IEC 61158-4-21:2023

IEC 61158-4-21 ®
Edition 3.0 2023-03
INTERNATIONAL
STANDARD
Industrial communication networks – Fieldbus specifications –
Part 4-21: Data-link layer protocol specification – Type 21 elements
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IEC 61158-4-21 ®
Edition 3.0 2023-03
INTERNATIONAL
STANDARD
Industrial communication networks – Fieldbus specifications –

Part 4-21: Data-link layer protocol specification – Type 21 elements

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 25.040.40; 35.100.20; 35.110 ISBN 978-2-8322-6555-0

– 2 – IEC 61158-4-21:2023 © IEC 2023
CONTENTS
FOREWORD . 7
INTRODUCTION . 9
1 Scope . 10
1.1 General . 10
1.2 Specifications . 10
1.3 Procedures . 10
1.4 Applicability . 10
1.5 Conformance . 10
2 Normative references . 11
3 Terms, definitions, symbols and abbreviated terms . 11
3.1 Reference model terms and definitions . 11
3.2 Service convention terms and definitions . 13
3.3 Common terms and definitions . 14
3.4 Additional Type 21 definitions . 17
3.5 Common symbols and abbreviations . 17
3.6 Additional Type 21 symbols and abbreviations . 18
4 Overview of the data-link protocol . 18
4.1 General . 18
4.2 Overview of medium access control . 19
4.3 Service assumed from the physical layer . 19
4.4 DLL architecture . 20
4.4.1 General . 20
4.4.2 DLL management (DLM) interface support function . 21
4.5 Data type . 22
4.5.1 General . 22
4.5.2 Boolean . 22
4.5.3 Unsigned integer . 22
4.5.4 Signed integer . 22
4.5.5 Octet String . 23
4.5.6 Visible String . 23
4.5.7 Time of day . 23
4.6 Local parameters and variables . 24
4.6.1 General . 24
4.6.2 DLE configuration parameters . 24
4.6.3 Queues to support data transfer . 25
4.6.4 Variables to support SAP management . 25
4.6.5 Variables to support local device information management . 27
4.6.6 Variables and counter to support network information management . 31
4.6.7 Variables and counter to support a device path information
management . 35
4.6.8 Variables, counters, timers, and queues to support path table
management . 39
5 General structure and encoding . 39
5.1 Overview. 39
5.2 MAPDU structure and encoding . 39
5.3 Common MAC frame structure, encoding and elements of procedure . 40
5.3.1 MAC frame structure . 40

5.3.2 Elements of the MAC frame . 40
5.3.3 Elements of the Type 21 DLPDU . 41
5.4 Order of bit transmission . 49
5.5 Invalid DLPDU . 49
6 DLPDU structure and procedure . 49
6.1 General . 49
6.2 Common DLPDU Field . 49
6.2.1 General . 49
6.2.2 Version . 50
6.2.3 Length . 50
6.3 DL-DATA Transfer . 50
6.3.1 DT DLPDU . 50
6.4 DL-SPDATA Transfer . 53
6.4.1 SPDT DLPDU . 53
6.5 Network control messages . 55
6.5.1 General . 55
6.5.2 NCM_LA DLPDU . 55
6.5.3 NCM_AT DLPDU . 56
6.5.4 NCM_LS DLPDU . 58
6.5.5 NCM_RS DLPDU . 59
6.5.6 NCM_AR_DLPDU . 60
6.5.7 NCM_AR DLPDU structure . 60
7 DLE elements of procedure . 61
7.1 Overall structure . 61
7.2 DL-protocol machine (DLPM) . 61
7.2.1 Overview . 61
7.2.2 Primitive definitions . 61
7.2.3 DLPM state table . 65
7.2.4 DLPM functions . 68
7.3 DLL management Protocol . 70
7.3.1 Overview . 70
7.3.2 Primitive definitions . 70
7.3.3 DLM state table . 73
7.3.4 DLM functions . 97
8 Constants and error codes . 106
8.1 General . 106
8.2 Constants . 106
8.3 Data-link layer error codes . 108
Bibliography . 109

Figure 1 – Interaction of PhS primitives with DLE . 19
Figure 2 – Data-link layer architecture . 21
Figure 3 – Relationships of DLSAPs, DLSAP-addresses, and group DL-addresses . 26
Figure 4 – Common MAC frame format for Type 21 DLPDU . 40
Figure 5 – MAC frame format for other protocols . 40
Figure 6 – Version and Length field . 41
Figure 7 – DST_addr field . 42

– 4 – IEC 61158-4-21:2023 © IEC 2023
Figure 8 – SRC_addr field . 43
Figure 9 – Frame Control Field . 43
Figure 10 – Extension field . 46
Figure 11 – DSAP field . 47
Figure 12 – Source service access point field . 48
Figure 13 – Length of group mask and extension information . 48
Figure 14 – Group mask option field . 48
Figure 15 – Common DLPDU field . 50
Figure 16 – Building a DT DLPDU . 50
Figure 17 – DT DLPDU structure . 50
Figure 18 – SPDT DLPDU structure . 54
Figure 19 – NCM_LA DLPDU structure . 55
Figure 20 – DLL structure and elements . 61
Figure 21 – State transition diagram of the DLPM . 65
Figure 22 – State transition diagram of DLM . 74

Table 1 – DLL components . 20
Table 2 – UNSIGNEDn data type . 22
Table 3 – INTEGERn data type . 23
Table 4 – DLE configuration parameters . 24
Table 5 – Queues to support data transfer . 25
Table 6 – Variables to support SAP management . 26
Table 7 – Variables to support device information management . 27
Table 8 – DL–entity identifier . 27
Table 9 – Device flags . 28
Table 10 – DLM state . 28
Table 11 – Device Unique Identification . 28
Table 12 – Unique identification of device connected to R-port1 . 29
Table 13 – Unique identification of device connected to R-port2 . 29
Table 14 – MAC address . 29
Table 15 – Port information . 30
Table 16 – Protocol version . 30
Table 17 – Device type . 31
Table 18 – Device description . 31
Table 19 – Hop count . 31
Table 20 – Variables to support managing network information . 32
Table 21 – Topology . 32
Table 22 – Collision count . 32
Table 23 – Device count . 33
Table 24 – Topology change count . 33
Table 25 – Last topology change time . 33
Table 26 – RNMP device UID . 33
Table 27 – RNMS device UID . 34

Table 28 – LNM device UID for R-port1. 34
Table 29 – LNM device UID for R-port2. 34
Table 30 – Network flags . 35
Table 31 – Variables and counter to support managing path information . 36
Table 32 – Hop count for R-port1 direction . 36
Table 33 – Hop count for R-port2 direction . 37
Table 34 – Preferred R-port . 37
Table 35 – Destination R-port . 37
Table 36 – In net count . 38
Table 37 – In net time . 38
Table 38 – Out net count . 39
Table 39 – Out net time . 39
Table 40 – Version and Length . 42
Table 41 – Destination DL–entity identifier . 42
Table 42 – Source DL–entity identifier . 43
Table 43 – Frame control . 44
Table 44 – Extension . 47
Table 45 – Destination service access point . 47
Table 46 – source service access point . 48
Table 47 – DT DLPDU parameters . 51
Table 48 – Primitives exchanged between DLS-user and DLE to send a DT DLPDU . 52
Table 49 – Primitives exchanged between DLS-user and DLEs to receive a DT DLPDU . 53
Table 50 – SPDT DLPDU Parameters . 54
Table 51 – Primitive exchanged between DLS-User and DLEs to send an SPDT
DLPDU . 54
Table 52 – Primitives exchanged between DLS-user and DLEs to receive an SPDT
DLPDU . 55
Table 53 – NCM_LA DLPDU parameters . 56
Table 54 – NCM_AT DLPDU parameters . 57
Table 55 – NCM_LS DLPDU parameters . 58
Table 56 – NCM_RS DLPDU parameters . 59
Table 57 – NCM_AR DLPDU parameters . 60
Table 58 – Primitives exchanged between DLPM and DLS-user. 62
Table 59 – Parameters exchanged between DLPM and DLS-user . 63
Table 60 – Primitives exchanged between DLPM and DLM . 64
Table 61 – Parameters used with primitives exchanged between DLPM and DLM . 65
Table 62 – DLPM state table . 66
Table 63 – DLPM functions table . 69
Table 64 – Primitives exchanged between DLM and DLS-user . 71
Table 65 – Parameters used with primitives exchanged between DLM and DLS-user . 72
Table 66 – Primitive exchanged between DLM and DMAC . 72
Table 67 – Parameters used with primitives exchanged between DLM and DMAC . 73
Table 68 – Primitive exchanged between DLM and DPHY . 73
Table 69 – Parameters used with primitives exchanged between DLM and DPHY . 73

– 6 – IEC 61158-4-21:2023 © IEC 2023
Table 70 – DLM state table . 75
Table 71 – DLM function table . 97
Table 72 – DLL constants . 107
Table 73 – Type 21 DLL error codes . 108

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 4-21: Data-link layer protocol specification –
Type 21 elements
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as "IEC Publication(s)"). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
Attention is drawn to the fact that the use of the associated protocol type is restricted by its
intellectual-property-right holders. In all cases, the commitment to limited release of intellectual-
property-rights made by the holders of those rights permits a layer protocol type to be used with
other layer protocols of the same type, or in other type combinations explicitly
authorized by its intellectual-property-right holders.
NOTE Combinations of protocol types are specified in the IEC 61784-1 series and the IEC 61784-2 series.
IEC 61158-4-21 has been prepared by subcommittee 65C: Industrial networks, of IEC technical
committee 65: Industrial process measurement, control and automation. It is an International
Standard.
This third edition cancels and replaces the second edition published in 2019. This edition
constitutes a technical revision.

– 8 – IEC 61158-4-21:2023 © IEC 2023
This edition includes the following significant technical changes with respect to the previous
edition:
a) changed Table 9, Table 15, Table 30, Table 34, Table 35, Table 43, Table 47, Table 48,
Table 49, Table 58, and Table 61;
b) changed Network Control Message Type;
c) miscellaneous editorial corrections.
The text of this International Standard is based on the following documents:
Draft Report on voting
65C/1202/FDIS 65C/1243/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts of the IEC 61158 series, published under the general title Industrial
communication networks – Fieldbus specifications, can be found on the IEC web site.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
INTRODUCTION
This part of IEC 61158 is one of a series produced to facilitate the interconnection of automation
system components. It is related to other standards in the set as defined by the "three-layer"
fieldbus reference model described in IEC 61158-1.
The data-link protocol provides the data-link service by making use of the services available
from the physical layer. The primary aim of this document is to provide a set of rules for
communication expressed in terms of the procedures to be carried out by peer data-link entities
(DLEs) at the time of communication. These rules for communication are intended to provide a
sound basis for development in order to serve a variety of purposes:
a) as a guide for implementors and designers;
b) for use in the testing and procurement of equipment;
c) as part of an agreement for the admittance of systems into the open systems environment;
d) as a refinement to the understanding of time-critical communications within OSI.
This document is concerned, in particular, with the communication and interworking of sensors,
effectors and other automation devices. By using this document together with other standards
positioned within the OSI or fieldbus reference models, otherwise incompatible systems could
work together in any combination.
The International Electrotechnical Commission (IEC) draws attention to the fact that it is claimed
that compliance with this document may involve the use of a patent. IEC takes no position
concerning the evidence, validity, and scope of this patent right.
The holder of this patent right has assured IEC that s/he is willing to negotiate licences under
reasonable and non-discriminatory terms and conditions with applicants throughout the world.
In this respect, the statement of the holder of this patent right is registered with IEC. Information
may be obtained from the patent database available at http://patents.iec.ch.
Attention is drawn to the possibility that some of the elements of this document may be the
subject of patent rights other than those in the patent database. IEC shall not be held
responsible for identifying any or all such patent rights.

– 10 – IEC 61158-4-21:2023 © IEC 2023
INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 4-21: Data-link layer protocol specification –
Type 21 elements
1 Scope
1.1 General
The DLL provides basic time-critical data communications between devices in an automated
environment. Type 21 provides priority-based cyclic and acyclic data communication using an
internal collision-free, full-duplex dual-port Ethernet switch technology. For wide application in
various automation applications, Type 21 does not restrict the cyclic/acyclic scheduling policy
in the DLL.
1.2 Specifications
This part of IEC 61158 describes:
a) procedures for the timely transfer of data and control information from one data link user
entity to a peer user entity, and among the data link entities forming the distributed data link
service provider;
b) procedures for giving communication opportunities based on ISO/IEC/IEEE 8802-3 MAC,
with provisions for nodes to be added or removed during normal operation;
c) structure of the fieldbus data link protocol data units (DLPDUs) used for the transfer of data
and control information by the protocol of this document, and their representation as
physical interface data units.
1.3 Procedures
The procedures are defined in terms of:
a) the interactions between peer data link entities (DLEs) through the exchange of fieldbus
DLPDUs;
b) the interactions between a data link service (DLS) provider and a DLS-user in the same
system through the exchange of DLS primitives;
c) the interactions between a DLS-provider and a physical layer service provider in the same
system through the exchange of Ph-service primitives.
1.4 Applicability
These procedures are applicable to instances of communication between systems that support
time-critical communications services in the data link layer of the OSI or fieldbus reference
models, and that require the ability to interconnect in an open systems interconnection
environment. Profiles provide a simple multi-attribute means of summarizing implementation's
capabilities, and thus its applicability to various time-deterministic communications needs.
1.5 Conformance
This document also specifies conformance requirements for systems implementing these
procedures. This document does not contain tests to demonstrate compliance with such
requirements.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
NOTE All parts of the IEC 61158 series, as well as the IEC 61784-1 series and the IEC 61784-2 series are
maintained simultaneously. Cross-references to these documents within the text therefore refer to the editions as
dated in this list of normative references.
IEC 61158-3-21:2019, Industrial Communication Networks – Fieldbus specifications – Part 3-21:
Data-link layer service definition – Type 21 elements
ISO/IEC 7498-1:1994, Information technology – Open Systems Interconnection – Basic
Reference Model: The Basic Model
ISO/IEC 7498-3, Information technology – Open Systems Interconnection – Basic Reference
Model: Naming and addressing
ISO/IEC/IEEE 8802-3:2021, Telecommunications and exchange between information
technology systems – Requirements for local and metropolitan area networks – Part 3: Standard
for Ethernet
ISO/IEC 10731, Information technology – Open Systems Interconnection – Basic Reference
Model – Conventions for the definition of OSI services
3 Terms, definitions, symbols and abbreviated terms
For the purposes of this document, the following terms, definitions, symbols, abbreviations and
conventions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1 Reference model terms and definitions
This document is based in part on the concepts developed in ISO/IEC 7498-1 and
ISO/IEC 7498-3, and makes use of the following terms defined therein.

– 12 – IEC 61158-4-21:2023 © IEC 2023
3.1.1 called-DL-address [ISO/IEC 7498-3]
3.1.2 calling-DL-address
[ISO/IEC 7498-3]
3.1.3 centralized multi-end-point-connection [ISO/IEC 7498-1]
3.1.4 correspondent (N)-entities [ISO/IEC 7498-1]
correspondent DL-entities  (N=2)
correspondent Ph-entities  (N=1)
3.1.5 demultiplexing [ISO/IEC 7498-1]
3.1.6 DL-address
[ISO/IEC 7498-3]
3.1.7 DL-address-mapping [ISO/IEC 7498-1]
3.1.8 DL-connection
[ISO/IEC 7498-1]
3.1.9 DL-connection-end-point [ISO/IEC 7498-1]
3.1.10 DL-connection-end-point-identifier [ISO/IEC 7498-1]
3.1.11 DL-connection-mode transmission [ISO/IEC 7498-1]
3.1.12 DL-connectionless-mode transmission
[ISO/IEC 7498-1]
3.1.13 DL-data-sink [ISO/IEC 7498-1]
3.1.14 DL-data-source
[ISO/IEC 7498-1]
3.1.15 DL-duplex-transmission [ISO/IEC 7498-1]
3.1.16 DL-facility
[ISO/IEC 7498-1]
3.1.17 DL-local-view [ISO/IEC 7498-3]
3.1.18 DL-name [ISO/IEC 7498-3]
3.1.19 DL-protocol [ISO/IEC 7498-1]
3.1.20 DL-protocol-connection-identifier
[ISO/IEC 7498-1]
3.1.21 DL-protocol-control-information [ISO/IEC 7498-1]
3.1.22 DL-protocol-data-unit
[ISO/IEC 7498-1]
3.1.23 DL-protocol-version-identifier [ISO/IEC 7498-1]
3.1.24 DL-relay [ISO/IEC 7498-1]
3.1.25 DL-service-connection-identifier [ISO/IEC 7498-1]
3.1.26 DL-service-data-unit
[ISO/IEC 7498-1]
3.1.27 DL-simplex-transmission [ISO/IEC 7498-1]
3.1.28 DL-subsystem
[ISO/IEC 7498-1]
3.1.29 DL-user-data [ISO/IEC 7498-1]
3.1.30 flow control
[ISO/IEC 7498-1]
3.1.31 layer-management [ISO/IEC 7498-1]
3.1.32 multiplexing
[ISO/IEC 7498-3]
3.1.33 naming-(addressing)-authority [ISO/IEC 7498-3]
3.1.34 naming-(addressing)-domain
[ISO/IEC 7498-3]
3.1.35 naming-(addressing)-subdomain [ISO/IEC 7498-3]
3.1.36 (N)-entity [ISO/IEC 7498-1]
DL-entity
Ph-entity
3.1.37 (N)-interface-data-unit [ISO/IEC 7498-1]
DL-service-data-unit  (N=2)
Ph-interface-data-unit  (N=1)
3.1.38 (N)-layer
[ISO/IEC 7498-1]
DL-layer  (N=2)
Ph-layer  (N=1)
3.1.39 (N)-service [ISO/IEC 7498-1]
DL-service  (N=2)
Ph-service  (N=1)
3.1.40 (N)-service-access-point
[ISO/IEC 7498-1]
DL-service-access-point  (N=2)
Ph-service-access-point  (N=1)
3.1.41 (N)-service-access-point-address [ISO/IEC 7498-1]
DL-service-access-point-address  (N=2)
Ph-service-access-point-address  (N=1)
3.1.42 peer-entities
[ISO/IEC 7498-1]
3.1.43 Ph-interface-control-information [ISO/IEC 7498-1]
3.1.44 Ph-interface-data [ISO/IEC 7498-1]
3.1.45 primitive name [ISO/IEC 7498-3]
3.1.46 reassembling
[ISO/IEC 7498-1]
3.1.47 recombining [ISO/IEC 7498-1]
3.1.48 reset
[ISO/IEC 7498-1]
3.1.49 responding-DL-address [ISO/IEC 7498-3]
3.1.50 routing [ISO/IEC 7498-1]
3.1.51 segmenting [ISO/IEC 7498-1]
3.1.52 sequencing
[ISO/IEC 7498-1]
3.1.53 splitting [ISO/IEC 7498-1]
3.1.54 synonymous name
[ISO/IEC 7498-3]
3.1.55 systems-management [ISO/IEC 7498-1]
3.2 Service convention terms and definitions
This document also makes use of the following terms defined in ISO/IEC 10731 as they apply
to the data-link layer:
– 14 – IEC 61158-4-21:2023 © IEC 2023
3.2.1 acceptor
3.2.2 asymmetrical service
3.2.3 confirm (primitive);
requestor.deliver (primitive)
3.2.4 deliver (primitive)
3.2.5 DL-confirmed-facility
3.2.6 DL-facility
3.2.7 DL-local-view
3.2.8 DL-mandatory-facility
3.2.9 DL-non-confirmed-facility
3.2.10 DL-protocol-machine
3.2.11 DL-provider-initiated-facility
3.2.12 DL-provider-optional-facility
3.2.13 DL-service-primitive;
primitive
3.2.14 DL-service-provider
3.2.15 DL-service-user
3.2.16 DLS-user-optional-facility
3.2.17 indication (primitive);
acceptor.deliver (primitive)
3.2.18 multi-peer
3.2.19 request (primitive);
requestor.submit (primitive)
3.2.20 requestor
3.2.21 response (primitive);
acceptor.submit (primitive)
3.2.22 submit (primitive)
3.2.23 symmetrical service
3.3 Common terms and definitions
For the purposes of this document, the following definitions also apply.
NOTE Many definitions are common to more than one protocol Type; they are not necessarily used by all protocol
Types.
3.3.1
active network
network in which data transmission between non-immediately-connected devices is dependent
on active elements within those intervening devices that form the connection path
[SOURCE: IEC 61918:2018, 3.1.3]
3.3.2
DL-segment
link
local link
single data link (DL) subnetwork in which any of the connected data link entities (DLEs) may
communicate directly, without any intervening data link relaying, whenever all of those DLEs
that are participating in an instance of communication are simultaneously attentive to the
DL-subnetwork during the period(s) of attempted communication

3.3.3
data-link service access point
DLSAP
distinctive point at which DL-services are provided by a single DLE to a single higher-layer
entity
Note 1 to entry: Definition derived from ISO/IEC 7498-1:1994, Clause 5.
3.3.4
DL(SAP) -address
either an individual DLSAP address designating a single DLSAP of a single data link service
(DLS) user (DLS-user), or a group DL-address potentially designating multiple DLSAPs, each
of a single DLS-user
Note 1 to entry: This terminology was chosen because ISO/IEC 7498-3 does not permit the use of the term
DLSAP-address to designate more than a single DLSAP at a single DLS-user.
3.3.5
(individual) DLSAP-address
DL-address that designates only one DLSAP within the extended link
Note 1 to entry: A single DL-entity may have multiple DLSAP-addresses associated with a single DLSAP.
3.3.6
data-link connection endpoint address
DLCEP-address
DL-address that designates either:
a) one peer DL-connection-end-point;
b) one multi-peer publisher DL-connection-end-point, and implicitly the corresponding set of
subscriber DL-connection-end-points, where each DL-connection-end-point exists within a
distinct DLSAP and is associated with a corresponding distinct DLSAP-address
3.3.7
DL–entity identifier
address that designates the (single) DLE associated with a single device on a specific local link
3.3.8
device
single DLE as it appears on one local link
3.3.9
end-station
system attached to a network that is an initial so
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