WiFi Insider - Power Save

Power consumption on the receive path is about a third lower than on the transmit path. It is more power efficient to shutdown the transmit path and just listen when no outgoing frames are present. Further power savings can be achieved by the STA when it indicates to the AP that it is entering power save mode and shutdown its receive path. The AP stores frames destined to an STA in power save mode and sends them to the STA when requested to do so. During association, an STA uses the Listen Interval parameter to indicate to the AP how many beacon intervals it shall sleep before it retrieves the queued frames from the AP. The AP shall not drop any queued frames until the STA's Listen Interval elapses.

Legacy Power Save Polling

In this scheme, an STA enters power-save mode by sending a Null frame to the AP with the Power Management bit set. From then on, the AP stores all packets destined to the STA in a per-STA queue and sets the TIM field in the beacon frame to indicate that packets destined for the STA have been queued at the AP. An STA wakes up from sleep every Listen Interval to receive the beacon frame and when it detects that the TIM field for it has been set, it sends a PS-Poll frame to the AP. In response, the AP sends the first queued frame to the STA. The STA receives the queued data frame and if the More Data field in this frame is set, it sends another PS-Poll frame to the AP. The STA continues to send PS-Poll frames to receive all the queued frames and when none are left, it goes back to sleep until the next Listen Interval.

It is apparent that this method is not a very efficient method of data transfer as polling is used and is suitable for light traffic conditions. An improvement over this method is to exit power-save mode upon detecting queued frames by sending a Null frame with Power Management bit turned off. The AP can then send all the queued frames to the STA. The STA can then re-enter power-save mode by sending a Null frame with Power Management bit turned on. The STA waits for a short period of time during which it has not received any frames from the AP to deduce that there are no outstanding queued frames for it at the AP.

Unscheduled Asynchronous Power Save Delivery (U-APSD)

Legacy power-save methods do not provide good power savings for periodic bi-directional data traffic consisting of short frames like in VoIP. The time to deduce that there are no outstanding queued frames from the AP is usually much longer than the time period between successive VoIP frames so that the STA does not go to power-save mode during the VoIP call. To handle this situation better, U-APSD mechanism was formulated as part of 802.11e.

U-APSD allows for an client to schedule in advance the communication pattern between the STA and AP based on the predictable traffic pattern of VoIP calls. For example, the client can negotiate with the AP to exchange VoIP frames every 20ms which is typical. Once this is negotiated, the client may sleep between the packet exchanges without notifying the AP. For non-VoIP traffic, an STA retrieves unicast traffic queued in the AP by sending trigger frames.

The AP indicates support for this mechanism by advertising this capability in beacon and probe response and association/reassociation response frames.

As in legacy power-save mode, an STA sets the Power Management bit in a Null frame to indicate to the AP that it is in power-save mode. An STA in power-save mode sends a QoS Null or QoS Data frame to trigger the AP to send queued frames. The AP acknowledges the trigger and then follows it up with up to a negotiated maximum number of frames. If there are no queued frames on the AP when the trigger is received, the AP will respond with a QoS NULL frame. The advantage of U-APSD over legacy power-save mode is that the exchange of frames occurs with SIFS separation so that the medium remains locked during the exchange. In legacy power-save mode, the frames are exchanged with DIFS separation so that other STAs can take over the medium.