Alleviating contention-induced spurious timeouts in QoS-aware MANETs

The bandwidth is one of the most precious resources in Mobile Ad Hoc Networks (MANETs); their network protocols should therefore be able to achieve an increase in efficiency of bandwidth utilisation. TCP, which implements flow control to regulate the network traffic, plays a significant role in shaping the workload of the network. However, spurious TCP retransmissions can increase the link utilisation, consequently limiting the amount of spare bandwidth available as extensively reported in the literature. This problem is exacerbated by the presence of high priority traffic and traffic differentiation introduced in IEEE 802.11e. In this paper, we focus on the optimisation of TCP for resource efficiency, by limiting the number of contention-induced retransmissions. Using a cross-layer technique, TCP can be notified of the medium contention state and adjusts its retransmission timer accordingly. Our simulation results demonstrate the effectiveness of our proposal in various scenarios. A 15% gain in resource utilisation is obtained, fewer packets are retransmitted thus reducing the overall power consumption and TCP goodput is improved.