Global effects of local transmission strategies in multihop wireless networks

Multihop wireless networks continue to grow in importance, especially in distributed device-networks. The shared nature of wireless media leads to interactions among the nodes that signficantly impact performance. Our research has been focused on modeling and analyzing these interactions. The most basic form of interaction arises from packet collisions, which lead to retransmissions and consequently shift the stability point. In our initial work, we quantied the stability point, taking queueing and packet retransmissions into account, for networks that use a simple channel access method called CSMA. A popular collision avoidance method is channel reservation using additional packets, called the RTS/CTS mechanism. In multihop networks, the RTS/CTS mechanism abates, but cannot eliminate packet collisions, even under ideal channel conditions. We analytically quantified the residual collision probability and confirmed the results by experiments on a real multihop network testbed. However, a more interesting effect of using the RTS/CTS mechanism is that it may create strong interdependencies among the nodes even when there is no packet collision, which unnecessarily refrains many nodes from transmitting. This effect propagates in the network like dominos, causing performance degradation such as network congestion and even deadlock situations. We described the problem and presented a simple, backward-compatible solution that increases the network throughput by as much as 50%. In a recent work, we have also analytically quantified the efficiency of the proposed solution.