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.
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