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A good design of MAC protocols should realize both minimum energy consumption as well as maximum data throughput. IEEE 802.11 power management scheme for an ad hoc network is based on the carrier sense multiple access with collision avoidance (CSMA/CA) procedure to transmit/receive data frames. CSMA/CA may waste the scarce energy and bandwidth due to frame collisions and lengthen the frame delay due to waiting backoff time. In addition, the IEEE 802.11 power management scheme does not specify how to tune the ATIM (announcement traffic indication message) window size in a beacon interval. However, the ATIM window size affects the power consumption and throughput of a network considerably. The fixed ATIM window size cannot always accommodate the various traffic conditions. To conquer these problems and to improve the performance of networks, we propose a novel energy efficient MAC protocol for IEEE 802.11 networks by scheduling transmission after the ATIM window and adjusting the ATIM window dynamically to adapt to the traffic status. Simulation results show that our protocol attains a better energy efficiency and throughput than existing protocols.