also provdes the hghest mnmum carred throughput on average, (as shown n fgure 4b), we have to note that provded the best results n most of the smulatons. 7. Conclusons 5 4 3 2 1 Saturated users (%) Fgure 2: Aggregate throughput: a) 65 saturated STAs. b) Saturated STAs and c = 55%. c) 65 STAs and c = 55% In ths paper we have presented a new dstrbuted load balancng algorthm for IEEE 82.11 WLANs, based on the dea of cell breathng. In our approach, the APs have the ablty to cooperate n order to redstrbute the load among neghborng cells, n a way that s transparent to the end user, who can be equpped wth standard devces. The most obvous concluson that can
Farness (APs) AAC mn (kbps) 1.9.8.7.6.5.4.3.2.1 18 16 14 12 1 8 6 4 2 a) 18 a) b) 16 14 12 mn carred throughput (kbps) 1 Fgure 4: a) mn AAC for an AP. b) mn carred throughput for a STA Farness (STAs) 1.9.8.7.6.5.4.3.2.1 8 6 4 2 Fgure 3: Jan s Farness ndex for a) APs and b) STAs. STAs n saturaton and c = 55% b) be derved from the evaluaton s that the absence of any knd of power control reduces the potental capacty of the network drastcally. Applyng an optmal for data exchange ensures a better utlzaton of the resources and therefore, the performance of the network s mproved. However, n scenaros wth a hgh densty of nodes, the average user experence can be further mproved, and the congeston level on APs allevated, f we ntroduce the ablty to dynamcally change the cell sze accordng to the envronment. Our approach not only provdes good network performance but also ensures an even share of bandwdth among clents and a balanced load among APs. Although t s not a strong requrement, the man mplementaton ssue arses wth the need to exchange nformaton between clent statons and APs. Snce the needed nformaton exchange s related to rado measurements, ths requrement wll be satsfed wth the advent of new IEEE standards: IEEE 82.11h and 82.11k. Acknowledgment Ths research work was funded by the ERDF, the Spansh Government through TEC26-454, and the 2CAT foundaton. References [1] T. Henderson, D. Kotz, and I. Abyzov, The changng usage of a mature campus-wde wreless network, n 1th Mobcom, pp. 187-21, September 24. [2] N. Prasad, A. Prasad, WLAN Systems and wreless IP for next generaton communcatons, Artech House, 21. [3] G. Banch and I. Tnnrello, Kalman Flter estmaton of the number of competng termnals n an IEEE 82.11 network, n 22nd Annual Jont Conference of the IEEE Computer and Communcatons Socetes, INFOCOM'3,vol. 2, pp. 844-852, Aprl 23. [4] I. B. Dhou, A Novel Load-Sharng Algorthm for Energy Efcent MAC Protocol Complant wth 82.11 WLAN," n IEEE 5th Vehcular Technology Conference, VTC 1999-Fall, vol. 2, pp. 1238{1242, September 1999. [5] G. Banch and I. Tnnrello, Improvng Load Balancng Mechansms n Wreless Packet Networks," n IEEE Internatonal Conference on Communcatons 22, ICC 22, vol. 2, pp. 891-895, Aprl 22. [6] A. Balachandran, P. Bahl, and G. M. Voelker, Hot-Spot Congeston Relef n Publc-area Wreless Networks," n Proc. of 4th IEEE Workshop on Moble Computng Systems and Applcatons, pp. 7-8, June 22.
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