This International Standard specifies the procedures, formats and

facilities at the Packet Layer for Data Terminal Equipment (DTE)

operating in conformance with ITU-T Recommendation X.25.

Both Virtual Call and Permanent Virtual Circuit modes of

operation are covered.

The Packet Layer protocol specified herein can be used in both

Open Systems Interconnection (OSI) and non-OSI environments.

When used within the context of OSI, the Packet Layer protocol

is encompassed in the Network Layer of the OSI Reference

Model, ITU-T Rec. X.200 | ISO/IEC 7498-1.

This International Standard covers DTE operation at the Packet

Layer when accessing a public or private packet-switched

network conforming to ITU-T Recommendation X.25 by means

of a dedicated path or a circuit-switched connection. It also

covers the additional Packet Layer procedures necessary for two

DTEs conforming to this International Standard to communicate

directly (i.e., without an intervening packet-switched network)

over a dedicated path, a circuit-switched connection, or a local

area network (LAN).

This International Standard also covers private networks that use

ITU-T Recommendation X.25 to connect to packet-switched

public data networks and that may also offer an X.25 interface to

a DTE (see annex A).

To evaluate conformance of a particular implementation, it is

necessary to have a statement of which capabilities and options

have been implemented. Such a statement is called a Protocol

Implementation Conformance Statement (PICS), as defined in

ITU-T Rec. X.290 | ISO/IEC 9646-1. Annex B provides the PICS

proforma in accordance with the relevant guidance given in ITUT

Rec. X.296 | ISO/IEC 9646-7.

The first edition of this International Standard was based on the

1984 CCITT Red Book text of Recommendation X.25. It also

contained the necessary provisions for compatibility with the

earlier 1980 CCITT Yellow Book text of Recommendation X.25.

The second edition was based on the 1988 CCITT Blue Book

text of Recommendation X.25. The third edition is based upon

the 1993 version of X.25. This fourth edition is based on the

1996 version of X.25. Retained within this fourth edition are the

necessary provisions for compatibility with the 1993, 1988, 1984

and 1980 versions of X.25. The differences between various

editions of this International Standard are summarized in annex

C.

CCITT Recommendation X.613 (1992) | ISO/IEC 10588 : 1993,

Information technology Use of X.25 Packet Layer Protocol in

conjunction with X.21/X.21bis to provide the OSI connectionmode

Network Service

CCITT Recommendation X.614 (1992) | ISO/IEC 10732 : 1993,

Information technology Use of X.25 Packet Layer Protocol to

provide the OSI connection-mode Network Service over the

telephone network