In 1976, the Consultative Committee for International Telegraph and Telephone (CCITT) formally approved "Recommendation X.25: Interface Between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) for Terminals Operating in the Packet Mode on Public Data Networks." This standard, which has been periodically revised throughout the 1980s and 1990s, corresponds (although not exactly) to OSI's first three layers. In the original standard, X.25 referred to these layers as levels, namely, level 1, level 2, and level 3. This terminology was discontinued in later revisions of the standard, but it is still occasionally used in the literature today. Although many people refer to it as one, X.25 is not a network. Instead, it is a formal set of specifications for connecting data terminal equipment (e.g., a computer or a terminal) to a public packet-switched public data network (see Figure E.1). The most recent version of this protocol, adopted in 1996, also specifies how this connection is established when access is made by a dedicated circuit. X.25 provides data transmission speeds up to 2 Mbps and provides error-free transmissions. X.25 is an international standard for data communications in wide area networking (it does not support voice communications, though) and is extremely robust in transmitting data over noisy lines. It is also one of the oldest WAN protocols.

Figure E.1: X.25 is a set of specifications that addresses the interface between the user's equipment and the network. In X.25 terminology, the user's equipment is called data terminal equipment (DTE) and typically comprises a...