ISO/IEC 7776:1995

INTERNATIONAL
ISO/IEC
STANDARD 7776
Second edition
1995-07-01
Information technology -
Telecommunications and information
exchange between Systems - High-level
data link control procedures - Description
of the X.25 LAPB-compatible DTE data link
procedures
Technologies de I ’informa tion - T6kcommunica tions et Gchange
d ‘in forma tion en tre sys tkmes - Prockdures de commande de liaison de
donnees de haut niveau - Description des procedures de liaison de
donnees ETTD compatibles X.25 LAPB
.
Reference number
ISO/1 EC 7776: 1995(E)
ISOhEC 7776 : 1995 (E)
Page
Contents
Foreword. iii
iv
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Scope .
Normative references. .
Framestructure .
Elements of procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Description of the procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Multilink procedure (MLP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1
Conformance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Annexes
A PICS Proforma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
B Repeated requirements from other International Standards . . . . . . . . . . . . 39
0 ISOAEC 1995
All rights reserved. Unless otherwise specified, no part of this publication may be
reproduced or utilized in any form or by any means, electronie or mechanical, including
photocopying and microfilm, without Permission in writing from the publisher.
ISO/IEC Copyright Office l Case Postale 56 l CH-121 1 Geneve 20 l Switzerland
Printed in Switzerland
ii
0 ISO/IEC
ISO/IEC 7776 : 1995 (E)
Foreword
ISO (the International Organization for Standardization) and IEC (the Inter-
national Electrotechnical Commission) form the specialized System for worldwide
standardization. National bodies that are members of ISO or IEC participate in the
development of International Standards through technical committees established
by the respective organization to deal with particular fields of technical activity.
ISO and IEC technical committees collaborate in fields of mutual interest. Other
international organizations, governmental and non-governmental, in liaison with
ISO and IEC, also take part in the work.
In the field of information technology, ISO and IEC have established a joint
technical committee, ISO/IEC JTC 1. Draft International Standards adopted by the
joint technical committee arc circulated to national bodies for voting. Publication
as an International Standard requires approval by at least 75 % of the national
bodies casting a vote.
International Standard ISOLIEC 7776 was prepared by Joint Technical Committee
ISO/IEC JTC 1, Information technology.
This second edition cancels and replaces the first edition (ISO 7776:1986), and
consolidates Amendment 1: 1992, as well as Technical Corrigenda 1, 2 and 3.
Annex A forms an i
ntegral part of this International Standard. Annex B is for
information only
. . .
0 ISO/IEC
ISOIIEC 7776 : 1995 (E)
Introduction
This document provides the ISO/IEC description of the ITU-T Recornmendation X.25
LAPB interface Operation as viewed by the DTB. It is the DTE counterpart of the X.25
LAPB DCE description.
This document also provides the ISO/IEC description of how two DTEs are capable of
communicating directly with one another at the Data Link Iayer using the X.25 LAPB
procedures without an intervening public data network.
The Data Link layer provides the DTE with three basic functions:
a) link initialization: necessary for the DTE to begin communication in a known state;
b) flow control: control of the flow of frames between the DTE and the other Station
@CE or DTE) to ensure that they are not sent more quickly than they tan be
received; and
c) error control: provided in two forrns:
a cyclic redu ndancy check using a Io-bit polynomial to detect mutilated
1) OW
frames, and
2) use of sequence numbers to ensure against losing entire frames.
(The Data Link layer endeavours to ensure correct receipt of all frames by
retransmission of mutilated or missing frames.)
This International Standard repeats requirements of other International Standards. Annex B
contains a list of these repeated requirements and references to the corresponding
International Standards.
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 ISO/IEC 9646-1.
This International Standard provides such a PICS proforma in compliance with the relevant
requirements, and in accordance with the relevant guidance, given in ISO/IEC 9646-2.
iv
INTERNATIONAL STANDARD @ ISOmC ISOhEC 7776 : 1995 (E)
Information technology - Telecommunications and
information exchange between Systems - High-level data
Description of the X.25 LAPB-
link control procedures -
compatible DTE data link procedures
1 Scope
This International Standard defines an application of the following HDLC Standards: ISO/IEC 3309, ISO/IEC 4335, ISO 7478, and ISO/IEC
7809. When there is difficulty in the interpretation of a reworded requirement from one of the other International Standards, the original
requirement of ISO/IEC 3309, ISO/IEC 4335, ISO 7478 or ISO/IEC 7809 is definitive. It also defines the structure, elements and procedures
for the Operation of a DTE using the X.25 LAPB protocol as specified in ITU-T Recommendation X.25.1) The procedures are applicable to
data interchange between a DTE and a DCE, or between two DTEs. The procedures are defined for use on duplex links, using synchronous
transmission or start/stop transmission.
Clause 3 describes two frame structures: one for basic (modulo 8) Operation and one for extended (modulo 128) Operation. Basic (modulo 8)
Operation is the ISO/IEC balanced asynchronous class of procedure with optional functions 2 and 8 (BAC, 2, 8). Extended (modulo 128)
Operation is the ISO/IEC balanced asynchronous class of procedure with optional functions 2, 8 and 10 (BAC, 2, 8, 10). For those DTE/DCE
connections that support both basic (modulo 8) Operation and extended (modulo 128) Operation, the choice is made at subscription-time only.
For those DTE/remote DTE connections that support both basic (modulo 8) Operation and extended (modulo 128) Operation, the choice is
made by bilateral agreement.
NOTE - The procedure herein described as basic (modulo 8) Operation is the only one available in all public data networks.
Clause 3 also describes two methods for encoding the frames, as sequences of bits when synchronous transmission is used, and as sequences
of octets when start/stop transmission is used. The start/stop encoding specifies optional mechanisms, for use in environments that are
sensitive to transmission of octets with values that could be interpreted as ISO/IEC 646 control characters, a.nd/or in environments that
support transfer of only seven data bits per start/stop Character. The choice of encoding is made by bilateral agreement, or other suitable
means, to suit the data transmission characteristics of the environment.
Clause 4 describes the elements of procedures. Same aspects are only operable for the basic (modulo 8) Operation and some for the extended
(modulo 128) Operation.
Clauses 5 and 6 describe the Single link procedure (SLP) which is derived fror-n the frame structure and the elements of procedures, and an
optional multilink procedure (MLP), respectively. The SLP is used for data interchange over a Single data link and the MLP is used for data
interchange over a multiple of parallel SLPs. An MLP is required if the effects of individual SLP failures are not to disrupt the higher layer
Operation. An MLP tan also be used over a Single SLP by Prior bilateral agreement. For DTE/DCE connections the choice of an MLP
Operation or not is made at subscription-time only. For DTE/remote DTE connections, the choice is made by bilateral agreement.
Where choices among alternative actions are indicated in the procedures, a recommended choice is usually indicated. Unless specifically
stated otherwise, the choice of action does not affect interoperability with other implernentations of this International Standard although
efficiency of Operation may be affected. Where such choices do affect interoperability, the procedures explicitly state that Prior bilateral
1) Future revisions of this International Standard will be made in accordance with revisions of KU-T Recommendation X.25. The pre.sent Version is based
on the 1993 ITU-T Recommendation X.25
0 ISO/IEC
ISO/IEC 7776 : 1995 (E)
agreement on the choice of procedure with the remote end is needed. An attempt has been made to minimize such choices consistent with the
need to satisfy a broad range of applications. A basic requirement for all implernentations of this International Standard is that they be
capable of responding, as specified, to any actions taken at the remote end that are permitted by this International Standard (except possibly
for those procedures whose use involves Prior bilateral agreement.)
Clause 7 covers the Static Conformance requirements, the Dynamit Conformance requirements, and the Protocol Implementation
Conformance Statement (PICS).
2 Normative references.
The following Standards contain provisions which, through reference in this text, constitute provisions of this International Standard. At the
time of publication, the editions indicated were valid. All Standards are subject to revision, and Parties to agreements based on this
International Standard are encouraged to investigate the possibility of applying the most recent editions of the Standards listed below.
Members of IEC and ISO maintain registers of currently valid International Standards.
ISO/IEC 646: 199 1, Information technology -ISO 7-bit coded Character set for iriformation interchange.
ISOfIEC 3309: 1993, Information technology - Telecommunications and information exchange between Systems - High-level data link
control (HDLC) procedures -Frame structure.
ISO/IEC 4335: 1993, Information technology - Telecommunications and information exchange between Systems - High-level data link
control (HDLC) procedures - Elements of procedure.
ISO 7478: 1987, Information processing Systems - Data communicatiorr - Multilink procedures.
ISO/IEC 7809: 1993, Information technology - Telecommurrications and information cxchange bctween systems - High-level data link
control procedures (HDLC) - Classes of procedures.
ISO/IEC 9646- 1: 1994, Information technology - Open Systems Interconnection - Conformance testing methodology and framework -
Part 1: General concepts.
ISO/IEC 9646-2: 1994, Information technology - Open Systems Interconnection - Conformance testing methodology and framework -
Part 2: Abstract Test Suite specijication.
ITU-T Recommendation X.25, Interface between data terminal equipment (DTE) and data circuit-terminating equi ’lment (DCE) for
terminals operating in the packet nzode and connected to public duta networks by dedicated circuit.
3 Frame structure
All transmissions on a SLP are in frames conforrning to one of the fomlats of table 1 for basic (modulo 8) Operation, or altematively one of
the formats of table 2 for extended (modulo 128) Operation. The flag preceding the address Geld is defined as the opening flag. The flag
following the FCS field is defined as the closing flag.
All transmissions from the DCE/remote DTE are expected to use this frame structure.
TabIe 1 - Frame formats - Basic (nnodulo 8) Operation
Bit Order of
transmission 12345678 12345678 12345678 16to 1 12345678
Flag Address Control FCS Flag
F A C FCS F
8-bi ts 16-bits 01111110
01111110 8-bits
FCS = Frame Check Sequence
Bit Order of
transmission 12345678 12345678 12345678 16to 1 12345678
Control Information FCS Flag
Flag Address
F A C 1 FCS F
01111110 8-bits 8-bits N-bits 16-bits 01111110
.
FCS = Frame Check Sequence
0 ISO/IEC ISOhEC 7776 : 1995 (E)
Table 2 - Frame formats - Extended (modulo 128) Operation
Bit Order of
transmission 12345678 12345678 1 to * 16to 1 12345678
Address Control
FCS
nag I nag 1
I I I I
/ olllF1llo / 8-tts mpmmr.h,,, 1 lZts 1 OlllF1l10 /
FCS = Frame Check Sequence
Bit Order of
transmission 12345678 12345678 12345678 16to 1 12345678
Flag Address Control Information FCS Flag
F A F
C 1 FCS
01111110 8-bits 8-bits N-bits 16-bits 01111110
I
.
FCS = Frame Check Sequence
I
16 for frame formats that contain sequence numbers; 8 for frame formats that do not contain sequence numtxrs.
3.1 Flag sequence
All frames shall Start and end with the flag sequence consisting of one “0” bit followed by six contiguous “1” bits and one “0” bit. The DTE
hunts continuously for this sequence on a bit-by-bit basis, and thus uses the flag sequence for frame synchronization. The DTE/DCE/remote
DTE may send one or more complete frag sequences between frames. The DTE shall only send complete eight-bit flag sequences when
sending multiple flag sequences (see 3.10). A Single flag may be used as both the closing flag for one frame and the opening flag for the next
frame.
3.2 Address field
The address field shall consist of one octet. The address field identifies the intended receiver of a command frame and the transmitter of a
response frame. The coding of the address field is described in 5.1.
3.3 Control field
For basic (modulo 8) Operation, the control field shall consist of one octet. For extended (modulo 128) Operation, the control field shall
consist of two octets for frame formats that contain sequence nurnbers, and one octet for frame formats that do not contain sequence
numbers. The content of this field is described in 4.1.
3.4 Information field
The information field of a frame, when present, follows the control Geld (see 3.3) and precedes the frame check sequence (see 3.6). (See
4.3.9 and 6.2 for the various codings and groupings of bits in the infomjation field that are defined for use in this International Standard.) The
coding and grouping of bits received from a higher layer are unrestricted, except
...