title: 802.11 - how can I tell which power saving scheme a STA implements?
body: <p>I am looking at how different mobile devices from different vendors implement the power save mechanisms defined in the 802.11 standard. My only way of viewing what these devices are doing, what their frame pattern is, is through wireshark captures. </p>
  
  <p>From what I understand,a common behavior to all mobile clients is for them to send:</p>
  
  <ul>
  <li>a Listen Interval value expressed as an integer in the (Re)Association Request</li>
  <li>a Null Function frame or a QoS Null Function frame with the power management bit set to 1 to announce the AP that they are going into PS mode, turning off their Tx ans Rx and that the AP needs to start buffering traffic directed at this now-dozing STA. </li>
  </ul>
  
  <p>Beyond this point there are several different mechanisms through which these devices get their traffic back once they're awake. By far the most common behavior I have seen in the captures is for them to send either a Null Function or a QoS Null Function frame with the power management bit set to 0 and the AP proceeds to send them the traffic, because the mobile device is now effectively operating in Awake Mode. </p>
  
  <p>The 802.11 standard, however, specifies that a mobile device should either send a PS Poll frame to specifically ask for its traffic or implement either S-APSD or U-APSD mechanisms. However, I have seen none of these happen. </p>
  
  <p>Does anyone have any experience with these mechanisms and can tell me how I can check which mechanism each device implements?  </p>