中文摘要：

针对DCF协议的初始竞争窗口不能随着节点数变化选择其最佳值的不足，提出一个具有两级退避的接入协4Z（OCW-DCF），并用Markov模型分析了协议性能，确定其最佳竞争窗口值。仿真结果表明，OCW-DCF协议能有效减小信道接入延时，且采用基本接入模式的总体性能比RTS／CTS接入模式的好。

英文摘要：

As the initial contention window could not derive its optimal value based on the network scale, a new medium ac- cess protocol with two-backoff stage named OCW-DCF is proposed. Its performance is analyzed with a two-dimensional Markov model. Simulation results show that OCW-DCF could effectively reduce the average channel access delay. The performance of OCW-DCF using basic access method is better than that of using RTS/CTS access method.