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How do devices save power on a Wi-Fi network? Here are the four main methods and how they work.
Key takeaways:
- Four methods for power management in client devices:
- PS-polling in legacy 802.11 a/b/g networks
- WMM unscheduled automatic power save delivery in 802.11n (U-APSD)
- VHT TXOP power save in 802.11ac (Wi-Fi 5)
- TWT in 802.11ax (Wi-Fi 6)
Power management is an important aspect of Wi-Fi management. It ensures that a radio isn't constantly active and can enter standby mode or go to sleep instead of using up energy unnecessarily. Power-saving modes allow Wi-Fi devices to stay connected to an access point (AP) while reducing power consumption.
So how has client radio power management changed, what are the most effective modes, and how do they impact connectivity and performance? Here is an overview of the four main methods corresponding to different Wi-Fi standards.
1. Legacy 802.11a/b/g: PS-polling
Legacy power management is still very much in use. It relies on something called power-save poll power management (PS-poll). PS-poll is the only power-saving mode specified in legacy 802.11 networks, and it's only available in the infrastructure mode network configuration.
The device always stays connected to the AP in the PS-poll mode. It periodically wakes up to hear the DTIM (delivery traffic indication map) elements sent by an access point (AP) in its beacon transmissions. This method may be old, but it can still save devices lots of power.
On the downside, the PS-poll mode may lead to a couple of hundred milliseconds of latency. So, it’s not great for VoIP use. The PS-supported APs have to buffer the packets, which takes extra time.
When a station is ready to go to sleep, it will send a null data frame with the power management bit set to the AP. It’s telling the AP, “I'm going to sleep; let me know if you have any frames waiting for me.” The device periodically wakes up, receives beacons, and examines the traffic indication map (TIM) element.
If the bit representing the station’s association ID (AID) in the partial virtual bit map is set, the AP buffers frames for the station. When the station associates with the AP, it gets an AID from 1 to 2,007, and it checks the bit that represents that AID.
If the station wakes up and sees that the bit for its AID is set, it will send a PS-poll to the AP, and the AP sends a frame that will either have more data bit set or not. If it is set, it’s going to continue to poll and retrieve those frames. If it’s not set, it will go back to sleep. That’s the basic logic behind legacy power management.
It can be hard to assess how much power can be saved with this method. However, one hardware manufacturer reportedly saw about 95% power savings, which is excellent.
Nevertheless, the expense with this method is a lot of latency. The process of the station waking up and receiving frames will slow things down. The device has to wait for the TIM element in the beacon, so the shortest sleep interval would be about 100 milliseconds. That presents a problem for VoIP and similar applications.
2. 802.11n: WMM automatic power save delivery
The most commonly used power-saving mechanism is the Wi-Fi multimedia (WMM) automatic power save delivery. It comes in both unscheduled and scheduled options. The unscheduled version is referred to as unscheduled automatic power save delivery, or U-APSD, and is implemented widely. This method simplifies things quite a bit compared to PS-polling.
The station still uses the power management bit to indicate that it’s going to sleep, just like the PS-poll power management mode. 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. The AP transmits buffered frames with the same quality of service values as the triggering frame immediately after acknowledging the station's transmission. No more waiting around—the station sends a frame, and the AP understands that it’s awake and starts sending back to it.
This greatly reduces latency because you don’t have to wait for a beacon. It works really well for VoIP applications, which usually transmit a frame every 20 milliseconds. This method works very well with applications that need to transmit periodically at less than the beacon interval.
How much power does it save? Unfortunately, not much documentation exists, but one source suggested that savings are about 75% for VoIP applications.
3. 802.11ac (Wi-Fi 5): VHT TXOP power save
Wi-Fi 5 introduced VHT TXOP Power Save, which means “very high throughput transmit opportunity” power save.
The bits are actually in the preamble part of the frame. As with PS-polling, there is an AID, but the algorithm to generate it is different. The station can check during the reception of the preamble whether or not it is a recipient of the frame. Since 802.11ac supports MU-MIMO (multi-user, multiple-input, multiple-output), even though it is rarely used, there can be multiple data recipients in that frame. If a station is not a receiver of the frame indicated in the VHT preamble, it can look at the duration field and shut down its receiver for the rest of the transmission.
This power management technique allows a station to receive the PPDU (physical layer protocol data unit) preamble, check it for its AID, and, if the station is not a recipient of the frame, turn off its receiver for the rest of the transmission.
This method is very simple. Essentially, the device says, “When another device is talking, and they aren't talking to me, I can just go to sleep.” It just makes sense.
4. 802.11ax (Wi-Fi 6): TWT
This latest Wi-Fi standard introduced target wake-up time (TWT). TWT is meant for low-power, long-sleep-time IoT-type devices. For example, if the device is some sort of meter that needs to report once or twice a day, this power-saving method is ideal.
TWT enables very long sleep times and requires negotiating with the AP to schedule a wake-up time. The AP can schedule its clients for non-overlapping times. So, there won’t be anyone else talking on the network when the client wakes up at the scheduled time.
There are three types of TWT:
- Individual: The AP and the station negotiate a wake-up time for the individual client. The client proposes a time, and the AP can agree or offer an alternate time.
- Broadcast: Best for multicasting. The AP will announce a wake-up time to multiple clients so they can all wake up to receive information like multicast.
- Opportunistic: The AP announces a wake-up time, and all the clients in a group wake up at that time. They will have to go through normal contention for the medium, as opposed to individual, where the AP schedules clients for individual wake-up times.
A significant distinction with TWT: where the previous power-save mechanisms gave you microseconds or milliseconds of sleep time to power down the radio, TWT is designed for IoT devices that might want to sleep for minutes, hours, or even as long as a full day.
Thus, it’s really designed for a narrowly defined class of IoT devices, like sensors, that just need to send and receive data infrequently on the network and are much better off preserving their batteries. TWT creates an immense opportunity for power savings when you can turn off the radio for hours at a time.
Power management is just one aspect of managing networks effectively
There are many ways to set up a large Wi-Fi network, and power management is an important piece of the puzzle that ensures devices aren’t unnecessarily wasting energy. All of the methods described here are still being used by today’s network engineers.
But many settings and tools are involved in effective WLAN management, and it’s impossible to make key network decisions without enough data. Common connectivity and performance issues won’t be solved for good without the right insights that identify root causes. But 7SIGNAL provides this visibility.
Our wireless network monitoring tools examine performance at the client level, allowing you to understand what's going on 24/7. Network managers can address problems in real-time and implement quick — and often proactive — solutions.
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