| IEEE Std 802.15.4, as one of the best standard for the present wireless sensor networks, specifies the PHY(physical layer) and MAC(medium access control layer) for low-rate wireless personal area networks which focuses on low-cost, low-complexity, low-power consumption and capability of self organization. However, it still needs further researches and optimizations to better meet the requests of WSN. Based on the analysis of IEEE Std 802.15.4, this paper researches its important performances such as the throughput, the average delay and the success rate, and proposes a optimal model according to different priority of signals.This paper, firstly, analyzes the MAC protocol of the IEEE 802.15.4 standard and studies the facts which impact the standard's performance, then establishes a simulation model using OPNET for the star-topology WSN which can simulate CSMA / CA mechanism accurately. Based on this simulation model, this paper analyzes under different loads the impacts of BO, BE, CW on the performances of IEEE Std 802.15.4 including the throughput, the average delay and the success rate. In the rest part of this paper, the concept of priority is introduced, the main point of which is using different channel contention mechanism depending on the priority of data packets in WSN in order to advance the real-reality of the whole network and optimize the performance of the standard. These analyses can be good references for the design of network parameters. In addition, this paper proposes an accurate Markov chain model and analyzes based on this model the impacts on the time-reality of nodes of different protocol parameters for superiority signal and common signal. The simulation results show that the application of the priority concept can promote the real-reality of superiority significantly with few negative impacts on the performance of the whole network. Moreover, the optimization based on the priority concept requires no modification on the hardware, thus is suitable for general WSN. |
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