Networking Explained, 2nd Edition

Appendix E

X.25

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 stan-

dard, 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, ...