ISO/IEC 13239:2002

INTERNATIONAL ISO/IEC
STANDARD 13239
Third edition
2002-07-15
Information technology —
Telecommunications and information
exchange between systems — High-level
data link control (HDLC) procedures
Technologies de l'information — Télécommunications et échange
d'information entre systèmes — Procédures de commande de liaison de
données à haut niveau (HDLC)
Reference number
©
ISO/IEC 2002
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©  ISO/IEC 2002
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ii © ISO/IEC 2002 – All rights reserved

Contents Page
Foreword.v
Introduction .vi
1 Scope.1
2 Normative references .2
3 Definitions, acronyms and abbreviations.3
3.1 Definitions.3
3.2 Acronyms and abbreviations.8
4 HDLC frame structure .10
4.1 Frame formats .11
4.2 Elements of the frame .12
4.3 Transparency.15
4.4 Transmission considerations .17
4.5 Inter-frame time fill .17
4.6 Invalid frame .17
4.7 Extensions .18
4.8 Addressing conventions .18
4.9 Frame format field .19
5 HDLC elements of procedures .21
5.1 Data link channel states .21
5.2 Modes .22
5.3 Control field formats.25
5.4 Control field parameters .27
5.5 Commands and responses .31
5.6 Exception condition reporting and recovery .53
6 HDLC classes of procedures.58
6.1 Types of data station .59
6.2 Configurations.60
6.3 Operational modes .60
6.4 Addressing scheme.60
6.5 Send and receive state variables .60
6.6 Fundamental classes of procedures .60
6.7 Optional functions.62
6.8 Consistency of classes of procedures .62
6.9 Conformance to the HDLC classes of procedures.62
6.10 Method of indicating classes and optional functions.63
6.11 Unbalanced operation (point-to-point and multipoint) .66
6.12 Balanced operation (point-to-point) .69
6.13 Unbalanced connectionless operation (point-to-point and multipoint) .73
6.14 Balanced connectionless operation (point-to-point).76
6.15 Uses of the optional functions.78
7 General purpose Exchange Identification (XID) frame.85
7.1 General purpose XID frame information field structure .85
7.2 General purpose XID frame information field encoding.85
7.3 Single-frame exchange negotiation process.91
7.4 Frame check sequence negotiation rules .92
7.5 Rules for negotiation use of the frame format field in non-basic frame format mode.93
8 Resolution/negotiation of data link layer address in switched environments .93
8.1 Operational requirements .93
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8.2 Address resolution . 94
Annex A (informative) Explanatory notes on the implementation of the frame checking sequence . 95
Annex B (informative) Example of the use of commands and responses . 97
Annex C (informative) Time-out function considerations for NRM, ARM and ABM . 118
Annex D (informative) Examples of typical HDLC procedural subsets. 120
Annex E (informative) Illustrative examples of 16/32-bit FCS negotiation . 123
Annex F (informative) Guidelines for communicating with LAPB X.25 DTEs. 125
Annex G (informative) Examples of information field encoding in multi-selective reject frames . 126
Annex H (normative) Frame format types. 127

iv © ISO/IEC 2002 – All rights reserved

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, ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
The main task of the joint technical committee is to prepare International Standards. 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.
Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of patent
rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.
ISO/IEC 13239 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology, Subcommittee SC 6,
Telecommunications and information exchange between systems.
This third edition cancels and replaces the second edition (ISO/IEC 13239:2000), which has been technically revised. It also
cancels and replaces ISO/IEC 3309:1993, ISO/IEC 4335:1993, ISO/IEC 7809:1993 and ISO/IEC 8885:1993.
Annex H forms a normative part of this International Standard. Annexes A to G are for information only.

© ISO/IEC 2002 – All rights reserved v

Introduction
This third edition adds a new frame format type to Annex H – Frame format types. This frame format type is used in those
environments where additional error protection, identification of both the source and the destination(s), and/or longer frame
sizes are needed.
High-level data link control (HDLC) procedures are designed to permit synchronous or start/stop, code-transparent data
transmission. 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.
Generally, 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.
In those situations that require it, data sequence integrity between the data source and the data sink is effected by means of a
numbering scheme, which is cyclic within a specified modulus and measured in terms of frames. An independent numbering
scheme is used for each data source/data sink combination on the data link.
The acknowledgement function is accomplished by the data sink informing the data source of the next expected sequence
number. This can be done in a separate frame, not containing information, or within the control field of a frame containing
information.
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 in unbalanced connection-mode data links and as the
control station in unbalanced connectionless-mode data links, 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 in unbalanced connection-mode data links and as
the tributary stations in unbalanced connectionless-mode data links, and the frames they transmit are referred to as response
frames.
For the transfer of data between the primary/control station and the secondary/tributary stations, two cases of data link control
are considered (see figures A and B). In the first case, the data station comprising the data source performs a primary/control
station data link control function and controls the data station comprising the data sink that is associated with a
secondary/tributary station data link control function, by select-type commands.
In the second case, the data station comprising the data sink performs a primary/control station data link control function and
controls the data station comprising the data source that is associated with a secondary/tributary 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 data link control may be combined so that the data link becomes capable of two-way alternate
communication, or two-way simultaneous communication.

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Select/information
Primary/
Secondary/
Control
Tributary
station station
Acknowledgement
Data source Data sink
Figure A — Unbalanced data link functions (case 1)
Poll/acknowledgement
Primary/
Secondary/
Control
Tributary
station station
Information
Data sink Data source
Figure B — Unbalanced data link functions (case 2)
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 in balanced connection-mode data links and
as a peer station in balanced connectionless-mode data links and is capable of transmitting and receiving both command and
response frames.
For the transfer of data between combined/peer stations, the data link control functions illustrated in figure C are utilized. The
data source in each combined/peer station controls the data sink in the other combined/peer 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/peer station to solicit acknowledgements and
status responses from the other combined/peer station.
Select/information/acknowledgement/poll
Combined/ Combined/
Peer Peer
station station
Select/information/acknowledgement/poll
Data sink/data source Data sink/data source

Figure C — Balanced data link functions
Data link configurations
HDLC classes of procedures describe methods of data link operation which permit synchronous or start/stop, code-transparent
data transmission between data stations in a variety of logical and physical configurations. The classes are defined in a
consistent manner within the framework of an overall HDLC architecture. One of the purposes of this International Standard is
to maintain maximum compatibility between the basic types of procedures, unbalanced, balanced and connectionless, as this is
particularly desirable for data stations with configurable capability, which may have the characteristics of a primary,
secondary, combined, control, tributary, or peer station, as required for a specific instance of communication.
Five fundamental classes of procedures (two unbalanced, one balanced, and two connectionless) are defined herein. The
unbalanced classes apply to both point-to-point and multipoint configurations (as illustrated in figure D using the
primary/secondary nomenclature) over either dedicated or switched data transmission facilities. A characteristic of the
unbalanced classes is the existence of a single primary station at one end of the data link plus one or more secondary stations at
the other end(s) of the data link. The primary station alone is responsible for data link management, hence the designation
"unbalanced" classes of procedures.
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Primary/
Control
station
Secondary/ Secondary/
Tributary Tributary
station station
A N
Figure D — Unbalanced data link configuration
The unbalanced connectionless class applies to point-to-point configurations over either dedicated or switched data
transmission facilities, or to multipoint configurations over dedicated data transmission facilities (as illustrated in figure D
using the control/tributary nomenclature). A characteristic of the unbalanced connectionless class is the existence of a single
control station at one end of the data link plus one or more tributary stations at the other end(s) of the data link. The control
station is responsible for determining when a tributary station is permitted to send. Neither the control station nor the tributary
station(s) support any form of connection establishment/termination procedures, flow control procedures, data transfer
acknowledgement procedures, or error recorvery procedures, hence the designation “connectionless” class of procedures.
The balanced class applies to point-to-point configurations (as illustrated in figure E using the combined nomenclature) over
either dedicated or switched data transmission facilities. A characteristic of the balanced class is the existence of two data
stations, called combined stations, on a logical data link, that may share equally in the responsibility for data link management,
hence the designation "balanced" class of procedures.
I
Combined/ Combined/
Peer Peer
station station
A B
Figure E — Balanced data link configuration
The balanced connectionless class applies to point-to-point configurations over either dedicated or switched data transmission
facilities (as illustrated in figure E using the peer nomenclature). A characteristic of the balanced connectionless class is the
existence of two data stations, called peer stations, on a data link, that are each independently in control of when they can send.
Neither peer station supports any form of connection establishment/termination procedures, flow control procedures, data
transfer acknowledgement procedures, or error recovery procedures, hence the designation "connectionless" class of
procedures.
For each class of procedures, a method of operation is specified in terms of the capabilities of the basic repertoire of commands
and responses that are found in that class.
A variety of optional functions are also listed. Procedural descriptions for the use of the optional functions are defined.
It is recognized that it is possible to construct symmetrical configurations for operation on a single data circuit from the
unbalanced classes of procedures which are defined in this International Standard. For example, the combination of two
unbalanced procedures (with I frame flow as commands only) in opposite directions would create a symmetrical point-to-point
configuration (as illustrated in figure F).
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Primary Secondary
station station
I
1 1
I
I
I
Secondary Primary
I
I
station station
2 2
Figure F — Symmetrical data link configuration
These HDLC procedures define the exchange identification (XID) command/response frame as an optional function for
exchange of data link information (identification, parameters, functional capability, etc.). The content and format for a general
purpose XID frame information field is defined.
These HDLC procedures also specify the parameters and procedures which may be employed by two data stations to mutually
determine the data link layer addresses to be used, prior to logical data link establishment.

© ISO/IEC 2002 – All rights reserved ix

INTERNATIONAL STANDARD ISO/IEC 13239:2002(E)

Information technology — Telecommunications and information exchange
between systems — High-level data link control (HDLC) procedures
1 Scope
This International Standard specifies the frame structures, the elements of procedures, the classes of procedures, the content
and format of the general purpose Exchange Identification (XID) frame, and a means for resolution/negotiation of a data link
layer address in switched environments for data communication systems using bit-oriented high-level data link control
(HDLC) procedures.
NOTE  The use of the phrase “bit-oriented”, referring to the HDLC control procedures, pertains to the allocation of a non-integral number
of bits to various subfields used for HDLC control purposes. However, the frame as an entirety may be constructed from octet-oriented units
(e.g., start-stop mode) for transmission purposes.
The frame structure portion defines the relative positions of the various components of the basic frame format and the non-
basic frame format. The mechanisms used to achieve bit pattern independence (transparency), where and when required, within
the frame are also defined. In addition, three frame checking sequences (FCS) are specified; the rules for address field
extension are defined; and the addressing conventions available are described.
The elements of procedures portion specifies elements of data link control procedures for synchronous or start/stop, code-
transparent data transmission using 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, a tributary station, a peer station, or a combined station.
This International Standard is intended to cover a wide range of applications; for example one-way, two-way alternate or two-
way simultaneous data communication between data stations which are usually buffered, including operations on different
types of data circuits; for example multipoint/point-to-point, duplex/half-duplex, switched/non-switched, synchronous/start-
stop, etc.
The defined elements of procedures are to be considered as a common basis for establishing different types of data link control
procedures. This International Standard does not define any single system and should not be regarded as a specification for a
data communication system. Not all of the commands or responses are required for any particular system implementation.
The classes of procedures portion describes the HDLC unbalanced classes of procedures, the HDLC balanced class of
procedures, and the HDLC connectionless classes of procedures for synchronous or start/stop data transmission.
For the unbalanced classes, the data link consists of a primary station plus one or more secondary stations and operates in
either the normal response mode or the asynchronous response mode in a point-to-point or multipoint configuration. For the
balanced class, the data link consists of two combined stations and operates in the asynchronous balanced mode in a point-to-
point configuration. For the unbalanced connectionless class, the data link consists of a control station plus one or more
tributary stations and operates in the unbalanced connectionless-mode in a point-to-point or multipoint configuration. For the
balanced connectionless class, the data link consists of two peer stations and operates in the balanced connectionless-mode in a
point-to-point configuration. In each class, a basic repertoire of commands and responses is defined, but the capability of the
data link may be modified by the use of optional functions.
Balanced operation is intended for use in circumstances which require equal control at either end of the data link. Operational
requirements are covered in accordance with the overall HDLC architecture.
The content and format of the Exchange Identification (XID) frame portion builds on the fact that the principal use of the XID
frame is to exchange data link information between two or more HDLC stations. For the purpose of this International Standard,
© ISO/IEC 2002 – All rights reserved 1

data link information shall include any and all essential operational characteristics such as identification, authentication and/or
selection of optional functions and facilities concerning each station. This International Standard defines a single-exchange
negotiation procedure for establishing operational characteristics when either one or more stations are capable of providing
multiple selections.
This International Standard provides a means for exchanging the necessary information to establish, at a minimum, a data link
connection between two correspondents wishing to communicate. It describes a general purpose XID frame information field
content and format for that purpose.
It defines encoding for information related to the basic HDLC standards only. Mechanisms are provided to permit the general
purpose XID frame information field to be used to negotiate private parameters in a single XID exchange simultaneously with
negotiation of the defined basic parameters.
This International Standard does not limit or restrict the use of the XID frame information field from defining other standard
formats for use in specific applications.
The following are examples of potential uses of the XID command/response frame interchange:
a) Identification of the calling and called stations when using circuit switched networks (including switched network backup
applications).
b) Identification of stations operating on non-switched networks requiring identification at start-up.
c) The XID command frame with an individual, group or all-station address may be used to solicit XID response frame(s)
from other station(s) on the data link, prior to or following data link establishment.
d) Negotiation of the Frame Check Sequence (FCS) to be used for subsequent information interchange, by stations that
support both 16-bit FCS and 32-bit FCS capabilities.
e) Convey higher layer information that may be required prior to data link establishment.
f) Transmission of an XID response frame at any respond opportunity to request an XID exchange to modify some of the
operational parameters (for example, window size) following data link establishment.
g) Negotiation of the number of protected bits in the frame when an Unnumbered Information with Header check (UIH)
frame is used.
The means for resolution/negotiation of a data link layer address in switched environments portion is applicable to data stations
employing HDLC balanced classes of procedures which provide the XID command/response capability with the two specific
parameter fields, identified below. It is used to select a pair of operational link addresses when preassigned, system designated
addresses are not known on an a priori basis; e.g., switched circuited data links. Additional XID frame functions (including the
exchange of operational parameters, command/response support, higher layer information, etc.) may be accomplished in
conjunction with data link layer address determination or following address determination, with additional XID frame
exchanges.
NOTE  Address resolution procedures for situations where the remote DTE does not support XID frames, the "all-station"
address, or complete address support capabilities as defined in clause 8 below are not within the scope of this International
Standard.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of this
International Standard. For dated references, subsequent amendments to, or revisions of, any of these publications do not
apply. However, parties to agreements based on this International Standard are encouraged to investigate the possibility of
applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the
normative document referred to applies. Members of ISO and IEC maintain registers of currently valid International Standards.
ISO/IEC 646:1991, Information technology  ISO 7-bit coded character set for information interchange
2 © ISO/IEC 2002 – All rights reserved

ISO/IEC 2382-9:1995, Information technology  Vocabulary Part 9: Data communication
ISO 7478:1987, Information processing systems  Data communication  Multilink procedures
ISO/IEC 7498-1:1994, Information technology  Open Systems Interconnection  Basic Reference Model: The Basic Model
ISO/IEC 7776:1995, 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
ISO/IEC TR 10171:2000, Information technology — Telecommunications and information exchange between systems — List
of standard data link layer protocols that utilize high-level data link control (HDLC) classes of procedures, list of standard
XID format identifiers, list of standard mode-setting information field format identifiers, and list of standard user-defined
parameter set identification values
3 Definitions, acronyms and abbreviations
3.1 Definitions
For the purposes of this International Standard, the following definitions apply.
3.1.1
abort
a function invoked by a sending primary, secondary, combined, control, tributary or peer station causing the recipient to
discard (and ignore) all bit sequences transmitted by the sender since the preceding flag sequence
3.1.2
accept
the condition assumed by a data station (primary, secondary, combined, control, tributary or peer station) upon accepting a
correctly received frame for processing
3.1.3
address field (A)
the sequence of eight (or any multiple of eight, if extended) bits identifying the secondary/combined or tributary/peer station
sending (or designated to receive) the frame
3.1.4
address field extension
enlarging the address field to include more addressing information
3.1.5
address resolution/negotiation
procedure for exchanging/determining the data link layer identity of each data link layer entity
3.1.6
basic status
a secondary/combined or tributary/peer station's capability to send or receive a frame containing an information field
3.1.7
centralized control
a control in which all the primary or control station functions of the data link are centralized in one data station
3.1.8
combined station
that part of a data station that supports the combined station control functions of the data link
NOTE  The combined station generates commands and responses for transmission and interprets received commands and responses.
Specific responsibilities assigned to a combined station include:
a) initialization of control signal interchange;
© ISO/IEC 2002 – All rights reserved 3

b) organization of data flow;
c) interpretation of received commands and generation of appropriate responses; and
d) actions regarding error control and error recovery functions at the data link layer.
3.1.9
command
in data communication, an instruction represented in the control field of a frame and transmitted by the
primary/combined/control/peer station, which causes the addressed secondary/combined/tributary/peer station to execute a
specific data link control function
3.1.10
command frame
a) All frames transmitted by a primary/control station.
b) Those frames transmitted by a combined/peer station that contain the address of the other combined/peer station.
3.1.11
contention mode
a mode of transmission in which a transmitter can send on its own initiative
3.1.12
control escape (CE)
the unique sequence of eight bits (10111110) employed to indicate the following octet has been modified according to the
transparency algorithm for start/stop transmission environments
3.1.13
control field (C)
the sequence of eight (or 16/32/64, if extended) bits immediately following the address field of a frame
NOTE  The content of the control field is interpreted by:
a) the receiving secondary/combined/tributary/peer station, designated by the address field, as a command instructing the performance of
some specific function; and
b) the receiving primary/combined/control/peer station as a response from the secondary/combined/tributary/peer station, designated by
the address field, to one or more commands.
3.1.14
control field extension
enlarging the control field to include additional control information
3.1.15
control station
the data station that supports the control station control functions of the data link
NOTE  The control station generates command for transmission and interprets received responses. Specific responsibilities assigned to the
control station include:
a) initialization of control signal interchange, and
b) organization of data flow.
3.1.16
data communication
see ISO/IEC 2382-9, term 09.01.03
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3.1.17
data link
see ISO/IEC 2382-9, term 09.04.08
3.1.18
data link connection
see ISO/IEC 7498-1 : 1994
3.1.19
data link layer
the conceptual layer of control or processing logic existing in the hierarchical structure of a data station (primary, secondary,
combined, control, tributary or peer station) that is responsible for maintaining control of the data link
NOTE  The data link layer functions provide an interface between the data station higher layer logic and the data link. These functions
include:
a) transparency;
b) address/control field interpretation;
c) command/response generation, transmission and interpretation; and
d) frame check sequence computation and interpretation.
3.1.20
data transmission
see ISO/IEC 2382-9, term 09.01.02
3.1.21
duplex transmission
see ISO/IEC 2382-9, term 09.03.01
3.1.22
exception condition
the condition assumed by a secondary/combined station upon receipt of a frame which it cannot execute due either to a
transmission error or to an internal processing malfunction
3.1.23
flag sequence (F)
the unique sequence of eight bits (01111110) employed to delimit the opening and closing of a frame
3.1.24
format identifier
designator of one of 128 different standardized formats or one of 128 user-defined formats of the Exchange Identification
(XID) frame information field
3.1.25
frame
the sequence of address, control, information, and FCS fields, bracketed by opening and closing flag sequences
NOTE  A valid frame is at least 24 bits in length and contains an address field, a control field and a frame check sequence. A frame may or
may not include an information field.
3.1.26
frame check sequence (FCS)
the field immediately preceding the closing flag sequence of a frame, containing the bit sequence that provides for the
detection of transmission errors by the receiver
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3.1.27
frame format identifier
an optional field in non-basic frame format mode that identifies the format of the frame
3.1.28
group identifier
classifier of data link layer characteristics or parameters by function (for example, address resolution, parameter negotiation,
user data)
3.1.29
half-duplex transmission
see ISO/IEC 2382-9, term 09.03.02
3.1.30
header check sequence (HCS)
a check sequence using one of the standard 8, 16, or 32 bit polynomials that is computed over the fields between the opening
flag sequence and the HCS field
3.1.31
HDLC-based protocol
a protocol which is a subset of the elements and classes of procedure and optional functions defined in the HDLC standard, and
adopted as a standard by ISO or a recognized international standards body (e.g., ITU-T)
3.1.32
higher layer
the conceptual layer of control or processing logic existing in the hierarchical structure of a data station (primary, secondary,
combined, control, tributary or peer station) that is above the data link layer and upon which the performance of data link layer
functions are dependent; for example device control, buffer allocation, station management, etc.
3.1.33
information field (INFO)
the sequence of bits, occurring between the last bit of the control field and the first bit of the frame check sequence
NOTE  The information field contents of I, UI, and UIH frames are not interpreted at the data link layer.
3.1.34
initiating combined station
a station that sends the initial XID command frame as part of the address resolution process
3.1.35
interframe time fill
the sequence or condition transmitted between frames
3.1.36
intraframe time fill
in start/stop transmission, the sequence or condition transmitted within a frame when the next octet is not available for
contiguous transmission immediately following the preceding octet (For synchronous transmission, there is no provision for
intraframe time fill)
3.1.37
invalid frame
a sequence of bits, following the receipt of an apparent opening flag sequence, that either
a) is terminated by an abort sequence; or
b) contains less than 32 bits before an apparent closing flag sequence is detected
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3.1.38
layer parameter
the specification of data link layer characteristics and parameters, and their values, available or chosen
3.1.39
non-initiating combined station
a station that waits for the other combined station to send the initial XID command frame as part of the address resolution
process
3.1.40
peer station
the data station that supports the peer station control functions of the data link
NOTE  The peer station generates commands for transmission and interprets received commands and responses.
3.1.41
primary station
the data station that supports the primary station control functions of the data link
NOTE  The primary station generates commands for transmission and interprets received responses. Specific responsibilities assigned to
the primary station include:
c) initialization of control signal interchange;
d) organization of data flow; and
e) actions regarding error control and error recovery functions at the data link layer.
3.1.42
primary/secondary station
the general case where the station may be either a primary station or a secondary station
3.1.43
private parameter
an implementation-specific data link layer parameter not defined in the basic HDLC standards
3.1.44
response
in data communication, a reply represented in the control field of a response frame that advises the
primary/combined/control/peer station with respect to the action taken by the secondary/combined/tributary/peer station to one
or more commands
3.1.45
response frame
f) all frames transmitted by a secondary/tributary station
g) those frames transmitted by a combined/peer station that contain the address of the transmitting combined/peer station
3.1.46
secondary station
the data station that executes data link control functions as instructed by the primary station
NOTE  A secondary station interprets received commands and generates responses for transmission.
3.1.47
secondary station status
the current condition of a secondary station with respect to processing the series of commands received from the primary
station
© ISO/IEC 2002 – All rights reserved 7

3.1.48
single-exchange negotiation procedure
the initiating station indicates its "menu" of capabilities in its command frame, and the responding station indicates its choices
from the menu in its response frame
3.1.49
tributary station
the data station that executes data link control functions as instructed by the control station
NOTE  The tributary station interprets received commands and generates responses for transmission.
3.1.50
two-way alternate data communication
see ISO/IEC 2382-9, term 09.05.03
3.1.51
two-way simultaneous data communication
see ISO/IEC 2382-9, term 09.05.02
3.1.52
unique identifier
a unique bit/character sequence (for example, global telephone number, station identification, or equivalent) associated with
each station
3.1.53
unnumbered commands
the commands that do not
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