差错信道下无线局域网的自适应MAC协议

深入分析了差错信道下802.11DCF在饱和状态下的条件冲突概率对吞吐量的影响.研究表明,在基本接入模式下,对于给定的网络配置,存在最优的条件冲突概率使饱和吞吐量最大,并且该最优值近似与站点数、误码率及分组负载长度(100～4000B)无关.在此基础上,提出一种信道自适应MAC协议,其中主要包括竞争窗口调整方法和帧长控制机制,前者主要通过调节竞争窗口使条件冲突概率接近最优值,后者根据信道的误码率选择最优帧长.分析和仿真结果表明,与标准DCF及其优化算法(SBEB和OPL)相比较,该协议在差错信道下不仅能有效改善吞吐量,同时对信道与负载条件的变化表现出良好的适应能力.

An Adaptive MAC Protocol for Wireless LANs Under Error-Prone Environment

The impact of conditional collision probability(pcl) on saturation throughput of 802.11 DCF(distributed coordination function) under error-prone channel is investigated through mathematical modeling.The study shows that under a certain network configuration in the basic CSMA CA mode there exists an optimal pcl to maximize the throughput,and the optimal pcl is approximately independent of the number of active nodes,the bit error rate and the length of the packet payload(100—4 000B).Based on the stated results,combining contention window and frame length adjustment methods,a channel adaptive protocol is proposed to enhance the performance of 802.11 DCF in case of error-prone environment.The contention window adjustment method adjusts the initial contention window to make pcl reach its optimum;moreover,the frame length adjustment scheme determines the optimal frame length according to channel condition.The analytical model is implemented based on NS-2 to evaluate the performance of the proposed protocol and the impact of channel condition.The results show that,as compared with legacy 802.11 DCF,the proposed protocol not only significantly improves the throughput in error-prone conditions,but also achieves the self-adapt character with the variation of channel condition and the node number.