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Based on the analysis of TCP performance of over the two ways of IEEE 802.11 DCF (RTS/CTS and CSMA/CA), TCP flows becomes unstable when WLAN is in high load. In this paper, we propose a novel backoff algorithm for IEEE 802.11 DCF named as ACWB (adaptive contention window backoff) with cross-layer optimization. Its basic idea is to dynamically adjust the size of the contention window by the estimation of the number of active nodes in the network according to information interaction between MAC-layer and transport-layer to avoid the hidden node problem in WLAN and alleviate the bad influence of MAC access mechanism on TCP congestion control. Simulation results show that ACWB algorithm can improve the TCP throughput and stability effectively.