SWRU423A July 2015 – May 2016 WL1801MOD , WL1805MOD , WL1807MOD , WL1831MOD , WL1835MOD , WL1837MOD
The unscheduled automatic power-save delivery (U-APSD) mechanism is also known as wireless multi-media (WMM) power-save. Legacy power-save methods can decrease the quality of periodic bi-directional traffic consisting of short frames as in VoIP. Because VOIP data should send data periodically on a fixed time (20 msec. for VOIP call), the legacy mechanism is not efficient enough. The U-APSD mechanism was built to optimize the legacy mechanism.
U-APSD is basically a polling scheme, similar to the legacy power-save delivery. However, in U-APSD mode, any transmitted frame, while in power-save mode, acts as a polling frame and triggers the AP to release a buffered frame from the same access category (AC) as the transmitted packet (the number of frames that are released by the AP is configurable and determined during the connection phase). For example, a voice packet releases only voice-buffered packets. If there are no transmitted packs, STA sends QoS null data packets (after the AP publicizes in its beacon that it has data for the specific associated station), which polls the buffered data. This is very efficient for bi-directional traffic streams, such as VOIP call.
As the STA awakes from power save to transmit the data, the STA then takes advantage of it to get any data buffered from the AP. This feature only works if the STA and AP are configured to WMM-enabled.