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For the nature of broadcast in wireless channel, any node sends a wireless signal which may be received by any other node within the scope of its communication in wireless sensor networks. Therefore, while more than two nodes transmit signal at the same time in the same communication area, it will form a mutual interference, resulting in unsuccessful transmission. In order to make numbers of sensor nodes share wireless channel resources effectively, avoiding conflict is one of the key issues in wireless sensor networks, which has important impact on the efficient using of wireless resources, network throughput and delay. Contention-based MAC protocols for wireless sensor networks mainly focuse on conflict resolution, which are suitable for traffic changing dynamically and has the advantage of distributed access control flexibility. So this paper argues it is more adaptive for the design of MAC protocol in wireless sensor networks.Firstly, this article describes the primary problem and pending solution of key technology for MAC protocol. Then we list and analyze two typical MAC protocol based on contention.Secondly, this article mainly focused on IEEE802.15.4 specification based on CSMA / CA mechanism. We emphasis on CSMA/CA access mechanism with time slot in MAC sub-layer. Next, we improve a mathematical model for IEEE802.15.4 in saturation conditions. Then, this paper proposed improved adaptive BE (ABE) and CW (ABW) algorithm according to the shortcoming of fixed parameters in CSMA/CA mechanism in IEEE802.15.4, such as macMinBE, CW.In the ABE algorithm, network coordinator can decide the minimum backoff exponent (macMinBE) for some nodes in a particular period, and make the counterpart nodes adjust its value of BE by means of Beacon. Meanwhile, we believe that numbers of times that the nodes listen idle channels should adjust according to the change of backoff range. Therefore, ABW algorithm changes the value of CW two times, one for the implementation of ABE algorithm,and the other for idle listening. In order to reduce the probability of conflict, we take a mechanism to increase or decrease the value of CW. In all, two algorithms for CSMA/CA were adjusted to accommodate the dynamic changes of network .Finally, the performances of the above two algorithms are simulated by using the network simulator NS2. In the different input traffic conditions, especially in heavy traffic case, compared with tranditional IEEE802.15.4, the combination of the two algorithms will increase the system throughput and decrease the network workload. This paper also simulated three functions of IEEE802.15.4 MAC sub-layer on hardware platform, that is, association, the process of data transmission and validation, CSMA/CA mechanism.
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