IEC 61158-3-21:2019

IEC 61158-3-21 ®
Edition 2.0 2019-04
INTERNATIONAL
STANDARD
Industrial communication networks – Fieldbus specifications –
Part 3-21: Data-link layer service definition – Type 21 elements

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IEC 61158-3-21 ®
Edition 2.0 2019-04
INTERNATIONAL
STANDARD
Industrial communication networks – Fieldbus specifications –

Part 3-21: Data-link layer service definition – Type 21 elements

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 25.040.40; 35.100.20; 35.110 ISBN 978-2-8322-6790-5

– 2 – IEC 61158-3-21:2019 © IEC 2019
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
1.1 Overview. 8
1.2 Specifications . 8
1.3 Conformance . 8
2 Normative references . 9
3 Terms, definitions, symbols, abbreviations, and conventions . 9
3.1 Reference model terms and definitions . 9
3.2 Service convention terms and definitions . 11
3.3 Common data-link service terms and definitions . 12
3.4 Additional Type 21 data-link specific definitions . 15
3.5 Common symbols and abbreviations . 15
3.6 Additional Type 21 symbols and abbreviations . 16
3.7 Common conventions . 16
3.8 Additional Type 21 conventions. 17
4 Data-link layer services and concepts . 17
4.1 General . 17
4.1.1 Overview . 17
4.1.2 Overview of full duplex flow control . 17
4.1.3 Types and classes of DL-layer service . 18
4.2 Detailed description of the data service . 21
4.2.1 General . 21
4.2.2 Sequence of primitives . 21
4.2.3 Transmit/receive data . 22
4.3 Detailed description of the sporadic data service . 23
4.3.1 General . 23
4.3.2 Sequence of Primitive . 24
4.3.3 Transmit/receive sporadic data . 24
4.4 Detailed description of network control message service . 25
4.4.1 General . 25
4.4.2 Sequence of Primitives . 25
4.4.3 Transmit/receive network control message . 25
5 Data-link management services . 28
5.1 General . 28
5.2 Data link management service (DLMS) facilities . 28
5.3 Data link management service (DLMS) . 28
5.3.1 Overview . 28
5.3.2 Reset . 29
5.3.3 Set-value . 29
5.3.4 Get-value . 29
5.3.5 SAP-allocation . 29
5.3.6 SAP-deallocation . 29
5.3.7 Get-SAP information . 29
5.3.8 Get-diagnostic-information . 29
5.3.9 Event . 29

5.3.10 Get-path . 29
5.4 Overview of interactions . 29
5.5 Detailed specification of service and interactions . 31
5.5.1 Reset . 31
5.5.2 Set-value . 31
5.5.3 Get-value . 32
5.5.4 SAP-allocation . 33
5.5.5 SAP-deallocation . 34
5.5.6 Get-SAP-information . 35
5.5.7 Get-diagnostic-information . 36
5.5.8 Event . 37
5.5.9 Get-path . 38
6 MAC control service . 39
6.1 General . 39
6.2 MAC control service . 39
6.2.1 Overview . 39
6.2.2 MAC-reset . 39
6.2.3 MAC-forward-control . 39
6.3 Overview of interactions . 39
6.4 Detailed specification of service and interactions . 40
6.4.1 MAC-reset . 40
6.4.2 MAC-forward-control . 41
7 Ph-control service . 42
7.1 General . 42
7.2 Ph-control service . 42
7.2.1 Overview . 42
7.2.2 Ph-reset . 42
7.2.3 Ph-get-link-status . 42
7.2.4 Ph-link-status-change . 42
7.3 Overview of interactions . 42
7.4 Detailed specification of service and interactions . 43
7.4.1 Ph-reset . 43
7.4.2 Ph-get-link-status . 44
7.4.3 Ph-link-status-change . 45
Bibliography . 46

Figure 1 – Full-duplex flow control . 18
Figure 2 – Sequence diagram of DL-DATA service . 18
Figure 3 – Sequence diagram of DL-SPDATA service . 19
Figure 4 – Sequence diagram of NCM service primitive . 19
Figure 5 – Relationships of DLSAPs, DLSAP-addresses, and group DL-addresses . 20
Figure 6 – DL-DATA service . 21
Figure 7 – Sequence diagram of Reset, Set-value, Get-value, SAP-allocation, SAP-
deallocation, Get-SAP information and Get-diagnostic information service primitives . 30
Figure 8 – Sequence diagram of Event service primitive . 31
Figure 9 – Sequence diagram of MAC-reset and MAC-forward-control service primitive . 40
Figure 10 – Sequence diagram of Ph-reset and Ph-get-link-status service primitive . 43

– 4 – IEC 61158-3-21:2019 © IEC 2019
Figure 11 – Sequence diagram of Ph-link-status-change service primitive . 43

Table 1 – Destination DL-address . 20
Table 2 – Primitives and parameters used in DL-DATA service . 22
Table 3 – DL-DATA Primitives and Parameters . 22
Table 4 – Primitives and parameters used in DL-SPDATA service . 24
Table 5 – DL-SPDATA Primitives and Parameters . 24
Table 6 – Primitives and parameters used on DL-NCM_SND service . 25
Table 7 – DL-NCM_SND Primitives and Parameters . 26
Table 8 – Summary of Network Control Message Type . 27
Table 9 – Summary of DL-management primitives and parameters . 30
Table 10 – DLM-RESET primitives and parameters . 31
Table 11 – DLM-SET_VALUE primitives and parameters . 32
Table 12 – DLM-GET_VALUE primitives and parameters . 33
Table 13 – DLM-SAP_ALLOC primitives and parameters . 34
Table 14 – DLM-SAP_DEALLOC primitives and parameters . 35
Table 15 – DLM-GET_SAP_INFO primitives and parameters . 35
Table 16 – DLM-GET_DIAG primitives and parameters . 36
Table 17 – DLM-EVENT primitives and parameters . 37
Table 18 – DLM event identifier . 38
Table 19 – DLM-GET_PATH primitives and parameters . 38
Table 20 – Summary of MAC control primitives and parameters . 40
Table 21 – MAC-RESET primitives and parameters . 40
Table 22 – MAC-FW_CTRL primitives and parameters. 41
Table 23 – Summary of Ph-control primitives and parameters . 42
Table 24 – Ph-RESET primitives and parameters . 43
Table 25 – Ph-GET_LINK_STATUS primitives and parameters . 44
Table 26 – Ph-LINK_STATUS_CHANGE primitives and parameters. 45

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 3-21: Data-link layer service definition –
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
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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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 IEC 61784-1 and IEC 61784-2.
International Standard IEC 61158-3-21 has been prepared by subcommittee 65C: Industrial
networks, of IEC technical committee 65: Industrial-process measurement, control and
automation.
This second edition cancels and replaces the first edition published in 2010. This edition
constitutes a technical revision.

– 6 – IEC 61158-3-21:2019 © IEC 2019
This edition includes the following significant technical changes with respect to the previous
edition:
• added Network Control Message Type;
• miscellaneous editorial corrections.
The text of this International Standard is based on the following documents:
FDIS Report on voting
65C/945/FDIS 65C/954/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This publication has been drafted in accordance with ISO/IEC Directives, Part 2.
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 publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

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.
Throughout the set of fieldbus standards, the term “service” refers to the abstract capability
provided by one layer of the OSI Basic Reference Model to the layer immediately above. Thus,
the data-link layer service defined in this document is a conceptual architectural service,
independent of administrative and implementation divisions.

– 8 – IEC 61158-3-21:2019 © IEC 2019
INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 3-21: Data-link layer service definition –
Type 21 elements
1 Scope
1.1 Overview
This part of IEC 61158 provides the common elements for basic time-critical messaging
communications between devices in an automation environment. The term “time-critical” in
this context means the prioritized full-duplex collision-free time-deterministic communication,
of which one or more specified actions are required to be completed with some defined level
of certainty. Failure to complete specified actions within the required time risks the failure of
the applications requesting the actions, with attendant risk to equipment, plant, and possibly
human life.
This International Standard defines in an abstract way the externally visible service provided
by the Type 21 data-link layer in terms of:
a) the primitive actions and events of the service;
b) the parameters associated with each primitive action and event, and the form that they
take; and
c) the interrelationships between these actions and events, and their valid sequences.
The purpose of this document is to define the services provided to:
• The Type 21 application layer at the boundary between the application and DLLs of the
fieldbus reference model;
• Systems management at the boundary between the DLL and the systems management of
the fieldbus reference model.
1.2 Specifications
The principal objective of this document is to specify the characteristics of conceptual DLL
services suitable for time-critical communications, and to supplement the OSI Basic
Reference Model in guiding the development of data link protocols for time-critical
communications. A secondary objective is to provide migration paths from previously existing
industrial communications protocols.
This document may be used as the basis for formal data link programming interfaces.
Nevertheless, it is not a formal programming interface, and any such interface will need to
address implementation issues not covered by this document, including:
a) The sizes and octet ordering of various multi-octet service parameters;
b) The correlation of paired primitives for request and confirm, or indication and response.
1.3 Conformance
This document does not specify individual implementations or products, nor do they constrain
the implementations of data-link entities within industrial automation systems.

There is no conformance of equipment to this data-link layer service definition document.
Instead, conformance is achieved through implementation of the corresponding data-link
protocol that fulfils the Type 21 DLL services defined in this document.
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 IEC 61784-1 and IEC 61784-2 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/IEEE 8802-3, Information technology – Telecommunications and information
exchange between systems – Local and metropolitan area networks – Specific requirements –
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, abbreviations, and conventions
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 http://www.electropedia.org/
• ISO Online browsing platform: available at http://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.

– 10 – IEC 61158-3-21:2019 © IEC 2019
[ISO/IEC 7498-3]
3.1.1 DL-address
[ISO/IEC 7498-1]
3.1.2 DL-address-mapping
[ISO/IEC 7498-3]
3.1.3 called-DL-address
[ISO/IEC 7498-3]
3.1.4 calling-DL-address
[ISO/IEC 7498-1]
3.1.5 centralized multi-end-point-connection
[ISO/IEC 7498-1]
3.1.6 DL-connection
[ISO/IEC 7498-1]
3.1.7 DL-connection-end-point
[ISO/IEC 7498-1]
3.1.8 DL-connection-end-point-identifier
[ISO/IEC 7498-1]
3.1.9 DL-connection-mode transmission
[ISO/IEC 7498-1]
3.1.10 DL-connectionless-mode transmission
[ISO/IEC 7498-1]
3.1.11 correspondent (N)-entities
correspondent DL-entities  (N=2)
correspondent Ph-entities  (N=1)
[ISO/IEC 7498-1]
3.1.12 DL-duplex-transmission
[ISO/IEC 7498-1]
3.1.13 (N)-entity
DL-entity  (N=2)
Ph-entity  (N=1)
[ISO/IEC 7498-1]
3.1.14 DL-facility
[ISO/IEC 7498-1]
3.1.15 flow control
[ISO/IEC 7498-1]
3.1.16 (N)-layer
DL-layer  (N=2)
Ph-layer  (N=1)
[ISO/IEC 7498-1]
3.1.17 layer-management
[ISO/IEC 7498-3]
3.1.18 DL-local-view
[ISO/IEC 7498-3]
3.1.19 DL-name
[ISO/IEC 7498-3]
3.1.20 naming-(addressing)-domain
[ISO/IEC 7498-1]
3.1.21 peer-entities
[ISO/IEC 7498-3]
3.1.22 primitive name
[ISO/IEC 7498-1]
3.1.23 DL-protocol
[ISO/IEC 7498-1]
3.1.24 DL-protocol-connection-identifier
[ISO/IEC 7498-1]
3.1.25 DL-protocol-data-unit
[ISO/IEC 7498-1]
3.1.26 DL-relay
[ISO/IEC 7498-1]
3.1.27 Reset
[ISO/IEC 7498-3]
3.1.28 responding-DL-address
[ISO/IEC 7498-1]
3.1.29 Routing
[ISO/IEC 7498-1]
3.1.30 Segmenting
[ISO/IEC 7498-1]
3.1.31 (N)-service
DL-service  (N=2)
Ph-service  (N=1)
[ISO/IEC 7498-1]
3.1.32 (N)-service-access-point
DL-service-access-point  (N=2)
Ph-service-access-point  (N=1)
[ISO/IEC 7498-3]
3.1.33 DL-service-access-point-address
[ISO/IEC 7498-1]
3.1.34 DL-service-connection-identifier
[ISO/IEC 7498-1]
3.1.35 DL-service-data-unit
[ISO/IEC 7498-1]
3.1.36 DL-simplex-transmission
[ISO/IEC 7498-1]
3.1.37 DL-subsystem
[ISO/IEC 7498-1]
3.1.38 systems-management
[ISO/IEC 7498-1]
3.1.39 DLS-user-data
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-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
– 12 – IEC 61158-3-21:2019 © IEC 2019
3.2.21 response (primitive);
acceptor.submit (primitive)
3.2.22 submit (primitive)
3.2.23 symmetrical service
3.3 Common data-link service terms and definitions
For the purpose 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, 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 source or a final destination of MAC frames
transmitted across that network
Note 1 to entry: A network layer router is, from the perspective of the network, an end-station. A switch, in its role
of forwarding MAC frames from one link to another, is not an end-station.
[SOURCE: IEC 61784-2, 3.1.5]
3.3.10
frame
unit of data transmission on an ISO/IEC/IEEE 8802-3 MAC (Media Access Control) that
conveys a protocol data unit (PDU) between MAC service users
[SOURCE: IEEE 802.1Q-2011]
3.3.11
Frame check sequence (FCS) error
error that occurs when the computed frame check sequence value after reception of all the
octets in a data link protocol data unit (DLPDU) does not match the expected residual
3.3.12
linear topology
topology where the nodes are connected in series, with two nodes connected to only one
other node and all others each connected to two other nodes (that is, connected in the shape
of a line)
Note 1 to entry: This topology corresponds to that of an open ring.
[SOURCE: IEC 61918, 3.1.51]
3.3.13
link
transmission path between two adjacent nodes
[SOURCE: derived from ISO/IEC 11801]
3.3.14
network management
management functions and services that perform network initialization, configuration, and
error handling
3.3.15
node
network entity connected to one or more links

– 14 – IEC 61158-3-21:2019 © IEC 2019
Note 1 to entry: A node may be either a switch, an end-station or an RTE end-station.
[SOURCE: IEC 61784-2, 3.1.17]
3.3.16
packet
logical grouping of information used to describe a unit of data at any layer to convey the
upper layer user data to its peer layer
Note 1 to entry: A packet is identical to the PDU at each layer in terms of the OSI reference model. A data-link
layer packet is a frame.
[SOURCE: IEC 61784-2, 3.1.18]
3.3.17
protocol
convention on the data formats, time sequences, and error correction for data exchange in
communication systems
3.3.18
real-time
ability of a system to provide a required result in a bounded time
[SOURCE: IEC 61784-2, 3.1.19]
3.3.19
real-time communication
transfer of data in real-time
[SOURCE: IEC 61784-2, 3.1.20]
3.3.20
Real-time Ethernet
RTE
ISO/IEC/IEEE 8802-3 based network that includes real-time communication
Note 1 to entry: Other communications can be supported, providing that the real-time communication is not
compromised.
Note 2 to entry: This definition is dedicated but not limited to ISO/IEC/IEEE 8802-3. It could be applicable to other
IEEE 802 specifications, for example IEEE 802.11.
[SOURCE: IEC 61784-2, 3.1.21]
3.3.21
ring
active network where each node is connected in series to two other nodes
[SOURCE: IEC 61918, 3.1.71]
3.3.22
RTE end device
device with at least one RTE end-station
[SOURCE: IEC 61784-2, 3.1.26]
3.3.23
RTE end-station
end-station with RTE capability

[SOURCE: IEC 61784-2, 3.1.27]
3.4 Additional Type 21 data-link specific definitions
3.4.1
device unique identification
unique 8 octet identification to identify a Type 21 device in a network. This ID is a combination
of a 6 octet ISO/IEC/IEEE 8802-3 MAC address and 2 octet DL-address
3.4.2
R-port
port in a communication device that is part of a ring structure
3.5 Common symbols and abbreviations
DL data link (used as a prefix or adjective)
DLC data link connection
DLCEP data link connection endpoint
DLE data link entity (the local active instance of the DLL)
DLL data link layer
DLPDU data link protocol data unit
DLPM data link protocol machine
DLM data link management
DLME data link management entity (the local active instance of DLM)
DLMS data link management service
DLS data link service
DLSAP data link service-access-point
DLSDU data link service-data-unit
FIFO first-in, first-out (queuing method)
NMT network management
OSI Open Systems Interconnection
Ph- physical layer (as a prefix)
PHY physical interface transceiver
PhL physical layer
RTE Real-time Ethernet
IEC International Electrotechnical Commission
IP Internet Protocol ( see RFC 791)
ISO International Organization for Standardization
MAC media access control
NRT non-real-time
PDU protocol data unit
SAP service access point
RT real-time
TCP Transmission Control Protocol (see RFC 793)
UDP User Datagram Protocol (see RFC 768)

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3.6 Additional Type 21 symbols and abbreviations
EFR extremely fast recovery
GD general device
LNM line network manager
PO power on
PnP plug and play
RNM ring network manager
RNMP primary ring network manager
RNMS secondary ring network manager
RNAC ring network auto configuration
UID device unique identification
Type 21 NMIB Type 21 network management information base

3.7 Common conventions
This document uses the descriptive conventions given in ISO/IEC 10731.
The service model, service primitives, and time-sequence diagrams used are entirely abstract
descriptions; they do not represent a specification for implementation.
Service primitives, used to represent service user/provider interactions (see ISO/IEC 10731),
convey parameters that indicate information available in the user/provider interaction.
This document uses a tabular format to describe the component parameters of the DLS
primitives. The parameters that apply to each group of DLS primitives are set out in tables
throughout the remainder of this document. Each table consists of up to six columns,
containing the name of the service parameter, and a column for each of those primitives and
parameter-transfer directions used by the DLS, including
• the request primitive’s input parameters;
• the request primitive’s output parameters;
• the indication primitive’s output parameters;
• the response primitive’s input parameters;
• the confirmation primitive’s output parameters.
NOTE The request, indication, response and confirmation primitives are also known as requestor.submit,
acceptor.deliver, acceptor.submit, and requestor.deliver primitives, respectively (see ISO/IEC 10731).
One parameter, or a portion of it, is listed in each row of each table. Under the appropriate
service primitive columns, a code is used to specify how the parameter is used, and its
direction:
M parameter: mandatory for the primitive;
U parameter: a user option that may or may not be provided depending on the
dynamic use of the DLS-user. When not provided, a default value for the
parameter is assumed;
C parameter is conditional upon other parameters or upon the environment of the
DLS-user;
(Blank) parameter is never present.
Some entries are further qualified by items in parentheses. These may be one of:

a) (=) A parameter-specific constraint indicating that the parameter is semantically
equivalent to the parameter in the service primitive to its immediate left in the table;
b) (n) An indication that following note n contains additional information pertaining to the
parameter and its use.
In any particular interface, not all parameters shall be stated explicitly. Some may be implicitly
associated with the DLSAP at which the primitive is issued.
In the diagrams illustrating these interfaces, dashed lines indicate cause and effect or time
sequence relationships, and wavy lines indicate that events occur at approximately the same
time.
3.8 Additional Type 21 conventions
In the diagrams illustrating the DLS and DLM interfaces, dashed lines indicate cause and
effect or time sequence relationships between actions at different stations, while solid lines
with arrows indicate cause and effect time sequence relationships that occur within the DLE
provider at a single station.
The following notation, a shortened form of the primitive classes defined in 3.7, is used in the
figures and tables.
req request primitive
ind indication primitive
cnf confirmation primitive (confirmation)
rsp response primitive
4 Data-link layer services and concepts
4.1 General
4.1.1 Overview
This document specifies the Type 21 data link services for an ISO/IEC/IEEE 8802-3 based
time-deterministic control network, which is one of the communication networks for RTE. The
communication services support timing demands typical of high-performance automation
applications. They do not change the basic principles of the Ethernet according to
ISO/IEC/IEEE 8802-3, but extend it toward RTE. Thus, it is possible to continue to use
standard Ethernet hardware, infrastructure components, or test and measurement equipment,
such as network analyzers.
The Type 21 DLL provides reliable and transparent data communication between two Type 21
end devices. The Type 21 DLL also guarantees abstract transparent data transfer between
DL-users so that DLL provides flexible and convenient network connectivity to network users.
4.1.2 Overview of full duplex flow control
A Type 21 device is based on an integrated switch with two ports (ring ports) connected to the
ring. Therefore, a Type 21 network system is made up of full-duplex, collision-free switching
devices configured as a ring or a line network. Figure 1 shows the full-duplex flow control
procedure in a Type 21 network system. Type 21 guarantees collision-free data transmission
between two devices linked by a full-duplex Ethernet connection so that the Type 21 DLL
provides reliable, transparent, and collision-free data transmission to the DLS-users.

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Type 21 Type 21
Rx Tx Rx Tx Rx Tx Rx Tx
MAC/DLL MAC/DLL
Forwarding Forwarding
MAC1 MAC2 MAC1 MAC2
Control Control
PHY1 PHY2 PHY1 PHY2
Figure 1 – Full-duplex flow control
4.1.3 Types and classes of DL-layer service
4.1.3.1 Overview
The DLS provides transparent and reliable data transmission between DLS-users over
Type 21. The DLS is based on services provided by the physical layer of
ISO/IEC/IEEE 8802-3 to the conceptual interface between the physical and data link layers.
Three types of data transmission services are provided.
Data service (DL-DATA)
Data service is used to transmit a Type 21 frame to a destination device or devices using the
priority option. DL-DATA service is a queued service using the RT-queue.
Sporadic data service (DL-SPDATA)
Sporadic data service is used to transmit a common protocol frame, such as TCP/IP or UDP.
Type 21 data link layer transmits without modification any received DLSDUs generated by
...