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U-PSMP and U-APSD

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Written by Devin Akin

Monday, 17 September 2007

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.

Comments (6) Add Comment

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written by Ben Miller, September 18, 2007

Do you think this could work if there is more than one BSS on the channel? It seems to me like it could be too close to HCCA. How does the timing work if stations are only going to be awake for their service periods? What happens if a STA or AP from another BSS starts using the channel? Would that mess up the service period timing?

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written by Devin Akin, September 18, 2007

APs are handing out schedules that indicate when the medium will be clear (as far as their BSS is concerned). If, during that scheduled transmission period, another BSS is transmitting while overlapping, then there would be a more limited period of time in which the STA could transmit during its schedule. This simply means less throughput during the scheduled period.

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written by Kevin Liu, May 20, 2008

I know that trigger frame from STA can start SP in U-PSMP,but I think the awake STA transmitting trigger frame to AP is only one(maybe other STAs is asleep),that is to say there is only one STA can join in the U-PSMP.

How can we take several STAs in U-PSMP SP after sending a tragger frame from a STA ? what is the mechanism?

Thanks!

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written by Devin Akin, May 21, 2008

With U-PSMP, each station sends a trigger frame whenever it decides to awake and check for queued data. With S-PSMP, all stations adhere to a common "schedule" where each takes a turn during an SP. There are currently no implementations of S-PSMP that I'm aware of.

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written by isaac, September 24, 2008

DO you know of a WLAN client supporting U-PSMP ?

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written by Devin Akin, September 24, 2008

I haven't seen anything yet, but will keep my eyes peeled.

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