An ITU-T standard protocol suite for the DTE-DCE interface in a packet-switched network, approved by ISO. X.25 defines standard physical layer, data link layer and network layers (layers 1 through 3). It was developed to describe how data passes into and out of public data communications networks. X.25 networks are in use throughout the world.Document: ISO 8208. Several other ITU-T recommendations are relevant to packet switching: X.3, X.28, X.29, X.75.
Last updated: 1996-08-10
X.224 ♦ X.225 ♦ X.226 ♦ X.227 ♦ X.229 ♦ X.25 ♦ X.28 ♦ X.29 ♦ X.3 ♦ X3J16 ♦ X3T10
Try this search on Wikipedia, Wiktionary, Google, OneLook.