IEC 61158-4-4:2007

IEC 61158-4-4
Edition 1.0 2007-12
INTERNATIONAL
STANDARD
Industrial communication networks – Fieldbus specifications –
Part 4-4: Data-link layer protocol specification – Type 4 elements

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester.
If you have any questions about IEC copyright or have an enquiry about obtaining additional rights to this publication,
please contact the address below or your local IEC member National Committee for further information.

IEC Central Office
3, rue de Varembé
CH-1211 Geneva 20
Switzerland
Email: inmail@iec.ch
Web: www.iec.ch
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigenda or an amendment might have been published.
ƒ Catalogue of IEC publications: www.iec.ch/searchpub
The IEC on-line Catalogue enables you to search by a variety of criteria (reference number, text, technical committee,…).
It also gives information on projects, withdrawn and replaced publications.
ƒ IEC Just Published: www.iec.ch/online_news/justpub
Stay up to date on all new IEC publications. Just Published details twice a month all new publications released. Available
on-line and also by email.
ƒ Electropedia: www.electropedia.org
The world's leading online dictionary of electronic and electrical terms containing more than 20 000 terms and definitions
in English and French, with equivalent terms in additional languages. Also known as the International Electrotechnical
Vocabulary online.
ƒ Customer Service Centre: www.iec.ch/webstore/custserv
If you wish to give us your feedback on this publication or need further assistance, please visit the Customer Service
Centre FAQ or contact us:
Email: csc@iec.ch
Tel.: +41 22 919 02 11
Fax: +41 22 919 03 00
IEC 61158-4-4
Edition 1.0 2007-12
INTERNATIONAL
STANDARD
Industrial communication networks – Fieldbus specifications –
Part 4-4: Data-link layer protocol specification – Type 4 elements

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
X
ICS 35.100.20; 25.040.40 ISBN 2-8318-9432-8

– 2 – 61158-4-4 © IEC:2007(E)
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 abbreviations.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
4 Data Link Protocol Definition .14
4.1 Overview of the DL-protocol .14
4.2 General structure and encoding of PhIDUs and DLPDUs, and related
elements of procedure.26
4.3 DLPDU-specific structure, encoding and elements of procedure.33
4.4 DL-service elements of procedure .37
4.5 Route mechanism.40
4.6 Link-access system .43
4.7 Local variables, counters and queues.44
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 .29
Figure 9 – Extended Type 4-route format .29
Figure 10 – Complex Type 4-route format .30
Figure 11 – Immediate Type 4-route format .30
Figure 12 – IP Type 4-route format .31
Figure 13 – Control-status format.32
Figure 14 – Data-field-format .32
Figure 15 – Source / destination designator .41
Figure 16 – Simple Type 4-route generation .41
Figure 17 – Extended Type 4-route generation .42
Figure 18 – Complex and IP Type 4-route generation .42

61158-4-4 © IEC:2007(E) – 3 –
Figure 19 – Simple DL-route generation.43
Figure 20 – Extended DL-route generation.43
Figure 21 – 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 – 61158-4-4 © IEC:2007(E)
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 provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
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.
NOTE  Use of some of the associated protocol types is restricted by their 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 particular data-link layer protocol type to be used with physical layer and application layer protocols in Type
combinations as specified explicitly in the IEC 61784 series. Use of the various protocol types in other
combinations may require permission from their respective intellectual-property-right holders.
International Standard IEC 61158-4-4 has been prepared by subcommittee 65C: Industrial
networks, of IEC technical committee 65: Industrial-process measurement, control and
automation.
This first edition and its companion parts of the IEC 61158-4 subseries cancel and replace
IEC 61158-4:2003. This edition of this part constitutes a minor revision. This part and its
companion Type 4 parts also cancel and replace IEC PAS 62412, published in 2005.
This edition of IEC 61158-4 includes the following significant changes from the previous
edition:
a) deletion of the former Type 6 fieldbus, and the placeholder for a Type 5 fieldbus data link
layer, for lack of market relevance;

61158-4-4 © IEC:2007(E) – 5 –
b) addition of new types of fieldbuses;
c) division of this part into multiple parts numbered -4-1, -4-2, …, -4-19.
The text of this standard is based on the following documents:
FDIS Report on voting
65C/474/FDIS 65C/485/RVD
Full information on the voting for the approval of this 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.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result 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.
NOTE  The revision of this standard will be synchronized with the other parts of the IEC 61158 series.
The 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.

– 6 – 61158-4-4 © IEC:2007(E)
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/TR 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 standard 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 standard is concerned, in particular, with the communication and interworking of sensors,
effectors and other automation devices. By using this standard together with other standards
positioned within the OSI or fieldbus reference models, otherwise incompatible systems may
work together in any combination.

61158-4-4 © IEC:2007(E) – 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 standard 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 standard, 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 standard also specifies conformance requirements for systems implementing these
procedures. This standard does not contain tests to demonstrate compliance with such
requirements.
– 8 – 61158-4-4 © IEC:2007(E)
2 Normative references
The following referenced documents are indispensable for the application 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.
IEC 61158-2 (Ed.4.0), Industrial communication networks – Fieldbus specifications – Part 2:
Physical layer specification and service definition
IEC 61158-3-4, Industrial communication networks – Fieldbus specifications – Part 3-4: Data-
link layer service definition – Type 4 elements
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 abbreviations
For the purposes of this document, the following terms, definitions, symbols and abbreviations
apply.
3.1 Reference model terms and definitions
This standard 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]
3.1.12 DL-connectionless-mode transmission [7498-1]
3.1.13 DL-data-sink [7498-1]
61158-4-4 © IEC:2007(E) – 9 –
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)
3.1.40 (N)-service-access-point [7498-1]
DL-service-access-point  (N=2)
Ph-service-access-point  (N=1)

– 10 – 61158-4-4 © IEC:2007(E)
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 standard 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
3.2.12 DL-service-primitive;
primitive
3.2.13 DL-service-provider
61158-4-4 © IEC:2007(E) – 11 –
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
3.3.1
broadcast-Node-address
address used to send broadcasts to all DLEs on a Link
NOTE  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  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
DL-segment, link, 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.
3.3.6
DLSAP
distinctive point at which DL-services are provided by a single DL-entity to a single higher-
layer entity.
NOTE  This definition, derived from ISO/IEC 7498-1, is repeated here to facilitate understanding of the critical
distinction between DLSAPs and their DL-addresses.
3.3.7
DL(SAP)-address
an individual DLSAP-address, designating a single DLSAP of a single DLS-user.

– 12 – 61158-4-4 © IEC:2007(E)
3.3.8
(individual) DLSAP-address
DL-address that designates only one DLSAP within the extended link
NOTE  A single DL-entity may have multiple DLSAP-addresses associated with a single DLSAP.
3.3.9
frame
denigrated synonym for DLPDU
3.3.10
IPNetID
identification of a unique IP network. The value of IPNetID shall be in the range of 0-127. The
values 0, 126 and 127 are reserved for special purposes
NOTE  An IPNetID is translated into an IP-address and a UPD port number.
3.3.11
IPNetTable
definition of the relation between IPNetID, IP address, UPD port number and Router
NodeAddress, where IPNetID is used as index in the table
3.3.12
IP Range net
is used 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
3.3.13
Nettype
an IP network is of a certain type, a Nettype that can be “Unused”, “IP Range net” or “UDP
Range net”
3.3.14
no-Confirm-Node-address
address used to indicate that a request or response is Unconfirmed
NOTE  The value of a No-Confirm-Node-address is 0.
3.3.15
node
single DL-entity as it appears on one local link
3.3.16
node-address
address which uniquely identifies a DLE on a Link
NOTE  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.
3.3.17
normal class device
device which replies to requests from other normal class devices, and initiates transmissions
NOTE  Such a device can act as a server (responder) and as a client (requestor) - this is also called a peer
3.3.18
Type 4-route
holds a sequence of Type 4-route-elements
NOTE  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.

61158-4-4 © IEC:2007(E) – 13 –
3.3.19
Type 4-route-element
an octet, holding a 7-bit DL-route-element or Remaining-route-length, and a 1-bit source/
destination designator
3.3.20
receiving DLS-user
DL-service user that acts as a recipient of DL-user-data
NOTE  A DL-service user can be concurrently both a sending and receiving DLS-user.
3.3.21
sending DLS-user
DL-service user that acts as a source of DL-user-data
3.3.22
service-Node-address
address reserved for service purposes only
NOTE  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 their receipt may or may not be acknowledged. 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.
3.3.23
simple class device
device which replies to requests from normal class devices, and can act as a server or
responder only
3.3.24
source-DL-route
holds a sequence of DL-route-elements, describing the complete route back to the source
3.3.25
UDP port number
a Server can receive requests on two different UPD port numbers: Normal UDP port and
Secure UDP port. The UDP port number shall be 34378 for Normal UDP port. The UDP port
number shall be 34379 for Secure UDP port
NOTE  These UDP port numbers are registered with the IANA (Internet Assigned Numbers Authority)
3.3.26
UDP range net
is used for remote access, where a node cannot be accessed directly on the same subnet as
the client. The IPNetTable holds a NAT Router IP address and access to the node is obtained
through this NAT Router
NOTE  The NAT Router shall hold a table that translates the UDP port number to the actual server node IP
address and UDP port number.
3.3.27
Virtual link-access token
basis for the link-access system
NOTE  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.

– 14 – 61158-4-4 © IEC:2007(E)
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
The DLL provides connectionless data transfer services for limited-size DLSDUs from one
DLS-user to one or more (broadcast) DLS-users.
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.

61158-4-4 © IEC:2007(E) – 15 –
Application
Layer
DLS-user DLS-user
DLSAP DLSAP
Data Link
DLE DLE
Layer
Physical
PhE PhE
Layer
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.
4.1.1 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).
4.1.2 Functions of the DLL
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):
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.
INK-IDLE, and use that to time-out when waiting for a
b) Receive a single PhIDU specifying L
DLS-user request primitive.
– 16 – 61158-4-4 © IEC:2007(E)
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.
4.1.2.1 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.
4.1.2.2 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.

61158-4-4 © IEC:2007(E) – 17 –
Indication to DLS-
user
Error OK
Receive DLPDU
Queue DLPDU
START-OF-ACTIVITY
indication from PhE
Request from DLS-user
Idle
Token received and
queue not empty
Send DLPDU
from queue
Figure 2 – DLE state diagram for confirmed and unconfirmed, unacknowledged DLPDUs

– 18 – 61158-4-4 © IEC:2007(E)
Wait for request
or response
from DLS-user
Response from DLS-
user or 30 bit idle
Indication to DLS-
Request from
user
DLS-user
Send Acknowledge
DLPDU
Error OK
Send Immediate-
Receive DLPDU reply DLPDU
Queue DLPDU
START-OF-ACTIVITY
indication from PhE
Request from DLS-user
Idle
Error indication to
DLS-user
Error indication to
DLS-user
Token received and
queue not empty
Retransmission
not allowed
Retransmission
not allowed
Retransmission
Send DLPDU
allowed
from queue
Retransmission
allowed
Queue DLPDU for
retransmission if
allowed
Retransmit DLPDU
Wait for Immediate-
immediately if
reply or Acknowledge
allowed
Indication to DLS-
DLPDU
user
Received RCL/ACK
35 bit idle
Received Wait Received
START-OF-ACTIVITY
immediate reply
indication from PhE
Receive DLPDU
Error
Figure 3 – DLE state diagram for confirmed acknowledged DLPDUs

61158-4-4 © IEC:2007(E) – 19 –
Indication to DLS-
user
Send Acknowledge
Error OK
DLPDU
Receive DLPDU
Queue DLPDU
START-OF-ACTIVITY
indication from PhE
Request from DLS-user
Idle
Error indication to
DLS-user
Error indication to
DLS-user
Token received and
queue not empty
Retransmission
not allowed
Retransmission
not allowed
Retransmission
Send DLPDU
allowed
from queue
Retransmission
allowed
Queue DLPDU for
retransmission if
allowed
Retransmit DLPDU
Wait for Acknowledge
immediately if
DLPDU
allowed
Received RCL/ACK
35 bit idle
Received Wait
START-OF-ACTIVITY
indication from PhE
Receive DLPDU
Error
Figure 4 – DLE state diagram for unconfirmed acknowledged DLPDUs

– 20 – 61158-4-4 © IEC:2007(E)
Indication to DLS-
user
Error OK
Receive DLPDU
START-OF-ACTIVITY
indication from PhE
Idle
Figure 5 – Full duplex DLE receive state diagram
Queue DLPDU
Request from DLS-user
Idle
Queue not empty
Send DLPDU
from queue
Figure 6 – Full duplex DLE transmit state diagram
4.1.2.3 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.

61158-4-4 © IEC:2007(E) – 21 –
c) Acknowledge - used by DLEs to acknowledge receipt of Confirmed or Unconfirmed
DLPDUs. The receipt of Acknowledge DLPDUs must never be acknowledged.
d) Immediate-reply - used to send responses between DLS-users. The receipt of Immediate-
reply DLPDUs must never be acknowledged.
4.1.2.4 SPDU types
Only one type of SPDU (Support Protocol Data Unit) is defined.
a) Sync - used to send Link access synchronization information between DLEs. An SPDU
holds the Node-address of the DLE holding the Virtual Link-access token. An SPDU can
be "stand-alone" or part of an Acknowledge or Immediate-reply DLPDU.
4.1.2.5 Responder role, receiving a DLPDU from the PhE
This action includes a sequence of steps, as described in the following.
a) Receive a single PhIDU specifying START-OF-ACTIVITY. This PhIDU holds a Node address.
This address is examined to determine, whether its value is equal to the Node-address of
this DLE, or equal to the Broadcast-Node-address (BNA) or the Service-Node-Address
(SNA). If not, ignore this sequence and wait for the next PhIDU specifying START-OF-
ACTIVITY.
b) Receive a sequence of PhIDUs from the PhE, specifying DATA, concatenate them to a
received DLPDU, compute a frame check sequence over the entire sequence of received
data as specified by the value of V(FCM) - FrameCheckMethod, and, if necessary, check
for the proper value. If the value is not correct, ignore the DLPDU and wait for the next
PhIDU specifying START-OF-ACTIVITY.
c) Convert the received DLPDU into its DL-protocol control information and data
components.
d) Generate a DLS-user indication primitive.
e) If the DLPDU received from the remote DLE is of type Confirmed, and the receipt of the
DLPDU must be acknowledged, according to the rules described in 4.1.2.1, wait for a
request or response primitive from the local DLS-user.
If no request or response primitive is issued from the local DLS-user in time (before a
PhIDU specifying "LINK-IDLE for 30 bit periods" is received from the PhE), generate and
immediately send an Acknowledge DLPDU. This DLPDU must specify "Wait" if this DLE is
of Simple class, and "Response Comes Later / Acknowledge" ("RCL/ACK") if this DLE is of
Normal class.
If a response primitive is issued from the local DLS-user in time, generate and
immediately send an Acknowledge DLPDU, specifying "Wait" if this DLE is of Simple
class, and "RCL/ACK" if this DLE is of Normal class.
If a request primitive is issued from the local DLS-user in time, convert it into an
Immediate-reply DLPDU and send it immediately. After sending, wait for the next PhIDU
specifying START-OF-ACTIVITY.
f) If the DLPDU received from the remote DLE is of the Confirmed type, and the receipt of
the DLPDU shall not be acknowledged, wait for the next PhIDU specifying START-OF-
ACTIVITY.
g) If the DLPDU received from the remote DLE is of the Unconfirmed type, and the receipt of
the DLPDU shall be acknowledged, according to the rules described in 4.1.2.1, generate
and immediately send an Acknowledge DLPDU, specifying RCL/ACK. After sending, wait
for the next PhIDU specifying START-OF-ACTIVITY.
h) If the DLPDU received from the remote DLE is of the Unconfirmed type, and the receipt of
the DLPDU shall not be acknowledged, wait for the next PhIDU specifying START-OF-
ACTIVITY.
– 22 – 61158-4-4 © IEC:2007(E)
4.1.2.6 Responder role, receiving a PhIDU specifying LINK-IDLE
As a responder, when waiting for a request or response primitive from the local DLS-user, the
receipt of a PhIDU from the PhE specifying "LINK-IDLE for 30 bit periods" is used to timeout
waiting for the DLS-user. The possible actions resulting from the timeout are defined in
4.1.2.5.
4.1.2.7 Initiator role, managing request primitives from the local DLS-user
This action includes a sequence of steps, as described in the following:
a) Convert a request primitive from the local DLS-user into a DLPDU, queue it, and send it to
a remote DLE (or all DLEs on the Link if broadcast) at the first opportunity.
b) If the DLPDU sent is of type Unconfirmed, and the receiving DLE should acknowledge the
receipt, according to the rules defined in 4.1.2.1, wait for an Acknowledge DLPDU from
the remote DLE specifying RCL/ACK. If no acknowledge is received in time (before a
PhIDU specifying "LINK-IDLE for 35 bit periods" is received from the PhE), immediately re-
transmit the DLPDU if the permitted number of transmission retries have not been sent. If
the permitted number of transmission retries have failed, do nothing, and this action is
completed.
c) If the DLPDU sent is of type Unconfirmed, and the receiving DLE should not acknowledge
the receipt, this action is completed.
d) If the DLPDU sent is of type Confirmed, and the receiving DLE should acknowledge the
receipt, wait for an Immediate-reply DLPDU holding the response, or an Acknowledge
DLPDU, from the remote DLE.
If an Acknowledge DLPDU is received from the remote DLE in time (before a PhIDU
specifying "LINK-IDLE for 35 bit periods" is received from the PhE), and the acknowledge
specifies "RCL/ACK", this action is completed. If the acknowledge specifies "Wait", queue
the DLPDU for retransmission if the associated retry timer has not expired. If the retry
timer has expired, generate a DLS-user indication primitive with the appropriate error
information.
If an Immediate-reply DLPDU holding the response is received in time from the remote
DLE, convert the received DLPDU into its DL-protocol control information and data
components, and generate a DLS-user indication primitive.
If neither acknowledge nor response is received from the remote DLE in time, re-transmit
the DLPDU immediately (while this DLE still holds the Virtual Link-access token) if the
permitted number of transmission retries have not been sent. If the permitted number of
transmission retries have failed, generate a DLS-user indication primitive with the
appropriate error information.
e) If the DLPDU sent is of type Confirmed, and the receiving DLE should not acknowledge
the receipt, this action is completed.
4.1.2.8 Initiator role, link-access
The Link-access system is based on a so-called Virtual Link-access token. Virtual because
the token is not explicitly sent from one Normal class DLE to another, but implicitly passed as
the Link is idle.
The following DLE variables and counters are used by the Link-access system.
V(NA) - Node-address. Each DLE on a Link is uniquely identified by its Node-address, the
value of which is stored in V(NA). The value of V(NA) must be different in all DLEs on the
Link.
61158-4-4 © IEC:2007(E) – 23 –
V(NDLE) - Number of DLEs - holds the maximum number of Normal class DLEs on the Link.
The value of V(NA) must be lower than or equal to the value of V(NDLE). The value of
V(NDLE) must not exceed 32. The value of V(NDLE) must be the same in all DLEs on the
Link.
C(LAC) - Link Access Counter - holds the Node-address of the DLE holding the Virtual Link-
access token. The value of C(LAC) will be the same in all DLEs on the Link.
C(LIC) - Link Idle Counter - holds information on, for how long the Link has been idle. The
value of C(LIC) will be the same in all DLEs on the Link.
Figure 7 illustrates the functionality of the Link-access system. The "Action" line describes the
use of the Link. The first action is that the DLE having Node-address 2 sends a Confirmed
DLPDU, and receives the corresponding Immediate-reply DLPDU. The second action is that
the DLE having Node-address 3 sends an Unconfirmed DLPDU. Then, after a long idle period,
the DLE with Node-address 2 sends a Sync SPDU.
The DLE having Node-address 4 is not present. Had it been present, DLE4 should have sent
the Sync SPDU, as the
...