MSU structure is not the same in LSL, HSL and ATM links.
I will show you how to calculate the average signalling speed in LSL:
MSU length is varaible and its length depends on the number of SIF/SIO fields.
the calculation is:
((no. of MSU*6)+(no. of SIF/SIO))*8 bits
then divide the number by the measurement duration in seconds.