Investigation of the TCP simultaneous-send problem in 802.11 wireless local area networks

This paper investigates the TCP simultaneous-send problem (SSP) which arises in infrastructure mode WLANs. In particular it has been observed that for file transfer traffic, 802.11 wireless nodes have a sustained supply of packets to send and hence experience a relatively high rate of MAC contention. For TCP, this results in competition among data and ACK packets for channel access which causes the SSP that deteriorates flow throughput. This SSP can be alleviated by skipping TCP ACKs. Detailed simulation results are presented to demonstrate the usefulness of ACK skipping in various network scenarios such as with MAC retries and multiple TCP flows. The largest improvement is seen for the case of a single TCP flow, and moderate gains are also achieved in cases with multiple streams. For the single TCP stream case with 1 ACK skip and no MAC retries, TCP throughput improves 30% for short-lived and 98% for long-lived TCP transmissions. The paper concludes with potential cross-layer solutions that potentially provide further improvements, including the use of the point coordination function (PCF) to reduce contention between multiple TCP streams and returning ACK packets.