The ISUP task maintains a database of circuits and circuit groups that are controlled by the application and keeps track of the state of each circuit. The initial characteristics of each circuit (group), such as the circuit identification code (CIC), direction, destination point code, and routing instructions are specified in the ISUP configuration file. The ISUP task reads the ISUP configuration file at startup.
For outgoing call setup requests, the ISUP task does not select a circuit for the connection. The application must specify the circuit to be connected.
For incoming calls, the ISUP task verifies that the circuit state and characteristics, such as bearer capability, are compatible with the incoming call request parameters before passing the incoming call indication to the application.
For both incoming and outgoing calls, the ISUP task provides all necessary connection timers. The ISUP task notifies both the application and the far exchange with necessary indications, such as connection clearing, when critical timers expire.
The ISUP task:
Provides circuit supervision for the duration of the connection.
Adjusts the circuit state as needed based on requests from the application and ISUP messages received from the far exchange.
Provides connect and disconnect timing.
Handles circuit (group) blocking and unblocking, updating the state of the affected circuits as needed.
Detects protocol errors on behalf of the application.