Abstract: For the problem that a traditional congestion signal such as the average length of a queue cannot indicate network congestion effectively in the ad hoc network, an analysis has been made of the frame service delay(FSD) of IEEE 802.11 MAC protocol, and the relation between the average FSD and the average length of the frame, the collision probability, and the number of saturated stations. Analytical results show that FSD increases sharply with the increase in transmission probability or the number of saturated stations if the transmitting probability of stations is larger than the critical one, and that FSD has a high sensitivity to the link congestion. Finally, the effectiveness of FSD as the congestion signal is validated by simulation. Compared with the average length of the queue, FSD can indicate network congestion more effectively. Together with TCP, the use of FSD as the congestion signal can prominently improve the throughput of TCP connection and decrease the ratio of packet loss.

Key words: frame service delay, congestion control, IEEE 802.11, ad hoc network