U-PSMP and U-APSD
Unscheduled PSMP (U-PSMP) extends the U-APSD (also known as WMM-Power Save or WMM-PS) mechanism.
In a U-PSMP enabled WLAN, if there is no unscheduled service period (SP) in progress, the unscheduled SP begins when the AP receives a trigger frame from a station, which is a QoS data or QoS Null frame associated with an AC the station has configured to be trigger-enabled. An unscheduled SP ends after the AP has attempted to transmit at least one MSDU, A-MSDU, or MMPDU associated with a delivery-enabled AC and destined for the station, but no more than the number indicated in the Max SP Length field of the QoS Info field in the trigger frame (if the field has a non-zero value). The Max SP Length subfield is 2 bits in length and indicates the maximum number of total buffered MSDUs and MMPDUs the AP may deliver to a station during any SP triggered by the station.
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