|  | 5.2 | How ISDN Relates to the OSI Reference Model
| 5.2.4 | The ISDN network layer
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.