Power save functionality has been gaining importance since the introduction of Internet of Things (IoT) devices connected over Wi-Fi. As the demand for higher power efficiency & in turn longer battery life has increased, new approaches have been identified to address this requirement.
In a previous blog that we published during the early days of 802.11ac (Wi-Fi 5), we have seen the details of the Unscheduled Automatic Power Save Delivery (U-APSD). Now, let us see two distinct mechanisms for managing power consumption in Wi-Fi networks: PS-Poll and Non-PS-Poll.
Legacy Power Save
In Legacy power-save mode, a STA sets the Power Management bit in a Null frame to indicate to the AP that it is in power save mode. An STA in power save mode sends a QoS Null or QoS Data frame to trigger the AP to send queued frames.
There are two types of legacy power save:
- Power save Poll [PS Poll]
- Non Power save Poll [Non PS Poll]
Note: Clients can support either one of the two legacy power save mechanisms at one time.
PS-Poll Mechanism
As the name suggests PS POLL stands for Power Save Polling. The Access point uses the TIM information element to indicate to the station that there is unicast data buffered for the WLAN station to the AP.
In PS Poll mode, the device may wake up at intervals to check for incoming data or to initiate data transmission, which can result in lower power consumption compared to the Non PS Poll mode.
Steps for PS Poll Mechanism
Assumptions:
- One client/station is connected to AP
- Client supports PS Poll mechanism from legacy power save.
Steps:
- After the Association Response, the client sends one Null Data packet with power management bit 1 to AP and AP replies back with an ACK.
This indicates that the client is going to sleep and AP agrees the same.
2. When the AP has buffer data, it signals in the Beacon frame that the beacon frame has the TIM [Traffic Indication Map]. There is a Partial Virtual Bitmap under TIM that indicates which AID AP holds buffer data.
3. Client sends PS-Poll frame to AP & AP sends ACK for PS-Poll.
4. Client is ready to receive buffer data, AP start’s sending buffer data to client.
5. AP removes buffer data indication from beacon and Partial Virtual Bitmap will be 0 which indicates AP does not have any buffer data to send.
Non PS Poll Mechanism
The Non PS Poll mechanism refers to situations where a device does not use the PS Poll mechanism to wake up from its power saving mode and instead relies on other mechanisms to wake up and communicate with the access point.
In Non PS Poll mode, the device may wake up more frequently to check for incoming data or to initiate data transmission, which can result in higher power consumption compared to the PS Poll mode. However, it provides more flexibility and responsiveness for the device to communicate when needed.
Steps for Non PS Poll Mechanism
Assumptions:
- One client/station is connected to AP
- Client supports Non PS Poll mechanism from legacy power save.
Steps:
- After the Association Response, the client sends one Null Data packet with power management bit 1 to AP and AP replies back with an ACK.
2. 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.
3. As Client supports Non PS-Poll mechanism, client sends NULL frame with PM=0 that means client is in awake state.
4. Now AP sends all buffer data one by one without expecting any frames like PS Poll from the client.
5. As long as More Data bit is set 1 client should be in awake state. Once More Data bit is set 0, then client sends NULL frame to AP indicating that client is going to sleep ie.. going to power save mode.
6. AP removes AID from Partial Virtual Bitmap in Beacon.
Conclusion:
We have delved into the world of Legacy Power Save mechanisms, exploring two distinct approaches: PS Poll and Non PS Poll. These methods play a crucial role in managing power consumption in devices connected to Wi-Fi networks, offering different trade-offs between power efficiency and responsiveness.
The PS Poll mechanism allows devices to periodically wake up to check for data, leading to lower power consumption but maintaining a certain level of responsiveness. On the other hand, the Non PS Poll mechanism provides greater flexibility and responsiveness but at the cost of higher power usage due to more frequent wake-ups.
In our upcoming blog, we will explore the WMM Power Save Test using 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 sales@aletheatech.com and we will be happy to support you
