A reliability layer for ad-hoc wireless sensor network routing

We have been studying communication in wireless ad-hoc sensor networks. One of the authors has designed the multipath on-demand routing (MOR) protocol (Shu Hui Chen, 2003), which makes use of all possible paths to a given destination. Each node in MOR has, wherever possible, a choice of next hops for a given destination. We have designed and implemented a MOR reliability layer to take advantage of this. The basic strategy is to use a different node on each retransmission, and to keep track of which transmissions are successful. The main benefit is that a packet is likely to be delivered even if a given neighbor is temporarily unavailable, thus improving the delivery ratio or decreasing the number of end-to-end retransmissions. A further benefit is that nonresponsive nodes are removed from the routing table after a number of consecutive failures. In a sensor network transmission may be unreliable for a number of reasons, but particularly when congestion results in collisions. Without our reliability layer, collisions would cause packet loss and route loss. If the transport protocol is reliable, this results in end-to-end retransmission, which requires additional energy. We simulated transmission in congested conditions in different realistic sensor networks and compared MOR to available protocols. In our tests, the reliability layer helped MOR deliver data faster and less energy usage than the other protocols.