Several exchanges must occur for one router to connect to another using ISDN. To establish an ISDN call, the D channel is used between the router and the ISDN switch. Signal System 7 (SS7) signaling is used between the switches within the service provider network.

The D channel between the router and the ISDN switch is always up. Q.921 describes the ISDN data-link processes of LAPD, which functions like Layer 2 processes in the OSI reference model. The D channel is used for call control functions such as call setup, signaling, and termination. These functions are implemented in the Q.931 protocol. Q.931 specifies OSI reference model Layer 3 functions. The Q.931 standard recommends a network layer connection between the terminal endpoint and the local ISDN switch, but it does not impose an end-to-end recommendation. Because some ISDN switches were developed before Q.931 was standardized, the various ISDN providers and switch types can and do use various implementations of Q.931. Because switch types are not standard, routers must have commands in their configuration specifying the ISDN switch to which they are connecting.

The following sequence of events occurs during the establishment of a BRI or PRI call:

1. The D channel is used to send the called number to the local ISDN switch.
2. The local switch uses the SS7 signaling protocol to set up a path and pass the called number to the remote ISDN switch.
3. The remote ISDN switch signals the destination over the D channel.
4. The destination ISDN NT-1 device sends the remote ISDN switch a call-connect message.
5. The remote ISDN switch uses SS7 to send a call-connect message to the local switch.
6. The local ISDN switch connects one B channel end-to-end, leaving the other B channel available for a new conversation or data transfer. Both B channels can be used simultaneously.

Interactive Media Activity Drag and Drop: ISDN Functions When the student has completed this activity, the student will be able to correctly identify the ISDN establishment cycle.

Web Links Integrated Services Digital Network (ISDN) http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ito_doc/isdn.htm