X.25 is an ITU-T standard protocol suite for the DTE-DCE interface in a packet switched network, approved by ISO. It defines standard physical layer, data link layer and network layers (layers 1 through 3) of the OSI model. It was developed to describe how data passes into and out of public data communications networks. It is connection oriented and has rigorous error correction.

X.25 networks are still in use throughout the world although in decline, being largely supplanted by newer layer 2 technologies such as Frame relay, ISDN and ATM and the ubiquitous layer 3 Internet Protocol.

Document: ISO 8208.

based on FOLDOC