Automatically adjust LAPDm timer values with 'fn-advance' parameter
Currently we use fn-advance=20 by default, so the burst scheduler is working 20 TDMA frames in advance. This advance gives the transceiver some time to receive a burst, process it (i.e. modulate), and send to the Tx buffer of SDR. As a side effect, this increases the delay between UL and DL: 20 TDMA frames => 92.3ms of delay.
Higher fn-advance values cause a longer delay, resulting in LAPDm frame retransmissions.
OsmoBTS should automatically adjust LAPDm timer values with the configured 'fn-advance' value.