Unscheduled automatic Power Save delivery (U-APSD) is a mechanism which allows an 802.11 station to wake up when the 802.11 station wants to wake up and ping the Access Point to send buffered unicast data packets meant for it.
The Power saving mechanism is good in cases such as VoWLAN connections wherein a voice packet comes every few milliseconds or so. The U-APSD mechanism provides two mechanisms to obtain the data buffered at the Access point. They are
- Trigger enabled U-APSD mechanism
- Delivery enabled U-APSD mechanism
The U-APSD support is provided in the QoS Info field in the QoS Capability Information field sent in the association (re-association) request. This provides an indication on whether the particular Access category supports U-APSD or not. By setting a 1 in the particular bit – the access category would support both delivery enabled and trigger enabled U-APSD mechanism.
The U-APSD bit will be set by the Access Point to indicate the support of U-APSD
A TSPEC (Transmit Specification) frame needs to be sent by the 802.11 station separately if a particular Access Category has to be made exclusively delivery enabled or Trigger enabled.
The QoS Capability Element and the QoS information field is provided below for an 802.11 station
FIG COURTESY: 802.11-2012TM Standard
Each bit set in the above QoS Info field indicates that the Access category on the 802.11 Station supports both Trigger enabled and Delivery enabled U-APSD mechanism
The Max Service Period Length field is 2 bits wide and conveys the below information
FIG COURTESY: 802.11-2012TM Standard
The QoS Information field when sent by the Access Point is given below
The Wireshark capture indicating the support of U-APSD in the Access Point and the QoS information field sent by the station indicating support for both delivery/trigger based U-APSD is shown below
FIG: AP broadcasting U-APSD support
FIG: Station setting all the access categories to be both Delivery enabled and Trigger enabled
The two mechanisms of U-APSD will be described in the following sections