ISDN utilizes a suite of ITU-T
standards spanning the physical, data link, and network layers of the
OSI reference model:
- The physical layer -- The ISDN Basic
Rate Interface (BRI) physical-layer specification is defined in
ITU-T I.430. The ISDN Primary Rate Interface (PRI) physical-layer
specification is defined in ITU-T I.431.
- The data link layer -- The ISDN data
link-layer specification is based on LAPD and is formally
specified in ITU-T Q.920, ITU-T Q.921, ITU-TQ.922 and ITU-T
Q.923.
- The ISDN network layer -- The ISDN network
layer is defined in ITU-T Q.930 (also known as I.450) and
ITU-T Q.931 (also known as I.451). Together these
two standards specify user-to-user, circuit-switched, and
packet-switched connections.
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