In a wireless network, stations (mobile devices) usually go into a power save mode, when this happens the configuration of the power save mode attributes in the Station and the Access Point have to be carefully tuned in order to have a better performance specially when some VoIP application is in place.
In this article we will discuss about 2 power save attributes, perhaps the most relevant but unfortunately these attributes are highly misunderstood by the majority. I’m talking about the Station Listen Interval and the SSID (AP) Delivery Traffic Indication Message (DTIM)
What is the Listen Interval and the DTIM?
Listen Interval indicates how long the station will be “sleeping” without listen to any Beacon transmitted by the AP’s when the station enter in power save mode, those beacons includes the DTIM information. The relevant point here is that if we don’t tuned properly these 2 attributes it could happen that when the AP transmit the DTIM letting the Station know that there is some information in the buffers to be transmitted, the Station could be not listening because the Listen Interval is so high for instance.
DTIM is the Delivery Traffic Indication Message that is configured in the SSID and indicates how often in terms of number of beacons intervals the AP will inform the Sleeping Stations that there is some traffic in the buffers to be transmitted to those specific stations.
These attributes use the same measure unit which is the number of Beacons Intervals. The default beacon interval for the majority of the WIFI vendors is 0.1024 seconds or 102.4ms and this is very important to know because it is a good practice to make these 2 attributes fit in time. Let’s see 2 examples below, the first one is an example how a bad DTIM configuration could impact the power save mode operation of a mobile device tripling the time needed to be aware that there is some buffered information to be processed. The second example shows how a good DTIM configuration reduce that time to the Listen Interval value, please keep in mind that we can configure and adjust the DTIM in the SSID but we can’t change the Listen Interval coming in the mobile device OS. Usually the Listen Interval is an even value like 10, 14 or 20 therefore it is recommended to have an even DTIM value or some odd value that ensure that the DTIM will be transmitted when the next Listen Interval allow the Station to wake and listen for beacons.
Example 1, DTIM and Listen Interval are not properly tuned to match as quick as possible, instead of that the Station will need to run over 3 listen cycles to be able to listen the DTIM information, whch means that instead of receive the information every 2 seconds it will take 6 seconds to be aware of what is going on during the sleep mode time. Example 2, DTIM and Listen Interval are properly tuned to match as quick as possible, that means that every Station Listen cycle (2 seconds in this example) it will be fully aware if there is some buffered information to be processed.
Listen Interval and DTIM in a 802.11 frame capture
You will find the Listen Interval in every Association Request indicating to the network how often the station will wake to listen for beacons with a DTIM indicating if there is some traffic awaiting to be delivered or not to this particular station.
Listen Interval equal to 10 Beacon Intervals expressed in hexadecimal 0x000a. This means that the station will wake to listen for new beacons every 10 beacon intervals or 1 second
You will find the DTIM in the Beacons and Probe Responses
DTIM equal 3 in a Beacon indicating that every 3 beacon intervals the AP will transmit a DTIM which means that every 0.3 seconds the AP will transmit a DTIM but not necessarily will reach the station in a wake mode based on the value of the Listen Interval,
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