ISO/IEC 4335:1993/Amd 6:1995

INTERNATIONAL
ISO/IEC
STANDARD
Fifth edition
1993-12-15
AMENDMENT 6
1995-11-15
Information technology - Telecommunications
and information exchange between Systems -
High-level data link control (HDLC) procedures -
Elements of procedures
AMENDMENT 6: Extension of HDLC sequence
number moduius beyond 128
Technologies de I ’information - T&kommunications et behange d ’information
en tre systkmes - Prockdures de commande de liaison de donnkes 2 haut niveau
(HDLC) - &ments de prockdures
AMENDEMENT 6: Extension du module du numkro de skquence HDLC au-del2
de 728
Reference number
ISO/lEC 4335: 1993/Amd.6: 1995(E)

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ISO/IEC 4335:1993/Amd.6:1995(EI)
Foreword
ISO (the International Organization for Standardization) and IEC (the lnter-
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, 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 6 to International Standard lSO/IEC 4335:1993 was prepared
by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 6, Telecommunications and information exchange
between Systems.
0 ISO/IEC 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-1 211 Geneve 20 l Switzerland
Printed in Switzerland
ii

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ISO/IEC 4335:1993/Amd.6:1995(E)
OISOAEC
Introduction
This amendment increases the modulus number (i.e. the sequence number) in Steps up to a
maximum of 2 147 483 648 which tan be represented in 31 bits. This is done by the
introduction of a new “Set Mode” command that tan be used to negotiate or indicate the
modulus in absence of, or to override, a default value. This uses an optional information
field in the mode-setting commands/responses.
This amendment also introduces the information field in mode-setting
commands/responses.
. . .
Ill

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OISO/IEC ISO/IEC 4335: 1993iAmd.6: 1995(E)
Information technology - Telecommunications and information
High-level data link control (HDLC)
exchange between Systems -
procedures - Elements of procedures
AMENDMENT 6: Extension of HDLC sequence number modulus beyond 128
Page 4
Subclause 5.2
Change item (a) of the fifth Paragraph to read as follows:
a) accepting and responding to one of several appropriate mode-setting commands [set
normal response mode (SNRM), set asynchronous response mode (SARM), set normal
response mode extended (SNRME), set asynchronous response mode extended (SARME),
set mode (SM) or set mode (SM) with the mode in the optional information field selected as
normal response mode or asynchronous response mode, set initialization mode (SIM), and
disconnect (DISC)];
In the first sentence of the sixth Paragraph, insert the following after ‘: . . . . extended
(SABME), ” and before WM . . . , . “:
set mode (SM) or SM with the mode in the optional information field selected as balanced
mode,
Page 5
Subclause 5.2.2
In the first sentence of the third Paragraph, Change the Phrase in the parentheses to read
as follo ws:
(SABM, SABME, SM or SM with the mode in the information field selected as balanced
mode, SIM) . . . . . . . . . . . .
Page 6
Subclause 6.2.1
ßeplace the second sentence of the first Paragraph to read as follows:
The modulus equals 8, 128,32 768 or 2 147 483 648.
Add the following at the end of the first Paragraph.
The control field formats for modulo 32 768 are shown in table 6 (see 7.5).
The control field formats for modulo 2 147 483 768 are shown in table 7 (see 7.6).
Page 9
Subclause 7.1
Add the following unnumbered command to table 3.
SM - Set Mode

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ISOhEC 4335: 1995/Amd.6: 1995(E)
@lSO/IEC
Page 10
Subclause 7.2.4
Add the following after the third sentence in Paragraph 2.
For modulo 32 768 sequence numbering the N(R) value for each designated I frame shall
occupy bit positions 2-16 of the two-octet field, as per the encoding of the N(R) field in
octets 3-4 of the control field for modulo 32 768, with bit Position 1 in the first octet of the
two-octet field set to “O ”, as illustrated in Figure 8. For modulo 2 147 483 648 sequence
numbering the N(R) value for each designated I frame shall occupy bit positions 2-32 of the
four-octet field, as per the encoding of the N(R) field in octets 5-8 of the control field for
modulo 2 147 483 768, with bit Position 1 in first octet of the four-octet field set to “O ”, as
illustrated in figure 9.
Page 11
Subclause 7.2.4
Insert
new figures 8 and 9, as shown below. ßenumber present figures 8 to 13 as figures
18
to 23.
Control Field Information Field
I ’ 16 17 18 1 2 16 1 2
16;
3q
1 1 0 0 . . . . . . . . . . . 0 P/F N(R) 0 N(R) 0 N(R) j FCS
; 0 1
I I
Receive sequence Sequence number(s) of
number of first I frame a later I frame(s)
to be retransmitted to be retransmitted
Figure 8 - Control field and information field encoding in SREJ frame for modulo 32
768 numbering
I
I I
I I I
I I
I
Control Field Information Field
I l I
I I I
1 2
34 1 2 32
I ’ 32 33
3r
64;
0 0 N(R) ; FCS
1 1 0 0 . . . . . . . . . . . 0 P/F N(R)
; 1 0 N(R)
I I
Receive sequence Sequence number(s) of
number of first I frame a later I frame(s)
to be retransmitted to be retransmitted
Figure 9 - Control field and information field encoding in SREJ frame for modulo 2
147 483 648 numbering

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ISO/IEC 4335:1993/Amd.6:1995(E)
0lSOAEC
Page 11
Subclause 7.3
ßeplace the last sentence of the first Paragraph by the following:
Fourteen command functions and eight response functions are defined below; all others are
reserved.
Page 12
Subclause 7.3
Add the following at the end of 7.3 and renumber 7.3.1 as 7.3.4 and 7.3.2 as 7.35:
Several unnumbered commands and responses (e.g. SABM, SABME, SNRM, SNRME,
SARM, SARME, SM, UA, DISC, DM) may have an optional information field.
7.3.1 Information field structure
The general structure of the information field is illustrated in figure 10. The first octet of the
information field, when present, shall be a format identifier subfield. One or more data link
layer subfields may follow the format identifier subfield. This may be followed by a User data
subfield.
Format
Identifier
Su bfield
F A Control Field FCS F
kp Information Field ,-d
Figure 10 - Format of the optional information field in specified unnumbered
command and responses
7.3.1 .l Format identifier subfield
The format identifier subfield is a fixed length of one octet and is illustrated in figure 10.
7.3.1.2 Data link layer subfields
The general structure of a data link layer subfield is illustrated in figure 11.
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ISO/IEC 4335: 199WAmd.6: 1995(E) @lSO/IEC
\
Group Identifier (GI) Group Length (GL) Parameter Field
L
Group Identifier (GI): 1 Octet
Group Length (GL): 1 Octet (to indicate the length of the associated Parameter field in octets)
Parameter Field: n Octets
Figure 11 - Data link layer subfield
The data link layer subfields specify various data link layer characteristics and Parameters.
The contents of these subfields are generated and consumed by the data link layer logic.
The length of these subfields is limited by the maximum length restrictions on the HDLC
frame information field, taking into account the lengths of the Fl subfield and the User data
subfield.
In terms of figure 11, a data link layer subfield consists of
Group Identifier (1 octet);
Group Length (1 octet); and
Parameter field (n octets).
The Group Identifier (GI) identifies the function of that data link layer subfield. Three data
link layer subfield identifiers are defined:
Mode and Modulus Group;
Parameter Negotiation Group; and
Private Parameter Group.
The Parameter field consists of a series of Parameter Identifier (Pl) (1 octet), Parameter
Length (PL) (1 octet), and Parameter Value (PV) (m octets) Sets, one set for each defined
data link layer subfield element. The structure of the defined PI/PUPV sets is detailed in
7.3.3.
A data link layer subfield, therefore, has the general organization depicted in figure 12.
Gl GL Pl PL PV . . . Pl PL PV
1 Octet 1 Octet 1 Octet 1 Octet m Octets
Where
.
Pl . Parameter Identifier, expressed as a decimal value
.
.
PL Parameter Length, expressed as a decimal value
Figure 12 - Data link layer subfield for the Parameter negotiation group
7.3.1.3 User data subfield
The optional information field may contain a User data subfield that contains User
information. This data link User information is transported transparently across the data link
layer to the user of the data link. The amount of information (number of bits) that tan be
accommodated is limited only by the maximum length restrictions on the HDLC frame
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OISO/IEC ISO/IEC 4335: 1993IAmd.6: 1995( E)
information field, taking into account the lengths of the Fl subfield and the data link layer
subfields.
The User data subfield is composed of
User data identifier (1 octet), and
User data field (n bits).
The User data subfield, therefore, has the organization illustrated in figure 13.
1 User data identifier 1 User data field 1
Figure 13 - User data subfield
7.3.2 Information field encoding
The format identifier subfield is always the first octet of the optional information field. The
data link layer subfields, if present, follow in ascending Order according to their GI values.
Except where noted, specific data link layer subfields may appear only once. The absence
of a particular data link layer subfield should be interpreted to mean default values. The
User data subfield, if present, is always the last subfield.
7.3.2.1 Format identifier subfield encoding
The format identifier subfield tan be encoded to have the capability of designating 256
different standardized formats. The format identifier shall be encoded as illustrated in figure
14 .
First bit transmitted
, 1 OO~~;rOd~bit, .
Figure 14 - Format identifier subfield encoding
All other values of the format identifier are reserved.
7.3.2.2 Data link layer subfield encoding
7.3.2.2.1 Group identifier encoding
Group identifiers identify various functions that pertain to the data link layer. Figure 15
indicates the GI encodings for the data link layer subfields covered in this International
Standard.
First bit transmitted High Order bit
0 0 0 0 1 Mode and Modulus Identifier
1 0 0
0 0 0 0 1 Parameter Negotiation Identifier
0 0 0
Private Parameter negotiation identifier
0 0 0 0 1 1 1 1
Figure 15 - Data link layer subfield encodings
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ISO/IEC 4335: 1995/Amd.6: 1995(E) @lSO/IEC
The Parameter negotiation data link layer subfield and the private Parameter negotiation data
NOTE -
link layer subfield may each appear more than once in the information field. This allows the
Station to convey multiple menus in the information field.
7.3.2.2.2 Group length encodings
Group length indicates the length of the associated Parameter field. This length is
expressed as a one-octet binary number representing the length of the associated
Parameter field in octets.
NOTE - The Group Length value does not does not include the lengths of either itself or its associated
identifier.
A Group Length value of Zero indicates that there is no associated Parameter field and that
all Parameters within the subfield specified by the associated Group Identifier should
assume their default values.
7.3.2.2.3 Parameter field encoding
A Parameter field contains a series of Parameter Identifier (Pl), Parameter Length (PL) and
Parameter Value (PV) set structures in that Order. Esch Pl identifies a Parameter and is
one octet in length. Esch PL indicates the length of the associated Parameter value (PV)
and is one octet in length. Esch PV contains the Parameter value and is m octets in length.
NOTE - The value of PL does not include the lengths of either itself or its associated PI.
The value of PL is expressed as a one-octet binary number representing the length of the
PV in octets. A PL value of zero indicates that the associated PV is absent and that the
Parameter shall assume the default value.
A PI/PL/PV set may be omitted if it is not required for conveying information or if default
values for the Parameters are to be used. A Parameter field containing a Pl that is not
specified in this International Standard is defined as invalid and shall be ignored (except
within the private negotiation subfield, in which PIS other than PI=0 may be defined by a
...