On the Performance of Expected Transmission Count (ETX) for Wireless Mesh Networks

The Expected Transmission Count (ETX) metric is an advanced routing metric for finding high-throughput paths in multi-hop wireless networks. However, it has been determined that ETX is not immune to load sensitivity and route oscillations in a single radio environment. Route oscillations refer to the situation where packet transmission switches between two or more routes due to congestion. This has the effect of degrading performance of the network, as the routing protocol may select a non optimal path. In this paper we avoid the route oscillation problem using a route stabilization technique which forces data transmission on a fixed route. We implement this solution in a popular routing protocol, AODV, by disabling both error messages and periodic updating messages. Therefore, packet transmissions will stay on the routes initially found by AODV. ETX is compared with a widely used routing metric, HOPS, for reference purposes. We find ETX greatly improves initial route selection in AODV compared to HOPS in networks in which only single flows exists. For networks in which there are multiple simultaneous flows, ETX behaves similar to HOPS in initial route selection. Although the known cause of performance degradation is eliminated, the ETX metric still shows anomalous behavior. We determine that a major cause of the poor performance of ETX is additional collisions due to extra overhead. We propose a modified solution in which we repeatedly broadcast RREQ (Route Request) packets. Simulation results show that our modified solution improves ETX in the initial route selection in both single flows and multiple flows cases.