Backoff mechanism is a key component of contention-based medium access control(MAC) layer protocol.It has been shown that the backoff mechanism of IEEE 802.11 standard may be very inefficient especially when the network is congested.Numbers of methods have been proposed to tune the contention window(CW) with the aim to achieve the optimal throughput in IEEE 802.11 WLANs.However,the mechanisms do not specifically address proper settings for the variable packet length influence and CW diverging problem.This paper proposes a novel four-way handshaking full-feedback backoff algorithm named adoptive contention window backoff(ACWB) to overcome these drawbacks.The performance of the proposed algorithm is investigated through analysis and simulation.Simulation results demonstrate that the ACWB algorithm provides a remarkable performance improvement in terms of short-term fairness,packet delay and delay jitter,while maintaining an optimal throughput close to the theoretical throughput limit of the IEEE 802.11 distributed coordination function(DCF) access scheme. Backoff mechanism is a key component of contention-based medium access control (MAC) layer protocol. It has been shown that the backoff mechanism of IEEE 802.11 standard may be very inefficient especially when the network is congested. Number of methods have been proposed to tune the contention window (CW) with the aim to achieve the optimal throughput in IEEE 802.11 WLANs.However, the mechanisms do not specifically address proper settings for the variable packet length influence and CW diverging problem.This paper proposes a novel four-way handshaking full -feedback backoff algorithm named adoptive contention window backoff (ACWB) to overcome these drawbacks. The performance of the proposed algorithm is investigated through analysis and simulation. Simulation results demonstrate that the ACWB algorithm provides a remarkable performance improvement in terms of short-term fairness, packet delay and delay jitter, while maintaining an optimal throughput close to the theoretical throughput limit of the IEEE 802.11 distributed coordination function (DCF) access scheme.