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
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
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