TABLE OF CONTENTS The ISUP protocol uses message types to indicate the type of message being carried as well as the format of the message. Each message type has a distinct format that has mandatory and optional parameters (Figure 8.10). The parameters depend on the message type.
As in the TCAP protocol, the ISUP uses mandatory fixed parts and mandatory variables. These are parameters that always must exist, depending on the type of ISUP message. Again, the parameters used depend on the message type.
The circuit identification code (CIC) identifies the circuit that is being set up or released (Figure 8.11). The CIC may be a voice trunk or any other transmission medium in the PSTN.
Currently, there are no defined standards for allocating circuit identifiers. These are determined by an agreement between the telephone companies. The CIC is provided to the originator of the ISUP message (SSP) by the end switch. The end switch may be incorporated into the SSP because many of these systems are fully integrated. This means that a voice subsystem provides the switching functionality for the voice circuits, whereas the SS7 subsystem provides all the circuit control.
