X.25 White Paper

Overview

In the 1970s, a set of protocols was needed to provide users with wide-area network (WAN) connectivity across public data networks (PDNs). PDNs such as TELENET and TYMNET had achieved remarkable success, but it was felt that protocol standardization would increase subscription to PDNs by providing improved equipment compatibility and lower cost. The result of the ensuing development effort was a group of protocols, the most popular of which is X.25. X.25 was developed by the common carriers (telephone companies, essentially) rather than any single commercial enterprise. The specification is therefore designed to work well regardless of a user’s system type or manufacturer. Users contract with the common carriers to use their packet-switched networks (PSNs) and are charged based on PSN use. Services offered (and charges levied) are regulated by the Federal Communications Commission (FCC). One of X.25’s unique attributes is its international nature. X.25 and related protocols are administered by an agency of the United Nations called the International Telecommunications Union (ITU). The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) (formerly CCITT) is the ITU committee responsible for voice and data communications.