Two Layer 3 specifications are used for
ISDN signaling: ITU-T I.450 (also known as ITU-T Q.930) and ITU-T
I.451 (also known as ITU-T Q.931). Together, these protocols support
user-to-user, circuit-switched, and packet-switched connections. A
variety of call establishment, call termination, information and
miscellaneous messages are specified, including setup, connect,
release, user information, cancel, status and disconnect. The figure shows the typical stages of an
ISDN circuit-switched call.
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