This is the 2nd part in a 3-part series on blogs exploring various power-saving methods available in modern Wi-Fi systems. In the first blog post titled “Exploring Legacy Power Save Methods with Managed Clients,” we took a deep dive into the realm of Legacy Power Save mechanisms. Specifically, we explored two distinct approaches: PS-Poll and Non-PS-Poll. These methods play a pivotal role in managing power consumption ESPECIALLY IN OLDER DEVICES connected to Wi-Fi networks, offering unique trade-offs between power efficiency and responsiveness.

Now, let’s shift our focus to the “WMM Power Save Test Using Managed Client” blog, where we delve into a technology designed to enhance data transmission efficiency in wireless networks. With this technology, client devices have the ability to conserve power by introducing pauses between data packets, while the access point intelligently buffers incoming data. Additionally, we’ll introduce you to WMM/U-APSD, an innovative feature that automates power-saving for unscheduled data. This is a refresh on an existing blog we had published 5 years ago (just an example of how long-lived & ubiquitous Wi-Fi remains in this fast-changing environment)

WMM Power Save


WMM-Power Save increases the efficiency and flexibility of data transmission. Specifically, the client device can doze between packets to save power, while the access point buffers downlink frames.


WMM Power Save



 U-APSD introduces the concept of automatic power save delivery for unscheduled data frames. This means that, in addition to scheduled wake-ups, devices can be notified when there is incoming data that requires immediate attention. For example, a VoIP call can trigger a device to wake up promptly to receive the voice packets.

     U-APSD is particularly useful for real-time applications like VoIP and video conferencing, where low latency and prompt response to incoming data are critical. By allowing unscheduled wake-ups for important traffic, U-APSD ensures that devices can conserve power while still providing an excellent user experience for time-sensitive applications.   

Check if AP Supports WMM Power Save:

  1. Check the U-APSD field from AP’s beacon.
  • Beacon -> Tagged Parameter -> WMM/WME -> WME QOS Info -> U-APSD


  • WMM Power Save also refers to U-APSD (Unscheduled Automatic Power Save Delivery), U-APSD is widely supported in all devices.

Steps for WMM Power Save: 

  1. Client and AP both should support WMM/QoS.


Power Save Mode 4


2. Association Request should be WMM Association, check packet for WMM: Information Element. 


Power Save Mode 5


3. For client WMM Power Save, check in Association Request frame -> WME -> WME QOS Info -> Set 4  AC’s [ VO, VI, BE, BK ] as 1 which means AC is delivery and trigger enabled.


Power Save Mode 6


4. After the Association process client sends one NULL Data frame to AP where PWR MGT as 1, this means the client is going to sleep.


Power Save Mode 7


5. When AP has buffer data, it indicates in Beacon frame, beacon frame has TIM [Traffic Indication Map]. Under TIM there is Partial Virtual Bitmap to indicate for which AID AP has buffer data.


Power Save Mode 8


6. AP starts sending buffer data one by one to the client.


Power Save Mode 9


7. Client sends ACK for each Data frame.


Power Save Mode 10



In conclusion, WMM Power Save, also known as U-APSD (Unscheduled Automatic Power Save Delivery), enhances the efficiency and flexibility of data transmission by allowing client devices to conserve power during periods of inactivity. It achieves this by enabling devices to enter a doze state between packets while the access point buffers downlink frames. U-APSD is particularly valuable for real-time applications like VoIP and video conferencing, as it ensures low latency and prompt responses to incoming data.


To utilize WMM Power Save/U-APSD effectively, both the client device and access point must support WMM/QoS. The client indicates its intention to use Power Save by setting four Access Categories (ACs) as ‘1’ in the Association Request frame, indicating that ACs are delivery and trigger enabled. After the Association process, the client sends a NULL Data Frame to the AP with PWR MGT set to 1, signifying its intention to enter a sleep state.


When the AP has data buffered for the client, it communicates this in the Beacon frame, including a Traffic Indication Map (TIM) with a Partial Virtual Bitmap to indicate which AIDs have buffered data. The AP then begins sending the buffered data to the client, with the client sending ACKs for each received Data frame.


In our upcoming blog (the final part of this series), we will explore the concept of Target Wake Time (TWT) Power Save with a Managed Client in more detail. Stay tuned for valuable insights and information on this topic. In case, you need further information or discussion on how Alethea Communications Technologies can help you validate the power save mechanisms in your Access Points, Routers or Client Devices, please contact and we will be happy to support you.