On demand routing protocols for ad hoc networks discover and maintain routes on a reactive, "as-needed" basis. These protocols are attractive for their low routing overheads. We develop a technique to make these protocols energy-aware in order to increase the operational lifetime of an ad hoc network where nodes are operating on battery power alone and batteries cannot be recharged. Our techniques uses a new routing cost metric which is a function of the remaining battery level in each node on a route and the number of neighbors of this node. The idea of the cost metric is to be able to route around the nodes that are running low in battery for which alternate routes are available. In addition, rerouting is done proactively when any node en route starts running low on battery while the route is being actively used. Further, we save energy by switching off the radio interfaces dynamically during the periods when the nodes are idle. Simulation results using AODV protcol show that combination of these techniques results in a significant improvement of the energy budget of the network as a whole resulting in increased operational life time. Generally speaking, the improvement is higher for higher traffic loads and modest mobilities.

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