Systems of Systems (SoS)

Definition and characteristics of Systems of Systems

System of systems engineering (SoSE) is not necessarily a new discipline, but it is an opportunity for the systems engineering community to define the complex systems of the 21st Century (Jamshidi 2009a, 1). While systems engineering is a fairly established field, SoSE represents a challenge for the present systems engineers at the global level. In general, SoSE requires considerations beyond those usually associated with engineering to include socio-technical and sometimes socio-economic phenomena.

There are many definitions of System(s) of Systems, some of which are dependent on the particularity of an application area. (Maier 1998) postulated five key characteristics of SoS: Operational independence of component systems, Managerial independence of component systems, Geographical distribution, Emergent behavior, and Evolutionary development processes. (Jamshidi 2009) has reviewed more than seven potential definitions of SoS and, although there is not all are universally accepted by the community, the following has received substantial attention:

A SoS is an integration of a finite number of constituent systems which are independent and operatable, and which are networked together for a period of time to achieve a certain higher goal.

It should be noted that according to this definition, formation of a SoS is not necessarily a permanent phenomenon, but rather a matter of necessity for integrating and networking them in a central way for specific goal such as robustness, cost, efficiency, etc. DeLaurentis (DeLaurentis find ref) has added to the five SoS criteria above for SoS Engineering to include: inter-disciplinarity, heterogeneity of the systems involved, and networks of systems. Not all SoS will exhibit all of the characteristics, but it is generally assumed that a SoS is characterised by exhibiting a majority of the Maier Criteria. Although the individual systems in a SoS are usually considered to have independent operational viability, it is sometimes the case that the SoS must contain some systems the only purpose of which is to enable the interoperation of the other component systems; i.e. the enabling systems cannot operate outside of the SoS.

Types of SoS

SoS can take different forms. Based on a recognized taxonomy of SoS, there are four types of SoS (Maier 1998; Dahmann and Baldwin 2008). These are:

Directed. Directed SoS are those in which the system-of-systems is created and managed to fulfil specific purposes and the constituent systems are subordinated to the SoS. The component systems maintain an ability to operate independently, but their normal operational mode is subordinated to the central managed purpose.

Acknowledged. Acknowledged SoS have recognized objectives, a designated manager, and resources for the SoS; however, the constituent systems 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.

Collaborative. In collaborative SoS the component systems interact more or less voluntarily to fulfil agreed upon central purposes. The central players collectively decide how to provide or deny service, thereby providing some means of enforcing and maintaining standards.

Virtual. Virtual SoS lack a central management authority and a centrally agreed upon purpose for the system-of-systems. Large-scale behavior emerges—and may be desirable—but this type of SoS must rely on relatively invisible mechanisms to maintain it.

This characterization offers a framework for understanding SoS based on the origin of the SoS capability objectives and the relationships among the stakeholders for both the SoS and the systems.

Emergence

The emergent behavior of a SoS is, put simply, the behaviors that are due either to the internal relationships among the parts of the SoS or as a response to its external environment. The behavior emerges because of interactions and it cannot arise from any of the constituent systems acting alone. Because of the strong element of humans and organizations in SoS, emergent behavior in a SoS can result from changes in the way systems are employed by users now that they are operating in a new and expanded context. Consequences of the emergent behavior may be viewed as negative/harmful, positive/beneficial, or neutral/unimportant by stakeholders of the SoS.

There is much concern about emergent behavior which is unexpected or cannot be predicted by knowledge of the system’s constituent parts. For the purposes of a SoS, unexpected means unintentional, not purposely or consciously designed-in, not known in advance, or surprising to the developers and users of the SoS. In a SoS context, not predictable by knowledge of its constituent parts means the impossibility or impracticability (in time and resources) of subjecting all possible logical threads across the myriad functions, capabilities, and data of the systems to a comprehensive SE process.

Application domains and the difference between System of System Engineering and Systems Engineering

Application of SoS is broad and expanding into almost all walks of life. Originally addressed in military applications, the defense sector has provided a base for some initial approaches to conceiving and engineering SoS which offer intellectual foundation, technical approaches and practical experience to this field. However, SoS is far from limited to defense. In fact, as you look at the world through a SoS lens it becomes clear that SoS concepts and principles apply across other governmental, civil and commercial domains. These include

Transportation; e.g. integrated ground transportation, cargo transport, air traffic, highway management, space systems
Energy; e.g. smart grid, smart houses, integrated production/consumption
Health Care; e.g. regional facilities management, emergency services, personal health management
Natural Resource Management, e.g. global environment, regional water resources, forestry, recreational resources
Disaster Response; forest fires, floods, terrorist attacks
Consumer Products; integrated entertainment, household product integration
Media; film, radio, television

Understanding the environment in which a system or SoS will be developed and employed is central to understanding how best to apply SE principles within that environment. Observations regarding differences between individual or constituent systems and SoS are listed in Table 1 (Dahmann and Baldwin 2008; Neaga et. al. 2009). In each case, the degree of difference varies in practice and with complexity of current systems and system development environments - many of the SoS characterizations may apply to systems in certain circumstances.

**Table 1** - Differences between Systems and Systems of Systems as They Apply to Systems Engineering
	Systems Engineering	Systems of Systems Engineering
Management and Oversight
System	Physical engineering	Socio-technical management and engineering
Stakeholder Involvement	Clear set of stakeholders	Multiple levels of stakeholders with mixed and possibly competing interests
Governance	Aligned management and funding	Added levels of complexity due to management and funding for both SoS and systems; SoS does not have control over all constituent systems
Operational Environment
Operational focus (goals)	Designed and developed to meet common objectives	Called upon to meet new SoS objectives using systems whose objectives may or may not align with the SoS objectives
Implementation
Acquisition/Development	Aligned to established acquisition and development processes	Cross multiple system lifecycles across asynchronous acquisition and development efforts, involving legacy systems, developmental systems, and technology insertion
Process	Well established	Learning and adaptation
Test and evaluation	Test and evaluation of the system is possible	Testing is more challenging due to systems' asynchronous life cycles and given the complexity of all the parts
Engineering and design considerations
Boundaries and interfaces	Focuses on boundaries and interfaces	Focus on identifying systems contributing to SoS objectives and enabling flow of data, control and functionality across the SoS while balancing needs of the systems. OR Focus on interactions between systems Difficult to define system of interest
Performance and Behavior	Performance of the system to meet performance objectives	Performance across the SoS that satisfies SoS use capability needs while balancing needs of the systems
Metrics	Well defined (e.g. INCOSE handbook)	Difficult to define, agree, and quantify

Topics

The topics contained within this knowledge area include:

References

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Citations

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Primary References

All primary references should be listed in alphabetical order. Remember to identify primary references by creating an internal link using the ‘’’reference title only’’’ (title). Please do not include version numbers in the links.

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Jamshidi, Mo (ed.) 2009b. Systems of Systems Engineering - Principles and Applications: CRC Press, ISBN 978-1-4200-6588-6

A Comprehensive set of chapters authored by leading experts in the SoS field covering theory and practical applications with examples from energy, medical, microgrid, sensor networks, defense, vehicles, space systems, and airport operations.

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