Ref: 99980057
Title: X.25 BACKGROUND
Date: 2/1/86

Copyright 3Com Corporation, 1991.  All rights reserved.

What Is X.25?

X.25 is a set of CCITT recommendations that defines a 3-layer
protocol for interfacing to a packet switched network.  Although
some networks use the X.25 protocol internally, it is primarily
an interface standard and is not an end-to-end protocol across
the network.  In addition to the basic 3-layer X.25 protocol,
three protocols have been defined to provide terminal access
across public data network (PDN).  These protocols are defined by
CCITT as X.3, X.28, and X.29 recommendations.

X.25 and Public Data Networks

Prior to 1974, there were five independent and incompatible
public data networks throughout the world.  So as to provide a
standard interface for computers into the various packet
switching networks, in March 1976, the Comite Consulatif
International pour Telegraphique et Telephonique (International
Advisory Committee for Telegraphy and Telephony (CCITT)
recommended the use of X.25.

In August 1975, Telenet introduced the first public data network
to use X.25.  In 1977, Tymnet in the United States was approved
as a public data network carrier.  During the same period, both
PSS in the United Kingdom and DATAPAC in Canada became
operational.  By 1978, TRANSPAC in France, and NTT and DX-2 in
Japan were also in operation; followed in late 1979 by EURONET,
sponsored by nine European countries.  All of these networks use
X.25 as the interface for computer equipment.

Every four years, CCITT reviews its X.25 specification.  In 1984,
additional features were added to the X.25 protocols.  For more
detailed descriptions of these new features, refer to the 1984
CCITT X.25 Specification.  Note that the current Bridge X.25
software is not compatible with the 1984 CCITT X.25
specification.  Most public data networks and host computers
still support the 1980 version of the standard with which Bridge
is compatible.

X.25 Layers

X.25 consists of three layers: X.25 Packet Layer (layer 3), X.25
Frame Layer (layer 2), and X.25 Physical Layer (layer 1).  The
layer 2 protocol is often referred to as High Level Data Link
Control (HDLC) and the layer 1 protocol most commonly used is
X.21 bis which is equivalent to an RS-232.  Many networks also
support RS-422 or V.35 at the physical level for higher speed
connections.

The following figure illustrates the 3 layers of X.25 protocols
and the protocols that run above X.25 for terminal access.

                   X.25 Layers
______________________________________________________________
| Layer 4 |            |  X.3/X.28/X.29 or Transport Protocol |
|-------------------------------------------------------------|
| Layer 3 |  Network   |  X.25 Packet Level                   |
|-------------------------------------------------------------|
| Layer 2 |  Data Link |  HDLC Frames                         |
|-------------------------------------------------------------|
| Layer 1 |  Physical  |  X.21 or X.21 bis                    |
|         |            |  RS-232, RS-422, or V.35             |
|_________|____________|______________________________________|

Packet Assembly/Disassembly (PAD)

X.25 is used to connect computers to a PDN.  However, to support
terminal access across a PDN, a device is required that converts
terminal traffic into packets for transmission across the PDN.
This device is called a Packet Assembly/Disassembly (PAD) unit.
A PAD may be provided as a dial-up service by the PDN or may be a
device located on the customer premises.  PAD devices use 3 CCITT
standards as follows: X.3 defines the set of parameters that can
be modifed by the terminal user.  X.28 defines the communication
between the terminal device and the PAD.  X.20 defines the
communication between the host and the PAD to set or read the X.3
parameters.
