ISO/IEC 7776:1995/Amd 1:1996

INTERNATIONAL
lSO/IEC
STANDARD 7776
Second edition
1995-07-01
AMENDMENT 1
1996-11-15
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
AMENDMENT 1: Modul0 32 768 and
multi-selective reject Option
Technologies de I’information -
Tekommunica tions et khange
d ‘informa tion en tre sys temes - Procedures de commande de liaison de
donnees a haut niveau - Description des procedures de liaison
d ’equipemen t terminal de transmission de donnees ETTD compatible X. 25
LAPB
AMENDEMENT 1: Option de rejet modulo 32 768 et multiselective
Reference number
ISO/1 EC 7776: 1995/Amd. 1: 1996(E)

ISO/IEC 7776: 1995/Amd. 1: 1996( E)
Foreword
ISO (the international Organization for Standardization) and IEC (the
International 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, lSO/IEC JTC 1. Draft International Standards adopted
by the joint technical committee are circulated to national bodies for voting.
Publication as an International Standard requires approval by at least 75 %
of the national bodies casting a vote.
Amendment 1 to International Standard ISO/IEC 7776:1995 was prepared
by Joint Technical Committee lSO/IEC JTC 1, Information technology,
Subcommittee SC 6, Telecommunications and information exchange
between Systems.
0 ISO/IEC 1996
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.
lSO/IEC Copyright Office l Case postale 56 l CH-l 211 Geneve 20 l Switzerland
Printed in Switzerland
ii
0 ISOAEC ISOAEC 7776:lggUAmd.l: 1996(E)
Introduction
This amendment to ISOAEC 7776:1995 adds the multi-selective reject Option (Option 3.3)
and modulo 32 768 (Option 10.2) from ISOAEC 7809:1993. This amendment enhances
ISOAEC 7776:1995 in the following areas:
l it allows more efficient recovery of errored or lost I frames by selectively requesting
retransmissions of one or more I frames with a Single request; this improved efficiency
is especially beneficial over links that have high data rates or large propagation delays.
l link efficiency is
improved by retransmitting only errored or lost I f rames; correctly
received I frames are saved by the DTE for deferred delivery;
l al lows for a much larger modulo for Operation over links that have high data rates or
Ia rge propagation delays;
l the maximum r of outsta .nding k tan be as large as the modulus number
minus 1;
l the additional procedures are fairly simple to implement.

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lSO/lEC 7776:1995/Amd.l: 1996(E)
lSO/IEC
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
AMENDMENT 1: Modul0 32 768 and multi-selective reject Option
Page 1
Clause 1
ßeplace Paragraph 2 with the following:
Clause 3 describes three frame structures: one for basic (modulo 8) Operation, one for extended (modulo 128)
Operation, and the third for modulo 32 768. Basic (modulo 8) Operation is the ISO/lEC 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 10.1 (BAC, 2, 8, 10.1) or 3.3, 8 and
IO. 1 (BAC, 3.3, 8, 10.1). Modul0 32 768 Operation is the ISO/IEC balanced asynchronous class of procedure
with optional functions 3.3, 8 and 10.2 (BAC, 3.3, 8, 10.2). -
The following pertains to the selection of the above combinations:
a. basic (modulo 8) Operation may optionally be supported - if supported, then the REJ recovery mechanism
shall be supported with this operating mode; support of SREJ-with-span-list recovery mechanism is
prohibited;
b. extended (modulo 128) Operation may optionally be supported - if supported, then either or both of the
REJ and SREJ-with-span-list recovery mechanism shall be supported but only one recovery mechanism
shall be used at a time;
C. modulo 32 768 Operation may optionally be supported - if supported, then SREJ-with-span-list recovery
mechanism shall be supported with this operating mode; support of the REJ recovery mechanism is
prohibited.
For those DTE/DCE connections that support basic (modulo 8) Operation, extended (modulo 128) Operation
and/or modulo 32 768 Operation, the choice of modulo and, when applicable, recovery mechanism is made at
subscription-time only. For those DTE/remote DTE connections that support basic (modulo 8) Operation,
extended (modulo 128) Operation and/or modulo 32 768 Operation, the choice of modulo and, when applicable,
recovery mechanism 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.
Page 7
Clause 1
ßepiace Paragraph 4 with the following:

ISO/lEC 7776:1995/Amd.l: 1996(E)
0 ISO/IEC
Clause 4 describes the elements of procedures. Some aspects are only operable for the basic (modulo 8)
Operation, some for the extended (modulo 128) Operation and some for modulo 32 768 Operation.
Page 2
Clause 3
ßeplace Paragraph 1 with the following:
All transmissions on a SLP are in frames conforming to one of the formats of table 1 for basic (modulo 8)
Operation, or one of the formats of table 2 for extended (modulo 128) Operation or alternatively one of the formats
of table 3 for modulo 32 768 Operation. The flag preceding the address field is defined as the openina flaa. The
flag following the FCS field is defined as the closing flag.
Page 3
Clause 3
Add the following new Table 3, and renumber all the other tables that follow:
Table 3 - Frame formats - modulo 32 768 Operation
Bit Order of
transmission
12345678 12345678 1 to * 16tol 12345678
c
Flag Address Control FCS Flag
F A C FCS F
*
01111110 8-bits -bits 16-bits 01111110
FCS = Frame Check Sequence
Bit Order of
transmission
12345678 12345678 1 to * 16tol 12345678
Flag Address Control Information FCS Flag
F A C I FCS F
*
01111110 8-bits -bits N-bits 16.bits 01111110
FCS = Frame Check Sequence
* 32 for frame formats that contain sequence numbers; 8 for frame formats that do not contain sequence numl
Page 3
Subclause 3.3
Paragraph 1, ßeplace with the following:
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 numbers, and one octet for
frame formats that do not contain sequence numbers. For modulo 32 768 Operation, the control field shall
consist of four octets for frame formats that contain sequence numbers, and one octet for frame formats that do
not contain sequence numbers. The content of this field is described in 4.1.
ISO/IEC 7776:1995/Amd.l: 1996(E)
0 ISOAEC
Page 3
Subclause 3.4, Paragraph 1, Line 2:
Change 4.3.9 to 4.3.10
Page 3
Subclause 3.4, Paragraph 3:
Change 4.3.9 to 4.3.10 and 5.7.3 to 5.8.3.
.
Page 5
Subclause 3.7.1
Replace Paragraph 3 with the following:
The FCS shall be transmitted to the Iine commencing with the coefficient of the highest term, which is found in bit
Position 16 of the FCS field (see tables 1, 2 and 3).
Page 5
Subclause 3.7.1, NOTE
Change “8” to “15 ”.
Page 7
Subclause 3.11.2.2
Change 5.7.1.5 to 5.8.1.5.
Page 7
Subclause 3.11.2.2, Note
Change 5.7.7.3 to 5.8.7.3.
Page 7
Subclause 4.1 .l
Replace the second Paragraph with the following:
Three types of control field formats are used to perform numbered information transfer (1 format), number
supervisory functions (S format) and unnumbered control functions (U format). The control field formats for basic
(modulo 8) Operation are depicted in table 4, control field formats for extended (modulo 128) Operation are
depicted in table 5 and control field formats for modulo 32 768 Operation are depicted in table 6.
ISO/lEC 7776:1995/Amd.l: 1996(E) 0 ISOAEC
Page 8
Renumber Tables 3 and 4 as Tables 4 and 5 and insert the following new Table 6:
Table 6 - Control field formats - modulo 32 768 Operation
Control field bits
Control
field First two octets Next two octets
format
12 8 16 17 18 32
P/F
I format 0 .
NW N(R) *
S format
1 ossxxxxx. x P/F
N(R)
111 MMP/FMMMI
U format
N(S) = transmitter send sequence number (bit 2 = low-Order bit)
N(R) = transmitter receive sequence number (bit 18 = low-Order bit)
S = supervisory function bit
M = modifier function bit
P/F = pell bit when issued as a command; final bit when issued as a response (1 = PoWFinal)
P = pell bit (1 = Pall)
o ISO/IEC ISO/lEC 7776:1995/Amd.l: 1996(E)
Page 8
Subclause 4.1 .1.3
Replace the last sentence with the following:
The unnumbered frames shall have the same control field length (one octet) in the basic (modulo 8) Operation,
extended (modulo 128) Operation and modulo 32 768 Operation.
Page 8
Subclause 4.1.2.1
.
Replace the last sentence with #he fol/owing:
The modulus equals 8, 128 or 32 768 and the sequence numbers cycle through the entire range.
Page 9
Subclause 4.1.2.2.1
Replace the last sentence with the following:
The value of k is defined in 5.8.4.
Page 9
Subclause 4.1.2.2.4
Replace with the following:
shall contain N(R), the expected
All I frames and supervisory frames, except SREJ frames with F bit set to “O ”,
sequence number of the next received I frames. Prior to transmission of a frame of the above types of the DTE,
N(R) indicates that the
the value of N(R) shall be set equal to the current value of the DTE receive state variable.
transmitter of the N(R) has correctly received all I frames numbered up to N(R) - 1 inclusive.
Page 9
Subclause 4.3
Replace with the following:
The commands and responses supported by the DTE are represented in table 7 for basic (modulo 8) Operation,
table 8 for extended (modulo 128) Operation and table 9 for modulo 32 768 Operation. For purposes of this
International Standard, those encodings of the modifier function bits in tables 4, 5 and 6 not identified in tables 7,
8 and 9 are identified as “undefined or not implemented” command and response control fields. The commands
and responses in tables 7, 8 and 9 are defined as follows”

0 ISOAEC
lSO/lEC 7776:1995/Amd.l: 1996(E)
Page 9-10
Renumber Tables 5 and 6 as Tables 7 and 8. Replace Table 8 with the following. Insert the following new Table
9:
Table 8 - Commands and responses - Extended (modulo 128) Operation
Encoding
Format Commands Responses
1 2 3 4 5 678 9 lOtol6
Information transfer 1 I (information) 0
I p I N(R) I
I I I N(S)
Supervisory RR (receive ready) RR (receive ready) 1 0 0 0 0 0 0 0 P/F WR)
RNR (receive not ready) RN R (receive not ready) 1 0 1 0 0 0 0 0 PIF WR)
REJ (reject) REJ (reject) 1 0 0 1 0 0 0 0 PIF N(R)
SREJ (selective 1 0 1 1 0000 F N(R)
.
. reject)
Unnumbered SABME (set asynchronous
baianced mode extended) 11 Ill ,/Pi1 1 01
1 110 OIPIO 1 01
1 DISC (disconnect)
I
UA (unnumbered
acknowledgement) 1 1 OOFIIO
DM (disconnected mode) 1 1 1 1 F 0 0 0
FRMR (frame reject) 1 1 1 0 F 0 0 1
Table 9 - Commands and responses - modulo 32 768 Operation
Encoding
Format Commands Responses
18to32 1
1 2 3 4 5 .8 . 16 17
Information transfer I (information)
N(S) / p 1 N(R) I
0 I
Supenrisory RR (receive ready) RR (receive ready)
1 0000 0 00 PIF N(R)
RNR (receive not ready) RN R (receive not ready)
10100 0 00 P/F N(R)
SREJ (selective
10110 000 F N(R)
reject)
1 1 0 0 P Oll
Unnumbered SM (set mode)
DISC (disconnect) 1 1 0 0 P 010
UA (unnumbered
acknowledgement) 1 1 0 0 F 110
DM (disconnected mode) 1 1 1 1 F 000
~1 1 1 0 F 001
FRMR (frame reject)
Page 11
Insert a new subclause 4.3.5 as follows and renumber all subclauses that follow:
4.3.5 Selective reject (SREJ) response
The selective reject, SREJ, frame shall be used by a DTE to request retransmission of one or more (not
necessarily contiguous) I frames. The N(R) field of the control field of the SREJ frame shall contain the sequence
number of the earliest I frame to be retransmitted and the information field shall contain the sequence numbers of
additional I frame(s), if any, in need of retransmission. (The DTE shall create a list of sequence numbers N(X),
N(X+l), N(X+2), N(Y), N(Z+3), N(Z+4), . . . , N(S)-1, where N(X) is greater than or equal to V(R) and none of the I
frames N(X) to N(S-1) have been received. The N(R) field of the SREJ frame shall be set to N(X) and the
information field set to the list N(X)+l, . . . . N(S)-1 .)
lSO/lEC 7776:1995/Amd.l: 1996(E)
0 ISOAEC
For extended (modulo 128) Operation, the information field shall be encoded such that there is an octet for each
standalone I frame in need of retransmission, and a two-octet span list for each sequence of two or more
contiguously numbered I frames in need of retransmission, as depicted in table 10. In the case of standalone I
frames, the sequence number of each designated I frame shall occupy bit positions 2-8 of an octet, with bit
Position 1 set to Zero, as depicted in table 10. In the case of span lists, the sequence number of the first
designated I frame in need of retransmission is encoded in the first octet (of the two octet field for span lists) and
the second octet contains the sequence number of the last designated I frame in the sequence in need of
retransmission. Sequence numbers in both octets are encoded in bits 2-8 with the bit Position 1 in both octets set
to 1 as depicted in table 10.
Table 10 - Control and Information field encoding of SRE
...