MTP level 2 frame formats
F
CK
SIF
SIO
LI
Control
F
F
CK
SF
LI
Control
F
F
CK
LI
Control
F
MSU (Message Signal Unit)
LSSU (Link Status Signal Unit)
FISU (Fill-In Signal Unit)
Level 3 signalling message
 Network:
 National
 International
 User part:
 TUP
 ISUP
 SCCP
 Network   
    management