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:

This is an older power-saving mechanism used in Wi-Fi networks, where the station (STA) (like a smartphone or laptop) tells the Access Point (AP) that it will go into sleep mode to save battery.

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. (PWR MGT Set to 1)

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. (PWR MGT Set to 0)

4. When AP has finished sending all buffered data to the station, the station goes back into the doze state. (PWR MGT Set to 1)

Real-Time Example:

• A smartphone in a Wi-Fi network goes to sleep to save battery. While it’s asleep, the router (AP) holds onto notifications or emails for that device. Once the smartphone wakes up (for example, when you unlock it), it asks the router for the data, retrieves the messages, and then goes back to sleep.

                                   Legacy Power Management Mechanism

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

This is a more advanced and efficient power-saving method, commonly used in devices that support Wi-Fi Multimedia (WMM), like smartphones or tablets.

Steps:

1. The STA still sends a signal (like in legacy mode) to say it's going into sleep mode.
2. Instead of waiting for regular network beacons and checking the TIM (as in older methods), the STA can send a packet at any time to the AP.
3. By sending this packet, the STA triggers the AP to immediately send any buffered data without waiting for a scheduled time.
4. The AP instantly sends the stored data once it acknowledges the STA’s packet.

Real-Time Example:

• While using a messaging app, your smartphone goes to sleep between sending or receiving messages to save power. But as soon as you send a new message (the packet), the router immediately sends back any waiting messages (buffered data), ensuring no delay in communication.

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

This is a power-saving feature introduced in Wi-Fi 5 (802.11ac), where a STA can save power during a TXOP (Transmit Opportunity), which is a period when it has the chance to send data without interruption.

Steps:

1. During the TXOP, the STA can briefly go to sleep if there’s a pause in data transmission, even during active communication.
2. When data transmission resumes, the STA wakes up and continues sending/receiving data.

Real-Time Example:

• Imagine you're streaming a video on your laptop via Wi-Fi. During small pauses in the video (like while buffering), your laptop temporarily sleeps to save power. Once the video resumes, it wakes up and continues receiving the data.

4) TWT(Target Wake Time) in 802.11ax (Wi-Fi 6) :

TWT (Target Wake Time) is a power-saving mechanism introduced in 802.11ax (Wi-Fi 6). It allows devices (STAs) and access points (APs) to negotiate specific wake-up schedules for data transmission, enabling more efficient use of energy. TWT helps improve battery life in devices by minimizing the amount of time they need to stay awake, which is especially useful for IoT (Internet of Things) devices, smartphones, and other battery-powered gadgets.

Key Features of TWT:

1. Scheduled Communication:

   • Devices negotiate with the AP to define a specific "wake-up" time when they will send or receive data. Outside of these scheduled times, the device can remain in a low-power state (doze mode).

2. Flexible Operation:

   • TWT can be used to schedule transmissions based on different priorities (e.g., high-priority data like streaming vs. low-priority background tasks).

3. Enhanced Power Efficiency:

   • By avoiding unnecessary wake-ups, devices conserve energy, which is especially beneficial for battery-powered devices. This is a key feature for improving device longevity in Wi-Fi 6 networks.

4. Target Wake Time Agreements:

   • The AP and the STA establish a TWT agreement, specifying when and how long the device should wake up to communicate with the network.

Real-World Use Cases:

1. IoT Devices:

   • Devices like smart sensors or smart home gadgets benefit from TWT as they don't need to stay awake for long periods and can transmit data during predefined intervals.

2. Mobile Devices:

   • Smartphones and tablets can negotiate TWT with an AP to extend battery life, especially when background data transmission is low.

3. Enterprise and Industrial Networks:

   • In environments with many devices (like offices, factories, etc.), TWT helps in reducing congestion and improving overall network efficiency by controlling when devices access the network.

In summary, TWT in Wi-Fi 6 allows devices and APs to agree on specific times for data transmission, improving both power efficiency and network performance.

Real-Time Example:

• A smart home device (like a Wi-Fi-enabled thermostat) sleeps most of the time and only wakes up at specific intervals to check for updates from the Wi-Fi network. This significantly saves its battery life while still being able to operate smoothly.