5.2 How ISDN Relates to the OSI Reference Model
5.2.1 The ITU-T standards of the first three layers of ISDN
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.