By CWNP On 09/17/2007 - 23 Comments

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.


Settings of the Max SP Length subfield:

Bit 5     Bit 6     Usage
0             0             AP may deliver all buffered MSDUs and MMPDUs.
1             0             AP may deliver a maximum of two MSDUs and MMPDUs per SP.
0             1             AP may deliver a maximum of four MSDUs and MMPDUs per SP.
1             1             AP may deliver a maximum of six MSDUs and MMPDUs per SP.

Now we know how a station indicates to the AP how much downlink data to send, but what about how a station indicates to the AP how much data it needs to send upstream?  A station can indicate the queue size or TXOP duration required to transmit its queued data to the AP in the QoS control field (bits 4 and 8-15) of the trigger frame.  This information can be used by the AP to estimate how much time to allocate to the station as part of its PSMP-UTT.

There are two noteworthy changes to U-APSD introduced by 802.11n.  First, PSMP allows stations to sleep during a PSMP sequence (during PSMP-DTTs and PSMP-UTTs) that doesn't interest them.  This is to say that these stations aren't scheduled to receive or transmit any data, so there's no need for them to stay awake.  This further extends battery life.  Second, in addition to the End of Service Period (EOSP) mechanism, the AP can indicate the end of a service period through the transmission of a PSMP frame with the More PSMP field set to 0 or by transmission of a CF-End frame when a PSMP frame was expected.  Keep in mind that stations cannot send CF-End frames to end the service period because they are not TXOP holders during a PSMP sequence.

With the changes in the 802.11n amendment, trigger frames can be one of the following:

1) A QoS Data or QoS Null frame associated with an Access Category (AC) the station has configured to be trigger-enabled
2) A PS-Poll frame
3) An Aggregate MPDU (A-MPDU) can be sent as a trigger frame containing QoS Data associated with an AC the station has configured to be trigger-enabled.

Blog Disclaimer: The opinions expressed within these blog posts are solely the author’s and do not reflect the opinions and beliefs of the Certitrek, CWNP or its affiliates.

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