Inte
1-58705-001-3
C H A P T E R 17
Chapter Goals
• Discuss the history and development of the X.25 protocol.
• Describe the basic functions and components of X.25.
• Describe the frame formats of X.25.
X.25
Introduction
X.25 is an International Telecommunication Union–Telecommunication Standardization Sector (ITU-T)
protocol standard for WAN communications that defines how connections between user devices and
network devices are established and maintained. X.25 is designed to operate effectively regardless of the
type of systems connected to the network. It is typically used in the packet-switched networks (PSNs)
of common carriers, such as the telephone companies. Subscribers are charged based on their use of the
network. The development of the X.25 standard was initiated by the common carriers in the 1970s. At
that time, there was a need for WAN protocols capable of providing connectivity across public data
networks (PDNs). X.25 is now administered as an international standard by the ITU-T.
X.25 Devices and Protocol Operation
X.25 network devices fall into three general categories: data terminal equipment (DTE), data
circuit-terminating equipment (DCE), and packet-switching exchange (PSE). Data terminal equipment
devices are end systems that communicate across the X.25 network. They are usually terminals, personal
computers, or network hosts, and are located on the premises of individual subscribers. DCE devices are
communications devices, such as modems and packet switches, that provide the interface between DTE
devices and a PSE, and are generally located in the carrier’s facilities. PSEs are switches that compose
the bulk of the carrier’s network. They transfer data from one DTE device to another through the X.25
PSN. Figure 17-1 illustrates the relationships among the three types of X.25 network devices.
17-1
rnetworking Technologies Handbook
Chapter 17
X.25
X.25 Devices and Protocol Operation
Figure 17-1 DTEs, DCEs, and PSEs Make Up an X.25 Network
Packet Assembler/Disassembler
The packet