ISDN connection establishment

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The establishment of a telephone connection within an ISDN takes the following steps:

Connection establishment
  1. The calling user ( A subscriber ) usually causes his telephone A1 to send a SETUP message to the exchange by lifting the receiver .
  2. If a B-channel is free, it responds with a SETUP ACK message. The phone then generates the dial tone .
  3. The selected by the A subscriber number is sent each either one digit at a INFO message or as a block (in the so-called block choice) from the phone via the exchange towards target exchange.
  4. If a valid number has been dialed, the exchange sends a CALL PROCEEDING message to the telephone. This then does not allow any further dialing digits to be entered.
  5. The destination exchange sends a SETUP message to all of the terminals connected to the called user ( B subscriber ).
  6. Every terminal that has been programmed to accept calls for the specified destination number sends an ALERT message and begins to ring.
  7. The destination exchange sends the ALERT message back through the network to the A-subscriber's telephone. This then generates the ring tone .
  8. When the handset is lifted, the destination telephone B2 sends a CONNECT message to the destination exchange.
  9. The destination exchange forwards this message through the network and confirms it with a CONN ACK message to telephone B2 .
  10. It sends a REL message to all other terminals that are still ringing, which is confirmed by REL COM. The terminals then stop ringing.
  11. Telephone A1 confirms receipt of the CONNECT message with a CONN ACK message.
  12. The voice or data connection is established.


  • Otfried Georg: Telecommunication technology. A practical introduction. Springer Verlag, Berlin / Heidelberg 1996, ISBN 3-540-61381-1 .
  • Hubert Zitt: ISDN & DSL for PC and telephone. Verlag Markt + Technik, Munich 2005, ISBN 3-8272-6987-3 .
  • Peter Bocker: ISDN. Digital networks for voice, text, data, video and multimedia communication. 4th edition. Springer Verlag, Berlin / Heidelberg 1997, ISBN 3-642-64360-4 .
  • Volker Jung, Hans-Jürgen Warnecke (Hrsg.): Handbook for telecommunications . Springer Verlag, Berlin / Heidelberg 1998, ISBN 3-642-97703-0 .
  • Thomas Joos: PC networks. Verlag Markt + Technik, Munich 2008, ISBN 978-3-8272-4386-7 .

See also

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