A Markov Decision Process(MDP)-Based Congestion-Aware Medium Access Strategy for
IEEE802.15.4
Bharat Shrestha,Ekram Hossain,Kae Won Choi,and Sergio Camorlinga
Abstract—IEEE802.15.4is a popular technology for short-range wireless networking due to the features such as low duty cycle operation,low power consumption,and both contention-based and contention-free transmissions.This standard can be enhanced to provide an optimal medium access mechanism in presence of congestion in the network.We present a Markov decision process(MDP)-based medium access control(MAC) model for IEEE802.15.4for the optimal use of contention and contention-free period to minimize energy consumption in repeated transmissions and carrier sensing without degrading latency in packet transmission.The simulation results show that the MDP strategy works more efficiently in presence of congestion when compared to a non-optimal(i.e.,traditional) slotted CSMA/CA scheme.
Keywords:IEEE802.15.4MAC,slotted CSMA/CA,GTS transmissions,Markov decision process,dynamic program-ming,network congestion.
I.I NTRODUCTION
We model and analyze a distributed channel access strategy that uses both contention-based and contention-free periods in the IEEE802.15.4MAC superframe in beacon-enabled mode in an optimal manner.To determine the optimum strategy for data transmissions during a superframe,we formulate a Markov decision process(MDP)[1]to decide whether to transmit data packet or request packet to access GTS or defer transmission.In the literature,[2],[3],[4],and[5]are some of the work which used MDP for optimizing channel access in a wireless network.The channel access policy developed in this paper is for the IEEE802.15.4-based MAC where each device is able to decide the best task by studying the environment. The main contributions of this paper are:i)Development of an optimal transmission strategy based on packet buffer level for transmissions using guaranteed time slots during CFP,ii) dynamic sleep and wake up for energy saving in presence of congestion by optimal use of contention and contention-free access,and iii)performance evaluation of the proposed channel access strategy using Network Simulator(NS2).
To realize the MDP model,we have to estimate the IEEE 802.15.4MAC parameters such as the probabilities of channel being idle duringfirst CCA and second CCA and probabilities of collision in the network(Section II).Using the MAC parameters,we formulate the MDP problem in Section III. In Section IV,we present numerical performance evaluation results for the proposed MDP-based channel access strategy.
GTS i
Contention access period
Active period Sleep period for the rest of the devices
For a Device i and coordinator
Sleep period
(CAP)
beacon Fig.1.Modified superframe structure for the IEEE802.15.4MAC.
II.S YSTEM M ODEL AND A SSUMPTIONS
A.IEEE802.15.4Superframe Structure,GTS Allocation,and MAC Parameters
We consider a star network topology with N wireless devices and a network coordinator.The superframe structure is shown in Fig.1.Unlike in the standard,we assume that each packet contains two bits of overhead.Thefirst bit is set if GTS request is sent and the second bit is set if the GTS de-allocation request is sent.The coordinator allocates a GTS slot to a device and can allocate a maximum of seven GTS slots in a superframe.The coordinator can use any algorithm (e.g.,thefirst-comefirst-served algorithm or the algorithm proposed in[6])for GTS allocation.Note that the use of GTS in the standard is for time-critical data transmission. In our case,the purpose of GTS transmission is to reduce the congestion during contention access period(CAP).Also, the sending methods for GTS request and GTS de-allocation request are different compared to those in the standard.
We use the following notations for the MAC parameters. Let P cs,i be the probability of carrier sensing for device i,αi be the probability that channel is idle atfirst CCA,and βi be the probability that channel is idle at second CCA providedfirst CCA is idle.The probability that channel will be free during carrier sensing period isαiβi for device i.We express the probability of channel being idle at thefirst clear channel assessment(CCA)in terms of the probability that at least one among the N−1devices starts carrier sensing, and the probability that one of themfinds channel free.This assumption simplifies the approximation ofα.However,we can also refer to the accurate method[7]which requires huge computational effort to consider every possible combination of(N−1)heterogeneous devices.We refer to[8]for the derivation ofαandβignoring deferred transmissions and the case when the channel becomes busy due to the transmission
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