| Energy efficient algorithms are important in ad hoc networks since battery life for wireless devices is limited. MAC layer power management is focused on in the article. It is assumed that all nodes are synchronous, time is divided into so-called beacon interval and all nodes stay awake for the whole Ad Hoc Traffic Indication Message (ATIM) window.Power Saving Mode (PSM) is the power managemnet algorithm of IEEE 802.11, and fixed ATIM window is used. Control Packets are exchanged for the node which has packets to transmit. If this process is done successfully, the pair staies awake and packets are transmitted after the ATIM window, otherwise enter into doze state. It is implemented relatively easily because of simplicity of the algorithm. However, the algorithm needlessly wastes energy when traffic is relatively light in a network due to the fixed ATIM window.Dynamic PSM (DPSM) algorithm choses dynimicly ATIM window from a set of ATIM windows according to traffic load in a network, and the problem which exists in PSM is improved. However, it becomes complicated because a buffer queue is allocated for each ATIM window and different ATIM window may be used by each node.An improved algorithm, Adaptive PSM (APSM), is proposed based on the analysis of PSM and DPSM. A predictable timer is used to dynamicly vary the ATIM window in the algorithm, and the maximal ATIM window is restricted by the ATIM window timer of PSM for best network performance. No other overhead is increased significantly because olny a predictable timer is added comparable with PSM.Conclusively, NS-2 simulation results are given. APSM don't significantly decrease network throghput, and at best (with 50 nodes, 8 pkts/s, no movement and 1 connection), 52.6% joule per node is saved in comparison with PSM's and 10% in comparison with DPSM's; at worst (with 50 nodes, 8 pkts/s, maximal mobile rate 2 m/s, 25 connections, pause time 0 s), 13.6% joule per node is saved in comparison with PSM's and 5% in comparison with DPSM's. |
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