5.2 ISDN Protocol Layers
5.2.5 ISDN call teardown
Similar to call setup, the call teardown request is not an end-to-end function, but instead is processed by the switch. The release procedures are based on a three-message approach:
  • Release
  • Released
  • Release complete

The release message is transmitted through the network as quickly as possible. The Figure assumes that the called party is generating the release. The process is triggered by a disconnect message on the D channel, between the calling and called parties. After receipt of this message, the exchange immediately starts the release of the switch path that supports the B-channel circuit, and sends a release message to the succeeding exchange at the same time. The message is passed through the network from all intermediate exchanges to the terminating exchange.

In the Figure, all the exchanges are characterized at the telco network box. The release message is intended to inform the exchanges involved in the call as quickly as possible that the call-related circuit connections are to be released. As the involved exchanges release the call, a released message is eventually transmitted to the terminating exchange, and this transmission causes the following actions:

  • It issues a disconnect message to the calling party.
  • It starts a timer T12 to ensure receipt of a released message.
  • It connects the switched path.
  • When a released message is received from the preceding exchange, it returns a release complete message to the preceding exchange.

The T timers are used through the exchanges to ensure that the exchange will repeat the release message if a release complete message is not received within the timer period. If not acknowledged, a reset circuit occurs.