Admission control issues in sensor clusters

Transcription

1 Admissio cotrol issues i sesor clusters Jelea Mišić, Shairmia Shafi, ad Vojislav B. Mišić Departmet of Computer Sciece, Uiversity of Maitoba Wiipeg, Maitoba, Caada PACS umbers: Valid PACS appear here Cotets I. Itroductio: admissio cotrol i wireless sesor ad ad hoc etworks 1 II. Operatio of the MAC layer 2 III. Performace uder asymmetric packet arrival rates 3 IV. Calculatig the admissio coditio 4 V. Performace of admissio cotrol 6 VI. Coclusio 7 Refereces 8 I. INTRODUCTION: ADMISSION CONTROL IN WIRELESS SENSOR AND AD HOC NETWORKS Wireless sesor etworks are expected to provide reliable evet sesig at the etwork sik for the logest possible time. These requiremets are somewhat coflictig, esp. i the case of etworks that use medium access protocols based o the ubiquitous CSMA-CA mechaism, where collisios may substatially affect trasmissio efficiecy ad thus impair etwork lifetime. This is the case with beaco eabled IEEE etwork clusters [2], despite the fact that the low rate WPAN (LR-WPAN) stadard is cosidered as a importat eablig techology for widespread, stadardized deploymet of sesor etworks [1]. I clusters, eergy is wasted i packet collisios ad active measures must be take to couter this effect. A geeral solutio to the above problem may be foud i the mechaism of admissio cotrol [12], i which a cetral authority most likely, a etwork coordiator or cluster head admits or rejects requests by idividual odes to joi the etwork or cluster. Most commo form of admissio cotrol is a cotetio-based mechaism which strives to maitai high utilizatio i the etwork uder cosideratio, whilst simultaeously miimizig cotetio. This goal Electroic address: jmisic@cs.umaitoba.ca; This research is partly supported the NSERC Discovery Grat. Electroic address: vmisic@cs.umaitoba.ca may be achieved through simple meas such as limitig the umber of odes or devices i the cluster, but more complicated schemes that use a combiatio of traffic characteristics ad Quality-of-Service (QoS) idicators, such as delay ad throughput, have bee proposed as well. The fact that fairess is aother desirable goal further complicates the desig of suitable admissio cotrol techiques. It is worth otig that admissio cotrol techiques have log bee studied as a promisig approach to solvig the geeric problem of improvig badwidth utilizatio whilst maitaiig the desired QoS idicators withi prescribed limits. The problem was first idetified i cellular etworks (i.e., mobile telephoy), where traffic is predomiatly composed of voice calls with well kow ad stadardized traffic characteristics, ad issues related to admissio cotrol ca be treated i isolatio from issues related to PHY ad other layers. As a result, a umber of proposals for admissio cotrol i this eviromet have bee described. More recetly, ad hoc ad sesor etwork applicatios have received much less attetio o accout of their higher complexity caused by users mobility ad eergy cosideratios. Moreover, admissio cotrol i wireless ad hoc ad sesor etworks is usually treated together with other aspects of the Medium Access Cotrol (MAC) layer fuctioality, ofte uder the geeric label of resource allocatio or resource maagemet. Time-Divisio Multiple Access (TDMA) techiques, bearig strog resemblace to cellular etworks, probably offer the simplest way i which admissio cotrol ca be eforced. The mai challege, i this case, is to calculate the impact of the traffic geerated by a ode which requests admissio, while actual badwidth allocatio is coducted by the cetral cotroller which allocates a certai umber of time uits to idividual odes. For example, this approach ca be used i Bluetooth picoets, where all commuicatios are sychroized to, ad cotrolled by, the picoet master [3]. I other types of etworks sychroizatio may be much more difficult to achieve, which meas that TDMA-based approach may ot be a viable optio for implemetig admissio cotrol. Admissio cotrol is much more complex i CSMAbased ad hoc etworks where all odes, oce admitted, have to coted for medium access. Effective admissio cotrol ecessitates the availability of a suitable traffic moitorig ad shapig (policig) mechaism, which is used i two ways. First, the ode which requests admis-

2 2 sio must advertise its traffic parameters, hece the eed for moitorig. Secod, the ode which gets admitted to the etwork must be able to maitai its traffic rate withi the allocated portio of the total available badwidth; hece the eed for traffic shapig. This is a commo theme i most, if ot all, related proposals: the Admissio Cotrol ad Dyamic Badwidth Maagemet scheme [11], the Cotetio-Aware Admissio Cotrol (CACP) [12], both of which are described i the cotext of IEEE etworks, as well as i the schemes for peer to peer etworks [9, 10]. Sesor etworks pose a slightly differet set of challeges. I most cases, sesor odes are ot mobile; but o the other had, sesor odes are ofte required to operate o battery power for prologed periods of time. What this meas is, that the etwork topology is agai ostatioary ot because of mobility, but because of the fact that sesor odes will evetually exhaust their power source ad cease fuctioig. Eergy efficiecy becomes oe of the mai factors that affect the desig of the etwork ad dictate the choice of commuicatio techology ad operatig regime, if ot the mai such factor. It is worth otig that eergy efficiecy is importat both for idividual odes ad for the etwork as a whole. I this case, badwidth utilizatio may simply be regulated through a sleepig mechaism that is applied to the large umber of odes preset i the cluster after beig admitted by the cluster coordiator. I this case, each ode is free to udertake the trasmissio of its packets (provided there are some) wheever it wakes up from sleep; this sleep may be coordiated i a cetralized maer (i.e., through explicit commads set by the cluster coordiator) or i a distributed fashio, where each ode goes to sleep of its ow volitio, based o some aggregate iformatio broadcast by the cluster coordiator. A example of the latter solutio was origially proposed i [6]. It is also possible to combie the two approaches described above ito a sigle scheme. I this cases, odes go to sleep, but are ot automatically allowed to trasmit whe they wake up. Istead, they first check with the cluster coordiator whether they are permitted to trasmit; if ot, they go to sleep agai. The cluster coordiator, o the other had, will oly admit (ad allow trasmissio by) the umber of odes that allows the etwork to perform the sesig task without excessive collisios. A suitable admissio scheme ca be developed based o the activity maagemet model from [6]. However, this model is rather complex ad requires umerical solutio of a large umber of o-liear equatios. While this solutio is acceptable for off-lie performace aalysis, it is virtually itractable whe admissio decisios have to be made i real time. I order to fid a viable solutio for the sesor etwork eviromet, we have developed a simplified cluster model that ca be repeatedly solved usig limited computatioal resources available to the cluster coordiator. The model uses umber of odes, average packet arrival rate, ad packet legth as idepedet variables; from these, it calculates packet service time, which is the used as the major admissio coditio. This simplified cluster model is developed usig the results of simulatio experimets coducted o clusters with a variable umber of odes, assumig that the packet arrival rates are ot symmetric but follow a uiform distributio istead, with small to moderate deviatios from the average value. The remaiig part of the chapter is orgaized as follows. I Sectio II we briefly discuss the operatio of MAC layer. I Sectio III, we discuss the performace of the cluster ad idetify the mai performace idicators that ca be used for admissio cotrol. The admissio cotrol scheme is derived i Sectio IV; it is computatioally lightweight ad thus suitable to be applied i resource-costraied sesor cluster. The performace of the admissio cotrol scheme is evaluated i Sectio V. Sectio VI cocludes the chapter. II. OPERATION OF THE MAC LAYER I beaco-eabled etworks, the chael time is divided ito superframes delimited by the trasmissio of etwork beacos by the etwork (or PAN) coordiator. The superframe cosists of a active portio, durig which the coordiator iteracts with the odes, ad a optioal iactive portio, durig which all devices may eter a low power mode to reduce power cosumptio. The active portio of each superframe is divided ito equally sized slots. Each slot cosists of 3 2 SO backoff periods which gives the shortest active superframe duratio abasesuperframeduratio of 48 backoff periods. Whe the cluster operates i the ISM bad at 2.4GHz, the duratio of the backoff period is 10 bytes, the duratio of oe slot is bytes, ad the maximum data rate is 0kbps. The active portio of the superframe is divided ito a Cotetio Access Period (CAP) ad a optioal Cotetio-Free Period (CFP), as show i Fig. 1. The access mode i the CAP period is slotted CSMA-CA, similar to I this case, the trasmissio of a packet begis with a backoff coutdow, with the iitial cout chose at radom to avoid cotetio. If the active portio of the superframe eds while the coutdow is i progress, the coutdow will be froze durig the iactive portio of the superframe ad will resume immediately after the beaco i the ext superframe. After the coutdow, the device listes to the chael to make sure it is idle; this is referred to as the Clear Chael Assessmet (CCA). The stadard prescribes two CCAs, both of which must successfully pass for the trasmissio to begi. A successful trasmissio is optioally ackowledged withi a predefied time period. The absece of ackowledgmet (ACK) idicates that the trasmissio has failed (which may be due to a collisio with aother trasmissio, or packet blockig because of buffer overflow) ad

3 3 Beaco cotetio-access period (CAP) cotetio-free period (CFP) guarateed time slot (GTS) GTS superframe duratio (SD) beaco iterval (BI) iactive Beaco FIG. 1: The compositio of the superframe i a cluster (adapted from [2]). must be repeated. If the remaiig time after the coutdow does ot suffice for the two CCAs, the packet trasmissio, ad subsequet ackowledgmet, all of those activities are deferred to the ext superframe. (A more detailed discussio of the operatio of the MAC layer i the slotted CSMA-CA mode ad its performace limitatios ca be foud i [5].) I this work, we assume that the cluster operates i beaco eabled, slotted CSMA-CA mode uder the cotrol of a cluster (PAN) coordiator. We assume that all trasmissios are set from the sesor odes to the cluster coordiator, i.e., there is uplik traffic oly, which is commo i simple sesor etworks. We also assume that all trasmissios are ackowledged. III. PERFORMANCE UNDER ASYMMETRIC PACKET ARRIVAL RATES I order to develop the admissio cotrol algorithm, we have simulated a IEEE cluster usig the object-orieted Petri et simulatio egie Artifex by RSoft desig, Ic. [8]. The goal of this aalysis is to ivestigate the impact of differet parameter values o cluster performace, i particular the effect of the asymmetry of the load origiatig from idividual sesors, ad to idetify parameters that might be suitable cadidates for the implemetatio of a admissio cotrol scheme. For these simulatios, we assumed that the etwork operates i the ISM bad at 2.4 GHz, with the maximum data rate of 0kbps. The cluster had the coordiator ad = 5, 15, ad ordiary sesor odes, respectively. Packet arrival rates at each of the ordiary odes were uiformly distributed i the rage of (1 δ)λ to (1+δ)λ, where the variatio spa δ took values from 0.1 to 0.8, ad λ was the arrival rate averaged over all odes. (For example, the load variatio of δ = 0.5 correspods to uiformly distributed arrival rates i the rage betwee 0.5λ, ad 1.5λ.) We assume the packet size is fixed at 90 bytes, which icludes all PHY ad MAC layer headers. Ordiary odes had buffers that ca hold L = 3 packets. All other parameters were set to default values prescribed i the stadard [2]. I this setup, we have measured the throughput ad the service time. The correspodig values are show i top ad middle rows of Fig. 2 as fuctios of the average packet arrival rate for the etire cluster λ ad the load variatio spa δ. Note that the mea packet service time does iclude the effect of packet retrasmissios: amely, if the sedig ode does ot receive a ackowledgmet from the cluster coordiator, it will assume that the packet is lost due to a collisio. The sedig ode will the repeat the trasmissio util the proper ackowledgmet is received. From these diagrams, two importat observatios ca be made. First, cluster performace is predomiatly depedet o the total traffic load i the cluster, but virtually idepedet of the load asymmetry, i.e., the load variatio amog idividual cluster odes. This observatio holds for both throughput ad mea packet service time, uder a rather wide rage of idividual ode traffic. Note that a variatio of δ = 0.8 meas that the values of traffic load for idividual odes is uiformly distributed i the rage from λ i to 1.8λ i, where λ i is the mea packet arrival rate per ode. The secod observatio is that the cluster effectively goes ito saturatio beyod a certai traffic load. Saturatio is caused by the CSMA-CA algorithm which allows collisio of packets set by differet odes. As log as the umber of odes is small ( = 5, 15), the overall rage of traffic loads i the diagrams is small ad the cluster does ot saturate. However, i the cluster with = odes, the probability of collisios is much higher. Whe the traffic load exceeds a certai level, the collisios become prevalet. Whe this happes, the majority of the badwidth is take up by retrasmissios ad the overall cluster efficiecy decreases. At the same time, the sesor odes are forced to reject or drop ewly arrived packets, sice the iput buffers are occupied by the packets awaitig retrasmissio. The cumulative effect of those pheomea is that the throughput experieces a sharp decrease, Fig. 2(c), while the packet service time shows a large icrease due to packet retrasmissio, Fig. 2(f). Note that collisios, effectively, waste badwidth; the presece of a large umber of collisios meas that the odes are usig up their eergy resources without actually maagig to sed useful data. Therefore, the umber of collisios must be kept as low as possible i order to coserve eergy. A detailed aalysis of eergy cosumptio is beyod the scope of this chapter; a suitable activity maagemet algorithm for etworks ca be foud i [7]. I order to verify that the aforemetioed effects are ideed caused by overwhelmig umber of collisios, we have also measured the probability of successful trasmissio γ, which is show i the bottom row of Fig. 2. As ca be see, the success probability is over 0.9 i the cluster with five odes (ote that seemigly large variatios are due to the reduced vertical rage for γ), ad drops to about 0.55 i the cluster with 15 odes. However, whe saturatio occurs i the cluster with odes, the success probability virtually drops to zero, as show

4 Th lambda delta (a)throughput with = 5 odes Th lambda delta (b)throughput with = 15 odes Th lambda delta (c)throughput with = odes lambda delta (d)mea packet service time with = 5 odes delta 150 lambda (e)mea packet service time with = 15 odes lambda delta (f)mea packet service time with = odes gamma lambda delta (g)success probability with = 5 odes gamma gamma lambda delta lambda delta (h)success probability with = 15 odes. (i)success probability with = odes. FIG. 2: Cluster performace uder asymmetric traffic: cluster throughput (top row), mea packet service time (middle row), ad probability of successful trasmissio (bottom row). i Fig. 2(i), which cofirms our aalysis. IV. CALCULATING THE ADMISSION CONDITION The aalysis preseted above ca be summarized as follows. First, the mai factor affectig performace is the total cluster load, rather tha the degree of asymmetry amog the odes i the cluster. Cosequetly, the aalysis results, ad admissio schemes derived thereof, for the case of symmetric cluster traffic ca safely be utilized for clusters with asymmetric traffic. Iterestigly eough, similar coclusio were made i the cotext of Bluetooth picoets [3]; however, the uderlyig mechaism that leads to this idepedece is quite differet sice Bluetooth uses TDMA. Secod, the mai objective of the admissio cotrol i a cluster is to prevet the oset of the saturatio coditio. I view of this, the actual admissio algorithm is quite simple. The cluster coordiator moitors the operatio of the cluster, i particular the traffic load. Whe the ew ode requests admissio, the cluster coordiator calculates whether the added traffic will cause the cluster to move ito saturatio. If the aswer is egative, the ew ode is admitted to the cluster. The ext step is to fid a suitable idicator variable, ad to devise the actual algorithm for the ecessary calculatio. I a earlier work, the performace of the etwork uder symmetric load was cosidered [4], ad it was foud that the forthcomig saturatio regime is reliably

5 5 predicted by ay of the followig coditios: packet service times which are loger tha a certai value (approx. 50 backoff periods); trasmissio success probability lower tha about 70%; fially, access probability at the ode larger tha a certai value (aroud 0.005). While ay of these descriptors ca be used for the purpose of admissio cotrol, the packet service time appears to be the most accurate idicator of the overall cluster load. Therefore, we will focus o calculatig or approximatig the packet service time as the mai idicator of the cluster operatig regime. A promisig startig poit is the aalytical model for a cluster with uplik traffic described i [6]. But before we proceed, let us first itroduce the ecessary otatio. Let the PGF of the data packet legth be G p (z) = 12 p k z k, where p k deotes the probability of the packet k=2 size beig equal to k backoff periods or 10 k bytes. The, the mea data packet size is G p(1) backoff periods. Let the PGF of the time iterval betwee packet trasmissio ad subsequet ackowledgemet (ACK) be t ack (z) = z 2 ; actually its value is betwee aturaroud- Time ad aturaroudtime + auitbackoffperiod [2], but we roud the expoet to the ext higher iteger for simplicity. Also, let G a (z) = z stad for the PGF of the ACK packet duratio. We ote that the timig prescribed by the stadard precludes the possibility that a ACK will collide with the trasmissio of a packet from aother device. The, the PGF for the total trasmissio time of the data packet will be deoted with D d (z) = z 2 G p (z)t ack (z)g a (z), while its mea value is D d = 2 + G p(1) + t ack (1) + G a(z). Let us deote the probabilities that the medium is idle o first ad secod CCA with α ad β, respectively, ad the probability that the trasmissio is successful with γ. Note that the first CCA may fail because of a packet trasmissio i progress (origiatig from aother device), ad this particular backoff period may be at ay positio with respect to that packet. The secod CCA, however, fails oly if some other device has just started its trasmissio i.e., this must be the first backoff period of that packet. Sice the correspodig probabilities differ, we eed two differet variables. The simplified probability that the medium is busy at the first CCA is 1 α = (1 (1 τ) 1 (G p(1) + G a(1))) D d (1) where τ deotes the access probability i.e., the probability that the packet will be trasmitted. (γ deotes the probability that the trasmissio will be successful.) If the cluster operates i o-saturated regime, we are able to igore the potetial cogestio effects at the begiig of superframe due to delayed trasmissios from previous superframe, as described i Sectio II. I this case, the access probability τ is uiform throughout the superframe ad the above expressio holds. The probability that the medium is idle o the secod CCA for a give ode is, i fact, equal to the probability that either oe of the remaiig 1 odes has started a trasmissio i that backoff period. The the probability of success of secod CCA becomes: β = (1 τ) 1 (2). By the same toke, the probability of success of the whole trasmissio attempt becomes: γ = β D d (3) The PGF of the time eeded to coduct oe trasmissio attempt (potetially usuccessful) is where A(z) = m i=0 j=0 i (B j (z)r ud ) z 2(i+1) (αβt d (z)) m i=0 j=0 i R ud αβ (4) T d (z) = G p (z)t ack (z)g a (z) represets the trasmissio ad ACK time without the backoff procedure; R ud = 1 αβ is the probability that oe CCA will ot be successful; B j (z) = W j 1 k=0 1 z k = zwi 1 W j W j (z 1) represets the PGF for the duratio of j-th trasmissio attempt; ad 0.. W i 1 represets the backoff couter value i i-th trasmissio attempt. If the battery savig mode is ot tured o the W 0 = 7, W 1 = 15, W 2 = W 3 = W 4 = 31. If we assume that trasmissio will evetually succeed i five attempts, which is reasoable uder light to moderate load, the probability distributio of the packet service time follows the geometric distributio ad its PGF is T (z) = (A(z)(1 γ)) k A(z)γ = k=0 γa(z) 1 A(z) + γa(z) (5)

6 6 I this case, mea packet service time ca simply be writte as T = T (1) = A (1) γ (6) The period betwee two cosecutive packet service times also depeds o the state of the iput buffer of the device i questio. If the buffer is ot empty after packet departure (whe positive ackowledgemet is received), the the ext service time will start immediately; if the buffer is foud to be empty after a successful packet trasmissio, the ext service period starts whe the ext packet arrives. If the probability that the buffer is empty after a packet trasmissio is deoted with π 0, the mea time period betwee two packet departures is P = T + π 0, ad the period betwee two trasmissio λ i attempts by the same device is p = A + π 0γ λ i (7) I a previous work [6], the probability distributio of the legth of device queue were derived usig the M/G/1/K queueig model coupled with the Markov chai model of the MAC algorithm. Due to the large computatioal complexity of the model, the probability π 0 could be obtaied oly through elaborate umerical calculatios. While this approach is appropriate for offlie performace evaluatio, it is defiitely usuitable for real-time admissio cotrol. The limited computatioal resources of sesor odes ecessitates a much simpler solutio. I order to arrive at a suitable algorithm, we have made two simplifyig approximatios. First, the expressio for the mea time for a trasmissio attempt was simplified by retaiig just the first two members of the Taylor series expasio aroud poit τ = 0. The accuracy of the approximate solutio was foud to be quite sufficiet. Secod, we have assumed that the product π 0 γ is approximately costat ad equal to. The, the approximate period betwee trasmissio attempts is p = Ã + λ i (8) I this case the access probability for ode becomes: τ =. (9) 1 p lambda (a)packet service time lambda (b)success probability. FIG. 3: Approximatio of performace idicators. 50 ad we ca solve equatio 9 algebrically for τ usig, λ i ad D d as parameters. Thus, we ca obtai approximate values for Ã, γ, ad as fuctios of, λ i ad D d. The latter two are show i Fig. 3, for D d = 9. V. PERFORMANCE OF ADMISSION CONTROL I order to verify the performace of the admissio cotrol, we have augmeted the simulator with the calculatio of approximate expressios τ, γ, ad. The cluster was set with variable umber of odes, startig with =15 odes ad addig oe ode at a time. The packet arrival rate of each ode is featured both i symmetric ad i asymmetric approach. I symmetric approach, we assumed the packet arrival rate λ = 2 packets per secod for each ad every ode. I asymmetric case, each of the odes featured a variable packet arrival rate λ i for i=1.. where the load variatio spa δ took value of 0.5 ad the average packet arrival rate λ = 2 packets per secod. I both cases, the packet size was set to ie backoff periods. After every 120 secods, a ew ode applied for admissio. The cluster coordiator the performed the approximate calculatios outlied above to obtai the approximate packet service time. The ode is admitted based o a desirable admissio coditio. The admissio coditio is set as the smallest duratio of a superframe 48 backoff periods i terms of packet service

7 7 50 Measured ad calculated service times Measured ad calculated service times (a)symmetric traffic (b)asymmetric traffic. FIG. 4: Measured ad calculated service times of idividual odes. time. The mea packet service time is below 48 backoff periods meas most of the packets will be processed withi a sigle superframe, which is certaily desirable from the stadpoit of eergy coservatio. The algorithm that the coordiator executes to admit a ew ode is as follows: Data: umber of existig odes ; packet arrival rates λ i ; ew ode packet arrival rate λ Result: admissio decisio calculate the ew umber of odes = + 1 ; if traffic load is asymmetric the calculate the ew average arrival rate λ avg = (λ 1 + λ λ + λ )/ ; else calculate λ avg = λ ; ed obtai approximate packet service time usig ad λ avg ; if 48 the allow the ew device to joi ; update =, λ +1 = λ ; else reject admissio ; retai existig value of ; ed Algorithm 1: Admissio policy. The calculated ad measured values for packet service time uder symmetric ad asymmetric traffic are show i Fig. 4(a) ad Fig. 4(b), respectively. Short horizotal lies show approximate values of service times as calculated by the algorithm, while circle poits show measured values (averaged over a five secod time widow) after admissio. The superimposed lie at T = 48 is set as a desirable admissio coditio. We observe very close match betwee calculated ad simulated values for both symmetric ad asymmetric traffic. We also observe that the admissio coditio is same i both cases, allowig 31 odess to get admitted with a average packet arrival rate λ = 2 packets per secod. Iterestigly, measured values of packet service time do ot icrease all the time with icreasig umber of odes i asymmetric case sometimes simulatio results with higher umber of odes fall slightly behid the simulatio results with lower umber of odes. The reaso for such behaviour will be clear if we observe the diagram show i Fig. 6, which shows the distributio of packet arrival rates amog the odes i the case of asymmetric traffic. Although we assume that the distributio of load variatio is uiform i the rage of 50% aroud the mea, the umber of samples (which is betwee 15 to 31) is ot sufficietly high eough to geerate a steady average arrival rate λ. As a result, whe a ew ode is admitted, the ew average arrival rate may vary withi a few percet of the average arrival rate λ. Thus, after admittig a ew ode, the ew measured service time is affected by the curret umber of odes as well as the ew average arrival rate. For example, the avearge service time geerated by a cluster of size 20 odes with λ = 2.03 packets per secod may be slightly greater tha the service time geerated by 21 odes with λ = 1.98 packets per secod. VI. CONCLUSION We have developed simplified aalytical expressios for the packet service time that ca be used for admissio cotrol at the cluster coordiator. Simulatio results for symmetric traffic show that simplified admissio coditio is slightly more coservative tha the accurate value.

8 8 Service time distributio of idividual odes Service time distributio of idividual odes (a)symmetric traffic (b)asymmetric traffic. FIG. 5: Packet service time distributio of idividual odes. [1] E. H. Callaway, Jr. Wireless Sesor Networks, Architecture ad Protocols. Auerbach Publicatios, Boca Rato, FL, [2] adard for part 15.4: Wireless MAC ad PHY specificatios for low rate WPAN. IEEE d , IEEE, New York, NY, Oct [3] J. Mišić, K. L. Cha, ad V. B. Mišić. Admissio cotrol i Bluetooth picoets. IEEE Trasactios o Vehicular Techology, 53(3): , May [4] J. Mišić, S. Shafi, ad V. B. Mišić. Performace of beaco eabled PAN with uplik trasmissios i o-saturatio mode - access delay for fiite buffers. I Proc. BroadNets 2004, pages 416 4, Sa Jose, CA, Oct [5] J. Mišić, S. Shafi, ad V. B. Mišić. Avoidig the bottleecks i the MAC layer i low rate WPANs. I Proc. HWISE2005 (ICPADS 2005 Workshops), pages , Fukuoka, Japa, July [6] J. Mišić, S. Shafi, ad V. B. Mišić. Maitaiig reliability through activity maagemet i sesor etworks. I Proc. Qshie 2005, Orlado, FL, Aug [7] V. B. Mišić, J. Fug, ad J. Mišić. Cross-layer activity maagemet i a sesor etwork. IEEE Commuicatios Magazie, 44(1): , Ja [8] RSoft Desig, Ic. Artifex v Sa Jose, CA, [9] N. Saxea, G. Tsudik, ad J. H. Yi. Admissio cotrol i Peer-to-Peer: desig ad performace evaluatio. I Proc. SASN 03, pages , Fairfax, VA, [10] N. Saxea, G. Tsudik, ad J. H. Yi. Access cotrol i ad hoc groups. I Iteratioal Workshop o Hot Topics i Peer-to-Peer Systems, pages 2 7, Voledam, The Nederlads, Oct [11] S. H. Shah, K. Che, ad K. Nahrstedt. Dyamic Badwidth Maagemet i Sigle-Hop Ad Hoc Wireless Networks. Mobile Networks ad Applicatios, 10: , [12] Y. Yag ad R. Kravets. Cotetio-aware admissio cotrol for ad hoc etworks. IEEE Trasactios o Mobile Computig, 4(4): , July-August 2005.

9 9 180 Packet arrival rate of idividual ode T FIG. 6: Packet arrival rate of idividual odes uder asymmetric traffic.