Thursday, 8 August 2013


Unscheduled Automatic Power Save Delivery (U-APSD)

The uplink data frames sent by stations (STA → AP) are used as indications (triggers) to AP that the power saving stations are awake. Here is a nice article on NULL frame which is being sent from STA to AP to inform that it is planning to switch to power save mode:

AP then sends the buffered data frames (buffered when the stations where in doze mode).

In APSD a station is awake during a Service Period (SP). An unscheduled SP begins when the AP receives a trigger frame from a station and ends when the station receives a QoS Data or QoS Null frame indicating the end of the service period (EOSP).

QOS Info Field when sent from AP(beacon)

AP needs inform to stations whether it supports the 

  1.  U-APSD(UN-scheduled) or 
  2. Scheduled power save mode 
by setting and resetting U-APSD flag bit in QoS info field. If U-APSD bit is 1, then it supports U-APSD and if U-APSD =0 then it does not supports U-APSD

QOS Info Field when sent from STA ((re)association request frame)

Max SP length (2 bit field) indicates max number of total buffered MSDUs and MMPDUs the WMM AP may deliver to WMM STA during the service period triggered by WMM STA.

  Max SP length

Setting 0 in AC (BE, BK, VI and VO) indicate that the corresponding AC is neither trigger-enabled nor delivery-enabled.

 TIM in the Beacon might not indicate whether frames are buffered at the AP, it requires to periodically send QoS Null frames if no other triggers are sent in order to learn the actual buffer status of the AP.

If U-APSD is configured through ADDTSs each time a new application starts which requires its usage, then the stations can rely on the TIM information of the Beacons to be informed about new traffic at the AP, enter into U-APSD mode through ADDTS when a communication starts and revert back to legacy
power save mode when it ends.

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