A wireless data communication system has a first station or mobile unit is linked to a second station configured as an access unit to support packet communication, voice or data, where the voice packets are transmitted in the Continuously Aware Mode (CAM) mode while other packets are buffered by the access point and held until asked for by the first station when in a Power Saving-Poll (PSP) mode. A monitoring apparatus at the access point monitors all transmitted packets and sorts the packets to the mobile unit according to CAM or PSP mode. Voice packets are sent out immediately to the mobile unit. Other packets are stored at the access point. The packet arrival rate may vary during transmission and due to random packet delays introduced by propagation characteristic and processing apparatus. The packet arrival rate and delays are taken into account by the first station in an algorithm to determine and extend the normal safe period in which the station receiver may be powered off.