X.25 Overview

X.25 is a communications subsystem based on packet-switching technologies. Packet-switching technologies are protocols in which messages are divided into packets before they are sent. Each packet is then transmitted individually and can even follow different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message.

Due to the original application of X.25 i.e. over potentially unreliable analog telephone lines, X.25 carries a significant error-checking overhead, as every device in the X.25 network acknowledges each packet it receives. This slows the transfer of information and uses up bandwidth, but does provide a highly reliable service. To use native X.25, an X.25 communications adapter is required. This connects to your computer and enables it to communicate directly with a synchronous modem. The synchronous modem then drives a leased line to the packet switched exchange. The modem and leased line are generally provided by your X.25 provider although the modem may be independently purchased.

X.25 benefits

Unlike asynchronous access, native X.25 access allows the packets of data to be transferred across the leased line and into the adapter connected directly to your machine. The packets of data are under the control of the HDLC error checking protocol, so the need for special logic extensions to check the validity of the data is eliminated, and data integrity is assured.

The second and probably more important benefit of native X.25 connection is that multiple simultaneous connections may be established across a single line. This means that you can be in communication with trading partners A, B and C and still have room left over for trading partner D to make an incoming call to you to send you urgent data.

The number of simultaneous connections on each line will depend upon the line’s characteristics as it was purchased from your X.25 service provider, but typical values are 4 sessions, 8 sessions, 32 sessions and above. The maximum theoretical limit is 255 sessions on a single line.

X.25 performance

Typical line speeds for a native X.25 connection would be 9600 baud up to 64K baud. What must be remembered when considering speed and number of sessions is that if the connection is 9600 baud and there are, for example, 4 concurrent sessions active, each session will be sharing the total bandwidth of 9600 baud. In other words, if all 4 are transmitting data each will get approximately 2400 baud worth of access time as the packets for each session are queued across the line.

This must be remembered if a 9600 line is used with a very large number of circuits, say 30. On a busy system the data throughput rate may be quite drastically reduced if a large number of users start communicating at the same time. This type of scenario will lead to time-outs and other communications congestion problems. The alternative is to reduce the number of circuits on the line so that the line capacity is reached and further users attempting to call you will get a line busy message, the X.25 equivalent of the telephone’s engaged signal.

Those members of the ODEX family that support native X.25 will, of course, allow you to define more than one X.25 line so, at a cost, network congestion need never happen.

X.25 and the OSI 7-layer reference model

X.25 defines layers 1, 2, and 3 in the OSI Reference Model. OSI is an ISO standard for worldwide communications that defines a networking framework for implementing protocols in seven layers.

Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.