Knowing its AP buffer status enables the station to send another trigger frame or PS-Poll to
retrieve more of its buffered frames.
As with the legacy power-save mechanism, a station can learn about its buffer status
by listening to the beacons for its TIM [21]. This is needed only while not receiving frames
from the AP, as the More Data control subfield in downlink frames to the station conveys
the same information.
To use full U-APSD, a station sets the first four bits of the QoS Info subfield of the
QoS Capability element in the (re-) association request all to 1. The Max SP Length
WLAN QoS
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subfield of the QoS Info field is used to place a limit on the maximum number of frames to
be delivered during a service period. For unrestricted delivery, this subfield should be set to
0.
3.4.2.3 Hybrid Unscheduled APSD
The hybrid U-APSD mechanism allows a station to choose between legacy power save
delivery and APSD based on access category [22]. Trigger frames are used to initiate the
delivery of buffered frames associated with access categories that have been designated as
???delivery enabled??™. Buffered frames of access categories not so designated, can be retrieved
with PS-Polls only.
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