Increase your cell phone battery life with a small change in the Wi-Fi network

This may sound silly, but yes, you can really improve your cell phone battery life with a small change in the access point. The change has no drawbacks and is easy to make. It has even more impact in your home Wi-Fi where your cell phone spends most of its time in sleep mode.

Buried deep in the advanced settings of the Wi-Fi access point there is a setting with a friendly title like DTIM Interval or DTIM Period. The default setting is typically one. Change it to three, five or slightly larger, but don’t go over ten. Often you can set it separately for 2.4GHz and 5GHz, but use the same value for both. This is the short answer. Read on to find out what this is all about.

Background

To extend the battery life all cell phones and tablets spend most of their time in different sleep modes. Switching off radio transmitters is one of the most efficient ways to save power since the transmitters are very power hungry. Even if the display is on and there is a game running the Wi-Fi radio may be off. The Wi-Fi radio will be powered on only when the user browses the net or some background app checks for messages, then it will go off again. As a matter of fact, the radio is actually switched off multiple times during browsing. It is switched on only momentarily as needed. It really makes a difference in the battery life.

Most network connections are opened from the client device. For example the device will periodically check for emails. In some apps the initiative is on the server side: the server will send a message that the device should perform some action. For example a VoIP call is coming in. If the Wi-Fi radio is off there is now way to receive such messages.

Broadcast and multicast messages are similar. The server will send a single message that is addressed to all or a group of devices. However, the server doesn’t know which devices are sleeping and which aren’t. Somehow the sleeping devices should also receive the message.

Wi-Fi access points send a beacon ten times a second. In the beacon there is a map of client devices which have buffered packets addressed to them. All devices wake up every tenth of a second to check if they should start receiving data. These wake-ups are very short and the display is not powered up, but still they consume power.

The DTIM Interval setting controls which beacons contain this information about upcoming data packets. If you set it to three then only every third beacon will contain the info. This means the dozing devices can sleep over two beacons and only wake up on the third. That means they will wake up only three times a second. With larger DTIM values they will sleep longer. For example with the value of 5 they will wake up just twice a second and with 10 just once every second.

So what does DTIM stand for? It is an acronym for Delivery Traffic Indication Message (or Map). The Message just Indicates that there is some Traffic to be Delivered to Mapped devices. The setting this article is about is DTIM Interval or Period, which is the multiplier, but it is commonly referred to as DTIM value.

Apple’s solution

Apple iPhones won’t wake up more often than to every third beacon, even if the DTIM value for the network is one. Apple’s customers are very concerned about their battery life and the delay of a few tenths of a second is a small price to pay. Apple has made the decision for its customers.

What will happen to data packets destined to an iPhone if the DTIM value is one but the iPhone acts like it were three? Nothing much. Unicast packets are buffered at the access point until the iPhone wakes up. The buffering will take up some memory space in the AP but no data is lost. Some broadcast and multicast packets will be lost, but most of the time they are not important for the dozing device. By setting the DTIM value to at least three you can avoid the loss of data and give Android devices the same advantage of better battery life.

Recommendations

During daytime phones are typically used actively, which will consume much more energy than waking up from sleep. At home the devices spend much more time sleeping. Or they would, if they didn’t wake up ten times a second. That’s why you should increase the DTIM value in your home Wi-Fi. The larger the value the less the battery will drain during the night.

Are there any drawbacks to increasing the DTIM value? An incoming VoIP call alarm may be delayed by a fraction of a second – that shouldn’t matter. Broadcasts and multicasts are buffered at the AP until they are delivered. The AP memory is limited and in theory the buffer could grow by a hundred megabytes every second. That’s why ten is a good rule of thumb for the upper limit of DTIM. Many APs won’t even accept larger values. Of course you can test and see if you notice any drawbacks.

There are consumer grade Wi-Fi APs that won’t let you adjust the DTIM Interval, but most APs appear to support it. It is hidden amongst the scary “advanced settings” but it is really safe to modify. You can always go back if you need. If you can’t find the setting try searching the web for your AP model name and the word DTIM.

Another way to decrease battery consumption during the night is to place the phone as close to the AP as possible. The phone will use lower transmit power level which will save energy. Even a small change will help. Just by leaving the phone on the other side of the bed is an improvement, if it is closer to the AP.

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