In MANET,it’s easy to lead to severe collision in data transmission,for the nodes are easily accessed and are mobile.Or,it may be misunderstood as the conflict caused by the breakdown of link.These situations affect the performance of TCP protocol.In light of this problem,we designed a distributed coordination function (DCF)mechanism,it adjusts the size of initial contention window on MAC layer by dynamically estimating the active nodes in wireless network,and adjusts contention window size based on the number of active nodes and the active feature of nodes.Simulation is conducted by using NS2,and the experimental results verify the effectiveness of the improved DCF mechanism,moreover,it is indicated that in contrast to standard DCF mechanism,the throughput gains an increase of 10% to 15% under the acceptable end-to-end delay.%MANET 由于节点的易接入性和移动性，容易导致数据发送冲突严重，或者因链路断开而误认为是冲突，影响网络 TCP协议的性能。为此设计一种动态估计无线网络中的活动节点数来调整 MAC 层的初始竞争窗口的大小，并根据活动节点数及节点的活动特性调整竞争窗口尺寸的 DCF 机制。通过使用 NS2进行仿真，实验结果验证了该改进的 DCF 机制的有效性，并表明相对于标准 DCF 机制，在可接受的端到端延迟下，吞吐量得到10％到15％的提升。
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