EN IEC 61158-4-4:2023

SLOVENSKI STANDARD
01-maj-2024
Nadomešča:
SIST EN IEC 61158-4-4:2019
Industrijska komunikacijska omrežja - Specifikacije za procesna vodila - 4-4. del:
Specifikacija protokola na ravni podatkovnih povezav - Elementi tipa 4 (IEC 61158-
4-4:2023)
Industrial communication networks - Fieldbus specifications - Part 4-4: Data-link layer
protocol specification - Type 4 elements (IEC 61158-4-4:2023)
Industrielle Kommunikationsnetze - Feldbusse - Teil 4-4: Protokollspezifikation des Data-
Link Layer (Sicherungsschicht) - Typ 4-Elemente (IEC 61158-4-4:2023)
Réseaux de communication industriels - Spécifications des bus de terrain - Partie 4-4:
Spécification du protocole de la couche liaison de données - Eléments de type 4 (IEC
61158-4-4:2023)
Ta slovenski standard je istoveten z: EN IEC 61158-4-4:2023
ICS:
25.040.40 Merjenje in krmiljenje Industrial process
industrijskih postopkov measurement and control
35.100.20 Podatkovni povezovalni sloj Data link layer
35.110 Omreževanje Networking
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 61158-4-4

NORME EUROPÉENNE
EUROPÄISCHE NORM April 2023
ICS 25.040.40; 35.100.20; 35.110 Supersedes EN IEC 61158-4-4:2019
English Version
Industrial communication networks - Fieldbus specifications -
Part 4-4: Data-link layer protocol specification - Type 4 elements
(IEC 61158-4-4:2023)
Réseaux de communication industriels - Spécifications des Industrielle Kommunikationsnetze - Feldbusse - Teil 4-4:
bus de terrain - Partie 4-4: Spécification du protocole de la Protokollspezifikation des Data-Link Layer
couche liaison de données - Eléments de type 4 (Sicherungsschicht) - Typ 4-Elemente
(IEC 61158-4-4:2023) (IEC 61158-4-4:2023)
This European Standard was approved by CENELEC on 2023-04-26. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2023 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.

Ref. No. EN IEC 61158-4-4:2023 E

European foreword
The text of document 65C/1202/FDIS, future edition 4 of IEC 61158-4-4, prepared by SC 65C
"Industrial networks" of IEC/TC 65 "Industrial-process measurement, control and automation" was
submitted to the IEC-CENELEC parallel vote and approved by CENELEC as EN IEC 61158-4-4:2023.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2024-01-26
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2026-04-26
document have to be withdrawn
This document supersedes EN IEC 61158-4-4:2019 and all of its amendments and corrigenda (if any).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 61158-4-4:2023 was approved by CENELEC as a
European Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standard indicated:
IEC 61158-1 NOTE Approved as EN IEC 61158-1
IEC 61158-2 NOTE Approved as EN IEC 61158-2
IEC 61158-3-4 NOTE Approved as EN IEC 61158-3-4
IEC 61158-5-4 NOTE Approved as EN IEC 61158-5-4
IEC 61158-6-4 NOTE Approved as EN IEC 61158-6-4
IEC 61784-1-4 NOTE Approved as EN IEC 61784-1-4
IEC 61784-2-4 NOTE Approved as EN IEC 61784-2-4
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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 1 Where an International Publication has been modified by common modifications, indicated by (mod), the
relevant EN/HD applies.
NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cencenelec.eu.
Publication Year Title EN/HD Year
ISO/IEC 7498-1 - 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 10731 - Information technology - Open Systems - -
Interconnection - Basic Reference Model -
Conventions for the definition of OSI
services
IEC 61158-4-4 ®
Edition 4.0 2023-03
INTERNATIONAL
STANDARD
Industrial communication networks – Fieldbus specifications –

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

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 25.040.40; 35.100.20; 35.110 ISBN 978-2-8322-6574-1

– 2 – IEC 61158-4-4:2023 © IEC 2023
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
1.1 General . 7
1.2 Specifications . 7
1.3 Procedures . 7
1.4 Applicability . 7
1.5 Conformance . 7
2 Normative references . 8
3 Terms, definitions, symbols and abbreviated terms . 8
3.1 Reference model terms and definitions . 8
3.2 Service convention terms and definitions . 10
3.3 Terms and definitions . 11
3.4 Symbols and abbreviations . 14
3.4.1 Constants, variables, counters and queues . 14
3.4.2 Miscellaneous . 14
4 Data Link Protocol Definition . 14
4.1 Overview of the DL-protocol . 14
4.1.1 General . 14
4.1.2 Functional classes . 15
4.1.3 Functions of the DLL . 15
4.1.4 Service assumed from the PhL . 24
4.2 General structure and encoding of PhIDUs and DLPDUs, and related
elements of procedure . 26
4.2.1 PhIDU structure and encoding . 26
4.2.2 Frame check sequence . 27
4.2.3 Common DLPDU structure, encoding and elements of procedure . 28
4.3 DLPDU-specific structure, encoding and elements of procedure . 33
4.3.1 DLPDU types . 33
4.3.2 Confirmed DLPDU . 33
4.3.3 Unconfirmed DLPDU. 34
4.3.4 Acknowledge DLPDU . 36
4.3.5 Immediate-reply DLPDU . 36
4.4 DL-service elements of procedure . 37
4.4.1 Receipt of a DL-UNITDATA request primitive . 37
4.4.2 Receipt of a DL-UNITDATA response primitive . 38
4.4.3 Autonomous DLE acknowledge . 39
4.4.4 Generation of a DL-UNITDATA indication primitive . 39
4.5 Route mechanism . 40
4.5.1 Type 4-route function . 40
4.5.2 Request Type 4-route generation . 41
4.5.3 DL-route generation . 42
4.6 Link-access system . 43
4.7 Local variables, counters and queues . 44
4.7.1 V(ACPDU) – acknowledge confirmed PDU . 44
4.7.2 V(AUPDU) – acknowledge unconfirmed PDU . 44
4.7.3 V(NA) – node-address . 44

IEC 61158-4-4:2023 © IEC 2023 – 3 –
4.7.4 V(NDLE) – number of DLEs . 44
4.7.5 V(PNR) – permitted number of retries . 44
4.7.6 V(DC) – device class (simple or normal) . 44
4.7.7 V(BR) – bit rate . 44
4.7.8 V(MID) – max indication delay . 45
4.7.9 V(DMRT) – default max retry time . 45
4.7.10 Q(UR) – user request queue . 45
4.7.11 C(LAC) – link access counter. 45
4.7.12 C(LIC) – link idle counter . 45
Bibliography . 46

Figure 1 – Relationship of PhE, DLE and DLS-user. 15
Figure 2 – DLE state diagram for confirmed and unconfirmed, unacknowledged
DLPDUs . 17
Figure 3 – DLE state diagram for confirmed acknowledged DLPDUs . 18
Figure 4 – DLE state diagram for unconfirmed acknowledged DLPDUs . 19
Figure 5 – Full duplex DLE receive state diagram . 20
Figure 6 – Full duplex DLE transmit state diagram . 20
Figure 7 – Link access example . 23
Figure 8 – Simple Type 4-route format . 28
Figure 9 – Extended Type 4-route format . 29
Figure 10 – Complex Type 4-route format . 29
Figure 11 – Immediate Type 4-route format . 30
Figure 12 – IP Type 4-route format . 30
Figure 13 – Control-status format . 31
Figure 14 – Data-field-format, one octet . 32
Figure 15 – Data field format, two octets . 32
Figure 16 – Source / destination designator . 41
Figure 17 – Simple Type 4-route generation . 41
Figure 18 – Extended Type 4-route generation . 42
Figure 19 – Complex and IP Type 4-route generation . 42
Figure 20 – Simple DL-route generation . 43
Figure 21 – Extended DL-route generation . 43
Figure 22 – Complex and IP DL-route generation . 43

Table 1 – Summary structure of DLPDUs . 33
Table 2 – Structure of confirmed DLPDUs . 34
Table 3 – Structure of unconfirmed DLPDUs . 35
Table 4 – Structure of acknowledge DLPDU. 36
Table 5 – Structure of immediate-reply DLPDU . 36

– 4 – IEC 61158-4-4:2023 © IEC 2023
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 4-4: Data-link layer protocol specification –
Type 4 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 61158-4-4 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 fourth edition cancels and replaces the third edition published in 2018. This edition
constitutes a technical revision.

IEC 61158-4-4:2023 © IEC 2023 – 5 –
This edition includes the following significant technical change with respect to the previous
edition:
a) Use of extended data size for DLS-user data. This extension is restricted to nodes operating
on a P-NET IP network.
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 the parts of the IEC 61158 series, 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.
– 6 – IEC 61158-4-4:2023 © IEC 2023
INTRODUCTION
This document 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.

IEC 61158-4-4:2023 © IEC 2023 – 7 –
INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 4-4: Data-link layer protocol specification –
Type 4 elements
1 Scope
1.1 General
The data-link layer provides basic time-critical messaging communications between devices in
an automation environment.
This protocol provides a means of connecting devices through a partial mesh network, such
that most failures of an interconnection between two devices can be circumvented. In common
practice the devices are interconnected in a non-redundant hierarchical manner reflecting
application needs.
1.2 Specifications
This document specifies
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) the structure of the fieldbus 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 DL-entities (DLEs) through the exchange of fieldbus DLPDUs;
b) the interactions between a DL-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 Ph-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 which
support time-critical communications services within the data-link layer of the OSI or fieldbus
reference models, and which require the ability to interconnect in an open systems
interconnection environment.
Profiles provide a simple multi-attribute means of summarizing an implementation’s capabilities,
and thus its applicability to various time-critical 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.
– 8 – IEC 61158-4-4:2023 © IEC 2023
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.
ISO/IEC 7498-1, 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 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 and abbreviated
terms 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.
3.1.1 called-DL-address [7498-3]
3.1.2 calling-DL-address [7498-3]
3.1.3 centralized multi-end-point-connection [7498-1]
3.1.4 correspondent (N)-entities [7498-1]
correspondent DL-entities  (N=2)
correspondent Ph-entities  (N=1)
3.1.5 demultiplexing [7498-1]
3.1.6 DL-address [7498-3]
3.1.7 DL-address-mapping [7498-1]
3.1.8 DL-connection [7498-1]
3.1.9 DL-connection-end-point [7498-1]
3.1.10 DL-connection-end-point-identifier [7498-1]
3.1.11 DL-connection-mode transmission [7498-1]

IEC 61158-4-4:2023 © IEC 2023 – 9 –
3.1.12 DL-connectionless-mode transmission [7498-1]
3.1.13 DL-data-sink [7498-1]
3.1.14 DL-data-source [7498-1]
3.1.15 DL-duplex-transmission [7498-1]
3.1.16 DL-facility [7498-1]
3.1.17 DL-local-view [7498-3]
3.1.18 DL-name [7498-3]
3.1.19 DL-protocol [7498-1]
3.1.20 DL-protocol-connection-identifier [7498-1]
3.1.21 DL-protocol-control-information [7498-1]
3.1.22 DL-protocol-data-unit [7498-1]
3.1.23 DL-protocol-version-identifier [7498-1]
3.1.24 DL-relay [7498-1]
3.1.25 DL-service-connection-identifier [7498-1]
3.1.26 DL-service-data-unit [7498-1]
3.1.27 DL-simplex-transmission [7498-1]
3.1.28 DL-subsystem [7498-1]
3.1.29 DL-user-data [7498-1]
3.1.30 flow control [7498-1]
3.1.31 layer-management [7498-1]
3.1.32 multiplexing [7498-3]
3.1.33 naming-(addressing)-authority [7498-3]
3.1.34 naming-(addressing)-domain [7498-3]
3.1.35 naming-(addressing)-subdomain [7498-3]
3.1.36 (N)-entity [7498-1]
DL-entity
Ph-entity
3.1.37 (N)-interface-data-unit [7498-1]
DL-service-data-unit  (N=2)
Ph-interface-data-unit  (N=1)
3.1.38 (N)-layer [7498-1]
DL-layer  (N=2)
Ph-layer  (N=1)
3.1.39 (N)-service
[7498-1]
DL-service  (N=2)
Ph-service  (N=1)
– 10 – IEC 61158-4-4:2023 © IEC 2023
3.1.40 (N)-service-access-point [7498-1]
DL-service-access-point  (N=2)
Ph-service-access-point  (N=1)
3.1.41 (N)-service-access-point-address
[7498-1]
DL-service-access-point-address  (N=2)
Ph-service-access-point-address  (N=1)
3.1.42 peer-entities
[7498-1]
3.1.43 Ph-interface-control-information [7498-1]
3.1.44 Ph-interface-data [7498-1]
3.1.45 primitive name [7498-3]
3.1.46 reassembling [7498-1]
3.1.47 recombining [7498-1]
3.1.48 reset [7498-1]
3.1.49 responding-DL-address [7498-3]
3.1.50 routing [7498-1]
3.1.51 segmenting [7498-1]
3.1.52 sequencing [7498-1]
3.1.53 splitting [7498-1]
3.1.54 synonymous name [7498-3]
3.1.55 systems-management [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:
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-provider-initiated-facility
3.2.11 DL-provider-optional-facility

IEC 61158-4-4:2023 © IEC 2023 – 11 –
3.2.12 DL-service-primitive;
primitive
3.2.13 DL-service-provider
3.2.14 DL-service-user
3.2.15 DL-user-optional-facility
3.2.16 indication (primitive)
acceptor.deliver (primitive)
3.2.17 multi-peer
3.2.18 request (primitive);
requestor.submit (primitive)
3.2.19 requestor
3.2.20 response (primitive);
acceptor.submit (primitive)
3.2.21 submit (primitive)
3.2.22 symmetrical service
3.3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.3.1
broadcast-Node-address
address used to send broadcasts to all DLEs on a Link
Note 1 to entry: All DLEs on a Link receive all DLPDUs where the first Node-address is equal to the Broadcast-
Node-Address. Such DLPDUs are always Unconfirmed, and their receipt is never acknowledged. The value of a
Broadcast-Node-address is 126.
3.3.2
destination-DL-route
holds a sequence of DL-route-elements, describing the complete route to the destination
Note 1 to entry: This includes both the destination DLSAP and a local component meaningful to the destination
DLS-user.
3.3.3
DL-route
combination of a Destination-DL-route and a Source-DL-route
3.3.4
DL-route-element
octet holding a Node-address or an address used by the DLS-user
3.3.5
DLSAP
distinctive point at which DL-services are provided by a single DL-entity to a single higher-layer
entity
Note 1 to entry: This definition, derived from ISO/IEC 7498-1, is repeated here to facilitate understanding of the
critical distinction between DLSAPs and their DL-addresses.

– 12 – IEC 61158-4-4:2023 © IEC 2023
3.3.6
DL(SAP)-address
an individual DLSAP-address, designating a single DLSAP of a single DLS-user
3.3.7
(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.
3.3.8
frame
denigrated synonym for DLPDU
3.3.9
IPNetID
identification of a unique IP network
Note 1 to entry: An IPNetID is translated into an IP-address and a UPD port number.
3.3.10
IPNetTable
definition of the relation between IPNetID, IP address, UPD port number and Router
NodeAddress, where IPNetID is used as index in the table
3.3.11
IP Range net
definition of the use of the IP network for local access, where nodes can be accessed directly
on the same subnet as the client, or through a local Router where the subnets are configured
in the local Router
3.3.12
Local link
single DL-subnetwork in which any of the connected DLEs may communicate directly, without
any intervening DL-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
3.3.13
no-Confirm-Node-address
address used to indicate that a request or response is Unconfirmed
Note 1 to entry: The value of a No-Confirm-Node-address is 0.
3.3.14
node
single DL-entity as it appears on one local link
3.3.15
node-address
address which uniquely identifies a DLE on a Link
Note 1 to entry: The value of a Node-address can be in the range of 0 to 127, with the values 0, 126 and 127
reserved for special purposes.
3.3.16
normal class device
device which replies to requests from other normal class devices, and initiates transmissions

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Note 1 to entry: Such a device can act as a server (responder) and as a client (requestor) – this is also called a
peer.
3.3.17
Type 4-route
a route that holds a sequence of Type 4-route-elements
Note 1 to entry: A Type 4-route is defined as an encoded DL-route, with one of the formats used when transmitting
the DLPDU on the Link. The Type 4-route format can be Simple, Extended, Complex, Immediate or IP.
3.3.18
Type 4-route-element
octet, holding a 7-bit DL-route-element or Remaining-route-length, and a 1-bit source/
destination designator
3.3.19
receiving DLS-user
DL-service user that acts as a recipient of DL-user-data
Note 1 to entry: A DL-service user can be concurrently both a sending and receiving DLS-user.
3.3.20
sending DLS-user
DL-service user that acts as a source of DL-user-data
3.3.21
service-Node-address
address reserved for service purposes only
Note 1 to entry: All DLEs on a Link receive all DLPDUs where the first Node-address is equal to the Service-Node-
Address. Such DLPDUs can be Confirmed or Unconfirmed, and it is possible that their receipt can be acknowledged
or not. The Service-Node-Address can be used on Links with only two DLEs – the requesting Normal class DLE and
the responding Simple or Normal class DLE. The value of the Service-Node-Address is 127.
3.3.22
simple class device
device which replies to requests from normal class devices, and can act as a server or
responder only
3.3.23
source-DL-route
route that holds a sequence of DL-route-elements, describing the complete route back to the
source
3.3.24
UDP port number
port number from where a Server can receive requests
Note 1 to entry: The UDP port number is 34378 for Normal UDP port. The UDP port number is 34379 for Secure
UDP port.
Note 2 to entry: These UDP port numbers are registered with the IANA (Internet Assigned Numbers Authority).
Note 3 to entry: There are two different UPD port numbers: Normal UDP port and Secure UDP port.
3.3.25
UDP range net
definition of the use of the IP network for remote access, where a node cannot be accessed
directly on the same subnet as the client
Note 1 to entry: The IPNetTable holds a NAT Router IP address and access to the node is obtained through this
NAT Router.
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Note 2 to entry: The NAT Router shall hold a table that translates the UDP port number to the actual server node
IP address and UDP port number.
3.3.26
virtual link-access token
basis for the link-access system
Note 1 to entry: It is called virtual because the token is not explicitly sent from one normal-class DLE to another,
but implicitly passed as the link is idle.
3.4 Symbols and abbreviations
3.4.1 Constants, variables, counters and queues
3.4.1.1 BNA broadcast node address
3.4.1.2 C(LAC) link access counter
3.4.1.3 C(LIC) link idle counter
3.4.1.4 SNA service node address
3.4.1.5 NCNA no confirm node address
3.4.1.6 Q(UR) user request queue
3.4.1.7 V(ACPDU) acknowledge confirmed PDU
3.4.1.8 V(AUPDU) acknowledge unconfirmed PDU
3.4.1.9 V(BR) bit rate
3.4.1.10 V(DC) device class (simple or normal)
3.4.1.11 V(DMRT) default max retry time
3.4.1.12 V(MID) max indication delay
3.4.1.13 V(NA) node address
3.4.1.14 V(NDLE) number of DLEs
3.4.1.15 V(PNR) permitted number of retries
3.4.1.16 IPNetTable Table to convert IPNetID to IP-addresses

3.4.2 Miscellaneous
3.4.2.1 RCL/ACK response comes later / acknowledge

4 Data Link Protocol Definition
4.1 Overview of the DL-protocol
4.1.1 General
The DLL provides connectionless data transfer services for limited-size DLSDUs from one DLS-
user to one or more (broadcast) DLS-users.

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A DLE is implicitly connected to one PhE and to a single DLSAP. This means that when a local
DLS-user issues a service primitive at a certain DLSAP, the DLE and hence the Link is implicitly
selected.
A DLE always delivers received DLSDUs at the same DLSAP, and hence to the same DLS-
user.
This concept is illustrated in Figure 1.

Figure 1 – Relationship of PhE, DLE and DLS-user
Each DLE has a Node-address. Node-addresses uniquely identify DLEs within the same Link.
A DL-route-element is an octet, which can hold a Node-address, or an address used by the
DLS-user.
A Destination-DL-route holds a sequence of DL-route-elements, describing the complete route
to the destination.
A Source-DL-route holds a sequence of DL-route-elements, describing the complete route back
to the source.
A DL-route is defined as a Destination-DL-route and a Source-DL-route.
4.1.2 Functional classes
The functional class of a DLE determines its capabilities, and thus the complexity of conforming
implementations. Two functional classes are defined:
• Simple class, including only responder functionality (server).
• Normal class, including initiator and responder functionality (client and server, also called
peer).
4.1.3 Functions of the DLL
4.1.3.1 General
The functions of the DLL are those necessary to bridge the gap between the services available
from the PhL and those offered to DLS-users. The functions are:
As a responder (in Simple class or Normal class DLEs):

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a) Receive a DLPDU from a remote DLE, perform frame check, parse the received DLPDU into
its DL-protocol information and data components, and generate a DLS-user indication
primitive. Possibly wait for a DLS-user request or response primitive, convert it to a DLPDU,
and send that DLPDU to the remote DLE.
b) Receive a single PhIDU specifying LINK-IDLE, and use that to time-out when waiting for a
DLS-user request primitive.
As an initiator (in Normal class DLEs):
c) Convert a DLS-user request primitive to a DLPDU, queue it, and send it to a remote DLE
(or all DLEs at the Link if broadcast) at the first opportunity. Possibly wait for an
Acknowledge or Immediate-reply DLPDU from the remote DLE, and (if an Immediate-reply
DLPDU is received) generate a DLS-user indication primitive.
d) Receive an SPDU, and use the associated data to check or gain Link-access
synchronization.
e) Receive a single PhIDU specifying LINK-IDLE, use that to keep Link-access synchronized,
and possibly to initiate sending a DLPDU from the queue if the queue is not empty, or if the
queue is empty, to send an SPDU for Link-access synchronization.
These functions are illustrated in Figure 2 to Figure 4.
4.1.3.2 Acknowledged vs. confirmed
The terms acknowledged and unacknowledged are used to describe whether the receiving DLE
must acknowledge the receipt of a DLPDU or not. The terms confirmed and unconfirmed are
used to describe whether the receiving DLS-user must confirm the receipt of a DLSDU or not.
The variable V(ACPDU) – Acknowledge Confirmed PDU – defines whether the DLE must
acknowledge the receipt of Confirmed DLPDUs. The variable V(AUPDU) – Acknowledge
Unconfirmed PDU – defines whether the DLE must acknowledge the receipt of Unconfirmed
DLPDUs.
A special case is when the first Node-address in a received DLPDU is equal to the Broadcast-
Node-address (BNA). In this case, the receiving DLE shall never acknowledge the receipt of
the DLPDU.
4.1.3.3 Half-duplex and full duplex
Unless otherwise stated, the PhL is assumed to support half-duplex transfer. However, a PhL
supporting full duplex is allowed.
Full duplex systems allow up to 125 DLEs on a Link, all of Normal class. Each DLE is allowed
to transmit immediately, that is, there is no Link Access system. DLEs supporting full duplex
PhEs have separate state machines for receive and transmit, as illustrated in Figure 5 and
Figure 6.
In full duplex systems, Confirmed as well as Unconfirmed DLPDUs are unacknowledged.
PhLs supporting full duplex shall not provide Link-Idle indications.

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Figure 2 – DLE state diagram for confirmed and unconfirmed, unacknowledged DLPDUs

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Figure 3 – DLE state diagram for confirmed acknowledged DLPDUs

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Figure 4 – DLE state diagram for unconfirmed acknowledged DLPDUs

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Figure 5 – Full duplex DLE receive state diagram

Figure 6 – Full duplex DLE transmit state diagram
4.1.3.4 DLPDU types
Four different types of DLPDUs are defined.
a) Confirmed – used to send confirmed requests between DLS-users.
b) Unconfirmed – used to send responses or unconfirmed requests between DLS-users.
c) Acknowledge – used by DLEs to acknowledge receipt of Confirmed or Unconfirmed
DLPDUs. The receipt of Acknowledge DLPDUs shall ne
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