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The popularity of wireless LANs, due to their low cost provision of high speed wireless Internet access, has resulted in a strong trend toward the integration of this technology in the upcoming all-in-one mobile devices that could include, for instance, cellular, wireless LAN and personal digital assistant (PDA) capabilities. Such devices, though, require power saving mechanisms in order to guarantee a reasonable battery duration. The 802.11 standard provides a power save mode that reduces the wireless LAN technology power consumption, however, this mode can result in downlink delays (AP to station) unacceptable for the QoS of some applications, e.g., VoIP. To overcome this problem, we propose an adaptive power save mode algorithm (APSM) that adapts the data frames MAC downlink delay of a certain station according to the downlink frame interarrival time observed at the AP MAC layer. We conducted an evaluation of our proposal with respect to downlink delay, power efficiency and signaling load using the OPNET simulator and compared its performance with the 802.11 standard power save mode and two different static alternatives. The results show the effectiveness of our algorithm in providing a soft upper bound to the MAC downlink delay while significantly decreasing the power consumption and requiring a signaling load similar to that of the standard power save mode.