The following sequence of diagrams shows the process when a user requests a call through an ISDN network.

• The SETUP is a request to create a connection by the user on the left to a user on the right.  The SETUP message usually includes at least the following information:
  • The called number (the destination number)
  • The call type (Bearer Capabilities)
  • It may also include other information, such as the calling number (the origin of the call).
• The network chooses a B channel for this half of the B channel and uses the SETUP_ACK to tell the user which B channel it is.
• The network sends the originating user a SETUP_ACK. This only acknowledges the SETUP. It carries no information about the outcome of the call.