IEC 61158-5-21:2019

IEC 61158-5-21 ®
Edition 2.0 2019-04
INTERNATIONAL
STANDARD
Industrial communication networks – Fieldbus specifications –
Part 5-21: Application layer service definition – Type 21 elements

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IEC 61158-5-21 ®
Edition 2.0 2019-04
INTERNATIONAL
STANDARD
Industrial communication networks – Fieldbus specifications –

Part 5-21: Application layer service definition – Type 21 elements

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 25.040.40; 35.100.70; 35.110 ISBN 978-2-8322-6752-3

– 2 – IEC 61158-5-21:2019 © IEC 2019
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
1.1 Overview . 8
1.2 Specifications . 9
1.3 Conformance . 9
2 Normative references . 9
3 Terms, definitions, symbols, abbreviations, and conventions . 10
3.1 Terms and definitions from other ISO/IEC standards . 10
3.1.1 ISO/IEC 7498-1 terms. 10
3.1.2 ISO/IEC 8822 terms . 10
3.1.3 ISO/IEC 8824-1 terms. 10
3.1.4 ISO/IEC 9545 terms . 11
3.2 Fieldbus data link layer terms . 11
3.3 Fieldbus application layer specific definitions . 11
3.4 Abbreviations and symbols . 17
3.5 Conventions . 17
3.5.1 Overview . 17
3.5.2 General conventions . 18
3.5.3 Conventions for class definitions . 18
3.5.4 Conventions for service definitions . 19
4 Concepts . 20
4.1 Common concepts . 20
4.1.1 Overview . 20
4.1.2 Architectural relationships . 21
4.1.3 Fieldbus application layer structure . 23
4.1.4 Fieldbus application layer naming and addressing . 34
4.1.5 Architecture summary . 35
4.1.6 FAL service procedures . 36
4.1.7 Common FAL attributes . 37
4.1.8 Common FAL service parameters . 37
4.1.9 APDU size . 38
4.2 Type specific concepts . 38
4.2.1 Node, AP, and object dictionary . 40
4.2.2 APO ASEs . 41
5 Data type ASE . 41
5.1 General . 41
5.1.1 Overview . 41
5.1.2 Basic type overview . 42
5.1.3 Fixed-length type overview . 42
5.1.4 Constructed type overview . 43
5.1.5 Specification of user-defined data types . 43
5.1.6 Transfer of user data . 43
5.2 Formal definition of data type objects . 44
5.2.1 Data type class . 44
5.3 FAL defined data types . 45

5.3.1 Fixed-length types . 45
5.3.2 String types . 48
5.4 Data type ASE service specification . 49
6 Communication model specification . 49
6.1 ASEs . 49
6.1.1 Application process ASE . 49
6.1.2 Service data object ASE . 55
6.1.3 Process data object ASE . 65
6.1.4 Application relationship ASE . 68
6.2 ARs . 75
6.2.1 Point-to-point user-triggered confirmed client/server AREP (PTC-AR) . 75
6.2.2 Multipoint network-scheduled unconfirmed publisher-subscriber AREP
(MSU-AR) . 76
6.2.3 Multipoint user-triggered unconfirmed publisher-subscriber AREP
(MTU-AR) . 78
6.3 Summary of FAL classes . 79
6.4 Permitted FAL services by AREP role . 79
Bibliography . 80

Figure 1 – Relationship to the OSI Basic Reference Model . 21
Figure 2 – Architectural positioning of the fieldbus application layer . 22
Figure 3 – Client/server interactions . 24
Figure 4 – Pull model interactions . 25
Figure 5 – Push model interactions . 26
Figure 6 – APOs services conveyed by the FAL . 28
Figure 7 – Application entity structure . 30
Figure 8 – FAL management of objects . 31
Figure 9 – ASE service conveyance . 32
Figure 10 – Defined and established AREPs . 34
Figure 11 – FAL architectural components . 36
Figure 12 – Interaction between FAL and DLL . 39
Figure 13 – Publisher-subscriber communication model . 39
Figure 14 – Client-server communication model . 40
Figure 15 – Object model . 40
Figure 16 – ASEs of a Type 21 application . 41
Figure 17 – Data type class hierarchy example . 42
Figure 18 – The AR ASE conveys APDUs between APs . 68

Table 1 – Types of timeliness . 27
Table 2 – Overall structure of the OD . 40
Table 3 – Identify service . 52
Table 4 – Status service . 54
Table 5 – Access rights for object . 56
Table 6 – Read service . 57
Table 7 – Write service . 59

– 4 – IEC 61158-5-21:2019 © IEC 2019
Table 8 – Write and Read service . 61
Table 9 – Write and Read Multiple service . 63
Table 10 – TB-transfer . 67
Table 11 – COS-transfer . 67
Table 12 – Conveyance of service primitives by AREP role . 69
Table 13 – Valid combinations of AREP roles involved in an AR . 69
Table 14 – AR-unconfirmed send . 73
Table 15 – AR-confirmed send . 74
Table 16 – FAL class summary . 79
Table 17 – Services by AREP role . 79

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 5-21: Application 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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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 61158-5-21 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 61158-5-21:2019 © IEC 2019
This edition includes the following significant technical changes with respect to the previous
edition:
• added Write and Read service;
• miscellaneous editorial corrections.
The text of this International Standard is based on the following documents:
FDIS Report on voting
65C/947/FDIS 65C/950/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.
The application service is provided by the application protocol making use of the services
available from the data-link or other immediately lower layer. This document defines the
application service characteristics that fieldbus applications and/or system management may
exploit.
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 application layer service defined in this document is a conceptual architectural service,
independent of administrative and implementation divisions.

– 8 – IEC 61158-5-21:2019 © IEC 2019
INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 5-21: Application layer service definition –
Type 21 elements
1 Scope
1.1 Overview
The Fieldbus Application Layer (FAL) provides user programs with a means to access the
fieldbus communication environment. In this respect, the FAL can be considered a window
between corresponding application programs.
This part of IEC 61158 provides the common elements for basic time-critical and non-time-
critical messaging communications between application programs in an automation
environment as well as material specific to the Type 21 protocol. The term “time-critical” is
used to represent the presence of a time-window within which one or more specified actions
are required to be completed with some defined level of certainty. Failure to complete
specified actions within the time window risks 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 FAL in terms of:
a) an abstract model for defining application resources (objects) capable of being
manipulated by users via the FAL service;
b) the primitive actions and events of the service;
c) the parameters associated with each primitive action and event, and the form that they
take;
d) the interrelationship between these actions and events, and their valid sequences.
The purpose of this document is to define the services provided to:
a) the FAL-user at the boundary between the user and the application layer of the fieldbus
Reference Model;
b) systems management at the boundary between the application layer and systems
management of the fieldbus Reference Model.
This document describes the structure and services of the IEC FAL, in conformance with the
OSI Basic Reference Model (ISO/IEC 7498) and the OSI Application layer Structure
(ISO/IEC 9545).
FAL services and protocols are provided by FAL application entities (AEs) contained in the
application processes. The FAL AE is composed of a set of object-oriented Application
Service Elements (ASEs) and a Layer Management Entity (LME) that manages the AE. The
ASEs provide communication services that operate on a set of related application process
object (APO) classes. One of the FAL ASEs is a management ASE that provides a common
set of services for management of the instances of FAL classes.
Although these services specify how requests and responses are issued and delivered from
the perspective of applications, they do not include a specification of what the requesting and
responding applications are to do with them. That is, these services only define what requests
and responses applications can send or receive, not the functions of the applications

themselves. This permits greater flexibility to the FAL-users in standardizing such object
behavior. In addition to these services, some supporting services are also defined in this
document to provide access to the FAL to control certain aspects of its operation.
1.2 Specifications
The principal objective of this document is to specify the characteristics of conceptual
application layer services suitable for time-critical communications, and thus supplement the
OSI Basic Reference Model in guiding the development of application layer protocols for time-
critical communications.
A secondary objective is to provide migration paths from previously existing industrial
communications protocols. This latter objective gives rise to the diversity of services
standardized as the various types of IEC 61158, and the corresponding protocols
standardized in subparts of IEC 61158-6.
This document may be used as the basis for formal application programming interfaces.
Nevertheless, it is not a formal programming interface, and any such interface must address
implementation issues not covered by this document, including:
a) sizes and octet ordering of various multi-octet service parameters;
b) correlation of paired primitives for request and confirmation, or indication and response.
1.3 Conformance
This document does not specify individual implementations or products, nor does it constrain
the implementations of application layer entities in industrial automation systems.
There is no conformance of equipment to this application layer service definition standard.
Instead, conformance is achieved through the implementation of conforming application layer
protocols that fulfill any given type of application layer services as 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/IEEE 60559:2011, Information technology – Microprocessor Systems – Floating-
Point arithmetic
IEC 61158-3-21:2019, Industrial communication networks – Fieldbus specifications –
Part 3-21: Data-link layer service definition – Type 21 elements
IEC 61158-4-21:2019, Industrial communication networks – Fieldbus specifications –
Part 4-21: Data-link layer protocol specification – Type 21 elements
IEC 61158-6-21:2019, Industrial communication networks – Fieldbus specifications –
Part 6-21: Application layer protocol specification – Type 21 elements
ISO/IEC 7498-1, Information technology – Open Systems Interconnection – Basic Reference
Model: The Basic Model
– 10 – IEC 61158-5-21:2019 © IEC 2019
ISO/IEC 7498-3, Information technology – Open Systems Interconnection – Basic Reference
Model: Naming and addressing
ISO/IEC 8822, Information technology – Open Systems Interconnection – Presentation
service definition
ISO/IEC 8824 (all parts), Information Technology – Abstract Syntax Notation One (ASN-1)
ISO/IEC 9545, Information technology – Open Systems Interconnection – Application layer
structure
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 Terms and definitions from other ISO/IEC standards
3.1.1 ISO/IEC 7498-1 terms
a) application entity
b) application process
c) application protocol data unit
d) application service element
e) application entity invocation
f) application process invocation
g) application transaction
h) real open system
i) transfer syntax
3.1.2 ISO/IEC 8822 terms
a) abstract syntax
b) presentation context
3.1.3 ISO/IEC 8824-1 terms
For the purposes of this document, the following terms as defined in ISO/IEC 8824-1 apply:
a) object identifier
b) type
3.1.4 ISO/IEC 9545 terms
a) application-association
b) application-context
c) application context name
d) application-entity-invocation
e) application-entity-type
f) application-process-invocation
g) application-process-type
h) application-service-element
i) application control service element
3.2 Fieldbus data link layer terms
For the purposes of this document, the following terms as defined in IEC 61158-3-21 and
IEC 61158-4-21 apply.
a) DL-Time
b) DL-Scheduling-policy
c) DLCEP
d) DLC
e) DL-connection-oriented mode
f) DLPDU
g) DLSDU
h) DLSAP
i) link
j) ISO/IEC/IEEE 8802-3 MAC address
k) DL–entity identifier
3.3 Fieldbus application layer specific definitions
3.3.1
application
function or data structure for which data are consumed or produced
3.3.2
application objects
multiple object classes that manage and provide a runtime exchange of messages across the
network and within the network device
3.3.3
application process
part of a distributed application on a network, which is located on one device and addressed
unambiguously
3.3.4
application process object
component of an application process that is identifiable and accessible through an FAL
application relationship
Note 1 to entry: Application process object definitions are composed of a set of values for the attributes of their
class (see the definition for “application process object class”). Application process object definitions may be
accessed remotely using the services of the FAL Object Management ASE. FAL Object Management services can

– 12 – IEC 61158-5-21:2019 © IEC 2019
be used to load or update object definitions, to read object definitions, and to create and delete application objects
and their corresponding definitions dynamically.
3.3.5
application process object class
class of application process objects defined in terms of the set of their network-accessible
attributes and services
3.3.6
application relationship
cooperative association between two or more application-entity-invocations for the purpose of
exchange of information and coordination of their joint operation
Note 1 to entry: This relationship is activated either by the exchange of application-protocol-data-units or as a
result of preconfiguration activities.
3.3.7
application relationship application service element
application-service-element that provides the exclusive means for establishing and
terminating all application relationships
3.3.8
application relationship endpoint
context and behavior of an application relationship as seen and maintained by one of the
application processes involved in the application relationship
Note 1 to entry: Each application process involved in the application relationship maintains its own application
relationship endpoint.
3.3.9
attribute
description of an externally visible characteristic or feature of an object
Note 1 to entry: The attributes of an object contain information about variable portions of an object. Typically,
they provide status information or govern the operation of an object. Attributes may also affect the behavior of an
object. Attributes are divided into class attributes and instance attributes.
3.3.10
behavior
indication of how an object responds to particular events
3.3.11
channel
single physical or logical link of an input or output application object of a server to the process
3.3.12
class
set of objects, all of which represent the same type of system component
Note 1 to entry: A class is a generalization of an object, a template for defining variables and methods. All objects
in a class are identical in form and behavior, but usually contain different data in their attributes.
3.3.13
class attributes
attribute shared by all objects within the same class
3.3.14
class code
unique identifier assigned to each object class

3.3.15
class-specific service
service defined by a particular object class to perform a required function that is not
performed by a common service
Note 1 to entry: A class-specific object is unique to the object class that defines it.
3.3.16
client
a) object that uses the services of another (server) object to perform a task
b) initiator of a message to which a server reacts
3.3.17
consume
act of receiving data from a producer
3.3.18
consumer
node or sink that receives data from a producer
3.3.19
consuming application
application that consumes data
3.3.20
conveyance path
unidirectional flow of APDUs across an application relationship
3.3.21
cyclic
repetitive in a regular manner
3.3.22
data consistency
means for coherent transmission and access of the input- or output-data object between and
within client and server
3.3.23
device
physical hardware connected to the link
Note 1 to entry: A device may contain more than one node.
3.3.24
device profile
collection of device-dependent information and functionality providing consistency between
similar devices of the same device type
3.3.25
diagnostic information
all data available at the server for maintenance purposes
3.3.26
end node
producing or consuming node
– 14 – IEC 61158-5-21:2019 © IEC 2019
3.3.27
endpoint
one of the communicating entities involved in a connection
3.3.28
error
discrepancy between a computed, observed, or measured value or condition and the specified
or theoretically correct value or condition
3.3.29
error class
general grouping for related error definitions and corresponding error codes
3.3.30
error code
identification of a specific type of error within an error class
3.3.31
event
instance of a change of conditions
3.3.32
FIFO variable
variable object class composed of a set of homogeneously typed elements, where the first
written element is the first element that can be read
Note 1 to entry: In a fieldbus system, only one complete element can be transferred as a result of one service
invocation.
3.3.33
frame
simplified synonym for data link protocol data unit (DLPDU)
3.3.34
group
general term for a collection of objects
3.3.35
group
when describing an address, an address that identifies more than one entity
3.3.36
invocation
act of using a service or other resource of an application process
Note 1 to entry: Each invocation represents a separate thread of control that may be described by its context.
Once the service completes, or use of the resource is released, the invocation ceases to exist. For service
invocations, a service that has been initiated but not yet completed is referred to as an outstanding service
invocation. For service invocations, an Invoke ID may be used to identify the service invocation unambiguously and
differentiate it from other outstanding service invocations.
3.3.37
index
address of an object within an application process
3.3.38
instance
actual physical occurrence of an object within a class that identifies one of many objects in
the same object class
EXAMPLE California is an instance of the object class US-state.
Note 1 to entry: The terms object, instance, and object instance are used to refer to a specific instance.
3.3.39
instance attributes
attribute that is unique to an object instance and not shared by the object class
3.3.40
instantiated
object that has been created in a device
3.3.41
logical device
specific FAL class that abstracts a software component or a firmware component as an
autonomous self-contained facility of an automation device
3.3.42
manufacturer ID
identification of each product manufacturer by a unique number
3.3.43
management information
network-accessible information that supports management of the operation of the fieldbus
system, including the application layer
Note 1 to entry: Managing includes functions, such as controlling, monitoring, and diagnosis.
3.3.44
network
set of nodes connected by some type of communication medium, including any intervening
repeaters, bridges, routers, and lower-layer gateways
3.3.45
object
abstract representation of a particular component within a device, usually a collection of
related data in the form of variables, and methods (procedures) for operating on that data that
have clearly defined interface and behavior
3.3.46
object dictionary
collection of definitions, communication-specific attributes and parameters, and application-
dependent data
3.3.47
object-specific service
service unique to the object class that defines it
3.3.48
physical device
automation or other network device
3.3.49
point-to-point connection
connection that exists between exactly two application objects

– 16 – IEC 61158-5-21:2019 © IEC 2019
3.3.50
pre-established AR endpoint
AR endpoint placed in an established state during configuration of the AEs that control its
endpoints
3.3.51
process data
object(s) that are already pre-processed and transferred cyclically for the purpose of
information or further processing
3.3.52
produce
act of sending data to be received by a consumer
3.3.53
producer
node that is responsible for sending data
3.3.54
property
general term for descriptive information about an object
3.3.55
provider
source of a data connection
3.3.56
publisher
role of an AR endpoint that transmits APDUs onto the fieldbus for consumption by one or
more subscribers
Note 1 to entry: A publisher may not be aware of the identity or number of subscribers.
3.3.57
publishing manager
role of an AR endpoint in which it issues one or more confirmed service request application
protocol data units (APDUs) to a publisher to request that a specified object be published.
Two types of publishing managers are defined by this document, pull publishing managers
and push publishing managers, each of which is defined separately.
3.3.58
push publisher
type of publisher that publishes an object in an unconfirmed service request APDU
3.3.59
push publishing manager
type of publishing manager that requests that a specified object be published using an
unconfirmed service
3.3.60
push subscriber
type of subscriber that recognizes received unconfirmed service request APDUs as published
object data
3.3.61
server
a) role of an application relationship endpoint (AREP) in which it returns a confirmed service
response APDU to the client that initiated the request

b) object that provides services to another (client) object
3.3.62
service
operation or function than an object and/or object class performs upon request from another
object and/or object class
3.3.63
station
host of one AP, identified by a unique data link connection endpoint (DLCEP)-address
3.3.64
subscriber
role of an AREP in which it receives APDUs produced by a publisher
3.4 Abbreviations and symbols
AE Application Entity
AL Application Layer
ALME Application Layer Management Entity
ALP Application Layer Protocol
APO Application Object
AP Application Process
APDU Application Protocol Data Unit
AR Application Relationship
AREP Application Relationship End Point
ASCII American Standard Code for Information Interchange
ASE Application Service Element
Cnf Confirmation
DL- (as a prefix) Data Link -
DLCEP Data Link Connection End Point
DLL Data Link Layer
DLM Data Link Management
DLSAP Data Link Service Access Point
DLSDU DL-service-data-unit
DNS Domain Name Service
FAL Fieldbus Application Layer
Ind Indication
Req Request
Rsp Response
3.5 Conventions
3.5.1 Overview
The FAL is defined as a set of object-oriented ASEs. Each ASE is specified in a separate
subclause. Each ASE specification is composed of two parts: its class specification and its
service specification.
– 18 – IEC 61158-5-21:2019 © IEC 2019
The class specification defines the attributes of the class. Access to these attributes is
beyond the scope of this document except where specified. The service specification defines
the services provided by the ASE.
3.5.2 General conventions
This document uses the descriptive conventions given in ISO/IEC 10731.
3.5.3 Conventions for class definitions
Class definitions are described using templates. Each template consists of a list of attributes
for the class. The general form of the template is as shown below:
FAL ASE: ASE name
CLASS:  Class name
CLASS ID: #
PARENT CLASS: Parent class name
ATTRIBUTES:
1 (o) Key Attribute: numeric identifier
2 (o) Key Attribute: name
3 (m) Attribute: attribute name(values)
4 (m) Attribute: attribute name(values)
4.1 (s) Attribute: attribute name(values)
4.2 (s) Attribute: attribute name(values)
4.3 (s) Attribute: attribute name(values)
5 (c) Constraint: constraint expression
5.1 (m) Attribute: attribute name(values)
5.2 (o) Attribute: attribute name(values)
6 (m) Attribute: attribute name(values)
6.1 (s) Attribute: attribute name(values)
6.2 (s) Attribute: attribute name(values)
SERVICES:
1 (o) OpsService: service name
2 (c) Constraint: constraint expression
2.1 (o) OpsService: service name
3 (m) MgtService: service name

(1) The FAL ASE: entry is the name of the FAL ASE that provides the services for the class
being specified.
(2) The CLASS: entry is the name of the class being specified. All objects defined using this
template will be an instance of this class. The class may be specified by this document, or
by a user of this document.
(3) The CLASS ID: entry is a number that identifies the class being specified. This number is
not used for Type 21 elements.
(4) The PARENT CLASS: entry is the name of the parent class for the class being specified.
All attributes defined for the parent class and inherited by it are inherited for the class
being defined, and therefore do not have to be redefined in the template for this class.
NOTE The parent-class TOP indicates that the class being defined is an initial class definition. The parent class
TOP is used as a starting point from which all other classes are defined. The use of TOP is reserved for classes
defined by this document.
(5) The ATTRIBUTES label indicates that the following entries are attributes defined for the
class.
a) Each of the attribute entries contains a line number in column 1; a mandatory (m),
optional (o), conditional (c), or selector (s) indicator in column 2; an attribute type label

in column 3; a name or a conditional expression in column 4; and an optional list of
enumerated va
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