|  | 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.