This is confirmed by looking at the source of their dissector.
The length can go up to 273 bytes apparently (again, according
to the source of their dissector).
For the MSC<->BSC connection we are going to use the same header
as used from BTS<->BSC but we are not having an E1-Link, a gsm network
or a gsm_bts available and can not use this part of the code.