service information octet (MSU) (SIO)

The Service Information Octet(SIO) is an 8 bit long data field in the Message Signal Unit( MSU).

Structure of the Message Signal Unit (MSU)

The SIO data field consists of a 4-bit subservice field and an equally long data field for the service indicator( SI).

The Subservice Field contains the message priority, the Service Subscriber Field( SSF) and the Network Indicator( NI). This data field indicates the protocol type at Message Transfer Part Level 4 (MPT4), such as Telephony User Part ( TUP), ISDN User Part( ISUP) and whether the protocol types are national or international standards.