The interested reader can refer <WMM Unscheduled Automatic Power Save Delivery (U-APSD)> as a precursor to this article.
In Delivery enabled U-APSD – the station will wake up on its own volition. When the 802.11 Station wakes up (possibly to send a packet), it sends a trigger frame to the Access point. The Trigger frame could be any frame – even the data frame that is sent by the 802.11 station can be the trigger frame. On receipt of the trigger frame – a service period is started by the Access Point – This is termed as an un-scheduled service period.
The Power Management bit is set to 1 in any trigger frame sent to the AP.
On receiving a trigger frame from the 802.11 station – the AP will dequeue packets for the particular TID for which a trigger frame was received. It will send packets as per the Max SP length sub-field setting negotiated during association.
When all the frames buffered in the Access Point for a particular access category is sent to the Station, The following steps can be taken
- The Access point will set the EOSP bit in the QoS Control Field and the More Data bit to zero.
- The Access point might set the EOSP bit and also set the More Data bit to 1 indicating that another delivery enabled access category has buffered data at the access point
Also, if the Access Point does not have any buffered data for the 802.11 station, it can also send a null data frame packet in response to a trigger – which suggests no data is buffered in the Access Point for which the 802.11 Station sent a trigger frame.
If the 802.11 station’s Transmit Opportunity (TXOP) finishes before all the buffered frames are received by the 802.11 station – the 802.11 station will stay awake and send a further trigger to obtain the remaining buffered frames from the Access point in a different Transmit Opportunity (TXOP).
Two pictorial examples of Delivery Enabled U-APSD is shown below
FIG: An unscheduled Power save Service period example
FIG: Example showing that the first trigger for an AC did not have any data at the AP but the next trigger for a different Delivery enabled AC had buffered data. AP indicated buffered data by setting more data to 1