On the Limits of Power Saving Techniques for Ad Hoc Networks

This paper analyzes the energy consumption of ad hoc nodes using IEEE 802.11 interfaces. The objective is to provide theoretical limits for the lifetime gains achievable by difierent power-saving techniques proposed in the literature. We assume that the lifetime is the most important metric from the user point of view. Our evaluation takes into account the properties of the medium access protocol and the packet forwarding process in ad hoc mode. The key point is to determine the node lifetime based on its average power consumption, which is estimated considering the time a node remains in the states sleeping, idle, receiving, or transmitting. We show that energy-aware routing may achieve a lifetime gain as large as 30% for nodes individually, allowing low-battery nodes to operate more time. The use of two-hops instead of direct transmission, when possible, reduces the total packet-transmission cost up to 50%, due to the smaller number of overhearing nodes. Nevertheless, despite the signiflcant gain for the network as a whole, the nodes themselves can not obtain lifetime gains larger than 35%. Furthermore, our results highlight the importance of the transition to sleep state and show that a scheme based on the transition of overhearing nodes to sleep state increases the node lifetime up to 48%, and that this improvement also applies to networks with a moderate node density.