On Analysis of the Contention Access Period of IEEE 802.15.4 MAC and its Improvement

The release of IEEE 802.15.4 medium access control (MAC) and physical layer specifications represents a significant milestone in promoting deployment of wireless sensor networks. This paper first analyzes the performance of the contention access period (CAP) specified in the IEEE 802.15.4 standard by integrating the discrete-time Markov chain models of the node states and the channel states. Then a modified CAP is analyzed, which could significantly improve the performance of the system. Based on the theoretical analysis of the CAP and the effect of the modification, we have that in the saturated network of IEEE 802.15.4, the collision probability is large, and the throughput is small. The main reasons are the employment of slotted carrier sense multiple access-collision avoidance and the mechanism that if a frame transmission cannot be completed before the end of the CAP, it has to wait until the start of the next CAP. Thus an enhanced collision-avoidance MAC protocol, which achieves a better performance and is compatible with the IEEE 802.15.4 standard, is proposed to improve the performance of the system. Extensive simulations validate our conclusions.