Wireless Radio and Battery Life

A wireless radio can perform one of four activities. It can be

• Asleep
• Idle and awake
• Receiving 
• Transmitting 

The goal of 802.11 power management, then, is to have wireless radios perform low-power activities, such as sleeping, as much as possible while performing high-power activities, such as transmitting, as little as possible. 

In 802.11, there are active mode and power save mode. 

Active mode : Stations and APs operate with the assumption that the station is always available to transmit and receive data

Power save mode: The AP is sleeping and unable to receive frame transmissions. 

When a station is in Power Save mode, it vacillates between one of two power stats, the doze state - saving the most battery life, and awake stat - either be idle, receiving, or transmitting. 

Every power management method is illustrated in the following steps and figures:

1. Before a station goes into the doze state, it sends a frame - null data frame, to indicate AP that power management is enabled. 

2. Once the station indicates that it is in Power Save mode, AP begins to buffer all frames destined to that station.

3. When the station goes into the awake state, it sends a frame to the AP in order to begin the data retrieval process.

4. When AP has finished sending all buffered data to the station, the station goes into the doze stat. 

802.11 power management 

802.11 power managment is the power management method from the original 80.11 standard. This affects the third and fourth steps of the basic power management.

Third Step. 

Fourth Step

Fifth Step

802.11e Unscheduled Automatic Power Save Delivery

U-APSD is the power management method from the 802.11e amendment.  It is part of the WMM-Power Save certification from Wi-Fi and is also required for 802.11n

above pictures from CWAP