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(protocol, standard)
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.
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The first international standard packet switching network developed in the early 1970s and published in 1976 by the CCITT (now ITU). X.25 was designed to become a worldwide public data network similar to the global telephone system for voice, but it never came to pass due to incompatibilities and the lack of interest within the U.S. It has been used primarily outside the U.S. for low speed applications (up to 56 Kbps) such as credit card verifications and automatic teller machine (ATM) and other financial transactions. It has also been used for signaling networks in first-generation cellular systems.

X.25 provides a connection-oriented technology for transmission over highly error-prone facilities, which were more common when it was first introduced. Error checking is performed at each node, which can slow overall throughput and renders X.25 incapable of handling real-time voice and video.

In the U.S., leased T1 lines were favored for internetworking offices together rather than public data networks. However, frame relay, which was modeled after X.25, has been successful as a public data network technology for meeting the high bandwidth demands of today's organizations. See packet switching, frame relay and SMDS.
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