A new access point selection policy for multi-rate IEEE WLANs

Transcription

1 Internatonal Journal of Parallel, Emergent and Dstrbuted Systems Vol. 23, No. 4, August 2008, 1 20 A new access pont selecton polcy for mult-rate IEEE WLANs Murad Abusubah, Sven Wethoelter, James Gross, Adam Wolsz Telecommuncaton Networks Group Techncal Unversty Berln Emal: {abusubah}@tkn.tu-berln.de (In fnal form August 2008) In wreless local area networks often a staton can potentally assocate wth more than one access pont. Therefore, a relevant queston s whch access pont to select best from a lst of canddate ones. In IEEE , the user smply assocates to the access pont wth the strongest receved sgnal strength. However, ths may result n a sgnfcant load mbalance between several access ponts. Moreover, the mult-rate flexblty provded by several IEEE varants can cause low bt rate statons to negatvely affect hgh bt rate ones and consequently degrade the overall network throughput. Ths paper nvestgates the varous aspects of best access pont selecton for IEEE systems. In detal, we frst derve a new decson metrc whch can be used for AP selecton. Usng ths metrc we propose two new selecton mechansms whch are decentralzed n the sense that the decson s performed by each staton, gven approprate status nformaton of each access pont. In fact, only few bytes of status nformaton have to be added to the beacon and probe response frames whch does not mpose sgnfcant overhead. We show that our mechansm mproves mean qualty of servce of all statons and better utlzes network resources compared to the conventonal one mplemented today n IEEE devces. Also, the schemes are appealng n terms of stablty and provde ther performance mprovement even for denser or lghter network confguratons. Keywords: IEEE ; Wreless LANs; Access Pont Selecton, Load Balancng 1 Introducton Over the last years wreless local area networks (WLANs) [1] have become qute popular. Due to decreasng costs of the equpments (wreless access ponts APs and wreless network cards) and fxed broadband connectons (dgtal subscrber lnes), WLANs have become the preferred technology of access n homes, offces, and hot-spot areas (lke arports and meetng rooms). Although orgnally several standards for WLAN have been competng, today vrtually all WLANs are based on the IEEE standard. Ths technology provdes users wth raw transmsson rates of up to 54 Mbps n IEEE a/g and 11 Mbps n IEEE b, whle even faster standard supplements are currently under dscusson. For the future, a further ncrease n wreless local

2 2 Murad Abusubah area networks can be expected as, for example, the cty of Chcago and the Bay Area currently consder the roll out of a cty-wde WLAN based on IEEE [2]. In current mplementatons of IEEE , a STA has to frst pck (and assocate wth) an AP before t can access data transmsson servces of the WLAN cell. Ths process can be performed actvely or passvely and s referred to as scannng. In actve scannng a STA sends a Probe Request frame and the AP reples wth a Probe Response frame. Ths frame exchange allows the STA to obtan basc nformaton about the cell lke sgnal strength, avalable transmsson modes, encrypton etc. The frame exchange s repeated for all APs n the vcnty, such that the STA has a lst of APs at the end of the scannng process. Alternatvely, n passve scannng a STA lstens to Beacon frames whch are perodcally transmtted by APs. After scannng (ether actvely or passvely) multple APs, a STA always assocates to the AP from whch t has receved the strongest sgnal. Afterwards, t stays assocated untl the STA s powered down or the AP shuts down ts servce. It s obvous that ths rather smple selecton process can lead to problems regardng the network performance of larger areas wth many STAs and several APs [3, 4]. Ths has ntated research actvty n an area commonly referred to as load balancng. Load balancng bascally tres to dstrbute statons (ether centrally or decentrally) over multple cells such that the network and staton performance ncreases (a tutoral dscusson of load balancng n packetswtched networks s provded n [5]). The major queston among the work n ths area has been how to measure load most realstcally n a WLAN cell. For example, n [6] t has been proposed to smply characterze the load per AP by the number of statons assocated to t. Whle ths s easy to mplement, the multple rates provded by several IEEE varants counteract ths load metrc. In prncple, the advantages of mult-rate protocols have been shown n [7]: Usually, a moble STA wth a relatvely low sgnal to nose (and nterference) rato chooses a low transmsson rate to balance ts frame error rate. However, the medum access control (MAC) protocol provdes perframe farness, meanng that n the long term STAs have the same chance to access the medum and send ther frames (all STAs should transmt wth an equal average frame rate over a longer tme horzon). As the tme duraton requred to transmt a frame wth a low transmsson rate s much longer than the duraton for the same frame sze wth a hgher transmsson rate, a low transmsson rate STA wll occupy the channel for a longer tme. Ths phenomena degrades the throughput of hgh rate STAs f they are assocated to the same AP. In [8], t has been shown that f a STA wth a transmsson rate of 11 Mbps shares the channel wth a STA at a transmsson rate of 1 Mbps, the throughput of the 11 Mbps STA s about the same as that of the 1 Mbps STA (assumng equal flow characterstcs of each STA as well as the saturaton

3 A new access pont selecton polcy for mult-rate IEEE WLANs 3 mode). A smple mathematcal model of ths effect and the consequence for centralzed, optmal load balancng has been dscussed by Kumar et al. n [9]. However, the authors suppose n ther study that the transmsson rate per statons equals the acheved goodput whch s not the case n general. A further centralzed soluton wth a smplfed load characterzaton can be found n [10]. The same problem of realstc load characterzaton apples also to a study recently publshed by Ekc et al. n [11]. In contrast to Kumar, the authors propose a totally decentralzed approach to load balancng where each staton decdes ndependently on ts assocaton. Ths decentralzed approach s also referred to as AP selecton. However, whle the authors take the mult-rate effect nto account, agan the transmsson rate per termnal s assumed to equal goodput. In ths paper we present a more realstc approach to AP selecton. We dentfy the core problem of AP selecton to be the choce of metrc to consder and whether the choce should be (perodcally) reevaluated or not. Obvously, a more effectve AP selecton mechansm sgnfcantly mproves the overall network performance whle also mprovng the STAs rates and delays. We frstly motvate a novel metrc for AP Selecton by showng that even under nonsaturated condtons, low-rate statons have a sgnfcant mpact on WLAN cell s performance. Secondly, a novel metrc s derved that s benefcal n combnaton wth a new statc AP selecton scheme for IEEE WLANs. Ths new metrc encapsulates several mportant cell and connecton parameters nto a sngle value. These parameters are easly dstrbuted by the APs va the Beacon and Probe Response frames. Thrdly, the statc scheme s enhanced towards a perodc reevaluaton of the current assocaton decson whch we refer to as dynamc AP selecton scheme n the followng. For the statc as well as the dynamc scheme, ths work demonstrates the effcency of both approaches for IEEE b WLANs. The remander of ths paper s organzed as follows: Secton 2 llustrates the mult-rate effect for an example IEEE b cell. Secton 3 descrbes the system model and the basc assumptons, derves the new metrc, and presents our statc as well as dynamc selecton mechansm. Fnally, we evaluate the performance ncrease of our schemes n Secton 4 before we conclude our paper n Secton 5. 2 Motvaton for novel metrc desgn In IEEE WLANs, statons choose the modulaton scheme for the transmsson of an MPDU dependent on channel condtons. Ths rate adaptaton has not been standardzed by IEEE, thus vendors use propretary schemes.

4 ML V 4 Murad Abusubah, + * ) ( ' -.0/ 1123/0456/ / 4 5 :;.6/611 /04:0<=3>@?A62 :;.0/ 110/ 4B6<=3>@?A62 :;.6/6110/04C <C3>@?A62 '!#"%$& * * D d TRS TU D V QP D F NOM f JK I e GH d h66j6k kl3j mn0j6 oqp j0m n r;0j kk0j m#r st@u@vw6l r;6j6k k j0mx st3u@vw l r;0j kk0j my6sy3u@vw6l D EF WXY V EF X[ZXY \] ^`_#a g Eg bc D D EF (a) G.729-coded VoIP wth 10 ms packetzaton (b) G.729-coded VoIP wth 20 ms packetzaton Fgure 1. Maxmum number of calls at homogeneous rates and call reducton due to sngle low-rate transmssons - refer to Secton 2 for a detaled descrpton Assumng even perfect adaptaton schemes,.e., neglectng effects due to false adaptaton, the modulaton choce of sngle statons has an mpact on the performance of the whole WLAN cell. Heusse et al. [8] present theoretcal analyss valdated by smulaton experments for TCP as well as UDP traffc. Here, we llustrate ths mpact by an example result from VoIP smulatons (see [17] for the detals and more results). Consder a sngle WLAN cell where multple statons are assocated to the AP. Every staton has one ongong VoIP call whch t tres to convey va the IEEE b system wth an acceptable qualty. Dependent on the average channel condton the statons set ther rate accordngly. Four transmsson rates wth 11, 5.5, 2.0 and 1 Mbps can be used by each staton. Intally, the maxmum number of calls at each transmsson rate has been obtaned assumng that each staton transmts wth the same rate. The maxmum number of calls s determned such that VoIP QoS lmts are not volated. Notce how the total number of VoIP calls that can be accommodated ncreases the hgher the transmt rate s whch all statons use. For example, n Fgure 1(a) a total of 6 VoIP calls can be served by the cell f all statons transmt wth 11 Mbps. Next, assume that among all statons n the cell one of them cannot select the same rate as all other statons (due to a lower SNR, for example), but apples a lower rate to transmt ts VoIP call. Clearly, the total amount of VoIP calls that can be served decreases, dependng on how low the rate selecton for the sngle staton s. For example, f the specfc staton can only acheve a rate of 1 or 2 Mbps, the total amount of VoIP calls served by the cell drops

5 A new access pont selecton polcy for mult-rate IEEE WLANs 5 n Fgure 1(a) to the level as f all statons would transmt wth 5.5 Mbps (despte the fact that the remanng statons even transmt wth 11 Mbps). These results show that especally statons wth low transmsson rates have a sgnfcant mpact on the total cell performance as well as on the ndvdual rate performance of other statons. Whle n ths example the mpact s moderate, t s much stronger for larger frame szes. 3 Improvng AP Selecton The key aspect for AP selecton s the desgn of an approprate decson metrc. In ths secton, we frst descrbe the system model and dscuss afterwards a much more approprate metrc for AP selecton. Then, we present two schemes and dscuss how to mplement the selecton mechansms based on the proposed metrc. 3.1 System model We consder an area where several dfferent access ponts complant to IEEE offer servce to STAs. Each AP forms a cell, whereby cells of adjacent APs overlap sgnfcantly. A STA k mght decde to request servce from some dstnctve access pont. In ths case t assocates to the access pont and may start to send or receve data afterwards. Denote the number of STAs assocated at tme t to access pont by U (t). We assume that all STAs contnuously have data frames queued for transmsson,.e., we consder the saturaton mode. For the medum access we consder only the dstrbuted coordnaton functon (DCF) (thus, the medum access s governed by the CSMA/CA protocol). Each cell s n nfrastructure mode, hence, all data transmsson nvolves the access pont even f one STA mght transmt data to some other STA n the same cell. For the physcal layer, we assume smlar wreless channel condtons n both uplnk and downlnk drectons. Dependng on the channel gan between any transmtter and recever par, the transmsson rate s selected from the avalable rates of the physcal layer. Ths rate s denoted by R k Mbps, where k denotes the STA. Tradtonally, f a STA s powered up, t frst scans the current avalable access ponts on all channels and chooses the one wth the best sgnal-to-nose rato (SNR). Ths choce s made once and no other parameters are consdered. We refer to ths scheme as the legacy AP selecton scheme.

6 6 Murad Abusubah 3.2 Decson Metrc for AP Selecton Gven a certan traffc characterzaton of a STA, ultmately a STA needs to jon the cell whch can serve ths traffc stream best. Ths mght be the access pont whch has the hghest SNR. However, f the cell s crowded wth other STAs operatng at a much lower SNR than the currently observed STA, t mght not be served very well even though the SNR ndcates a good servce. Furthermore, a STA mght jon a cell (whch has the best SNR regardng ths STA), but the cell s already servng some other STAs wth a much hgher SNR than the observed STA. Then, the new STA wll sgnfcantly degrade the throughput of the other STAs. That s because APs generally consume longer tme servng low rate STAs. Hence, a metrc s requred whch can capture the mpact of the STA jonng the cell on the performance of the other (already joned) STAs. In addton, the metrc should capture the expected throughput of the STA tself f t jons a cell. We start dervng such a metrc by consderng the traffc n a certan cell. There are currently U (t) STAs assocated to the access pont. Some new STA k consders the assocaton to cell. Hence, t has to evaluate the data rate t wll receve from jonng cell. However, t also has to take nto consderaton how much t wll degrade the data rate of other STAs, whch have already joned cell. We start wth dervng the data rate STA k wll receve f jonng cell. Followng the analyss n [13], STA k n cell successfully transmts a frame of length L bts after j consecutve unsuccessful transmssons wthn a tme perod of T k, (j), gven by: T k, (j) = T P + T H + T DIFS + L R k + T SIFS + T ack + T backoff (j) (1) T P and T H represent the tme duraton of the physcal layer preamble and header. T DIFS s the Dstrbuted Coordnaton Functon Inter-frame Space and T SIFS s the Short Inter Frame Spacng, L = (28 + L MSDU ) 8 s the length of the MAC packet n bts (where L MSDU s the MAC Servce Data Unt length and the 28 bytes stem from the MAC header), T ack = (T P + T H R k ) s the duraton of the ACK frame, and T backoff (j) s the average backoff nterval n µs after j consecutve unsuccessful transmsson attempts gven as: T backoff (j) = { 2j (T CWmn+1) 1 2 T Slot 0 j< 6 T CWmax 2 T Slot j 6 (2) where T Slot s the basc slot duraton, T CWmn and T CWmax are the mnmum and maxmum contenton wndow szes respectvely.

7 A new access pont selecton polcy for mult-rate IEEE WLANs 7 However, T k, (j) s only the raw average transmsson tme of a frame. The frame s stll subject to frame errors, whch requres one or several retransmssons. The average tme span that STA k requres to transmt a sngle frame correctly s [13]: T k, = T k, (0) + (1 P k, )P j k, j=1 [ j 1 m=0 T f (m) + T k, (j) where P k, s the frame error probablty 1 and T f (m) = T P + T H + T DIFS + T backoff (m) + L R k + T SIFS + T ack + T Slot s the tme between two consecutve transmssons f the frame transmsson fals. In fact the transmsson can fal due to frame errors or collsons. Assumng that frame errors are ndependent from collsons, we could wrte P k, as follows: ] (3) P k, = P e k, + P c k, P e k, P c k, (4) where P e k, s the frame error rate due to the channel and P c k, s the error rate due to collsons. In [14], Banch et al. derved as expresson for the P c k, as follows: P c k, = 1 (1 τ) U 1 (5) where τ represents the probablty that a staton transmts n a randomly chosen tme slot expressed as: τ = 2(1 2P c k, ) 2(1 2P c k, )(CW mn + 1) + P c k, CW mn (1 (2P c k, ) m ) (6) where CW max = 2 m CW mn. (.e m=5 f CW mn =32 and CW max =1024). Banch proposed to solve the non lnear system n 5 and 6 usng numercal technques. Assumng that STA k s the only STA n the cell, the fracton L/T k, would yeld the average throughput of STA k n the cell (assumng also that the access pont does not transmt any data). However, we assume that there are n general U (t) actve STAs n cell, therefore t s qute lkely that the channel s occuped by some other STA f STA k wants to start a 1 See Secton 3.3 for detals on the acquston of the frame error rate.

8 8 Murad Abusubah data transmsson. In general, STA k s throughput depends on the channel occupancy tme of other STAs n the cell. We capture ths effect by modelng the average rate of correctly transmtted bts by: G k, = µ k,l T k, (7) where µ k, s the fracton of channel tme consumpton left over for STA k, gven as: µ k, = T k, T k, + U (t) j=1 T j,. (8) Recall that STA k has not joned cell yet. Fnally, we obtan the average throughput of correct bts G k, (combnng Equatons (7) and (8)) as: G k, = L T k, + U (t) j=1 T j, (9) From Equaton (9) t s clear that STA k s throughput depends on the channel occupancy tme of frames transmtted by other STAs n cell (apart from other ssues lke the SNR and chosen rate). However, ths equaton does not consder the effect of STA k on the average throughput of all other STAs n cell. The current average channel occupancy tme of all STAs n cell s smply gven by: U (t) j=1 T j, U (t) (10) Hence, the STA can compute the new average channel occupancy tme once t joned the cell. If ths new value ncreases, the STA can deduce that t wll decrease the throughput of all STAs n the cell. If the new value decreases, the STA wll not harm other STAs but could acheve a hgher throughput f the other STAs had a better SNR, for example. Hence, the dfference between current average channel occupancy tme of the STAs n cell and the new average channel occupancy tme per STA n cell (f STA k joned ths cell)

9 A new access pont selecton polcy for mult-rate IEEE WLANs 9 s a measure for the mpact of STA k on cell. Ths measure s gven by: U (t) j=1 T j, U (t) T k, + (t) U j=1 T j, U (t) + 1 = U (t) j=1 T j, U (t) T k, ( ) (11) U (t) U (t) Statc AP Selecton To select an AP, a STA wll try to maxmze ts throughput n the cell t wshes to assocate to but should also mnmze ts mpact on already assocated STAs. Therefore, t computes the followng mpact value for all canddate APs and sends ts assocaton request frame to the AP that maxmzes ths functon. W () = α L T k, + U j=1 T j, + (1 α) U j=1 T j, U T k, U (U + 1) (12) where α s a weghtng factor between 0 and 1, whose value depends on the varance of (9). Therefore, a STA tres to mnmze ts negatve effect on other STAs f ts theoretcal throughput from the canddate APs does not dffer sgnfcantly. Note that the two parts of the cost functon represent dfferent quanttes. For ths reason, each part has been normalzed by the maxmum among all values obtaned from the dfferent APs. The frst part of the cost functon n (12) consttutes the theoretcal throughput a STA wll get n some cell, whle the second part s a measure of ts mpact on the other STAs n the same cell. To do so, a STA needs three peces of nformaton, whch are the number of STAs the AP currently accommodates, the summaton value n equaton (9) and the current SNR between AP and STA. The AP could nclude the frst two values n a new nformaton feld n the Beacon and Probe Response frames. Obvously, the length of ths feld s only a few bytes, so that t does not mpose a sgnfcant overhead. For computng T k, and T j,, P k, and P j, can be evaluated as descrbed n [15], where a STA can use the perceved SNR and the AP can use the uplnk SNRs for the statons (as we assume smlar wreless channel condtons n both uplnk and downlnk drectons). U (t) 3.4 Dynamc AP Selecton As the wreless envronment s dynamcally changng, after a perod of tme any selected AP may no longer reman the best one. Therefore, a STA should be requred to evaluate the functon W () after some tme perod T c and re-

10 10 Murad Abusubah assocate f t found a better AP. Unlke prevous solutons n whch the tme perod T c s constant and STAs always scan all supported channels [12] we propose an enhancement. Specfcally, the value of T c s dynamcally adjusted n order to avod unnecessary scannng and a frequent png-pong effect. Moreover, a STA scans all channels only after t has been powered up. Afterwards, a mask s used to flter out all channels from whch a Beacon or Probe Response frame have not been receved. However, the set of all non-overlappng channels (channels 1,6, and 11 n IEEE b) are not masked snce those channels are most lkely to be used by APs. Ths consderably reduces both the number of scans and the scannng tme, whch s obvously very crtcal for delay senstve applcatons. For example, consderng voce over IP (VoIP traffc), whch has very strct requrements, the maxmum tolerable end-to-end networkng delay s about 150 ms. The typcal scannng and re-assocaton tme wth the legacy approach s between 1 and 2 seconds, whch wll hardly allow a good VoIP qualty. Ths tme s reduced by usng the combnaton of dynamc selecton and maskng. However, ths reducton s stll not satsfactory. The scannng tme span can be mtgated further by spreadng the scannng process over tme. Specfcally, once the STA has a lst of neghborng APs, t can sequentally send probe request frames to the APs. The tme separaton between two successve probe request frames T w could be selected such that the qualty of servce of ongong calls s not degraded sgnfcantly. After recevng probe responses from all neghborng APs, the STA has the requred nformaton to decde whether to jon a new AP or not. Apart from that, the emergng k and the r standards as well as the power-save mechansm can further contrbute n resolvng the scannng latency problem k [18] enables a STA to request a lst of canddate neghborng APs from the AP to whch the STA s assocated r [19] defnes mechansms for fast and secure transton between APs. Dependent on the duraton of the scannng latency, a staton wll experence packet losses snce t s not able to receve frames beng sent by ts old AP. Wth the powersave mechansm of legacy , one s able to mnmze packet losses whle the staton scans for APs on other channels. Durng ths perod, the AP buffers frames dedcated to the scannng staton such that they can be conveyed after STA s reappearance. Huang et al. [16] used ths scheme to optmze selectve scannng pror to fast WLAN handoffs. Therefore, by combnng the spreadng scannng concept wth the power-save mechansm as well as wth IEEE802.11k and IEEE802.11r technologes, we expect dynamc selecton mechansm to be possble wth VoIP wthout harmng ongong calls. We summarze our Dynamc AP Selecton algorthm as follows:

11 A new access pont selecton polcy for mult-rate IEEE WLANs 11 Algorthm 1 Dynamc AP Selecton Algorthm 1: ChannelLst {All Supported Channels} 2: Non-Overlapped {Non-overlappng Channels} 3: Send Probe Request Frames or Lsten to Beacons over ChannelLst 4: AP new Select AP based on W () (as explaned n Secton 3). 5: for (tme = 0 to T c ) do 6: do normal communcaton 7: end for 8: Send Probe Request Frames or Lsten to Beacons over ChannelLst. Alternatvely: 9: for all Channels ChannelLst do 10: { 11: Send Probe Request over a Channel; 12: do normal communcaton for Some Tme T w ; 13: } 14: end for 15: ChannelLst Non-Overlapped {Channels over whch Probe Responses or Beacons were receved} 16: AP new Select AP based on W () 17: f (AP new better than current AP ) then 18: T c T c /2 19: Re-assocate 20: else 21: T c 2. T c 22: end f 23: go to step 5 4 Performance evaluaton Ths secton compares the performance of the proposed statc and dynamc selecton mechansms wth the default one mplemented n IEEE b WLAN cards, n whch STA-AP selecton s based on the Strongest Receved Sgnal. The performance results are obtaned by means of smulaton usng the NC- TUns [20] smulator. 4.1 Smulaton scenaro and metrcs We consder a large area lke a departure hall n an arport as basc settng. Due to a potentally hgh number of users, four b APs that operate on dfferent IEEE b channels are placed wthn ths area. Users appear n ths hall at dfferent ponts n tme and at dfferent places. Two dfferent user

12 12 Murad Abusubah types may be present: ether FTP or VoIP clents. Both have nomadc moblty degree,.e., users start ther devces and stay at a constant poston durng ther actve sesson. Dependng on the dstance between AP and STA the wreless channel s attenuated more ore less severely. However, we assume that rado sgnals are not only attenuated by path loss, but are also affected by fadng due to mult-path propagaton. In order to accurately model these effects, a path loss component as well as a Raylegh-dstrbuted fadng component s consdered. For the path loss we have used a two-ray ground reflecton model wth the receved power P rx gven as: P rx = P txg tx G rx h tx h rx d 2 (13) where P tx s the transmt power (n mw), G tx,g rx denote the transmtter and recever antenna gans respectvely, h tx and h rx are the antenna heghts of transmtter and recever, and d s the dstance between them. We assume that APs and STAs use the same transmsson power level. The fadng mpact s regenerated for every frame transmsson per staton. An nternal fadng process of the NCTUns smulaton tool s used whch takes as parameter a varance coeffcent. Statons choose ther transmsson rates dependng on the perceved, average SNR (.e. wthout the fadng mpact) and try to assure a bt error rate (BER) less than Ths rate remans constant durng the smulaton,.e, no rate adaptaton mechansm has been mplemented. For IEEE b the possble rates actually are 1 Mbt/s, 2 Mbt/s, 5.5 Mbt/s and 11 Mbt/s. As the throughput of the whole system should be maxmzed, every AP measures the throughput every second n up- as well as downlnk drectons. The sum of the four AP throughputs s the frst metrc, whch s denoted as aggregated throughput n the followng. Despte packet loss, the end-to-end delay s the most crtcal component for VoIP. In smulatons nvestgatng VoIP, the average round-trp tme s measured addtonally. The value averaged over all STAs s a second metrc. 4.2 Parameterzaton STAs are unformly dstrbuted n a square area wth an edge length of 500 meters whle ther arrval tme s unformly dstrbuted over 40 seconds. APs are placed 250m apart from each other. In all smulatons, the smulaton tme was set to 350 seconds. We consder two dfferent traffc mxtures: Frst, we nvestgate a settng wth 60 FTP users, whle n a second settng 30 VoIP users are present wthn the

13 A new access pont selecton polcy for mult-rate IEEE WLANs 13 WLAN area. FTP as well as VoIP sessons termnate at the wred part of the network at a sngle server. The latency for packets between APs and the server was set to 10µs. The cables connectng the APs to the server (va an swtch) have a 100 Mbt/s bandwdth. For each user traffc settng, smulatons have been carred out for 5 dfferent cases of random STA placements. Each case has been smulated 15 tmes where the appearance of users was randomly generated. In the case of the FTP settng, every user downloads a fle, whereby ts sze s ndefntely large. All TCP users utlze greedy TCP wth packet length of 1000 bytes. The TCP traffc s generated wth Jug s Traffc Generator (jtg) [21]. In the case of the VoIP settng, each VoIP call s modeled by a b-drectonal, sochronous audo flow. Wth ITU-T s G729 codec (whch s wdely used n devces) and an audo frame length of 10ms, ths results n an audo packet sze of 10 bytes. The VoIP traffc was generated usng the RTP/UDP traffc generator whch comes wth NCTUns smulator [20]. Table 1 lsts other values of the parameters used n the smulatons. Table 1. Constant parameters Parameter Value Parameter Value PLCP header T H 48 µs T SIFS 10 µs PLCP preamble T P 144 µs T DIFS 50 µs Cell overlap 20 % T Slot 20 µs Fadng Varance 10 db T CWmn 31 APs/STAs Tx Power 100 mw T CWmax 1023 APs/STAs Tx Range 300 m G tx, G rx 0 db h tx and h rx 1 m T c 20 s 4.3 Results In ths secton we present the smulaton results of the proposed AP selecton mechansms. Frst, we dscuss the results of the FTP settng and of the VoIP settng. Then, we present some deeper analyss of certan aspects of the new selecton schemes. Note that parts of these results have already been presented n [22]. Let us ntally consder the case of the FTP settng. We compare the aggregate throughput of the statc and dynamc selecton mechansms dscussed n ths paper versus the legacy selecton mechansm. The two graphs n Fgure 2 presents the mnmum and maxmum aggregate throughput mprovement (among the 5 dfferent placement nstances) obtaned for the FTP settng for the statc selecton scheme. We observe that the aggregate throughput obtaned from the proposed mechansm s almost always mproved, n the best case ths mprovement s about 14.7%. Next, consder the mnmum and maxmum mprovements n case of the dynamc selecton scheme for the FTP settng n Fgure 3. Notce that for the dynamc selecton scheme a sgnfcantly hgher throughput gan (up to 33%)

14 14 Murad Abusubah s acheved than n the case of the statc selecton scheme. Hence, perodcally reevaluatng the AP assocaton decsons appears to be qute benefcal for the performance of the consdered network. Table 2 shows mnmum, maxmum and average throughput of both new selecton mechansms compared to the legacy mechansm for all fve STA placements (notce that all presented results for a sngle placement are already the average over the 15 dfferent nstances of staton appearance tmes). Next, we consder the results regardng the VoIP settng. The two graphs n Fgure 4 present the mnmum and maxmum mprovements wth respect to aggregate throughput for the statc selecton scheme. Clearly, the aggregate throughput s mproved, however, the maxmum mprovement s only about 10%, whch s n contrast to the FTP settng. Despte ths small mprovement n aggregate throughput, notce the sgnfcant mprovement of the average round trp tmes. In Table 3 the average round trp tmes (n mllseconds) are gven for all fve staton placements regardng the statc and legacy selecton scheme. We observe that f the round trp tmes of the legacy scheme become qute hgh (above 100 ms), the statc selecton scheme mproves the round trp tmes substantally, snce large round trp tmes are an ndcaton for a hghly loaded network. Thus, the lower the round trp tmes are for the legacy scheme, the smaller becomes the space for potental mprovements wth the statc selecton. An mportant queston relates to the mpact of the dynamc selecton scheme on the performance of a staton durng ts scannng process. Graph (a) of Fgure 5 presents the effect of perodc selectve scannng (where only the three non-overlappng channels are consdered) compared to default scannng (where all channels are consdered) for a FTP clent. The graph shows clearly that the throughput s less nterrupted for the selectve scannng process as only a few channels are consdered. Ths leads to a greater mnmum throughput whle the recovery tme s also shorter when selectve scannng s used. In the above dscusson, the dynamc selecton scheme has not been evaluated n combnaton wth the VoIP settng. However, n Secton 3.4 we argued that by spreadng the scannng process n tme (every once n a whle a certan channel s scanned whle the assocaton to some AP s mantaned), t mght be feasble to apply dynamc selecton even for VoIP scenaros. Graph (b) of Fgure 5 shows the feasblty of ths spreadng soluton. The throughput of a VoIP sesson s almost not effected by the modfed dynamc selecton scheme n comparson to the dynamc selecton scheme wthout spreadng. Ths result shows that the dsrupton tme can be reduced by the spreadng soluton n comparson wth successve scannng. As especally the dynamc selecton scheme seems to provde a sgnfcant performance mprovement, an obvous ssue to consder s stablty. In order to capture the stablty of the dynamc selecton mechansm, we nvestgated the

15 A new access pont selecton polcy for mult-rate IEEE WLANs 15 percentage of dynamc selecton decsons n whch a STA was requred handoff from one AP to a new one. The results are gven n Fgure 6. They show that only about 15% of the re-evaluatons requre a STA to handoff from one AP to a new one. In order to further nvestgate the stablty characterstcs, we also consdered the STAs s total dstrbuton among the four APs durng one sample smulaton run for the dynamc selecton scheme. These results are plotted n Fgure 7, showng that wth the dynamc selecton scheme the total number of statons assocated to each AP stays roughly constant after some ntalzaton phase of the system. Furthermore, we have been nterested n the behavor of the statc selecton scheme n comparson to the legacy selecton f the network densty vares (ether gets lghter or denser). In order to do so, we keep the number of statons n the smulaton fxed and vary the dstance between the AP s. Then, we collect the assocaton decson by both approaches. Graph (a) of Fgure 8 shows the percentage of equal assocaton decsons between the two approaches for several dfferent denstes. The fgure shows clearly, that the lghter the network densty gets, the less often do the approaches perform equal assocaton decsons. Ths s due to a much hgher number of low-rate statons n the area, as the dstance between APs ncreases. As our decson metrc not only consders the throughput achevable per staton but also the mpact on other statons, the assocaton decsons become less smlar to the legacy decsons. Graph (b) of Fgure 8 shows the comparson of the correspondng throughputs showng that the aggregate throughput was always hgher wth the statc selecton approach regardless of the dstance between APs. It can be expected that the dynamc selecton scheme would further mprove ths throughput advantage. Whle a saturated network has been assumed n the presented analyss only, addtonal smulatons have been conducted also for a non-saturated network wth Posson traffc n order to determne the gan n the throughput performance for a more realstc settng. Fgure 9 plots the gan of the statc selecton polcy over the legacy selecton scheme for dfferent packet nter-arrval tmes. Agan, the results show that the statc polcy outperforms the legacy strategy even n a low loaded network whle the gan ncreases as the load ncreases. In order to capture the mpact of the weght parameter α (whch balances the throughput performance of each staton to the mpact to other statons n Equaton 12) on the overall performance of the proposed selecton polcy, we evaluated the throughput performance of the statc selecton polcy applyng dfferent values of α rangng from 0 to 1. Fgure 10 provdes the results of the smulaton experments carred out. The fgure shows that a maxmum throughput has been obtaned wth α = 0.4. However, the fluctuatons n the curve of fgure 10 ndcate that a precse estmate of an optmal value of α would requre further extensve smulatons whch are left for future work. Fnally, we have been nterested n comparng the performance of our selec-

16 16 Murad Abusubah ton polcy wth the ones proposed n [11] and [12] by Ekc et al. and Fukuda et al., respectvely. We have ncluded both polces n the smulator and conducted smulatons wth our scenaro and traffc patterns as descrbed above. In our mult rate envronment, the polcy of [12] does not mprove the throughput overall, whle the strategy of [11], however, has mproved the throughput by 6.46%. The dfferences between our values and the mprovements publshed n [11] are smply due to a dfferent scenaro choce. Wth a settng n whch most of the STAs are orgnally postoned close to some APs and few STAs were dstrbuted around the other APs, Ekc et al. were able to gan mprovements of up to 50 percent. Moreover, our dynamc selecton polcy s more stable than the dynamc one of [12]. Ths s due to the dynamc settng of the perodcty of the re-evaluaton of the best AP. Furthermore, wth selectve scannng our dynamc selecton polcy reduces the scannng tme for the polcy of [12] by almost 50%. Table 2. Mnmum, maxmum, and average APs throughput (FTP scenaro) Mn., Max., and Avg. Throughput (KB/s) Case Scheme Mn. Max. AVG. 1 Legacy Statc Dynamc Legacy Statc Dynamc Legacy Statc Dynamc Legacy Statc Dynamc Legacy Statc Dynamc Table 3. Average round trp delay of RTP packets (VoIP scenaro) Average Round Trp Delay of RTP packets (ms) Case Legacy Statc

17 A new access pont selecton polcy for mult-rate IEEE WLANs 17 5 Conclusons The smple selecton mechansm mplemented currently n IEEE WLANs does not effectvely utlze WLAN resources as t bases the selecton decson on RSSI values only and gnores other mportant parameters. In addton, as several IEEE physcal layer varants support multple transmsson rates, the currently mplemented selecton mechansm of IEEE WLANs also does not consder the mpact of assocaton on the effectve throughput of other (already assocated) statons. In ths paper we propose to take both ssues nto account when selectng an AP: the own effectve throughput as well as the mpact on other (already assocated) statons. We present a new AP Selecton polcy to mtgate ths problem where the selecton metrc encapsulates several cell and connecton parameters nto a sngle value. Bascally the mechansm tres to maxmze STA s throughput as well as mnmze ts negatve effect on hgh rate STAs currently accommodated by the AP to whch t wshes to assocate. We propose two selecton schemes based on ths metrc: a statc one where statons only consder ther assocaton once as well as a dynamc scheme where assocatons are reevaluated from tme to tme. Smulaton results show that the proposed mechansms can utlze WLAN resources much better and enhances users QoS (by mprovng aggregate network throughput and reducng the round trp delay). Especally the dynamc selecton scheme provdes a sgnfcantly performance mprovement, whle we have dscussed several ways to mtgate problems related to channel scannng latency wth the dynamc scheme. Regardng future work, we expect further mprovements when an optmal value of the weghng coeffcent α n the cost functon (equaton 12) s used. It s our future goal to nvestgate ths ssue. Also, we are nterested n the globally optmum performance of a WLAN network f all assocatons can be evaluated at a central controller n the network. Throughput (KB/s) APs Aggregate Throughput Statc Selecton Legacy Selecton Throughput (KB/s) APs Aggregate Throughput Statc Selecton Legacy Selecton Tme (Sec) Tme (Sec) (a) Mnmum mprovement (b) Maxmum mprovement Fgure 2. Statc selecton- FTP scenaro

18 18 Murad Abusubah APs Aggregate Throughput APs Aggregate Throughput Throughput (KB/s) Dynamc Selecton Legacy Selecton Throughput (KB/s) Dynamc Selecton Legacy Selecton Tme (Sec) Tme (Sec) (a) Mnmum mprovement (b) Maxmum mprovement Fgure 3. Dynamc selecton- FTP scenaro Throughput (KB/s) APs Aggregate Throughput Statc Selecton Legacy Selecton Throughput (KB/s) APs Aggregate Throughput Statc Selecton Legacy Selecton Tme (Sec) (a) Mnmum mprovement Tme (Sec) (b) Maxmum mprovement Fgure 4. Statc selecton- VoIP scenaro Throughput (KB/s) Clent Throughput Selectve Scannng Default Scannng Tme (Sec) (a) Comparson between selectve and default scannng Throughput (KB/s) VoIP Throughput wth Dynamc Selecton Modfed Dynamc Selecton Dynamc Selecton Tme (Sec) (b) VoIP wth dynamc AP selecton crteron Fgure 5. Impact of scannng on STA s throughput performance

19 ˆ Š ô ñð íóò ï ëèê ì j gfed h ch z Šƒ ˆ / +* $ & () &% '! ( Ÿ ª š A new access pont selecton polcy for mult-rate IEEE WLANs 19 ƒ z z z Ž0 z z Ž0z z%z Œ z%z Œ z z z z z 6 0 ~# 0 6 {} 9~# Ÿ š œ ž ¹ ¾ ;Ä ¾ ÁÀ ¾ ྠÃÅ ª ½ ¼ ¹ ³ «} # 0 ±³²% µ ³ º¹ ¹ º»³ Fgure 6. Hoppng statstcs for dynamc channel scannng Fgure 7. STAs s dstrbuton among the APs ìíî è çèé è öø öïë ê ø öïë õè ôè ê Î6Ç0È Í6Ç0È Ì0Ç6È Ë6Ç6È Ê0Ç6È É0Ç6È Æ Ç0È Ê0ÇÏÍ6ÇÑÐ6ÇÒÆ É0ÇÓÆ Ì0ÇÓÆ Ô0ÇÕÉÆ%Ç ÖØ Ù0Ú Û6Ü Ý0Þ3ß}Þ Ú àãþ0þ Ü@áãâÙqä å3æ $"# "!,-. ý%þ0ú0ú ý0ý%ü0ú ý%ú0ú0ú û6ü0ú ù0ú0ú þ6úïÿ0ú 0ú ý 0ú ý%ü0ú ý%û0ú ý%ú : : : ;6 6 : 6;< = ;<5= 7 (a) Percentage of same selecton decsons (b) Performance of legacy and statc selecton Fgure 8. Legacy and statc selecton n dense and lght WLANs bc jm hn h khel B C? B5@? B q? B >? A? q? >? DFENPOQRSTPUVT DHGDJIDHKDJLDJMD SW X YW W Z[O\ ]PZ ^S_ ^`a odjpd 5 ŽŒ ƒ ~ 5 5 r s t2u v w x5s ys v z { 5} Fgure 9. Statc selecton performance wth non-saturated traffc Fgure 10. Impact of α on the performance of the statc selecton polcy

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