Wi-Fi Station devices are usually Battery operated devices. WLAN radios consume a lot of battery power and hence efficient power saving schemes should be introduced which conserve battery power. The earliest 802.11 standard introduced a power save mechanism which is now termed as Legacy Power save in 802.11.

It is based on a polling mechanism that is termed as Power-save poll (PS-POLL).

An 802.11 station that intends to go down to power save mode can achieve the same by dozing in between beacons or a multiple of beacons transmitted by the Access point.

The 802.11 standard provides  mechanisms wherein the WLAN (802.11) station connecting to the access point can indicate the number of beacon intervals it wants to doze before waking up to receive a beacon and unicast data frames meant for it.

They are

• Listen Interval
• ReceiveDTIM Beacon setting

Listen Interval

Listen interval is part of the association request frame sent by the 802.11 station. The 802.11 standard’s definition for the listen interval is provided below

8.4.1.6 Listen Interval field

The Listen Interval field is used to indicate to the AP how often a STA in power save mode wakes to listen to Beacon management frames. The value of this parameter is the Listen Interval parameter of the MLMEASSOCIATE.request or MLME-REASSOCIATE.request primitive and is expressed in units of Beacon Interval. The length of the Listen Interval field is 2 octets. The Listen Interval field is illustrated in Figure 8-40.

An AP may use the Listen Interval information in determining the lifetime of frames that it buffers for a STA.

If the AP accepts the association request from the Access Point – then it needs to buffer frames till the number of beacon intervals as specified in the Listen interval for that station.

The Access Point also uses the TIM element to inform the station of buffered traffic. If no buffered traffic is present at the access point – then the station can go back to sleep immediately till the next listen interval beacon.

The Listen interval and the TIM element is pictorially depicted below

The listen interval depicted above is pretty aggressive power-save. Normally, The Listen interval is set to a value of 10.

ReceiveDTIMs Beacon setting

The 802.11 station can also be configured to wake up for every DTIM beacon by setting the ReceiveDTIMs 802.11 MIB flag to true.

The 802.11 Station will then wake up according to dot11DTIMPeriod setting. For example if dot11DTIMPeriod is set to 3, then the 802.11 Station will wake up every third beacon which is a DTIM beacon transmission from the Access Point.

The two mechanisms of Listen Interval and DTIM wake are shown pictorially below

The TIM element indicates to the WLAN station whether data is buffered in the Access Point and the TIM information field is explained in the next article.

Traffic Indication Map Explained