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-T Q.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
I don't know how to remember stuff that is sooooo anally uninteresting (AU)! I've done well so far, but ISDN seem to be a study of Terminology Without Reason (TRW), according to Vexen standard 2002 (Circa 2004, beta release known as 2003ra). TRWb (Also known as TRW-AU1) was adopted by LJ on 2002 Oct 29 RFC, however Cisco delayed release until 2005 due to a lack of TLAs, which led to the widespread adoption of No-one's TRWB-232 standard, except in Southern UK where it is known as ISTR-Nothing v29.