Recommendation X.25 is a specification of the interface between data terminal equipment (DTE) and data transmission equipment (DCE).
The ITU-T recommendation describes the synchronous operation of a packet-capable data terminal equipment on a public data network and covers both the timing and the transmission format. The first X.25 Recommendation was published in 1976 in the Orange Book; in the meantime, the Red Book was published in 1984, the Blue Book in 1988, and new ITU Recommendations in 1993.OSI Reference Model.
With X.25, virtual connections are operated between the connected terminal devices, which run on logical channels. There are permanent and switched virtual connections, so-called Permanent Virtual Circuits (PVC) and Switched Virtual Circuits (SVC). Up to 16 logical channel numbers can be formed at the DTE-DCE interface, resulting in 4,096 possible virtual connections for each DCE. Communication is handled by means of control and data packets.
X.25 layer 3 provides the application-oriented layers with connection establishment and termination of end system connections, data transport as well as resetting of a connection and, in special cases, fast signal transmission (interrupt). This layer defines the structure of the transmitted data packets. X.25 is still used in some countries, but it is increasingly being replaced by other technologies such as frame relay and ATM.