Improving TCP performance in integrated wireless communications networks

Many analytical and simulation-based studies of TCP performance in wireless environments assume an error-free and congestion-free reverse channel that has the same capacity as the forward channel. Such an assumption does not hold in many real-world scenarios, particularly in the hybrid networks consisting of various wireless LAN (WLAN) and cellular technologies. In this paper, we first study, through extensive simulations, the performance characteristics of four representative TCP schemes, namely TCP New Reno. SACK, Veno, and Westwood, under the network conditions of asymmetric end-to-end link capacities, correlated wireless errors, and link congestion in both forward and reverse directions. We then propose a new TCP scheme, called TCP New Jersey, which is capable of distinguishing wireless packet losses from congestion packet losses, and reacting accordingly. TCP New Jersey consists of two key components, the timestamp-based available bandwidth estimation (TABE) algorithm and the congestion warning (CW) router configuration. TABE is a TCP-sender-side algorithm that continuously estimates the bandwidth available to the connection and guides the sender to adjust its transmission rate when the network becomes congested. TABE is immune to the ACK drops as well as ACK compression. CW is a configuration of network routers such that routers alert end stations by marking all packets when there is a sign of an incipient congestion. The marking of packets by the CW-configured routers helps the sender of the TCP connection to effectively differentiate packet losses caused by network congestion from those caused by wireless link errors. Our simulation results show that TCP New Jersey is able to accurately estimate the available bandwidth of the bottleneck link of an end-to-end path; and the TABE estimator is immune to link asymmetry, bi-directional congestion, and the relative position of the bottleneck link in the multi-hop end-to-end path. The proactive congestion avoidance control mechanism proposed in our scheme minimizes the network congestion, reduces the network volatility, and stabilizes the queue lengths while achieving more throughput than other TCP schemes.