MSU uses principles from HDLC.
Layer 3 frames are conveyed by the MSU. Layer 2 uses FISU and LSSU.
You will need start / stop flags (8 bits), BIB, FSN, FIB, LI, CK, SIO and the key SIF “envelope”
Depending on the messages the SIF size can vary. The SIF is also related to call control,
network management, or databases query/response. SIF varies from 16 to 2176 bits.
Spent a few years with a K1198 decoding and testing SS7 (in partucular, NUP / BTNR_167)
CO / NCR signalling systems.