Improving Expected Transmission Time Metric in Multi-Rate Multi-Hop Networks

Routing metrics are critical to select paths with maximal throughput in multi-rate multi-hop wireless networks. Simple path selection based on minimal hop count often leads to poor performance due to the fact that paths with low hop count often have higher packet loss rates. Better paths can be obtained by characterizing the actual quality of wireless link. A number of link quality aware routing metrics such as per hop round trip time (RTT), expected transmission count (ETX), expected transmission time (ETT) have been developed and explored. This study highlights some shortcomings of these routing metrics and proposes the design of a novel metric called improved ETT metric, or iETT, that addresses the discussed weaknesses and works more efficiently under various link quality conditions. The performance of the iETT metric is compared against the normal ETT metric over a DSR-based routing protocol. In extensive simulations, iETT metric outperforms the normal ETT metric in terms of network throughput and average packet delay.