We focus on energy saving in 802.11-based WLANs. Previous work has shown that, on the one hand, 802.11 wireless interfaces consume a significant amount of energy, on the other hand the use of current power management schemes can severely degrade the QoS performance of several Internet-based applications. Furthermore, the energy spent by wireless devices may even increase when the the standard 802.11 power-saving mode (PSM) is implemented. These facts suggest that other solutions to energy saving are highly needed. In this paper, we consider the 802.11 distributed access scheme and we propose a novel ap-proach that enables a station to enter a low-power operational state during channel contention. More specifically, our technique exploits the virtual carrier sense mechanism and the backoff function specified in the IEEE 802.11 DCF, so that a station can dramatically reduce its energy consumption without significant degradation of the QoS performance. By using the network simulator ns-2, we evaluate the performance improvement that is obtained when the proposed mechanism is implemented, against the results attained through the standard DCF. The results show that we can achieve a reduction in energy consumption as large as 80 % and 28 % under UDP and TCP traffic, respectively, while still maintaining good performance in terms of packet delivery delay and throughput. Index terms – Wireless LANs, energy saving, quality of service provisioning, channel access. This work was supported by the Italian Ministry of University and Research through the PRIMO project.

Energy conservation is an important issue in wireless networks. We propose a method for estimating the least amount of energy needed for broadcasting a message to all nodes in the network. The method can work with any reasonable energy models. We prove that this least-energy problem is NP-complete by showing that the maximum-leaf spanning-tree problem is a special case of the least-energy problem. 1 Key Words and Phrases: graph theory; least-energy problem; maximum-leaf spanningtree problem; NP-complete; wireless network