It’s transparent because it’s just a “transport protocol”, meaning there’s not much room for improvement,IMO.
Some ppl here have been experimenting with longer T200 duration, and it seems to improve drop rate.
There is a mecanism in the LapDm : if the SACCH is not decoded succesfully in one occurence, then the timer T200 is started. At expiry, if SACCH are still not decoded, then a counter “n” is incremented by 1.
When n reaches the value set by N200, then the lapDm consider the session as “terminated”, which is counted as a “call drop cause radio” by the BSC.
This mechanism works independantly in UL and in DL, as far as I remember.
It works in parallel to the “typical” Radio Link Failure mechanism.