The performance of these access protocols decreases dramatically when the node number increases in WLAN.To resolve this problem

we propose an adaptive backoff algorithm by estimating the active node numbers with multiple thresholds.The proposed algorithm optimizes the delay model and gives a linear function between the contention window and the active node number

and an accurate contention window index is given for the optimal contention window scheme.Then

we realize the estimation of node number by using three thresholds to identify four levels of node number changes with a calculation of the idle slot intervals.The theoretical analysis shows that the proposed backoff algorithm holds low collision probability and larger throughput

and the node number has little impact on the performance.The simulation results demonstrate the validity and scalability of the algorithm.Moreover

it outperforms other algorithms in fairness as well.