Information Technology Reference
In-Depth Information
19
CHAPTER
Troubleshooting X.25 Connections
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 TYMNET had achieved remarkable success, but it
was felt that protocol standardization would increase subscriptions 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, 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 ITU
Telecommunication Standardization Sector (ITU-T; formerly CCITT, the Consultative Committee for
International Telegraph and Telephone) is the ITU committee responsible for voice and data
communications. ITU-T members include the FCC, the European Postal Telephone and Telegraph
organizations, the common carriers, and many computer and data communications companies. As a
result, X.25 is truly a global standard.
X.25 Technology Basics
X.25 defines a telephone network for data communications. To begin communication, one computer
calls another to request a communication session. The called computer can accept or refuse the
connection. If the call is accepted, the two systems can begin full-duplex information transfer. Either
side can terminate the connection at any time.
The X.25 specification defines a point-to-point interaction between data terminal equipment (DTE) and
data circuit-terminating equipment (DCE). DTEs (terminals and hosts in the user's facilities) connect to
DCEs (modems, packet switches, and other ports into the PDN, generally located in the carrier's
facilities), which connect to packet-switching exchanges (PSEs, or switches) and other DCEs inside a
PSN and, ultimately, to another DTE. The relationship between the entities in an X.25 network is shown
in Figure 19-1.
