U-APSD is a power-save mechanism that is an optional part of the IEEE amendment 802.11e, QoS. U-APSD is also known as WMM Power Save, and the behaviour and operation is exactly the same, except for coding of some information elements.
U-APSD is working in conjunction with WMM and is tied with usage of the 4 EDCA access categories that is used to differentiate packets and their priorities. U-APSD is very well suited for bi-directional data flows like voice.
The power savings is around 400% with Ascom i62 default settings compared to active mode, and extends the talk time from around 4 hours in active to around 16 hours in U-APSD mode with the standard battery.
This attached document aims to explain the logics and behaviour of 802.11 power-save mechanisms and U-APSD as implemented in the Ascom i62. The document will also suggest some methods to debug any anomalies or problems with U-APSD operation.