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.