A Distributed MAC Scheme to Avoid CollisionsAmong Multiple Wireless Personal Area Networks

Peng-Yong KongDept. of Electrical & Computer Engineering

Khalifa University of Science, Technology & Research (KUSTAR)Abu Dhabi, United Arab Emirates.E-mail: kongpy@ieee.org

Abstract—Wireless personal area network (WPAN) technologywill be used for cable replacement in connecting various devicesand sensors carried by a person for health monitoring. Withthe increasing need, every person will eventually have a WPANand each WPAN is an autonomous entity. When two WPANscome into the radio range of each other, a collision occurs andnetwork performance is affected. To manage the impact, theWPAN collision must be detected early and resolved quickly.This paper assumes the use of IEEE 802.15.4 WPAN. Then,we propose a MAC scheme, called Inter-Network CollisionAvoidance (INCA) to detect and avoid WPAN collision accuratelyin a distributed manner. The proposed INCA scheme does notrequire any central coordinator for transmission scheduling. In-stead, INCA successively separates each pair of colliding WPANsafter identifying which portion of an active period suffers fromthe collision. We have evaluated the proposed scheme throughextensive simulations. Evaluation results confirm that WPANcollisions can be effectively resolved leading to a significantlyhigher normalized throughput. Specifically, the probability ofsuffering from zero normalized throughput can be reduced fromabout 40% to less than 1%.

Index Terms—Wireless Personal Area Networks, WPAN, Col-lisions, MAC, IEEE 802.15.4.

I. INTRODUCTION

Health problems such as cardiac arrest, hypoxemic and badblood circulation can lead to sudden collapse of a person.These collapses claim thousands of lives every year. In viewof this, there is an interest to continuously monitor the bodycondition of a person with high risks to such health problemsso that a sudden fatal collapse can be prevented. In order toperform such a health monitoring, the person needs to carryvarious devices to record heart beat, respiration rate, bodytemperature, etc. We envisage that these devices are connectedthrough a wireless personal area network (WPAN).

Generally, WPAN is a person-centered network concept thatenhances our personal environment by connecting a variety ofpersonal and wearable devices within the space surrounding aperson, and providing the wireless communication capabilitieswithin that space and with the outside world. This person-centered network concepts has been developed from Zimmer-man’s [1] idea to use capacitive coupling electrical currentsthrough the human body for communication between thedevices attached to the body. This original idea of WPAN hasevolved significantly over the years leading to many different

variations. These different WPAN technologies include IEEE802.15.1/Bluetooth, IEEE 802.15.3, and IEEE 802.15.4/Zig-Bee ([2], [3]), etc. They offer different data rates and providedifferent transmission ranges.

Among the different WPAN technologies, IEEE 802.15.4has gained popularity for healthcare applications [4]. For thehealth monitoring scenario described earlier, every person willeventually carry one WPAN and each WPAN is an autonomousentity. The person who carries a WPAN should be able to movefreely and seamlessly, to enter an existing networking spacewithout disrupting the ongoing communications. Hence, theWPAN become a mobile domain capable of performing adhoc networking and potentially multi-hopping.

While the mobility support is essential, it leads to a WPANcollision when two WPANs that operate in the same wirelesschannel come in close proximity to each other. This collisionof co-located WPANs can be prevalent considering a personwalking down a busy street may pass by many other peoplewho also carry WPANs. When a collision occurs, two WPANscan degrade each other’s performance. Further, the collisioncan temporarily disrupt on-going transmissions, applicationsand network operations. Considering a pedestrian speed of0.4m/s and a radio range of 3m, even if it is temporarily, thedisruption can last for as long as 7.5 seconds [5]. This periodis too long for the life-saving health monitoring system andhence, the impact of such WPAN collision must be managed.In managing the impact, a WPAN should first detect thecollision with another WPAN so that a collision resolutioncan be performed as a reaction.

In the literature, collisions among co-located IEEE 802.15.4WPANs have been studied as a problem that affects cluster-tree topology establishment, where multiple WPANs cannotbe properly organized in a tree structure due to the loss ofsuperframe synchronization beacons. The beacons are lostas a result of direct collisions among adjacent WPANs. Toresolve the beacon collision problem, IEEE 802.15.4 TaskGorup has proposed two methods, namely beacon-only periodapproach, and time division approach. In the beacon-onlyperiod approach, a time window, denoted as Beacon-OnlyPeriod, is reserved at the beginning of each superframe forthe transmission of beacon frames in a contention-free fashion.The main complexity of this approach is the dimensioning of

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the duration of the beacon-only period for a given cluster-tree topology. On the other hand, the time division approachdivides time domain such that beacons and superframe dura-tion of a given WPAN are scheduled in the inactive period ofits neighboring WPANs. The idea is that each WPAN uses astart time, called beacon offset time to transmit its beacons,and the start time must be different from the start times ofits neighboring WPANs. Anis Koubaa et al have proposed ascheme to determine the beacon offset times for co-locatedWPANs in a cluster-tree topology [6]. Consider the sameproblem, J.-W. Kim et al have proposed to schedule the beaconoffset times in both the time domain and frequency domain,where different wireless channels occupy different frequencies[7]. Unfortunately, both [6] and [7] deal with static cluster-tree which is different from our scenario where each personis a mobile WPAN. Also, in [6] and [7], the WPAN collisiondetection and recovery process can take several seconds. Formobile WPANs, [8] has shown through simulations that aneighboring WPAN approaching at a speed of 1.5m/s or 2.5m/scan cause a substantial network performance degradation dueto transmissionm collisions. However, no solution has beenprovided by [8] in mitigating the performance degradation.

In view of the problem described above, this paper proposesa scheme for the IEEE 802.15.4 MAC protocol to facilitatecollision avoidance among co-located mobile WPANs. Theproposed scheme is called Inter-Network Collision Avoidance(INCA), and it works in a distributed manner where nocentral coordinator is needed for transmission scheduling. Theremainder of this paper is organized as follows. In Section II,we present the system model. The proposed INCA schemefor IEEE 802.15.4 WPAN is described in Section III. SectionIV presents and discusses performance evaluation results. Thispaper ends with concluding remarks in Section V.

II. SYSTEM MODEL

We consider a system model such that a WPAN consistsof a coordinator node and a collection of sensor nodes. Allthe sensors are within the radio range of the coordinator butmay not be within the range of each other. Regardless of theexistence of direct links, sensor-to-senor communications mustgo through the coordinator. We adopt the IEEE 802.15.4 MACprotocol [9] to govern medium access among the nodes in aWPAN.

Fig. 1. MAC superframe structure for IEEE 802.15.4 WPAN.

The MAC protocol supports two operational modes, namelythe non-beacon-enabled mode and the beacon-enabled mode.In the non-beacon-enabled mode, the MAC protocol is simply

the non-slotted CSMA/CA protocol. In the beacon-enabledmode, a star network topology is formed around a PAN coor-dinator which transmits synchronization beacons periodicallyto all other nodes within the network. Based on the systemmodel adopted above, we focus on the beacon-enabled modehereafter.

Fig. 2. Illustration of a collision between two WPANs. A collision occursbetween two WPANs when they are within the radio range of each otherand their superframe active periods overlap. Notice that BI and SD can bedifferent for different WPANs.

As illustrated in Fig. 1, synchronization beacons are trans-mitted at the beginning of each periodic superframe, wherethe period is called beacon interval (BI). Within each beaconinterval, superframe duration (SD) is the active portion ofthe superframe, during which packets can be transmitted.Notice that there is an inactive portion of a superframe duringwhich all nodes may go to sleep to conserve energy. ThePAN coordinator controls the beacon interval and superframeduration through two parameters, namely beacon order (BO)and superframe order (SO) as follows:

BI = aBaseSuperframeDuration × 2BO, (1)SD = aBaseSuperframeDuration × 2SO, (2)

where 0 ≤ SO ≤ BO ≤ 14. In the equations, aBaseSu-perframeDuration is 15.36ms assuming 250kbps transmissionrate at the 2.4 GHz frequency band. Following Fig. 1, thesuperframe duration (active period) is further divided into acontention access period (CAP) and a contention free period(CFP). The formation of the superframe, the values of BOand SO, as well as the allocation between CAP and CFP aredone by the PAN coordinator, and are announced throughthe synchronization beacons. As such, when two WPANscollide and the synchronization beacons are lost, all nodesin the colliding WPANs cannot transmit any packet leadingto a severe degradation in network performance. When thecollisions do not affect the synchronization beacons but onlythe superframe active periods, some packet transmissions maybe erroneous resulting in a lower throughput.

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In modeling collisions among multiple WPANs, we let eachWPAN be a node represented by its PAN coordinator consider-ing that within a WPAN on a human body, sensors are locatedvery close to the PAN coordinator relative to the distancebetween different PAN coordinators. As illustrated in Fig. 2,a collision occurs when two PAN coordinators are within theradio range of each other and part of their superframe activeperiods overlaps. We do not consider overlap between activeperiod and beacon as a collision because beacon transmissionshave absolute priority over data packet transmissions. It is forthis reason, we have ignored beacon in our figures thereafterfor simplicity. In the figure, the active period durations andinactive period durations are different for different WPANs.This is reasonable considering different persons may havedifferent communication needs and thus, their BO and SO areconfigured differently. Also, the superframe start time variesfrom one WPAN to another WPAN because different networksmay be powered up at different times.

Following Fig. 2, it is reasonable to think that the chancesof collision are higher when there are more neighboringWPANs. However, it is not trivial to find out the usual numberof neighboring WPANs. Instead of adopting an academicmobility model and deriving a corresponding node degreedistribution, we have determined the number of neighboringWPANs by examining the actual human traffic at the lobbyof a building within a university campus. We have recorded avideo of the human movement over a 10.5m × 7.5m region.We have examined the recorded video on a frame-by-framebasis. In each video frame, we identify the number of existingpersons and the distances between them. Let each person’shead be the location of a WPAN and the radio range be R, wedetermine the number of neighboring WPANs, N within R foreach person in each video frame. Fig. 3 show the probabilitydistribution of different numbers of neighboring WPANs whenR = 3m. In the figure, the probability is an average valuecalculated using the video data as follows:

P{N = n} =1

M

M∑i=1

total time person i has n neighborstotal time person i spends in video

, (3)

where M is the number of persons appear in the video. Inour video, there are M = 39 persons spend an average of5.8 seconds each within the region. We notice that there is noneighboring WPAN almost 45% of the times. The number ofneighboring WPANs never exceeds 4. However, when thereare 3 or 4 neighboring WPANs, the duration of the collisioncan last for up to 6.4 seconds each. If WPAN collision resultsin a complete communication breakdown as indicated by zerothroughput, 6.4 seconds of outage will have a severe effecton real-time health monitoring. In short, the impact of WPANcollisions will not be negligible.

III. WPAN COLLISION AVOIDANCE

In this section, we propose a distributed scheme, calledInter-Network Collision Avoidance (INCA) to resolve collisionamong multiple co-located WPANs. We first define a baseline

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scheme for performance benchmark before presenting theproposed scheme.

A. Baseline Scheme

We define the baseline scheme as the original IEEE 802.15.4MAC protocol that does not adapt to collisions amongWPANs. Let us assume that when a collision occurs, allthe transmissions within the overlapping active periods willfail. On the other hand, all the transmissions within the non-overlapping active periods are always correct. Then, for thebaseline scheme, the normalized throughput of WPAN i, Si isgiven as follows:

Si =

∑Tt=1 SDi,t

SDi × T, (4)

where SDi is the active period of WPAN i as given by (2),SDi,t is the non-overlapping duration of SDi in superframe t,and T is the number of superframes over which the throughputis calculated. Although there is no adaptivity to collision, SDi,t

still varies from a superframe to another superframe due to therandomness in BO, SO and start time among different WPANs.We notice that the normalized throughput of WPAN i can becomputed using only values known locally by i.

B. Inter-Network Collision Avoidance

We propose INCA to achieve a higher normalized through-put compared to that of the baseline scheme in the presenceof WPAN collisions. In INCA, normalized throughput is alsocomputed using (4). Here, the beacon interval is divided intotime slots with a fixed length equals to aBaseSuperframeDu-ration. These time slots are used solely for collision detectionand avoidance, and may not be the same as the time units usedin resource allocation. A bit map is created for the time slotssuch that each time slot is marked by a bit. The bit is reset tozero by default. The bit is set to 1 when a transmission hasfailed in the time slot, or a transmission from a foreign WPANis detected in the time slot. This is illustrated in Fig. 4. Noticethat the bit map is reset at the beginning of each superframe.

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Fig. 4. Bit map to mark each time slot according to its communicationoutcome.

With an assumption that each time slot with its bit map setto 1 has suffered from collision, INCA is able to identify at theend of a superframe the time instance at which a collision hasoccurred. INCA will determine the duration of a collision atthe beginning and the end of an active period. As illustrated inFig. 5, the duration of a collision is the number of consecutivetime slots with bit map set to 1.

Fig. 5. Collision durations at the beginning and the end of an active period,based on bit map.

Let b[t] and e[t] be respectively the duration of collision atthe beginning and the end of an active period of superframet. Then, INCA use these two values to successively separatetwo colliding WPANs. This is achieve by adjusting the starttime of the next superframe t+ 1 as much as o[t+ 1] whichis calculated as follows:

o[t+ 1] =

⌈b[t]

2

⌉−⌈e[t]

2

⌉. (5)

Notice that o[n + 1] is an integer number which can beeither positive or negative. A positive value means the nextsuperframe start time will be delayed by as many as o[t+ 1]time slots. On the other hand, a negative value mean the nextsuperframe start time is advanced by as many as o[t+1] timeslots. This process of successive separation of two collidingWPANs is best explained through illustrations in Fig. 6. Wehighlight that this is a distributed scheme since each WPANneeds only to know its own b[t] and e[t], and makes its ownadjustment in superframe start time without coordination withneighboring WPANs.

In Fig. 6, WPAN A and WPAN B have the same beaconinterval and active period duration but different superframestart times. In superframe 1, a collision occurs between the twoWPANs. For WPAN A, the collision affects only the end ofits active period for as many as 2 time slots. Therefore, b[1] =

Fig. 6. Illustration of the successive separation of two colliding WPANs.

0 and e[1] = 2. On the other hand, WPAN B suffers fromcollision for 2 time slots at the beginning of its active periodbut not at the end. Thus, b[1] = 2 and e[1] = 0. At the endof superframe 1, WPAN A computes its start time adjustmentas o[2] = b[1] − e[1] = −1 and advances superframe 2 for 1times slot. This advancement is illustrated by a left-pointingarrow and the vertical line at the start of the arrow indicates theoriginal superframe start time. A few time slots later, WPANB reaches its end for superframe 1 and computes its start timeadjustment as o[2] = b[1] − e[1] = 1. With a positive o[2],WPAN B delays the start time of its superframe 2 for 1 timeslot. This delay is illustrated by a right-pointing arrow from thevertical line which indicates the original start time. As depictedin the figure, by the time WPAN B starts its superframe 2, thecollision is resolved. We also notice from the figure that thestart time adjustment will lead to a change in beacon interval.However, we do see this as an issue because the change inbeacon interval is only transient in nature. The beacon intervalwill return to its original value after the collision is avoided.

In Fig. 6, the same steps used to resolve collision betweenWPAN A and WPAN B are performed to avoid collisionbetween WPAN C and WPAN D. The collision betweenWPAN A and WPAN B is resolved in just 1 iteration ofstart time adjustment compared to 2 iterations needed toresolve collision between WPAN C and WPAN D. While thenumber of iterations needed to resolve collision depends on thescenario, the two examples illustrate the simplicity of INCA.The simple INCA is capable of handling collisions amongmore than 2 WPANs using the same algorithm. Fig. 7 depicts

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INCA’s operation in avoiding collision among 3 WPANs. Inthe figure, the collision is eventually avoided after 3 iterationsof start time adjustment. We do not provide illustrations formore than 3 colliding WPANs due to limited space.

Fig. 7. Illustration of the INCA’s operation for three colliding WPANs.

In addition to the collisions that occur at the beginning andthe end of active periods, collisions may also happen com-pletely within an active period of a neighboring WPAN. Here,INCA detects the occurrence of this case when b[t] = e[t],and b[t] equals the duration of its own active period. In thiscase, as illustrated in Fig. 8, INCA sets o[t+ 1] to a positiveinteger randomly selected from the range [1, 2BO].

Fig. 8. Illustration of the INCA’s operation to avoid collision that occurscompletely within an active period of a neighboring WPAN.

IV. PERFORMANCE EVALUATION

We have performed extensive monte-carlo simulations toevaluate the performance of INCA. In the simulations, theBO for each WPAN is a random variable uniformly distributedwithin the range [0, 14] as specified by IEEE 802.15.4 MACprotocol. Given a BO, the WPAN’s SO is a random variableuniformly distributed within the range [0, BO]. We assumethere are always packets to be transmitted in each time slotof an active period. We further assume the transmission iserror free as long as there is no WPAN collision. On theother hand, when there is collision, all transmissions withinthe overlapped portion of an active period will fail. Withreference to the system model and Fig. 3, we have variedthe number of neighboring WPANs from 1 to 6, and collected

data to compute normalized throughput as the performancemetric. For a given number of neighboring WPANs, werun a large number of simulations such that each run willproduce a computed normalized throughput. For each of thesesimulations, we run it long enough to obtain a stable value ofnormalized throughput.

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Fig. 9. Comparison of the probability distribution of normalized throughputachieved by the baseline scheme and INCA. There are 4 neighboring WPANs.

Fig. 9 shows the probability distribution of normalizedthroughput when there are 4 neighboring WPANs. In the base-line scheme, the impact of WPAN collision is very severe suchthat more than 30% of the times the normalized throughputat each WPAN is zero. This is a complete breakdown incommunications, and is highly undesirable for critical healthmonitoring. Recall from Section II, this situation may lastfor about 6 seconds and it is long enough to cause a seriousconcern in real-time monitoring applications. The performancedisruption can be significantly reduced by INCA. Noticethat the probability of complete communication breakdownis reduced to about 1% in INCA. This similar improvementbrought about by INCA can be seen for different numbers ofneighboring WPANs but these results are not presented heredue to limited space. This is a clear evident that the proposedINCA, despite simple, can indeed help in avoiding collisionamong co-located WPANs.

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Fig. 10. Comparison of cumulative probability densities in achieving a normalized throughput between the baseline schemes and INCA.

Fig. 10 shows the cumulative probability density for normal-ized throughput. Comparing across the sub-figures, we noticethat the normalized throughput is lower with an increasingnumber of neighboring WPANs. This is understandable be-cause more neighboring WPANs implies a higher chance ofcollision and therefore a lower throughput. Apart from thisobservation, the results further confirm that INCA can signif-icantly reduce the probability of zero normalized throughput.The level of improvement is greater for a larger numberof neighboring WPANs. For example, when there is only1 neighboring WPAN, the probability of zero normalizedthroughput is reduced from about 10% to less than 1%. Whenthere are 6 neighboring WPANs, the reduction is from about42% to less than 1%.

V. CONCLUSION

This paper studies the problem of collision among multipleneighboring WPANs, where the collision can lead to a signifi-cant degradation in normalized throughput. We have proposeda distributed MAC scheme, called INCA that can detect andavoid WPAN collisions. INCA uses a bit map to mark eachtime slot depending on the communication outcomes of theslot. Then, INCA uses the bit map to determine the duration ofa collision that happens at the beginning or the end of an activeperiod. Based on the collision duration, INCA adjusts the starttime of the next superframe such that colliding WPANs aresuccessively separated. We have performed extensive simula-tions to confirm that INCA can indeed significantly reducethe occurrence of communication breakdown in the presenceof WPAN collisions. The level of improvement brought about

by INCA is greater when there are more interfering WPANs.When there are 6 neighboring WPANs, INCA can reduce theprobability of zero normalized throughput from about 42%to less than 1%.

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