802.11 Power Save Mechanism

Understanding Different types of 802.11 Power Save Mechanism Modes and How it Works?

1) PS- Polling in Legacy in 802.11
2) Unscheduled Automatic Power Save Delivery (U-APSD)
3) VHT TXOP Power Save Mode in 802.11ac (Wi-Fi 5)
4) TWT in 802.11ax (Wi-Fi 6)

1) PS- Polling in Legacy in 802.11:

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.

                                   Legacy Power Management Mechanism

2) Unscheduled Automatic Power Save Delivery (U-APSD):

1) Power save mechanism is the Wi-Fi Multimedia (WMM) automatic power save delivery . it comes in both scheduled and un scheduled options.

2) Just like power management mode the station still uses the power management bit to indicate that it is going to sleep mode.

3) But instead of waiting for a beacon and checking the TIM element , the station may trigger the  AP to send any buffered frames by transmitting any packet to the AP.

4) The AP transmits buffered frames with same quality of service values as the  triggering frame immediately after acknowledging the stations transmission.

5) No more waiting around-the station sends a frame , and the AP understands that's its awake starts sending back to it.

VHT TXOP Power Save Mode in 802.11ac (Wi-Fi 5)

Wi-Fi 5 introduced  VHT TXOP Power Save , Which means "Very high throughput Transmit Opportunity"

The VHT TXOP Power Save features allows a VHT client to enter the doze state when frames are delivered during a transmit opportunity (TXOP) to other clients . A clients can operate in VHT TXOP Power save mode when its power management in active mode 

This method is very simple . essentially the device says "when another device is taking and they aren't taking to me , I can just go sleep " It just makes sense i.e., The AP must not transmit to this VHT client during that TXOP.

The process is simple , if a STA sees that another STA has a TXOP  it will power down its radio during the duration the transmission. The longer the TXOP , the longer the sleep time, the more battery saved. 

TWT in 802.11ax (Wi-Fi 6)

Without Target Wake Time (TWT), since the AP can only communicate with one station at a time, the stations (which are mobile devices) have to “stay awake” to receive data packets from the AP one after another regardless of how long that process takes. When station 1 is exchanging data with AP, station 2 is in an idle state and waits until the AP finishes its communication with station 1. This causes extra power consumption and shorter battery lives for network devices.

Target Wake Time (TWT) is a new feature that allows an AP and stations to “wake up” at negotiated times. The stations and AP reach a TWT agreement that defines when a station is awake to receive and send data. Stations only wake up at TWT sessions and remain in sleep mode for the rest of the time.

It's like you are home waiting for a friend, and before, you have do go downstairs every 5 mins to check if you friend arrives, but now, you just stay at home waiting for you friend rings the bell, that's it. And also, since you have made an agreement with your neighbors, so when you go out, all resource is just for you, the elevator is just for you, the taxi is waiting for you in front of your apartment, you are the only customer in a restaurant. This is super-efficient, isn't it?