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.