Layer 3 Call Setup

The following are the various messages exchanged in the course of call establishment in ISDN.  In the following example, there are three points where messages are sent and received; 1) the Caller, 2) the ISDN Switch, and 3) the Receiver.
 


 
 
 
 

  1. Caller sends a SETUP to the Switch.
  2. If the SETUP is OK, the switch sends a CALL PROCeeding to the Caller, and then a SETUP to the Receiver.
  3. The Receiver gets the SETUP. If it is OK, then it rings the phone and sends an ALERTING message to the Switch.
  4. The Switch forwards the ALERTING message to the Caller.
  5. When the receiver answers the call, is sends a CONNECT message to the Switch
  6. The Switch forwards the CONNECT message to the Caller.
  7. The Caller sends a CONNECT ACKnowledge message to the Switch
  8. The Switch forwards the CONNECT ACK message to the Receiver.
  9. Done. The connection is now up.