Power Save Mechanisms – 802.11ax

Power save is one of the important requirements for mobile devices. WiFi can drain the battery of a mobile device quickly. Hence in 802.11 networks power save management is achieved based on active and doze states of a mobile device. In the active state, the mobile device is communicating to the AP and in doze state, the mobile radio is turned off. Although there are multiple mechanisms introduced in various 802.11 technologies, 802.11ax really considers the battery requirement and tries to have the mobile device in a doze state as much as possible along with taking care of the data communication with the mobile device as and when required. It should be noted that for every type of power save mechanism the AP never knows when would a device go to doze state. So this information should always be sent by the mobile device to the Access Point.

There are a few types of mechanisms that were introduced (listed below), however, lets recap through the legacy power save mechanism that has been used from the initial days of 802.11 networks. 

Legacy Power Save Mechanism

In the legacy power save mechanism, the WiFi driver does go to doze state but has its time synched with Target Beacon Transmission Time (TBTT) and Time Synch Function (TSF). This makes the client check for beacons with Traffic Indication Map (TIM) bit frequently. To do the same the Laptop/device keeps coming back to the awake state from the doze state.

Refer to the exhibit below for Legacy Power Save Mechanism:

Below are the steps:

Step 1: A normal association and authentication are completed between the AP and the WLAN Laptop/Device. AP allocates an Association ID (AID) to this particular device and the same is shared with respective WLAN Laptops/Devices.

Step 2: Laptop/Device that doesn’t have any information to transmit sends a null frame to the AP with power management bit set to 1. 

Step 3: The AP updates its user-entry in the list of devices that are in power-save mode.

Step 4: The AP receives some data for that particular Laptop which is in power save mode. The AP sends beacon with TIM set and AIDs of the users that have the data buffered.

Step 5: Laptop realizes that there is data stored on AP for itself. It then sends a PS-Poll Frame to the AP.

Step 6: This continues until all the data is sent and is received by the client. The communication completes and the Laptop/Device goes back to power save mode.

Issues with this mechanism:

  • A lot of overhead in the air as the communication includes a lot of WLAN frames. 
  • This mechanism is not efficient enough to save power because the station needs to be awake to check TIM in beacons even if there is no data for that particular station.

Legacy power save process always gives a hint of an idea on what we are trying to achieve with WLAN. However, its inefficiencies are more when relating to the type of operations these mobile devices can do. Automatic Power Save Delivery (APSD) like Unscheduled and Scheduled (U-APSD and S-APSD) does help achieve efficiency up to a certain extent. But, it is always better to have more. 802.11ax brings more to the plate. First, it extends the 802.11ac based microsleep mechanism, and second, it uses Target Wake Up Time (TWT) picked up from 802.11ah.

  1. Microsleep Power Save Mechanism: This was introduced in 802.11ac wherein the WLAN client which is not transmitting/receiving can be in doze/sleep state whenever there is a TXOP given to a Client. 802.11ax extends this to be used along with Uplink and Downlink communications. This is shared across all clients using its High-Efficiency PHY header. Whenever an 802.11ax enabled Client senses that there is a Transmit Opportunity (TXOP) provided to a station/client then it goes to doze state as during this period only the Clients whose information was shared in PHY header will be participating in the communication process.
  2. Target Wake Time (TWT): 802.11ax came up with this power save mechanism from IOT based standard 802.11ah. Here the station and the AP can negotiate time interval called Service Periods (SP) when they would send data packets to each other and the station would be in dozing state for the rest of the time. Station OR 802.11ax client is usually the requestion Station and AP is the responding station. The advantage of this is that the Station only needs to be in the awake state during the service period (SP) and can be in dozing state rest of the time even during the transmission of beacons. Please note that this doesn’t make the communication contention-free i.e. the SP established between the AP and the Station doesn’t forbid other users from trying to transmit/receive.

Furthermore, there are two types of TWT mechanisms. Individually agreed on TWTs and Broadcast TWTs. To start with the AP needs to set the TWT Required subfield in High Efficiency (HE) Operation Element to 1. On receiving this information, the stations will start negotiating the TWT SP with the AP.

Individually Agreed TWT SPs: 

As the name suggests here the 802.11ax enabled WLAN client and the 802.11ax AP takes part in the negotiation individually for the SP allocation. This depends on the TWT command setup value represented by the initiator. For our understanding, we can consider the initiator as the WLAN Client station (Note that the AP can also be an initiator in very rare cases).The TWT command field has various values that are used in the negotiation process. Below table explains the possible state and type of exchanges between AP and Client for forming Individually agreed TWT:

TWT Command Setup field in WLAN Client (Initiator)TWT Command field Setup in AP (Responder)TWT Condition after complete exchange
Request TWT or Suggest TWT or Demand TWTNo frame transmitted backThis means there was no individual TWT exchange done earlier and no new agreement happens either.
Demand TWTAccept TWTIndividual agreement exists. This is verified after TWT parameters are verified in the initiating frame post which AP sends Accept.
Suggest TWT or Request TWTAccept TWTIndividual agreement exists and the TWT parameters used are identified in the response frame.
Demand TWT or Suggest TWTAlternate TWTNo individual agreement exists. Responder is offering an alternative set of parameters. Initiator can send a new request with those set of parameters which could be accepted later by responder.
Demand TWT or Suggest TWTDictate TWTNo individual TWT exists. The responder doesn’t give option to initiator. The initiator TWT will only be accepted if the Dictated TWT parameters are used to send the request again.
Request TWT or Suggest TWT or Demand TWTReject TWTNo individual TWT exists. The responder will not create any TWT with the initiator.
Individually Agreed TWT Setup Messages

Broadcast TWT SPs:

Broadcast TWT SPs are similar to the Individual Triggered TWT. However, here AP will schedule TWT for WLAN clients and distribute that information with the help of Beacons. The WLAN clients that have received this information but have not done any TWT negotiation with the AP shall follow the SP mentioned in the beacons and exchange data only during the schedule mentioned by the AP. The Beacon may also specify on which beacons the station can wake up; however for the rest of the time the WLAN station will be always in doze state. The WLAN client can again negotiate with the AP on the interval during which the client may not want to wake up to receive beacons. So in general, the WLAN clients will sign up to the Beacon Broadcast TWT SP.

Opportunistic Power Save (OPS):

Another reason to recap the Legacy Power save mechanism was that 802.11ax does combine some part of legacy power save and TWT. This mechanism is an opportunistic Power Save Mechanism. Here, the AP splits the beacon interval into several sub-intervals. These sub-intervals are broadcast TWT SPs. At the beginning of the sub-interval, AP specifies which WLAN Client would be served during this Service Period. TIM is sent along with the TWT SP advertisement frames which list WLAN clients which will be served during a particular Service Period. A station not part of TIM can doze during the service period. Here the TWT SP negotiation will not happen and the client can try to access the medium as per TWT and TIM advertisements.

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