data link failure is a failure on the layer 2 (lapdm), which is caused by non-acknowledgment of 1 message. The message was repeated N200 times, waiting for the ack during T200 at every transmission.
So basically, it is a radio failure that is detected thanks to lapdm failures rather than radiolink timeout.
The value T200*N200 is usually longer than the time it takes to have a RadioLink Timeout, so it is rare to face a datalink failure before the radiolink failure, unless you have a high value of “radio link timeout”.
(other suggestions are welcomed !)