Sleeping, WiFi clients
DTIM stands for Delivery Traffic Indication Map. This is an information element inside the beacon that tells the associated client if the AP has Multicast traffic or Unicast traffic for the client.
TIM stands for Traffic Indication Map. This is an informational element inside the beacon that tells the associated client if the AP has any unicast traffic for the client.
APs send out Beacons to advertise the networks and the capabilities of these networks. Any wireless device will read these beacons and display the SSID of each network. These beacons are sent out every 102ms or so. A Beacon has either a TIM or a DTIM information element in the beacon. When the AP has unicast packets for a station the AP will send a beacon with TIM informational element in it. The Beacon will list the AID of that client. When the client wakes up it will read these beacons and send a (Null Data Frame ???) asking for these packets. The AP will send down the packets and set more flag (see screenshot) telling the client the AP has more data. The client will request the data the AP has, and this exchange will persist as long as the AP has data.
When the AP has multicast packets for a station the AP will send a beacon with DTIM information in it and the AID of the station. When the APs tell the station, it has Multicast packets it sends those packets out ASAP even if the station is asleep. The AP will hold Unicast packets, but it will not hold multicast packets longer than the DTIM period. The AP must dump these packets as soon as the DTIM expires.
When the DTIM is set to 1 the beacons with DTIM Informational Element will be sent out every beacon. If you set the DTIM value to 2 the Multicast packets will be sent out every 2 beacons. If you rely on Multicast to deliver audio and your DTIM is set to high you will hear choppy audio during Multicast sessions.
Some device manufacturers require a higher DTIM setting so their devices can sleep more often
This begs the question “Does a Lower DTIM make the device sleep less often?”
The obvious answer is yes. Since all devices must (per the IEEE) wake up at each DTIM Beacon. The less obvious answer is it depends (“It depends” was used with permission and all applicable fees and homage were paid to Sam Clements for use of this TM saying). Most devices will sleep longer than the DTIM period. Vocera badges for instance will sleep for 500ms. This means they may miss the 1st few (up to 4) Multicast packets. It’s tough to say how often this happens since it will be tough to tell the difference between 100ms and 500ms as the badge is waking up.
Other device manufactures are the same. Ben Miller from @sniffwifi has an excellent blog called Go to Sleep, Go to sleep, Got to sleep little iPhone.
Ben Miller’s Blog Go To Sleep, Go To Sleep, Go To Sleep Little iPhone