Layer 3 ISDN Session Establishment

The caller, switch, and receiver establish a Layer 3 ISDN connection as follows:

When a Layer 2 connection is initialized, the caller sends a SETUP message to the switch in the telephone network.
If the setup is message is valid, the switch responds with a call proceeding (CALL PROC) message to the caller and a SETUP message to the receiver.
When the receiver receives the SETUP message, it responds with an ALERTING message to the telephone switch.
This ALERTING message is then forwarded to the caller.
The receiver then accepts the connection by sending a CONNECT message to the switch.
The switch forwards the CONNECT message to the caller.
The caller responds with an acknowledgement message (CONNECT ACK).
When the CONNECT ACK message is received by the receiver, the ISDN call is set up and traffic can pass.