The CCITT, bearing in mind (a) the international user classes of service indicated in Recommendation X.1; (b) the overall user-to-user performance objectives; (c) the need to standardize the procedures for use over public synchronous data networks; (d) in the case of packet switching, the need to standardize several procedural levels.
ITU-T X.92-1988 history
1988ITU-T X.92-1988 HYPOTHETICAL REFERENCE CONNECTIONS FOR PUBLIC SYNCHRONOUS DATA NETWORKS