Introduction to Tandem X.25 Capabilities

Multi-Vendor International Standard

What is X.25?

065307 Tandem Computers Incorporated 1–3

as an international standard of data communications. It is customary for networks

and computer manufacturers to indicate which year(s) of the X.25 recommendation

they support. Tandem supports the 1980 and 1984 standards.

The X.25 standard was developed primarily for public networks, but it is also used for

connecting computer systems to private packet-switching data networks.

Packet-switching is the technique used by public data networks and many private data

networks to provide a large number of users with relatively inexpensive data transfer

through access to shared resources. Packet switching performs data transfer using

fixed-length blocks that are surrounded by control information, which controls the

routing of the blocks through the network and detects any errors that occur during

transmission.

Note that X.25 refers to the standard for connecting the user’s computer system to the

packet-switching network; it has nothing to do with how the data is treated within the

network. Nevertheless, PSDNs are often casually referred to as X.25 networks. The

PSDN is generally depicted as a cloud (Figure 1-2), because what happens inside the

cloud (such as routing or temporary format changes) does not matter to the computer

system sending or receiving the information.

Figure 1-2. X.25 Connection

X.25

PSDN

DTE DTE

DTE

DCE

Tandem System IBM System

Other System

002

DCE

In networking terminology, the user’s system is identified as data terminal equipment

(DTE), and the network is identified as data circuit-terminating equipment (DCE).

The DTE must be capable of sending and receiving information in packets. If the DTE