The development of the X.25 standard was initiated by the common carriers in the 1970s. This chapter covers the basic functions and components of X.25. 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.
Figure 17-1 illustrates the relationships between the three types of X.25 network devices. PADs are used when a DTE device, such as a character-mode terminal, is too simple to implement the full X.25 functionality. The PAD buffers data sent to or from the DTE device. It also assembles outgoing data into packets and forwards them to the DCE device. The PAD buffers, assembles, and disassembles data packets.