MTP level 2 frames
MSU (Message Signal Unit):
Contains signalling messages (User Part SIO)
The received frame is MSU if LI
> 2 (number of octets)
LSSU (Link Status Signal Unit):
Contains signalling messages for link supervision
The received frame is LSSU if LI
= 1 or 2
FISU (Fill-In Signal Unit):
Can be used to monitor quality of signalling link
The received frame is FISU if LI
= 0
