An ITU-Tstandardprotocol 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}. (1996-08-10)