Improving TCP Throughput Stability in Wireless Multi-Hop Ad Hoc Networks

To enable seamless integration of ad hoc networks with other Infrastructure LANs and to the Internet, TCP seems to be the natural choice for users of ad hoc networks that want to communicate reliably with each other and with the Internet. It is well-known that due to the nature of multi-hop wireless Ad hoc network applications, TCP throughput instability can have a devastating impact on the QoS requirements and is regarded as one of the main TCP problems. In this paper, we first illustrate the TCP throughput instability simulation results for a chain-topology network that uses the IEEE 802.11 medium access control protocol and ad hoc routing protocol. We then discuss why the performance of TCP is poor in this scenario. A simple solution is proposed that enables the DSR routing protocol to reduce its trigger of unnecessary route error process due to MAC collisions which interrupts and affects the TCP link operation, and forces the TCP congestion window to enter the slow start phase leading to deteriorated and unstable TCP throughput. Our proposed solution enforces the TCP sender to recover packet losses of data by fast retransmit / fast recovery TCP mechanism without resorting to the coarse grained Retransmission Time Out (RTO), which is often detrimental to throughput. It is shown that our modification in DSR has caused significant improvement in TCP throughput stability for the chain, grid, and random mobile topologies compared to TCPs over standard DSR.