ISDN User Part - Message Format

Message Format

The Signalling Information Field (SIF) for all ISUP Message Signal Units (MSU) contain the following components:

• Routing Label
• Circuit Identification Code
• Message Type
• Mandatory Fixed Part
• Mandatory Variable Part
• Optional Part

The Routing Label indicates the Point Codes of the originating and destination nodes in the network; it also includes the Signalling Link Selection field that is used to select between the multiple routes an MSU could take between two nodes.

The Circuit Identification Code is used to specify which trunk between two switches is used to carry a particular call. Note that some versions of ANSI ISUP permit a CIC with 14 significant bits instead of the 12 that are shown.

The Message Type indicates the ISUP message type. The presence and form of the remaining 3 components are determined by this message type. The Mandatory fixed part, when present, contains the mandatory, fixed-length parameters associated with the message type. The Mandatory variable part, when present, contains the mandatory, variable-length parameters associated with the message type. The Optional part, when present, contains the optional parameters permitted to be included in the message type.

When sent using the services of the Signalling Connection Control Part, ISUP messages passed to SCCP in the User Data parameter (NSDU) consist of only the last 4 components (Message Type, Mandatory fixed part, Mandatory variable part, Optional part""). The routing label and circuit identification code are not included in the user data passed to SCCP.