Systems and software engineering -- Taxonomy of systems of systems

This document defines a normalized taxonomy for systems of systems (SoS) to facilitate communications among stakeholders. It also briefly explains what a taxonomy is and how it applies to the SoS to aid in understanding and communication.

Ingénierie système et logiciel -- Taxonomie des systèmes de systèmes

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Publication Date
22-Jul-2019
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6060 - International Standard published
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25-Jun-2019
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23-Jul-2019
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INTERNATIONAL ISO/IEC/
STANDARD IEEE
21841
First edition
2019-07
Corrected version
2019-09
Systems and software engineering —
Taxonomy of systems of systems
Ingénierie système et logiciel — Taxonomie des systèmes de systèmes
Reference number
ISO/IEC/IEEE 21841:2019(E)
ISO/IEC 2019
IEEE 2019
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ISO/IEC/IEEE 21841:2019(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2019
© IEEE 2019

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© ISO/IEC 2019 – All rights reserved
ii © IEEE 2019 – All rights reserved
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ISO/IEC/IEEE 21841:2019(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

3.1 General terms ........................................................................................................................................................................................... 1

3.2 SoS types....................................................................................................................................................................................................... 2

4 Concepts and application ............................................................................................................................................................................ 3

4.1 Overview ...................................................................................................................................................................................................... 3

4.2 Importance of taxonomies to SoS ........................................................................................................................................... 3

4.3 Use of SoS taxonomies ...................................................................................................................................................................... 3

5 Taxonomies for systems of systems .................................................................................................................................................. 4

5.1 General ........................................................................................................................................................................................................... 4

5.2 Taxa: directed, acknowledged, collaborative and virtual ................................................................................... 4

5.2.1 Overview ................................................................................................................................................................................. 4

5.2.2 Description of the taxa ................................................................................................................................................ 4

5.2.3 Examples of potential application of the taxa ......................................................................................... 5

5.2.4 When to use .......................................................................................................................................................................... 5

5.2.5 How to use ............................................................................................................................................................................. 5

5.2.6 Why to use ............................................................................................................................................................................. 6

5.2.7 Limitations ............................................................................................................................................................................ 6

5.2.8 Benefits of use .................................................................................................................................................................... 6

Annex A (informative) Summary of SoS taxonomies ........................................................................................................................... 7

Bibliography ................................................................................................................................................................................................................................ 8

IEEE notices and abstract ............................................................................................................................................................................................... 9

© ISO/IEC 2019 – All rights reserved
© IEEE 2019 – All rights reserved iii
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ISO/IEC/IEEE 21841:2019(E)
Foreword

ISO (the International Organization for Standardization) and IEC (the International Electrotechnical

Commission) form the specialized system for worldwide standardization. National bodies that

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described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

rules given in the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

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This document was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,

Subcommittee SC 7, Systems and software engineering, in cooperation with the Systems and Software

Engineering Standards Committee of IEEE Computer Society, under the Partner Standards Development

Organization cooperation agreement between ISO and IEEE.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.

This corrected version of ISO/IEC/IEEE 21841:2019 incorporates the following correction:

— The publication date on the cover page has been corrected.
© ISO/IEC 2019 – All rights reserved
iv © IEEE 2019 – All rights reserved
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ISO/IEC/IEEE 21841:2019(E)
Introduction

Systems of systems engineering (SoSE) is a concept that is increasingly thought of as a discipline

important for the realization and sustainability of large and persistent sociotechnical systems in areas

as diverse as healthcare, transportation, energy, defense, corporations, cities and government.

While SoSE applies broadly to hardware, software, middle-ware as well as embedded, cyber-physical

and digital systems, the importance of SoSE has been heightened in the last fifteen years by the rapid

increase in the pervasiveness of information technology (IT), illustrated by new technologies and

paradigms such as Sensor Networks, Cloud Computing, the Internet of Things, Big Data, Smart Devices

and Artificial Intelligence. It is, for instance, the application of these technologies to cities that transform

them into “smarter” cities.

This pervasiveness of IT was not only driven by the availability of these technologies, but also more

importantly by the requirements in our resource and environmentally-constrained world for increased

and sustainable economic development and, ultimately, personal well-being.

SoSE goes well beyond IT and potentially applies to all types of systems, including hardware and

cyber physical systems where IT is an enabler. SoSE addresses functionality, performance and

interdependencies of the systems as well as their connectivity. The interconnectivity of systems has

become pervasive in large command and control systems, defense systems, communications systems,

transportation systems and medical/health systems, among others. The accelerating need to share

information and leverage capabilities from other systems has changed how systems need to be viewed

and engineered.

Taxonomies provide a means in many fields to classify and describe the relationships among the

relevant elements being studied. The elements of a taxonomy, or taxa, form a partitioning or means of

classification within that body of knowledge. In the context of systems of systems (SoS), the relevant

elements of the system of interest are, by definition, systems themselves. Using essential characteristics

to partition the various types of SoS provides an abbreviated nomenclature for thinking about SoS.

Based on taxonomies, different approaches to the engineering of systems of systems are possible,

improving the efficiency and effectiveness of systems of systems engineering.
© ISO/IEC 2019 – All rights reserved
© IEEE 2019 – All rights reserved v
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INTERNATIONAL STANDARD ISO/IEC/IEEE 21841:2019(E)
Systems and software engineering — Taxonomy of systems
of systems
1 Scope

This document defines a normalized taxonomy for systems of systems (SoS) to facilitate communications

among stakeholders. It also briefly explains what a taxonomy is and how it applies to the SoS to aid in

understanding and communication.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

NOTE For additional terms and definitions in the field of systems and software engineering, see

ISO/IEC/IEEE 24765, which is published periodically as a “snapshot” of the SEVOCAB (Systems and software

Engineering Vocabulary) database and is publicly accessible at www .computer .org/ sevocab.

ISO, IEC, and IEEE maintain terminological databases for use in standardization at the following

addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
— IEEE Standards Dictionary Online: available at http:// dictionary .ieee .org
3.1 General terms
3.1.1
constituent system
independent system that forms part of a system of systems (SoS) (3.1.4)

Note 1 to entry: Constituent systems can be part of one or more SoS. Each constituent system is a useful system

by itself, having its own development, management (3.1.3), utilization, goals, and resources, but interacts within

the SoS to provide the unique capability of the SoS.

[SOURCE: ISO/IEC/IEEE 21839:2019, 3.1.1, modified — The abbreviated term "CS" has been added.]

3.1.2
governance

process of establishing and enforcing strategic goals and objectives, organizational policies and

performance parameters
Note 1 to entry: This definition is adapted from Reference [8].
© ISO/IEC 2019 – All rights reserved
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ISO/IEC/IEEE 21841:2019(E)
3.1.3
management

system of controls and processes required to achieve the strategic objectives set by the organization's

governing body

Note 1 to entry: Management is subject to the policy guidance and monitoring set through corporate governance

(3.1.2).
[SOURCE: ISO/IEC/IEEE 24765:2017, 3.2338]
3.1.4
system of systems
SoS

set of systems or system elements that interact to provide a unique capability that none of the constituent

systems (3.1.1) can accomplish on its own

Note 1 to entry: System elements can be necessary to facilitate interaction of the constituent systems in the

system of systems.
[SOURCE: ISO/IEC/IEEE 21839:2019, 3.1.4]
3.1.5
systems of systems engineering
SoSE

process of planning, analyzing, organizing, developing and integrating the capabilities of a mix of

existing and new systems, including inter-system infrastructure, facilities, and overarching processes

into a system-of-systems capability that is greater than the sum of the capabilities of the constituent

systems (3.1.1)
Note 1 to entry: This definition is adapted from Reference [9].

Note 2 to entry: SoSE also includes testing, modification, maintenance and other post-integration activities.

3.1.6
taxonomy

scheme that partitions a body of knowledge and defines the relationships among the pieces

[SOURCE: ISO/IEC/IEEE 24765:2017, 3.4167, modified — Definition 2 has been removed; Note 1 to entry

has been removed.]
3.2 SoS types
3.2.1
acknowledged system of systems
acknowledged SoS

SoS (3.1.4) with recognized objectives, a designated manager, and resources for the SoS

Note 1 to entry: Constituent systems (3.1.1) retain their independent ownership, objectives, funding, and

development and sustainment approaches. Changes in the systems are based on cooperative agreements

between the SoS and the system.
Note 2 to entry: This definition is adapted from Reference [7].
3.2.2
collaborative system of systems
collaborative SoS

SoS (3.1.4) in which component systems interact more or less voluntarily to fulfill agreed-upon central

purposes

Note 1 to entry: Constituent systems (3.1.1) collectively decide how to provide or deny service, thereby providing

means of enforcing and maintaining consistency.
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ISO/IEC/IEEE 21841:2019(E)
Note 2 to entry: This definition is adapted from Reference [7].
3.2.3
directed system of systems
directed SoS

SoS (3.1.4) created and managed to fulfill specific purposes and the constituent systems (3.1.1) are

subordinated to the SoS

Note 1 to entry: Component systems maintain an ability to operate independently; however, their normal

operational mode is subordinated to the central managed purpose.
Note 2 to entry: This definition is adapted from Reference [7].
3.2.4
virtual system of systems
virtual SoS

SoS (3.1.4) that lacks a central management (3.1.3) authority and a centrally-agreed-upon purpose

for the SoS

Note 1 to entry: Large-scale behavior emerges—and can be desirable—but this type of SoS relies on relatively

invisible mechanisms to maintain it.
Note 2 to entry: Virtual SoS are typically self-organizing.
Note 3 to entry: This definition is adapted from Reference [7].
4 Concepts and application
4.1 Overview

Taxonomies provide a means in many fields to classify and describe the relationships between the

relevant elements being studied. The elements of a taxonomy, or taxa, form a partitioning or means of

classification within that body of knowledge. Partitioning based on essential characteristics provides

an abbreviated nomenclature to refer to a larger composite of characteristics, facilitating discussion

about the partitions (taxa) without having to refer to each of the characteristics.

4.2 Importance of taxonomies to SoS

In systems engineering (SE), the relevant pieces of the system of interest can be called subsystems,

elements or components. In the context of SoS, the relevant pieces of the system of interest are, by

definition, systems themselves. These are called constituent systems (CS) throughout this document.

That is, an SoS consists of some number of CS, plus any inter-system infrastructure, facilities and

processes necessary to enable the CS to integrate or interoperate. Relationships between CS affect

the SoS. Using essential characteristics to partition the various types of SoS provides an abbreviated

nomenclature for thinking about SoS. While Clause 5 elaborates one mature SoS taxonomy, Annex A

provides a list of SoS less-mature taxonomies. Based on taxonomies, different approaches to the

engineering of systems of systems are possible, improving the efficiency and effectiveness of SoSE.

NOTE 1 Taxonomies can have some overlap in their definition and need not be orthogonal to each other to

be useful. An SoS can be considered as belonging to several taxonomies as long as its characteristics meet the

definitions of the taxonomies.

NOTE 2 It is possible that inter-system infrastructure, facilities and processes do not meet the criteria for

being systems in their own right. From the perspective of the SoS, these could be system elements (or SoS

elements).
4.3 Use of SoS taxonomies

There are many characteristics such as scale and scope, around which taxonomies can be derived. The

SoS taxonomy in Clause 5 organizes the relevant aspects or essential characteristics of SoS, providing

© ISO/IEC 2019 – All rights reserved
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ISO/IEC/IEEE 21841:2019(E)

specific viewpoints that align with stakeholder concerns. This organization facilitates communications

between the various stakeholders that are involved with activities like governance, engineering,

operation and management of these SoS, and provides a reference for other related standards. The

taxonomy in Clause 5 meets the following criteria.
— External references are publicly available.
— The taxa are stable with evidence of having been applied in multiple systems.

Annex A lists additional SoS taxonomies that do not meet all of the above criteria. These developing

taxonomies can still be useful, but they lack sufficient maturity to be included in Clause 5. As the

discipline matures, it is likely that more taxonomies will evolve.
5 Taxonomies for systems of systems
5.1 General

SoS taxonomies organize the relevant aspects or essential characteristics of SoS, providing specific

viewpoints that align with stakeholder concerns. Taxonomies can have some overlap in their

construction, but the lack of orthogonality does not inhibit their application.

Since most of the taxonomies are not explicitly named, they are most often referenced in practice by the

author’s names or the names of the taxa. Consequently, the taxonomies are presented using the taxa

names themselves, using the author’s order.
5.2 Taxa: directed, acknowledged, collaborative and virtual
5.2.1 Overview

In this taxonomy, SoS are classified according to the degree of managerial and operational independence.

Four types are defined: directed, acknowledged, collaborative and virtual.
[6]

This taxonomy is mature, originating in the work of Maier (1998) , expanded by Dahmann and Baldwin

[9]

(2008) and published as an informative annex in ISO/IEC/IEEE 15288. Maier’s observation was that

SoS are not simply systems in which the constituents are also systems. One essential characteristic

is that constituent systems within the SoS are operationally independent. Operational i

...

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