ISUP configuration overview

The ISUP (ISDN User Part) layer provides the interface for applications to establish, maintain, and clear circuit switched connections through the SS7 network in accordance with the following recommendations:

The ISUP layer is also responsible for circuit group management such as blocking, unblocking, and resetting circuits and circuit groups.

ISUP implements services through the configuration of general parameters and the following entities:

Entity

Description

Circuits

Physical bearer circuits controlled by the ISUP layer. Circuits are identified by both a circuit index and a circuit identification code (CIC). The circuit index is a number unique across all circuits configured on a particular TX board. This number has only local significance. It is used between the ISUP layer and the local call processing application to identify a particular circuit.

The CIC (usually called the kick, or kick code) is used between signaling points (the ISUP layer and the far exchange that terminates the other end of the circuit) to uniquely identify a particular circuit. The CIC must be configured at both ends of the circuit to identify the exact same bearer facility (the same T1 span and timeslot). CICs need not be unique across circuits that terminate on different far exchanges.

Circuits are specified in the ISUP configuration file in groups. A group is one or more circuits with contiguous circuit indexes and contiguous CICs that terminate on the same far exchange and have common characteristics. A single circuit group is frequently used to represent all the timeslots on a single T1 or E1 span. When defining a circuit group, only the circuit index and CIC of the first circuit in the group, along with the number of circuits in the group, are specified. The ISUP layer derives the circuit index and CIC for subsequent circuits since they are considered to be contiguous. The starting circuit index and starting CIC for a group need not be the same value.

User service access points (SAPs)

Define the interface between the ISUP layer and the user applications.

Note: ISUP supports only a single user application; configure only one user SAP.

Network service access points (NSAPs)

Define the interface between the ISUP layer and the MTP layer. NSAPs identify the MTP network SAP to be used by the ISUP layer, allowing multiple user parts (for example, ISUP and SCCP) to share access to the MTP layer services.


The following illustration shows the concept of user SAPs and network SAPs: