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)

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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

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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:

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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.
2

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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.
3

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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)

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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”

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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 .)
6

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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 SREJ frame - Extended (modulo 128) Operation
12345678 12345678 1 to16 16to 1 12345678
Flag Address Control Information Flag
FCS
A C I F
F FCS
01111110
8-bits 16-bits N-bits
01111110 16.bits
iiY//
345678 12345678 12345678
0110000 F: N(R) i N(R) 1 : N(R) 1 :. N(R) 0: N(R) 1 %=
. . . . .
l
l >
Sequence number of standalone
Sequence number of last
I frame to be retransmitted
I frame in span list to be retransmitted
/
1
Sequence number of first
Sequence number of standalone
I frame in span list to be retransmitted
I frame to be retransmitted
For modulo 32 768 Operation, the information field shall be encoded such that there are two octets for each
standalone I frame in need of retransmission, and a four-octet span list for each sequence of two or more
contiguously numbered I frames in need of retransmission, as depicted in table 11. In the case of standalone I
frames, the sequence number of each designated I frame shall occupy bit positions 2-16 of the two octet field,
with bit Position 1 set to Zero, as depicted in table 11. In the case of span lists, the sequence number of the first
designated I frame in need of retransmission is encoded in the first two octets (of the four octet field for span
lists) and the second two octet field contains the sequence number of the last designated I frame in the sequence
in need of retransmission. Sequence numbers in both two octet fields are encoded in bits 2-16 with the bit
Position 1 in both two-octet fields set to 1 as depicted in table 11.

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ISO/lEC 7776: 1995/Amd.l: 1996(E) 0 ISO/IEC
Table 11 - Control and Information field encoding of
SREJ frame - modulo 32 768 Operation
12345678 12345678 1 to 32 16tol 12345678
Flag Address Information Flag
Control
FCS
A C I F
F FCS
8-bits
32.bits N-bits 01111110
01111110
16-bits
I frame in span list to be retransmitted
I
Sequence number of standalone
Sequence number of standalone
I frame to be retransmitted
/
I frame to be retransmitted
Sequence number of first
I frame in span list to be retransmitted
If the list of sequence numbers is too large to fit in the information field of the SREJ frame, then the list shall be
truncated to fit in one SREJ frame, by including only the earliest sequence numbers. The truncated sequence
numbers may be transmitted in another SREJ frame. The number of bits in an SREJ frame shall not exceed the
value of Parameter NI, the maximum number of bits in an I frame.
If the F bit in an SREJ frame is set to “l”, then I frames numbered up to N(R) - 1 inclusive (N(R) being the value
in the control field) are considered as acknowledged. If the F bit in an SREJ frame is set to “O ”, then the N(R) in
the control field of the SREJ frame does not indicate acknowledgement of 1 frames.
The procedures to be followed on receipt of an SREJ frame are specified in 5.56.
Page 10
Old Subclause 4.3.5
Replace with the followingr
4.3.6 Set asynchronous balanced mode (SABM) command/Set asynchronous balanced mode extended
(SABME) command/Set mode (SM) command
The SABM unnumbered command shall be used to place the addressed DCE or DTE in an asynchronous
balanced mode (ABM) information transfer Phase where all command/response control fields shall be one octet
in length.
The SABME unnumbered command shall be used to place the addressed DCE or DTE in an asynchronous
balanced mode (ABM) information transfer Phase where numbered command/response control fields shall be
two octets in length, and unnumbered command/response control fields shall be one octet in length.
8

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ISO/lEC 7776:1995/Amd.l: 1996(E)
0 ISO/IEC
The SM unnumbered command shall be used to place the addressed DCE or DTE in an asynchronous balanced
mode (ABM) information transfer Phase where numbered command/response control fields shall be four octets in
length, and unnumbered command/response control fields shall be one octet in length.
NOTE - For DTE/DCE connections, the mode of Operation of the data link [basic (modulo 8), extended (modulo 128) or
modulo 32 768 shall be determined at subscription time and shall only be changed by going through a new subscription
process. For DTE/DTE connections, the mode of Operation of the data link [basic (modulo 8), extended (modulo 128) or
modulo 32 7681 shall be determined by bilateral agreement.
No information field shall be permitted with the SABM, SABME or SM command. The transmission of a
SABM/SABME/SM command shall indicate the clearance of a busy condition that was reported by the earlier
transmission of an RNR frame by that same Station. The DTE confirms acceptance of SABM/SABME/SM [basic
(modulo 8) operation/extended (modulo 128) operation/modulo 32 768 Operation] command by the transmission
at the first opportunity of a UA response. Upon acceptance of this command, the DTE send state variable V(S)
and receive state variable V(R) shall be set to “0 ”.
Previously transmitted I frames that are unacknowledged when this command is actioned shall remain
unacknowledged (i.e., they are not retransmitted following link set-up). lt shall be the responsibility of a higher-
level protocol (for example, Network Layer or MLP) to recover from the possible loss of the contents (data units)
of such I frames.
Page 11
Old subclause 4.3.7 renumbered as 4.3.8
First sentence, replace with the following:
The UA unnumbered response shall be used by the DTE to acknowledge the receipt and acceptance of the
SABM/SABME/SM and DISC unnumbered commands.
Page 11
Old subclause 4.3.8 renumbered as 4.3.9
Second Paragraph, Replace with the follo wing:
A DTE in the disconnected Phase shall monitor received commands, react to a SABM/SABME/SM command as
outlined in 5.3 and respond with a DM response with the F bit set to “1” to any other command received with the
P bit set to “1 ”.
Page 11
Old subclause 4.3.9 renumbered as 4.3.10
Second Paragraph, Replace with the follo wing:
An undefined or not implemented control field shall be any of the control field encodings that are not identified in
tables 7, 8 or 9.
Page 11
Old subclause 4.3.9 renumbered as 4.3.10
Fourth Paragraph, replace with the following:

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lSO/lEC 7776:1995/Amd.l: 1996(E)
0 ISO/IEC
An information field which immediately follows the control field, and consists of 3 octets [basic (modulo 8)
Operation], 5 octets [extended (modulo 128) Operation], 9 octets [modulo 32 768 Operation], shall be returned
with this response to provide the reason for the FRMR response. These formats are given in table 12, table 13
and table 14.
Page 12
Renumber Tables 7 and 8 as Tables 12 and 13, Insert the following new table at the end of 4.3. IO.
Table 14 - FRMR information field format - modulo 32 768 Operation
.
Information field bits
1 to 32 33 34 to 49 50 to 65 66 67 68 69 70 71 72
64
48
Rejected
frame
0 N(S) C/R N(R) w x y z 0 0 0 0
control
f ield
Rejected frame control field shall be the control field of the received frame which caused the frame reject.
a)
When the rejected frame is an unnumbered frame, the control field of the rejected frame shall be
positioned in bit positions 1 to 8, with 9 to 32 set to “0 ”.
b) N(S) shall be the current send state variable value at the DCE or DTE reporting the rejection condition
(bit 34 = low-Order bit).
c) C/R set to “1” shall indicate the rejected frame was a response. C/R set to “0” shall indicate the rejected
frame was a command.
d) N(R) shall be the current receive state variable value at the DCE or DTE reporting the rejection condition
(bit 50 = low-Order bit).
w set to “1” shall indicate that the control field received and returned in bits 1 to 32 was undefined or not
implemented.
xset to “1” shall indicate that the control field received and returned in bits 1 to 32 was considered invalid
f )
because the frame contained an information field which is not permitted or is a supervisory frame with
incorrect length. Bit w shall be set to “1” in conjunction with this bit.
y set to “1” shall indicate that the information field received exceeded the maximum established capacity
g)
of the DTE or DCE reporting the rejection condition.
h) z set to “1” shall indicate that the control field received and returned in bits 1 to 32 contained an invalid
N(R)-
i) Bits 33 and 69 to 72 shall be set to “0 ”.
The w, x, y and z bits in the information field of the FRMR response may all be set to “O ”, indicating an
unspecified rejection condition of the frame for one or more of the conditions cited above.
Page 13
Subclause 4.4.1
IO

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0 ISO/IEC ISO/lEC 7776:1995/Amd.l: 1996(E)
First Paragraph, last sentence, replace with the following:
An indication that the busy condition has cleared shall be communicated by the transmission of a UA (only in
response to a SABM/SABME/SM command), RR, REJ, SREJ or SABM/SABME/SM frame.
Page 13
Subclause 4.4.2
Replace with the following:
If the multi-selective reject Option is not used the information field of all I frames received by the DTE whose N(S)
does not equal the DTE receive state variable V(R) shall be discarded.
If the multi-selective reject Option is used, the information field of all I frames received by the DTE whose N(S) is
not in the range V(R) and V(R) + k - 1 inclusive, shall be discarded. If the multi-selective reject Option is used,
the information field of all I frames received by the DTE whose N(S) is in the range V(R) and V(R) + k - 1
inclusive, shall be saved in the receive buffer.
An N(S) sequence exception condition occurs in the DTE when an I frame received contains the N(S) that is not
equal to the DTE receive state variable. The DTE shall not acknowledge (i.e., increment its receive state
variable) the I frame causing the sequence error, or any I frames which may follow, until an I frame with the
correct N(S) is received.
A DTE which receives one or more I frames having sequence errors or subsequent supervisory frames (RR,
RNR, REJ or SREJ (with F bit set to “1 ”)) shall accept the control information contained in the N(R) field and the
P or F bit to perform link control functions; for example, to receive acknowledgment for I frames previously
transmitted by the DTE and to respond (P bit set to “1 ”).
The means specified in, 4.4;2.1 to 4.4.2.4 shall: be avaiiabde for, init&ing the retransmission of lost or errored I
f,kames foll;mifig the occurrence of! a N(S) seqwence error comdititin,.
Page 73
Subclause 4.4.2.1
At the end of this subclause, add the following:
When using the multi-selective reject Option, Checkpoint retransmissions shall be performed only when an RR
response frame with the F bit set to “1” is received and no new I frames have been sent subsequent to the last
frame with the P bit set to “1 ”. In this case, the DTE shall retransmit all unacknowledged I frames, except those I
frames that were transmitted subsequent to the last frame with the P bit set to “1 ”.
Page 14
Subclause 4.4.2.3
Renumber 4.4.2.3 as 4.4.2.4 and insert the following new subclause:
4.4.2.3 SREJ recovery
Only SREJ recovery shall be used when the multi-selective reject Option is used; REJ recovery shall not be used.
The SREJ frame shall be used to initiate more efficient error recovery by selectively requesting the
retransmission of one or more (not necessarily contiguous) lost or errored I frame(s) following the detection of
sequence errors, rather than requesting the retransmission of all I frames. When a DTE receives an out-of-
11

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ISO/lEC 7776:1995/Amd.l: 1996(E) 0 ISO/IEC
sequence frame, the I frame shall be saved in a receive buffer. The I frame shall be delivered to the upper layer
only when all I frames numbered below N(S) are correctly received. If frame number N(S) - 1 has not been
received previously, then an SREJ response frame with the F bit set to “0” shall be transmitted, that contains the
sequence numbers of the block of consecutive missing I frames ending at N(S) - 1. The DTE on receiving such
an SREJ frame shall retransmit all requested I frames. After having retransmitted these l frames, the DTE may
transmit new I frames, if they become available.
When a DTE receives a command frame with the P bit set to “l”, if there are out-of-sequence I frames saved in
the receive buffer, it shall transmit an SREJ frame with the F bit set to “l”, that contains a complete list of
missing sequence numbers. The DTE on receiving such an SREJ frame shall retransmit all requested I frames,
except those that were transmitted subsequent to the command frame with the P bit set to “1 ”.
Page 14
Old subclause 4.4.2.3 renumbered as 4.4.2.4
Replace with the following:
4.4.2.4 Time-out recovery
If the DCUremote DTE, due to a transmission error, does not receive (or receives and discards) a transmission
consisting of a Single I frame or the last I frame in a sequence of I frames, it will not detect an out-of-sequence
error exception, and therefore, will not transmit a REJ or SREJ frame.
If the multi-selective reject Option is not used, then the DTE which transmitted the unacknowledged I frame(s)
shall, following the completion of a System specified time- out period (see 5.8.1) take appropriate recovery action
to determine at which I frame retransmission shall begin. The retransmitted I frames may contain an N(R) and a
P bit that are updated from, and therefore different from, the ones contained in the originally transmitted frames.
If the multi-selective reject Option is used, then the DTE which transmitted the unacknowledged I frame(s) shall,
following the completion of a System specified time-out period (see 5.8.1) , send a supervisory command frame
(RR or RNR) with the P bit set to “1 ”. I frames shall only be retransmitted on the receipt of an RR response frame
with the F bit set to 1 or an SREJ frame.
Page 14
Subclause 4.4.4
Replace with the following:
A frame rejection condition shall be established at the DTE upon the receipt of an error-free frame with one of the
conditions listed in 4.3.10.
This frame rejection exception condition shall be reported by the DTE by a FRMR response frame for appropriate
action by the DCE/remote DTE. Once the DTE has established such an exception condition, no additional I
frames shall be accepted, except for examination of the P bit, until the condition is reset by the DCE/remote DTE.
The FRMR response may be repeated by the DTE at each opportunity, as specified in 5.7.2, until recovery is
effected by the DCE/remote DTE and the data link is reset, or until the local DTE initiates its own recovery.
Page 15
Subclause 5.2
Replace with the following:
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0 ISO/IEC
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