Power Save has always been a part of the 802.11 standard since 1999. The devices it had targeted were a lot different from the wireless devices used today. Back then these devices were primarily laptops with big batteries moving between conference rooms. No thought was made that Wi-Fi could and would be added to small hand held devices being used in all aspects of our life. With the addition of Wi-Fi to mobile devices like cell phones, the power save requirements have changed.

               There is now a greater focus on power saving in Wi-Fi than any previous stage of its existence. The advancements in hardware and application have shaped the power save options available today in 802.11. Let’s look at the some of the techniques used to save power consumption.

               There are four basic operations that a radio can perform that leads to power consumption. Power consumed by each activity increases in the given order (1-4). By controlling how long the radio operates between the below states, a user has some control on how much power his radio consumes.

• Asleep
• Idle & awake
• Receiving
• Transmitting 

The power management structure of a wireless radio can be summarized into four simple steps:

1. Before a STA goes into the doze state, it sends a frame, usually null data frame, to the AP indicating that power management is enabled.
2. Once STA indicate, that it is in Power Save mode, the AP begins to buffer all frames destined to that station.
3. When the station goes into 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 back into the doze state.

 The two popular techniques that most radios rely for power save are:

PS-POLL:

PS-Poll is a legacy power save mechanism defined in the 802.11 spec. This mechanism allows the client to indicate to the AP that it is going to sleep until the next beacon. The AP buffers frames while asleep, then lets the client know that frames are buffered via an advertisement in the beacon.

Wake on Wireless (WoW):

The Wake-on-Wireless (WoW) feature enables the implementation of a more complete system level power save scheme. When enabled in the radio it allows the host system to enter deep sleep mode while there is no activity from the WLAN interface. When the radio detects there is traffic, related to the host system, it wakes the system via the use of a dedicated WoW signal sourced from the radio going to an interrupt on the host.

To learn more on how these techniques are used for our latest dual-band, single stream, 802.11a/b/g/n/ac plus Bluetooth radio module, download our application note below.