ISO/IEC 4335:1993

Information technology -- Telecommunications and information exchange between systems - High-level data link control (HDLC) procedures - Elements of procedures

Specifies elements of data link control procedures for synchronous or start/stop, code-transparent data transmission using the HDLC frame structure specified in ISO/IEC 3309 and independent frame numbering in both directions. These HDLC elements of procedures are defined specifically in terms of the actions that occur on receipt of commands at a secondary station or a combined station.

Technologies de l'information — Télécommunications et échange d'informations entre systèmes — Procédures de commande de liaison de données à haut niveau (HDLC) — Éléments de procédures

General Information

• Status: Withdrawn
• Publication Date: 22-Dec-1993
• Withdrawal Date: 22-Dec-1993
• ICS: 35.100.20 - Data link layer
• Technical Committee: ISO/IEC JTC 1/SC 6 - Telecommunications and information exchange between systems
• Drafting Committee: ISO/IEC JTC 1/SC 6 - Telecommunications and information exchange between systems
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 22-Aug-2002
• Completion Date: 30-Oct-2025

Relations

• Amended By: ISO/IEC 4335:1993/Amd 7:1995 - Information technology -- Telecommunications and information exchange between systems - High-level data link control (HDLC) procedures - Elements of procedures - Amendment 7: Enhanced multi-selective reject option
• Effective Date: 06-Jun-2022
• Amended By: ISO/IEC 4335:1993/Amd 6:1995 - 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 modulus beyond 128
• Effective Date: 06-Jun-2022
• Revises: ISO/IEC 4335:1991 - Information technology -- Telecommunications and information exchange between systems - High-level data link control (HDLC) procedures - Elements of procedures
• Effective Date: 15-Apr-2008
• Revised: ISO/IEC 13239:2002 - Information technology - Telecommunications and information exchange between systems - High-level data link control (HDLC) procedures
• Effective Date: 15-Apr-2008
• Parent: ISO/IEC 4335:1993/Amd 6:1995 - 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 modulus beyond 128
• Effective Date: 15-Apr-2008
• Parent: ISO/IEC 4335:1993/Amd 7:1995 - Information technology -- Telecommunications and information exchange between systems - High-level data link control (HDLC) procedures - Elements of procedures - Amendment 7: Enhanced multi-selective reject option
• Effective Date: 15-Apr-2008

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 4335
Fifth edition
1993-12-15
Information technology -
Telecommunications and information .
exchange between systems - High-level
data link control (HDLC) procedures -
Elements of procedures
Technologies de /‘information - T&?communications et Gchange
d’informations entre syst&mes - Pro&dues de commande de
liaison de don&es 9 haut niveau (HDLC) - &ments de proce’dures
Reference number
ISO/IEC 4335: 1993(E)
Page
Contents
. . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Foreword
iv
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction
1 scope .
2 Normative references .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Definitions
4 Data link channel states .
5 Modes .
..............................................................................
6 Control field and parameters
.................................................................................. 9
7 Commands and responses
8 Exception condition reporting and recovery .
Annexes
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A Time-out function considerations
B Examples of the use of commands and responses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0 ISO/IEC 1993
All rights reserved. Unless otherwise specified, no part of this publication may be
reproduced or utilized in any form or by any means, electronic or mechanical, including
photocopying and microfilm, without permission in writing from the publisher.
ISO/IEC Copyright Office l Case postale 56 l CH-1211 Genkve 20 l Switzerland
Printed in Switzerland
ii
0 ISO/IEC ISO/IEC 4335:1993(E)
Foreword
IS0 (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 IS0 or IEC participate in
the development of International Standards through technical committees estab-
lished by the respective organization to deal with particular fields of technical
activity. IS0 and IEC technical committees collaborate in fields of mutual inter-
est. Other international organizations, governmental and non-governmental, in
liaison with IS0 and IEC, also take part in the work.
In the field of information technology, IS0 and IEC have established a joint
technical committee, ISO/IEC JTC 1. Draft International Standards adopted by
the joint technical committee are circulated to national bodies for voting. Publi-
cation as an International Standard requires approval by at least 75 % of the na-
tional bodies casting a vote.
International Standard ISO/IEC 4335 was prepared by Joint Technical Committee
ISO/IEC JTC 1, Informution technology, Subcommittee SC 6, Telecommuni-
cations and information exchange between systems.
This fifth edition cancels and replaces the fourth edition (ISO/IEC 4335:1991),
and incorporates ISO/IEC 433YAmd. 4.
Annexes A and B of this International Standard are for information only.
. . .
o ISO/IEC
Introduction
General
High-level data link control (HDLC) procedures are designed to permit synchronous
or start/stop, code-transparent data transmission. This International Standard describes
the HDLC elements of procedures. Further studies are in progress to identify and
define additional elements of procedures which may be included at a later date.
In HDLC procedures, the normal cycle of the code-transparent data communication
between two data stations consists of the transfer of frames containing information
from the data source to the data sink acknowledged by a frame in the opposite
direction. Until the data station comprising the data source receives an
acknowledgement, it holds the original information in memory in case the need should
arise for retransmissions.
Data sequence integrity between the data source and the data sink is effected by means
of a numbering scheme, which is cyclic within a modulus specified in this
International Standard and measured in terms of frames. An independent numbering
scheme is used for each data source/data sink combination on the data link.
HDLC procedures are applicable to unbalanced data links and to balanced data links.
Unbalanced data links
An unbalanced data link involves two or more participating data stations. For control
purposes, one data station on the data link assumes responsibility for the organization
of data flow and for unrecoverable data link level error conditions. The data station
assuming these responsibilities is known as the primary station, and the frames it
transmits are referred to as command frames. The other data stations on the data link
are known as the secondary stations, and the frames they transmit are referred to as
response frames.
For the transfer of data between the primary station and the secondary stations, two
cases of data link control are considered (see figures 1 and 2). In the first case, the
data station comprising the data source performs a primary station data link control
function and controls the data station comprising the data sink that is associated with a
secondary station data link control function, by select-type commands.
iv
oISO/IEC
In the second case, the data station comprising the data sink performs a primary
station data link control function and controls the data station comprising the data
source that is associated with a secondary station data link control function, by poll-
type commands.
The information flows from the data source to the data sink, and the
acknowledgements are always transmitted in the opposite direction.
These two cases of control may be combined so that the data link becomes capable of
two-way alternate communication, or two-way simultaneous communication.
Balanced data links
A balanced data link involves only two participating data stations. For control
purposes, each data station assumes responsibility for the organization of its data flow
and for unrecoverable data link level error conditions associated with the
transmissions that it originates. Each data station is known as a combined station and
is capable of transmitting and receiving both command and response frames.
For the transfer of data between combined stations, the data link control functions
illustrated in figure 3 are utilized. The data source in each combined station controls
the data sink in the other combined station by the use of select-type commands. The
information flows from the data source to the data sink, and the acknowledgements are
always transmitted in the opposite direction. The poll-type commands may be used by
each combined station to solicit acknowledgements and status responses from the
other combined station.
Select/information
A
Secondary
Primary
station station
I
Acknowledgement
Data sink
Data source
Figure 1 - Unbalanced data link configuration (case 1)
Poll/acknowledgement
>
Secondary
Primary
station station
b
Information
Data source
Data sink
Figure 2 - Unbalanced data link configuration (case 2)
Select/infoxmation/acknowledgement/poll
D
Combined Combined
station
station
r=
I I
Select/information/acknowledgement/poll
Data sink/data source
Data sink/data source
Figure 3 - Balanced data link configuration
V
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INTERNATIONAL STANDARD @ Isomc
Telecommunications and
Information technology -
information exchange between systems - High-level data link
Elements of procedures
control (HDLC) procedures -
1 Scope ISO/IEC 3309 : 1993, Information technology -
Telecommunications and information exchange between
This International Standard specifies elements of data link
systems - High-level data link control {HDK)
control procedures for synchronous or start/stop, code-
procedures - Frame structure.
transparent data transmission using the HDLC frame structure
specified in ISO/IEC 3309 and independent f&me numbering
3 Definitions
in both directions.
For the purposes of this International Standard, the following
These HDLC elements of procedures are defined specifically
definitions apply (see also IS0 2382-9).
in terms of the actions that occur on receipt of commands at a
secondary station or a combined station. 3.1 abort: A function invoked by a sending primary,
secondary or combined station causing the recipient to
This International Standard is intended to cover a wide range
discard (and ignore) all bit sequences transmitted by the
of applications; for example one-way, two-way alternate or
sender since the preceding flag sequence.
two-simultaneous data communication between data stations
which are usually buffered, including operations on different 3.2 accept: The condition assumed by a data station
types of data circuits; for example multipointlpoint-to-point, (primary, secondary or combined station) upon accepting a
duplex/half-duplex, correctly received frame for processing.
switched/non-switched,
synchronous/start-stop, etc.
3.3 address field (A): The sequence of eight (or any
The defined HDLC elements of procedures are to be multiple of eight, if extended) bits immediately following the
considered as a common basis for establishing different types flag sequence of a frame identifying the
opening
of control procedures. This International Standard does not secondary/combined station sending (or designated to
define any single system and should not be regarded as a receive) the frame.
specification for a data communication system. Not all of the
3.4 address field extension: Enlarging the address field
commands or responses are required for a particular system
to include more addressing information.
implementation.
A secondary/combined station’s
3.5 basic status:
2 Normative references capability to send or receive a frame containing an
information field.
The following standards contain provisions which, through
reference in this text, constitute provisions of this 3.6 centralized control: A control in which all the
International Standard. At the time of publication, the
primary station functions of the data link are centralized in
editions indicated were valid. All standards are subject to one data station.
revision, and parties to agreements based on this International
3.7 combined: That part of a data station that supports the
Standard are encouraged to investigate the possibility of
combined station control functions of the data link.
applying the most recent editions of the standards indicated
below. Members of IEC and IS0 maintain registers of NOTE -The combined station generates commands and
currently valid International Standards. responses for transmission and interprets received commands and
responses. Specific responsibilities assigned to a combined station
IS0 2382-9 : 1984, Data processing - Vocabulary -Part 9:
include:
Data communication.
0 ISO/IEC
initialization of control signal interchange; b) address/control field interpretation;
organization of data flow; c) command/response generation, transmission and
interpretation; and
interpretation of received commands and generation of
appropriate responses; and d) frame check sequence computation and interpretation.
actions regarding error control and error recovery functions at 3.17 data transmission: See IS0 2382-9, term 09.01.02.
the data link level.
3.18 duplex transmission: See IS0 2382-9, term
3.8 command: In data communication, an instruction 09.03.01.
represented in the control field of a frame and transmitted by
3.19 exception condition: The condition assumed by a
the primary/combined station. It causes the addressed
secondary/combined station upon receipt of a frame which it
secondary/combined station to execute a specific data link
cannot execute due either to a transmission error or to an
control function.
internal processing malfunction.
3.9 command frame:
3.20 flag sequence (F): the unique sequence of eight bits
a) All frames transmitted by a primary station. (01111110) employed to delimit the opening and closing of a
frame.
b) Those frames transmitted by a combined station that
contain the address of the other combined station. 3.21 frame: The sequence of address, control, information,
and FCS fields, bracketed by opening and closing flag
3.10 contention mode: A mode of transmission in which
sequences.
a transmitter can send on its own initiative.
NOTE - A valid frame is at least 32 bits in length and contains an
3.11 control escape (CE): The unique sequence of eight
address field, a control field and a frame check sequence. A frame
bits (10111110) employed to indicate the following octet has
may or may not include an information field.
been modified according to the transparency algorithm for
start/stop transmission environments. 3.22 frame check sequence (FCS): The field
immediately preceding the closing flag sequence of a frame,
3.12 control field (C): The sequence of eight (or sixteen,
containing the bit sequence that provides for the detection of
if extended) bits immediately following the address field of a
transmission errors by the receiver.
hme.
3.23 half-duplex transmission: See IS0 2382-9, term
NOTE - The content of the control field is interpreted by:
09.03.02.
a) the receiving secondary/combined station, designated by the
3.24 higher level: The conceptual level of control or
address field, as a command instructing the performance of
processing logic existing in the hierarchical structure of a data
some specific function; and
station (primary, secondary or combined station) that is above
b) the receiving primary/combined station as a response from the data link level and upon which the performance of data
the secondary/combined station, designated by the address link level functions are dependent; for example device
field, to one or more commands. control, buffer allocation, station management, etc.
3.13 control field extension: Enlarging the control field 3.25 information field (INFO): The sequence of bits,
to include additional control information. occurring between the last bit of the control field and the first
bit of the frame check sequence.
3.14 data communication: See IS0 2382-9, term
09.01.03. NOTE - The information field contents of I and UI frames are not
interpreted at the data link level.
3.15 data link: An assembly of two or more terminal
installations and the interconnecting line operating according 3.26 interframe time fill: The sequence or condition
to a particular method that permits information to be transmitted between frames.
exchanged.
3.27 intraframe time fill: In start/stop transmission, the
NOTE - In this context, the term “terminal installation” does not sequence or condition transmitted within a frame when the
include the data source and the data sink. next octet is not available for contiguous transmission
immediately following the preceding octet. For synchronous
3.16 data link level: The conceptual level of control or
transmission, there is no provision for intraframe time fill.
processing logic existing in the hierarchical structure of a data
station (primary, secondary or combined station) that is 3.28 invalid frame: A sequence of bits, following the
responsible for maintaining control of the data link. receipt of an apparent opening flag sequence, that either
NOTE - The data link level functions provide an interface is terminated by an abort sequence; or
a)
between the data station high level logic and the data link. These
contains less than 32 bits before an apparent closing
b)
functions include:
flag sequence is detected.
a) transparency;
o ISO/IEC
4.1.2 Abort
3.29 primary station:
The data station that supports the primary station control 4.1.2.1 Synchronous transmission
functions of the data link, i.e. that generates commands for
Aborting a frame shall be accomplished by transmitting at
transmission and interprets received responses.
least seven contiguous “1” bits (with no inserted “0” bits) to
responsibilities
NOTE - Specific assigned to the primary end the frame. Receipt of seven contiguous “1” bits shall be
include: interpreted as an abort and the receiving data station shall
ignore the frame.
a) initialization of control signal interchange;
NOTE - If more than seven ” 1” bits are sent to abort, care should
b) organization of data flow; and
be taken because if 15 or more “1” bits are sent, including those
actions regarding error control
and error recovery functions at already transmitted at the time of the decision to abort, an idle data
Cl
the data link level. link channel state will result.
3.30 primary/secondary station: The general case 4.1.2.2 Start/stop transmission
where the station may be either a primary station or a
Aborting a frame shall be accomplished by transmitting the
secondary station.
two-octet sequence “control escape-closing flag”. Receipt of
3.31 response: In data communication, a reply represented this sequence shall be interpreted as an abort and the
in the control of a response frame that advises the receiving data station shall ignore the frame.
primary/combined station with respect to the action taken by
4.13 Interframe time fill
the secondary/combined station to one or more commands.
4.13.1 Synchronous transmission
3.32 response frame:
Interframe time fill shall be accomplished transmitting
bY
All frames transmitted by a secondary station.
contiguous flags between frames.
b) Those frames transmitted by a combined station that
4.1.3.2 Start/stop transmission
contain the address of the transmitting combined
station. Interframe time fill shall be accomplished by the transmission
of either continuous flags or marking condition (logical “1”
The data station that executes
3.33 secondary station:
state) between frames. Selection of the interframe time fill
data link control functions as instructed by the primary
method depends on systems requirements.
station.
4.1.4 Intraframe time fill
- A secondary station interprets received commands and
responses for transmission. 4.1.4.1 Synchronous transmission
generates
3.34 secondary station status: The current condition of There is no provision for time fill within a frame.
a secondary station with respect to processing the series of
4.1.4.2 Start/stop transmission
commands received from the primary station.
Inter-octet time fill within a frame shall be accomplished by
3.35 two-way alternate data communication: See
transmitting continuous mark-hold condition (logical “1”
IS0 2382-9, term 09.0503.
state). There is no provision for time fill within an octet (i.e.,
3.36 two-way simultaneous data communication: between the start element and stop element).
See IS0 2382-9, term 09.0502.
4.2 Idle data link channel state
commands: The commands that do
3.37 unnumbered
4.2.1 Synchronous transmission
not contain sequence numbers in the control field.
A data link channel is in an idle state when a continuous “1”
3.38 unnumbered responses: The responses that do not
state is detected that has persisted for at least 15 bit times;
contain sequence numbers in the control field.
detection of the idle state at the data link level shall be
considered to indicate that the remote data station has
4 Data link channel states
relinquished its right to continue transmission.
4.1 Active data link channel state
4.2.2 Start/stop transmission
4.1.1 General
A data link channel is in the idle state when a continuous
A data link channel is in an active state when the primary mark-hold condition persists for the period of time
determined by a system-specified timeout function. The
station, a secondary station or a combined station is actively
transmitting a frame, a single abort sequence, or interframe duration of this timer is not a subject of this International
Standard.
time fill. In the active state, the right to continue transmission
shall be reserved.
5 Modes
Three operation .a1 modes and three non-operational modes are
defined.
o ISO/IEC
These two disconnected modes (NDM and ADM) are
5.1 Operational modes
provided to prevent a secondary/combined station Tom
The three operational modes are:
appearing on the data link in a fully operational mode during
a) normal response mode (NRM); unusual situations or exception conditions since such
operation could cause:
asynchronous response mode (ARM); and
b)
a) unintended contention in ARM;
asynchronous balanced mode (ABM).
Cl
b) sequence number mismatch between the primary
5.1.1 Normal response mode (NRM)
station and the secondary station, or between
In NRM, which is an unbalanced data link operational mode, combined station; or
the secondary station shall initiate transmission only as the
ambiguity in the primary/combined station as to the
Cl
result of receiving explicit permission to do so from the
status of the secondary/other combined station.
primary station. After receiving permission, the secondary
station shall initiate a response transmission. The response A secondary station shall be system predefined as to the
transmission shall consist of one or more frames while condition(s) that causes it to assume a disconnected mode.
maintaining an active data link channel state. The last frame The disconnected mode (NDM or ADM) shall also be system
of the response transmission shall be explicitly indicated by predefined. A combined station shall be system predefined as
the secondary station. Following indication of the last frame, to the condition(s) that causes it to assume the asynchronous
the secondary station shall stop transmitting until explicit disconnected mode (ADM).
permission is again received from the primary station.
The secondary station capability in a disconnected mode shall
5.1.2 Asynchronous response mode (ARM) be limited to:
In ARM, which is an unbalanced data link operational mode, accepting and responding to one of several
the secondary station may initiate transmission without appropriate mode-setting commands [set normal
receiving explicit permission to do so from the primary response mode (SNRM), set asynchronous response
Such an asynchronous transmission may contain
station. mode (SARM), set normal response mode extended
single or multiple frames and shall be used for information (SNRME), set asynchronous response mode
field transfer and/or to indicate status changes in the extended (SARME), set initialization mode (SIM),
secondary station (for example, the number of the next and disconnect (DISC)];
expected information frame, transition from a ready to a busy
accepting and responding to an exchange
b)
condition or vice versa, occurrence of an exception
identification (XID) command;
condition).
accepting and responding to a test (TEST) command;
c)
5.13 Asynchronous balanced mode (ABM)
accepting and responding to an unnumbered poll
d)
In ABM, which is a balanced data link operational mode,
(UP) command;
either combined station may send commands at any time and
may initiate response frame transmission without receiving transmitting a disconnected mode (DM), request
explicit permission to do so from the other combined station. initialization mode (RIM), exchange identification
Such an asynchronous transmission may contain single or (XID), or request disconnect (RD) response frame at
multiple frames and shall be used for information field a respond opportunity to solicit a specific action on
transfer and/or to indicate status changes in the combined the part of the primary station;
station (for example, the number of the next expected
accepting an unnumbered information
f) WI)
information frame, transition from a ready to ‘a busy condition
command; and
or vice versa, occurrence of an exception condition).
transmitting a UI response at a respond opportunity.
g)
5.2 Non-operational modes
The capability of a combined station, as a receiver of
The three non-operational modes are:
commands, in the asynchronous disconnected mode, shall be
normal disconnected mode (NDM); the same as that stated above for a secondary station
a)
[appropriate mode setting commands for a combined station
asynchronous disconnected mode (ADM); and
b)
include the set asynchronous balanced mode (SABM), set
initialization mode (IM).
asynchronous balanced mode extended (SABME), SIM and
C)
DISC commands]. In addition, since the combined station
The disconnected modes (NDM and ADM) differ from the
has the ability to transmit commands at any time, the
operational modes in that the secondary/combined station is
combined station may transmit an appropriate mode setting,
logically disconnected from the data link; i.e. no information
XID, UI or TEST command.
(I) or supervisory frames are transmitted or accepted. The
initialization mode (IM) differs from the operational modes in A secondary/combined station in a disconnected mode (NDM
that the secondary/combined station data link control program or ADM) shall, as a minimum capability, be capable of
is either in need of regeneration or is in need of an exchange generating the DM response with the F bit set to “1” in
of the parameters to be used in an operational mode. response to a command frame received with the P bit set to
o ISO/IEC
response, as appropriate. Any command with the P bit set to
“O”, other than the implemented mode setting, XID, UI,
A secondary/combined station in a disconnected mode (NDM
TEST or UP commands as described above, may be ignored
or ADM) receiving a DISC command shall respond with the
by the secondary station in NDM.
DM responses. A sewndary/wmbined station in the
initialization mode receiving a DISC command shall respond 5.2.2 Asynchronous disconnected mode (ADM)
with the unnumbered acknowledgment (UA) response if it is
In ADM, which is an unbalanced data link or balanced data
capable of actioning the command. A secondary/command
link non-operational mode, the secondary/combined station
station in an operational mode receiving a DISC command
shall be logically disconnected from the data link and shall,
shall respond with the UA response.
therefore, not be permitted to accept information in I
Examples of possible conditions (in addition to receiving a command frames/I command or response frames,
DISC command) which shall cause a secondary/combined respectively, or to transmit information in I response frames/I
station to enter a disconnected mode are: command or response frames, respectively. The
secondary/combined station shall, however, be permitted to
the secondary/combined station power is turned on,
a)
accept information in UI command frames/U1 command or
or restored following a temporary loss of power;
response frames, respectively, or to transmit information in
the secondary/combined station data link level logic UI response frames/U1 command or response frames,
b)
is manually reset; and respectively. The secondary station, or combined station as a
receiver of commands, has asynchronous mode respond
the secondary/combined station terminal is manually
C>
opportunity and may initiate a response transmission in two-
switched from a local (home) condition to a
way alternate exchange upon detection of an idle data link
connected-on-the-data-link (on-line) condition.
channel state, and in two-way simultaneous exchange at any
A secondary/combined station in a non-operational time. Such a response transmission shall only consist of a UI
shall not establish a frame reject exception condition. response frame, a request for a mode setting command (DM),
a request for exchange of identification (XID), or a request
5.2.1 Normal disconnected mode (NDM)
for initialization (RIM) if the secondary station, or combined
station as a receiver of commands, determines it is unable to
In NDM, which is an unbalanced data link non-operational
function. The combined station, as a transmitter of command
mode, the secondary station shall be logically disconnected
from the data link and shall, therefore, not be permitted to frames, is also permitted to send a UI command frame at any
accept information in I command frames or to transmit asynchronous mode respond opportunity.
information in I response frames. The secondary station
In this mode a secondary station, or combined station as a
shall, however, be permitted to accept information in UI
receiver of commands, shall action only mode setting, XID,
command frames and to transmit information in UI response
UI and TEST commands. Mode setting commands, except
frames. The secondary station has normal mode respond
the DISC command, that can be actioned shall be responded
opportunity and shall initiate a single frame response
to with the UA response at the earliest respond opportunity.
transmission, indicating its status, as a result of receiving a
An XID or TEST command that can be actioned shall be
command frame with the P bit set to “1”; optionally, it may
responded to with the XID or TEST response, respectively, at
initiate such a response as the result of receiving a UP
the earliest respond opportunity. Receipt of an implemented
command with the P bit set to “0”.
mode setting, XID or TEST command that cannot be
In this mode, a secondary station shall action only mode actioned, or receipt of any other command (except a UI
setting, XID, UI and TEST commands. Mode setting command) with the P bit set to “l”, shall be responded to with
commands, except the DISC command, that can be actioned a DM response, or, if the secondary station, or combined
shall be responded to with the UA response at the earliest station as a receiver or commands, determines it is unable to ,
respond opportunity. An XID or TEST command that can be function, with the RIM response. Receipt of a UI wmmand
actioned shall be responded to with the XID and TEST with the P bit set to “1” shall cause a secondary/combined
response, respectively, at the earliest respond opportunity. station in ADM to respond at the earliest respond opportunity
Receipt of an implemented mode setting, XID or TEST with a UI response, with a DM response, or with a RIM
command that cannot be actioned, or receipt of any other response. Any command with the P bit set to “O”, other than
command (except a UI command) with the P bit set to “l”, the implemented mode setting, XID, UI, TEST or UP
shall cause a secondary station in NDM to respond at the commands as described above, may be ignored by the
secondary/combined station in ADM.
earliest respond opportunity with the DM response, or, if the
secondary station determines it is unable to function, with the
Because a combined station is also a generator of commands,
RIM response. Receipt of a UI command with the P bit set to
it can terminate a disconnected mode at any time by
“1” shall cause a secondary station in NDM to respond at the
transmitting an appropriate mode setting command (SABM,
earliest respond opportunity with a UI response, with a DM
SABME, or SIM). Such action can be taken spontaneously or
response, or with a RIM response. In the case where an
as a result of transmission received from the other combined
implemented mode setting, XID or TEST command has been
station (for example, a DM or RIM response).
received but cannot be actioned or a status condition is to be
reported, a UP command with the P bit set to “0” shall cause a
secondary station in NDM to respond with a DM or RIM

o ISO/IEC
5.2.3 Initialization mode (IM) = transmitting send sequence number
* N(S)
(bit 2 = low-order bit)
In IM, which is an unbalanced data link or balanced data link
non-operational mode, a secondary/one combined station data
N(R) = transmitting receive sequence number
link control program may be initialized or generated by the
(bit 6 = low-order bit)
primary/other combined station action, or other parameters to
be used in the operational mode may be exchanged. IM is
s = supervisory function bit
invoked when the primary/one combined station concludes
that a secondary/other combined station is operating
M = modifier function bit
abnormally and needs its data link control program corrected,
and for upgrading a secondary/other combined station data
Similarly, a secondary/one combined P/F = poll bit - primary station or combined station
link control program.
command frame transmissions/final bit -
station may determine it is unable to function due to program
secondary station or combined station response
checks and request IM to obtain a good program from the
frame transmissions
primary/other combined station.
(1 = poll/final)
A secondary/combined station shall enter IM upon sending a
UA response, at its system predefined respond opportunity, in
6.1.2 Information transfer (I) format
response to the receipt of a SIM command. A
The I format is used to perform an information transfer. The
secondary/combined station may request a SIM command by
functions of N(S), N(R) and P/F are independent; i.e. each I
sending a RIM response. In IM, the primary/one combined
frame shall have an N(S) sequence number, an N(R) sequence
station and a secondary/other combined station may exchange
number which may or may not acknowledge additional I
information in the predetermined manner specified for that
frames at the receiving data station, and a P/F bit that may be
secondary/each combined station (for example, UI or I
set to ” 1” or “0”.
fkimes).
6.13 Supervisory (S) format
IM shall be terminated when a secondary/combined station
receives and acknowledges (via a UA response) one of the
The S format is used to perform data link supervisory control
other mode setting commands, or when entering the
functions such as acknowledging I frames, requesting
disconnected mode caused by internal constraints such as loss
retransmission of I frames, and requesting a temporary
of power.
suspension of transmission of I frames. The functions of
N(R) and P/F are independent; i.e. each S format frame shall
6 Control field and parameters have an N(R) sequence number which may or may not
acknowledge additional I frames at the receiving data station,
6.1 Control field formats
and a P/F bit that may be set to ” 1” or “0”.
6.1.1 General
6.1.4 Unnumbered (U) format
The three formats defined for the control field (see table 1)
The U format is used to provide additional data link control
are used to perform numbered information transfer, numbered
functions and unnumbered information transfer. This format
supervisory functions and unnumbered control functions and
shall contain no sequence numbers, but shall include a P/F bit
unnumbered information transfer.
that may be set to “1” or “0”. Five “modifier” bit positions
Table 1 - Control field formats
are available, this allowing definition of up to 32 additional
command functions and 32 additional response functions.
Control field
format for
6.2 Parameters
6.2.1 Modulus
Information transfer command/
response (I format)
Each I frame shall be sequentially numbered with a number
supervisory commands/ which may have the value 0 to modulus minus one inclusive
responses (S format) (where modulus is the modulus of the sequence number).
N(R) 1
lhMPIFI
The modulus equals 8 or 128. The sequence numbers cycle
Unnumbered commands/
through the entire range. The control field formats for
responses (U format)
modulo 8 are shown in table 1. The control field formats for
modulo 128 are shown in table 4 (see 7.4).
The maximum number of sequentially numbered I frames that
secondary or combined station may have
a primary,
outstanding (i.e. unacknowledged) at any given time shall
never exceed one less than the modulus of the sequence
numbers. This restriction is to prevent any ambiguity in the
association of transmitted I frames with sequence numbers
during normal operation and/or error recovery action.

o ISO/IEC
TSO/IEC 4335:1993(E)
NOTE - The number of outstanding I frames may be further of the receive state variable. The N(R) indicates that the
restricted by the data station frame storage capability; for example, the station transmitting the N(R) has correctly received all I
number of I frames that can be stored for transmission and/or frames numbered up to N(R) - 1 inclusive.
retransmission in the event of a transmission error. Optimum data
In the case of the SREJ frame with the P/F bit set to “0”. the
link efficiency can only be obtained, however, if the minimum data
N(R) indicates only that the I frame with N(S) equal to N(R)
station frame storage capacity is sufficient for the maximum
has not been received.
anticipated round trip transmission delay.
As more than one SREJ frame with the P/F bit set to “0” may
6.2.2 Frame state variables and sequence numbers
be outstanding at any one time, it is necessary to ensure that
6.2.2.1 General all non-received I frames are ultimately correctly received.
This may be achieved by multiple variable counters or by
In HDLC operations, each data station shall maintain an
other means.
independent send state variable V(S) and an independent
(See 7.2.4 for definition of the use of the SREJ command and
receive state variable V(R) for the I frames it sends to and
receives from another data station. Each secondary station response.)
shall maintain a V(S) for the I frames it transmits to the
6.2.3 Poll/final (P/F) bit
primary station and an V(R) for the I frames it correctly
The poll (P) bit set to “1” shall be used by the
receives from the primary station. In the same manner, the
primary/combined station to solicit (poll) a response or
primary station shall maintain an independent V(S) and V(R)
sequence of responses from the secondary
for I frames sent to and received from, respectively, each
secondary station on the data link. Each combined station station(s)/combined station.
shall maintain a V(S) for the I frames it transmits to the other
The final (F) bit set to “1” shall be used:
combined station’ and a V(R) for the I frames it correctly
receives from the other combined station. a) by a secondary station in NRM to indicate the final
frame transmitted as the result of a previous
6.2.2.2 Send state variable V(S)
soliciting (poll) command; and
The send state variable denotes the sequence number of the
b) by a secondary station in ARM and by a combined
next in-sequence I frame to be transmitted. The send state
station in ABM to indicate the response frame
variable can take the value 0 to modulus minus one inclusive
transmitted as the result of a soliciting (poll)
(where modulus is the modulus of the sequence numbering
command.
scheme and the numbers cycle through the entire range). The
value of the send state variable shall be incremented by one 6.3 Functions of the poll/final (P/F) bit
with each successive I frame transmission, but shall not
The poll/final (P/F) bit shall serve a function in both
exceed N(R) of the last received frame by more than modulus
command frames and response frames. (In command frames,
minus one.
the P/F bit is referred to as the P bit. In response frames, it is
6.2.2.3 Send sequence number N(S) referred to as the F bit.)
Only I frames shall contain N(S), the send sequence number 63.1 Functions of the poll bit
of transmitted frames. Prior to transmission of an in-
6.3.1.1 General
sequence I frame, N(S) shall be set equal to the value of the
send state variable. The P bit set to ’ 1” shall be used to solicit a response frame
with the F bit set to “1” from the secondary/combined station.
6.2.2.4 Receive state variable V(R)
On a data link, only one frame with a P bit set to “1” shall be
The receive state variable denotes the sequence number of the
outstanding in a given direction at a given time. Before a
next in-sequence I frame expected to be received. The
primary/combined station issues another frame with the P bit
receive state variable can take the value 0 to modulus minus
set to “1 “, it shall have received a response frame from the
one inclusive (where modulus is the modulus of the sequence
secondary/combined station with the F bit set to “1”. If no
numbering scheme and the numbers cycle through the entire
valid response frame is obtained within a system-defined
rate). The value of the receive state variable shall be
time-out period, the retransmission of a command with the P
incremented by one on receipt of an error-free, in-sequence I
bit set to “1” for error recovery purposes shall be permitted.
frame whose send sequence number N(S) equals the receive
6.3.1.2 Functions of the poll bit in NRM
state variable.
6.2.2.5 Receive sequence number N(R) In NRM, the P bit shall be set to ‘1” to solicit response frames
The secondary station shall not
from the secondary station.
All I frames and S format frames shall contain N(R) which
transmit until it receives either a command frame with the P
with the exception of the selective reject (SREJ) supervisory
bit set to “1” or a UP command.
frame with the P/F bit set to “0” shall indicate the N(S)
sequence number of the next expected I frame. The secondary station may send I frames upon receipt of an I
frame with the P bit set to “l”, certain S frames (RR, REJ or
transmission of an I frame or S
With this exception, prior to
SREJ) with the P bit set to “l”, a UI command with the P bit
be set equal to the current value
format f&me, the N(R) shall
set to “l”, or a UP command with the P bit set to “1” or “0”.

o ISO/IEC
63.13 Functions of the poll bit in ARM and ABM 633 Use of the P/F bit to assist in error recovery (see also
clause 8)
In ARM and ABM, the P bit set to “1” shall be used to solicit
a response, at the earliest respond opportunity, with the F bit 633.1 General
set to “1”.
As the P and F bits set to “1” are always exchanged as a pair
NOTE - For example, if the primary/combined station requires
(for every P bit there shall be one F bit, and another P bit
positive acknowledgment that a particular command has been
shall not be issued until the previous P bit has been matched
received, it sets the P bit in the command to “1”. This forces a with an F bit, and, similarly, another F bit shall not be issued
respanse from the secondary/combined station as described in 6.3.2.2.
until another P bit is received), the N(R) contained in a
received frame with a P bit (see 8.2.lh) or F bit set to “1” can
63.2 Functions of the final bit
be used to detect that I frame retransmission is required. This
A response frame with the F bit set to “1” shall be used by the
capability provides early detection of I frames not received by
secondary/combined station to acknowledge the receipt of a
the remote data station
...