Optimization And Performance Analysis Of IEEE 802.15.4 For QoS Guarantee Of Real-time Traffics

IEEE 802.15.4 described the specifications of physical layer(PHY) and media access control sublayer(MAC) for wireless personal area LAN. This protocol has distinctive characteristics of low cost, low power consumption, simple networking and low data rate. Because of these advantages, 802.15.4 has been widely used in various communication scenarios, such as wireless sensor networks, industrial control networks, smart home, intelligent agriculture and environmental monitoring. However, practice shows that its MAC sublayer protocol has many defects. Whether the collision avoidance based carrier sense multiple access mechanism(CSMA/CA) in CAP, or the inflexible first-come-first-serve(FCFS) based guaranteed slot reservation mechanism in CFP does not provide differentiated services and priority guarantee. In the context of springing up of machine type communication(MTC), the traffic types and Qo S requirements in 802.15.4 networks differ greatly. Real-time traffics have a fixed time constraints. As exceeding time limits, these traffics will lose timeliness, such as video streaming, industrial control signal and disaster warning signal. Oppositely, non-real-time traffics, which are generated by majority of nodes, have characteristics of short packet size, burst arrive and frequent access attempts.In this paper, we are inspired to present optimization theory for 802.15.4 to ensure the transmission of real-time traffics. First, in CAP, GTS request commands of real-time nodes compete with data frames of non-real nodes for channel resource. Since standard protocol can’t provide distinguishing priority, enhancing the access probability of real-time node is our first purpose. Markov chain theory is the most common theory for modeling random access procedure. In this paper, we utilize Markov chain theory to establish the probability transmission mode of channel states and node states, and reveal the influence of different access parameters, such as contention window, backoff exponents and number of backoff stages, on network performance. Then, to guarantee the transmission of real time traffic, we configure different access parameters for heterogeneous traffics. As a result, we can provide differentiated services between GTS request commands and data frames, and ensure the GTS request commands getting higher access priority.Secondly, in CFP, huge number of real time traffics request for the limited number of GTS resources. How to maximize bandwidth utilization is another purpose of this paper. The FCFS based GTS allocation strategy is a static method, and lacks of fairness, which may result in reducing bandwidth utilization. Markov Decision Process(MDP) is the most efficient theory for optimal resource allocation problem in delay sensitive and resource limited systems. For the purpose of maximizing the bandwidth utilization in CFP, we formulate the GTS allocation problem during CFP in IEEE 802.15.4 as a MDP problem, and propose a dynamic GTS allocation scheme, namely Markov decision process-based Optimal Scheduling(MOS). The MOS is a consideration of cross-layer information consisting of flow specification information(such as random burst arrivals, delay requirements, channel qualities), channel state information, and the available bandwidth resources. Furthermore, the optimal policy is a tradeoff between current immediate reward and possible future reward. Namely, we consider not only how to maximize the reward at present, but also the how to get greater reward in the future. Finally, we provide some implementation consideration of MOS to ensure backward compatibility with the standard.