TCP performance in wireless multi-hop networks

We investigate the interaction between TCP and MAC layer in a wireless multi-hop network. Using simulation, we provide new insight into two critical problems of TCP over wireless multi-hop. The first is the conflict between TCP data packets and TCP ACKs, which causes performance to degrade for window sizes greater than 1 packet. The second is the interaction between TCP and MAC layer backoff timers which causes severe unfairness and capture conditions. We show that these effects are particularly pronounced in two families of MAC protocols that have been extensively used in ad-hoc network simulation and implementations, namely CSMA and FAMA (a descendent of MACA). We then demonstrate that these problems are in part overcome by using MACAW, a MAC layer protocol which extends MACA by adding link level ACKs and a less aggressive backoff policy. We argue that link level protection, backoff policy and selective queue scheduling are critical elements for efficient and fair operation of ad-hoc networks under TCP. These conclusions are supported by extensive simulation and measurement experiments.