5.1 ISDN Architecture
5.1.3 BRI call processing

The Figure shows the sequence of events that occur during the establishment of a BRI call.

  1. When the call is initiated, the D channel comes up. The called number is sent to the local ISDN switch.

  2. The local switch uses the Signaling System 7 (SS7) signaling protocols to set up a path and pass the called number to the terminating ISDN switch.

  3. The far-end switch brings up the D channel to the destination. The D channel is used for call setup, signaling, and call termination, which are the call-control functions.

  4. When the terminating end answers, the B channel is connected end to end. A B channel carries the conversation or data. Both B channels can be used simultaneously to the same or different destinations.

The maximum length of most ISDN local loops in North America is about 18,000 feet (5.5 kilometers [km]), using standard plain old telephone service (POTS) wiring. ISDN is a local-loop technology. After the ISDN switch processes the call, the communication uses the SS7 infrastructure to reach its destination.