Technology Reference Guide

	Technology Reference Guide 7. Wide-Area Networks D. X.25

What is it?
X.25 is a transmission protocol defined by the CCITT to specify standards for a packet-switched data network, and is commonly used to describe public data networks using this protocol. The X.25 standard was released in 1976, and formed the basis for all packet-switched networks until recent protocol innovations with higher rates of throughput.

What does it do?
Packet-switching refers to a method of routing packets of data based on the channel to which they are assigned. X.25 networks provide the ability to set up multiple logical channels, called virtual circuits. These circuits can be either permanent virtual circuits (PVC), similar to leased lines, or switched virtual circuits (SVC), which function like dial-up lines.
The data to be transmitted is segmented into packets, each of which has a channel identifier attached. The packets for all channels are multiplexed together, and routed throughout the network by the packet switches, based on their channel ID.

X.25 has several protocol levels. At the physical level, it uses transmission protocols similar to standard modems. For a data link protocol, it uses HDLC (high-level data link control), which is virtually identical to SDLC data link protocol used by IBMs SNA. The top level of X.25 controls virtual circuits, data flow control, and other parameters, and has some elements of both network layer and transport layer functionality. This protocol level allows negotiation between end-nodes for parameters such as packet size, data rate, and error control (see OSI reference model).

The host systems in an X.25 network were called DTEs (data terminal equipment). These interface with packet switches through DCEs (data circuit terminating equipment). The packet switches, which control the traffic through the network, are designated as DSEs (data switching equipment). Other X.25 devices include PADs (packet assemblers/disassemblers) that allow data from terminals and other non-X.25 devices to be grouped into packets for transmission across the network.

The data rates for an X.25 network range from 75 bits/sec (not often used) to 48 Kbits/sec.

How is it used?
X.25 can be implemented on either public or private networks. If a company wishes to implement a private network, they lease all the lines required, supply the switching equipment, and install the necessary X.25 interfaces. More commonly, they simply install the interfaces, and connect to a public X.25 network cloud. Although it has been superseded by faster technologies, X.25 is the most common type of public network service, and is available in many more areas than the new technologies. Examples of X.25 networks are Tymnet and Telenet.

Where do I get more information?
Contact your local CBV Office.