Sunday, September 6, 2015

UAPSD aka WMM Power Save

Problems in Legacy Power Save:

  • Retrieving frames using PS-Poll frame is inefficient as each PS-Poll frame fetch only single packet. 
  • Not suited to efficiently handle the needs of bi-directional traffic e.g. VoIP traffic.
  • Slow and hence not suitable for low latency applications such as voice. 

UAPSD Explanation
  • Legacy Power Save is more like a polling mechanism where polling interval is 100 msec (beacon interval) in the best case whereas UAPSD is more like interrupt (trigger) driven mechanism hence more fast. 
  • Uses up-link traffic (QoS Data Frame) as a trigger for the delivery of buffered frames. 
  • AP supporting UAPSD has two queues: Legacy Power save queue & UAPSD queue. For all the ACs which are marked delivery enabled, buffering is done in UAPSD queue & for the other queues it is done in legacy power save queue. Hence, if any AC needs to use UAPSD procedure it should be marked delivery enabled.
  • When a trigger frame is received, it will start buffering out frames from the UAPSD queue on FIFO basis. Its not that voice will be followed by video.
  • To improve power consumption of the station, AP must respond to trigger ASAP. 

  • Number of frames to be delivered by the AP is decided from Service Period length field which is decided by client. 
  • As the AP can send any number of frames less than or equal to that of service period, station does not know when service period will end. Hence, AP need to send EOSP information to the stations using QoS Info field in the last data frame. However, if AP does not have any data buffered when trigger is received then AP responds with QoS NULL frame with EOSP bit set.  

  • Works only for uni-cast traffic. Delivery of Multicast & Broadcast still follows legacy power save method.
  • Well suited for applications that have bidirectional traffic flow. For applications which do not have sufficient up-link traffic to meet the needs of down-link traffic needs to send QoS frames periodically.

  • Trigger Enabled vs Delivery Enable AC
    • Trigger Enabled: Frame for AC which is marked as trigger enabled act as a trigger to the AP.
    • Delivery Enabled: Frames for ACs marked as Delivery enabled are queued in UAPSD queue otherwise they go in legacy power save queue. Hence, during service period only frames from Delivery enabled queues will be transmitted.
    • During association, station just marks that WMM is enabled for one or more of the ACs (no provision to mark ACs as Delivery enabled or Trigger enabled). Hence, ACs supporting UAPSD are both triggered as well as delivery enabled by default during association. Disabling one of the property needs TSPEC which is beyond the scope for this blog post.
  • Hybrid Mode:
    • Few ACs supports UAPSD & other supports Legacy Power Save. 
    • In this mode, TIM indicates buffering of frames only for the non UAPSD ACs.

Working with 
  • Legacy Clients: APs supporting UAPSD still have to support legacy power save for one or more of AC. Hence, supporting legacy clients is as simple as marking station as legacy power save capable only.  
  • Legacy Access Points

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