System information

CHAPTER
Troubleshooting X.25 Connections 19-409
19
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 Telnet and 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 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 ITU Telecommunication Standardization Sector (ITU-T; formerly CCITT,
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.