Summary: <br> Power saving is a normal function of mobile devices. It’s required to conserve battery. <br> <br> <br> <br> In this episode, we summarize what we’ve read about Power Management from the 802.11 standard. <br> <br> <br> <br> Power save (PS) is a power management mode in which a station (STA) alternates between two states:<br> <br> <br> <br> * Awake: STA is fully powered* Dozing: STA is not able to transmit or receive and consumes very low power<br> <br> <br> <br> Stations may define their own power management states. For example, this is a configurable setting from a Wi-Fi scanner:<br> <br> <br> <br> * CAM – Constant Active Mode* Fast Power Save* MAX Power Save<br> <br> <br> <br> The Power Management Flag is located in the Frame Control Field located in the MAC header of the 802.11 frame:<br> <br> <br> <br> <br> <br> <br> <br> <br> <br> <br> <br> The Power Saving Flag is used by a STA to notify that it will go to sleep.<br> <br> <br> <br> That flag is sent within a Null Data Frame from the STA. The Null Data Frame contains no data at all but is used to specify whether the STA goes into Awake or Doze state.<br> <br> <br> <br> Null Data Frame sent when client is going to sleep<br> <br> <br> <br> Null Data Frames<br> <br> <br> <br> <br> <br> <br> <br> Power management flag<br> <br> <br> <br> When a STA goes into a doze state, traffic for the STA is buffered at the access point.<br> <br> <br> <br> How does the STA know there is data buffered for it? The Beacon frame includes a Traffic Indication Map (TIM) and Delivery TIM (DTIM) which tells the STA how often it should come out of doze state and check for buffered data.<br> <br> <br> <br> Traffic Indication Map<br> <br> <br> <br> The TIM contains a Partial Virtual Map and an Association ID for buffered data associated with a STA. <br> <br> <br> <br> <br> <br> <br> <br> The DTIM Count field indicates how many Beacon frames (including the current frame) appear before the next DTIM. A DTIM count of 0 indicates that the current TIM is a DTIM.<br> <br> <br> <br> The DTIM Period field indicates the number of beacon intervals between successive DTIMs. <br> <br> <br> <br> The Bitmap Control field contains the traffic indication virtual bitmap bit associated with an AID. This bit is set to 1 in TIM elements with a value of 0 in the DTIM Count field when one or more group addressed MSDUs/MMPDUs are buffered at the AP. <br> <br> <br> <br> Buffered data for multiple STAs<br> <br> <br> <br> <br> <br> <br> <br> Wireshark filters<br> <br> <br> <br> * Filter frames with the Power Management bit set to 1* wlan.fc.pwrmgt == 1* Filter null Data frames (null data + CF-Poll): * (wlan.fc.type == 2) && (wlan.fc.subtype == 4)* Filter beacon frame that indicates buffered data is available for doze STA: * (wlan.fc.type == 0) && (wlan.fc.subtype == 8) && (wlan.tim.partial_virtual_bitmap != 00)<br> <br> <br> <br> 802.11-2016 Sections<br> <br> <br> <br> * Section 6.3.2 – Power Management p.264* Section 9.2.4.1.7 – Power Management subfield p.642* Section 11.2 – Power Management (MLME) p.1599* Section 11.2.3 – Power management in a non-DMG infrastructure network p.1600* Figure 11-7 – Infrastructure power management operation (no PCF operating) p.1603<br> <br> <br> <br> PCAP Files<br> <br> <br> <br> * <a href="https://drive.google.com/drive/folders/1iDPMa47EzMmVo9gJmM5oK8hkxv8n12im?usp=sharing" target="_blank" rel="noreferrer noopener">Download here</a><br>
