A MAC layer congestion control method to achieve high network performance for EAST experiment

Abstract

Many applications would require fast data transfer in Wireless Local Area Networks (WLANs). A representative example is that EAST experiment data are retrieved by some physics researchers using the Transmission Control Protocol (TCP). However, due to the high contention degree and the high error rate in wireless networks, the packets may be loss for wireless reasons but not for congestion. This will greatly degrade the TCP performance. On one hand, the wireless packet loss is not congestion, but the traditional TCP assumes that every packet drop is congestion and thus decreases its congestion window, which will degrade its performance. On the other hand, due to the MAC layer retransmission policy employed by the IEEE 802.11 DCF mechanism, the lost packets at the MAC layer will be retransmitted for some times. Thus the waiting time of the packets in the MAC layer queue will be increased. So if we ignore all the packet loss for wireless reasons as the other improved mechanisms do, the network work congestion will be aggravated and its performance will be degraded. To alleviate the impact of the wireless packet loss to TCP in WLANs, this paper proposes a MAC layer congestion control method which is implemented at the end wireless nodes based on IEEE 802.11b DCF mechanism. At first, we propose a concept of MAC layer congestion window which means the MAC layer will send all the packets in a window when it gets access to the wireless channel, other than just sends only one packet as the traditional DCF mechanism does. Then our congestion control mechanism adjusts the MAC layer congestion window based on the contention degree and the MAC layer packet loss rate. If the MAC layer contention degree or packet error rate is high, we will increase the congestion window to improve the successful transmission rate, and we will decrease the congestion window when the packet loss rate is lower than the average wireless packet loss rate. We also use a threshold to control the increase of the congestion window. The threshold is set according to the number of wireless nodes. By performing wireless congestion control at the MAC layer, our mechanism can mitigate the effect of wireless loss to TCP, and therefore improve the TCP performance. The simulation and experiment results show that our mechanism can have better performance than traditional MAC layer mechanisms in WLANs.