Systems of Systems (SoS)

System of systems engineering (SoSE) is not a new discipline, but it is an opportunity for the systems engineering community to define the complex systems of the 21st Century (Jamshidi 2009a). 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.

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Topics

The topics contained within this knowledge area include:

• Architecting Approaches for Systems of Systems
• Socio-Technical Features of Systems of Systems
• Capability Engineering

Definition and characteristics of Systems of Systems

There are several 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 several potential definitions of SoS and, although 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 systems in a co-ordinated way for specific goals such as robustness, cost, efficiency, etc.

DeLaurentis (DeLaurentis 2005) has added to the five SoS characteristics 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 characteristics. 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

Whilst emergent behaviour is an issue for Systems, emergence in Systems of Systems is particularly problematic. This is due to:

• The strong human element in the system, where users may not understand the full implications of their actions
• The ability of SoS to dynamically re-configure, undermining the normal performance and safety management processes which rely on fixed configurations
• The lack of a single design or operating authority, making SoSE a collaborative, rather than directive, activity

There is much concern about emergent behavior which is unexpected or cannot be predicted by knowledge of the system’s constituent parts. As discussed in the DoD SoS SE Guide (US DoD 2008) “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 Systems Engineering and Systems Engineering

Application of SoSE is broad and is 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 one looks 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. the European rail network, 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. 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 (Figure developed for BKCASE, adapted from Dahmann and Baldwin 2008; Neaga et. al. 2009)

	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

References

Works Cited

Dahmann, J., and K. Baldwin. 2008. Understanding the Current State of US Defense Systems of Systems and the Implications for Systems Engineering. Paper presented at IEEE Systems Conference, 7-10 April, Montreal, Canada.

DeLaurentis, D., and W. Crossley. "A Taxonomy-Based Perspective for System of Systems Design Methods," Paper 925, IEEE 2005 Conference on Systems, Man, and Cybernetics, Waikoba, HI, Oct. 10-12, 2005.

Neaga, E.I., M. Henshaw, and Y. Yue. 2009. "The influence of the concept of capability-based management on the development of the systems engineering discipline." Proceedings of the 7th Annual Conference on Systems Engineering Research, 20th - 23rd April 2009, Loughborough University, UK.

Maier, M.W. 1998. "Architecting Principles for Systems-of-Systems." Systems Engineering. 1(4): 267-284.

Primary References

Dahmann, J., and K. Baldwin. 2008. "Understanding the Current State of US Defense Systems of Systems and the Implications for Systems Engineering." Paper presented at IEEE Systems Conference, 7-10 April, Montreal, Canada.

Jamshidi, M. (ed.) 2009a. Systems of Systems Engineering – Innovations for the 21st Century. Hoboken, NJ, USA: Wiley. (ISBN 978-0-470-19590-1)

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

Maier, M.W. 1998. "Architecting Principles for Systems-of-Systems." Systems Engineering. 1(4): 267-284.

Additional References

Checkland, P.B. 1999. Systems Thinking, Systems Practice. Chichester, UK: John Wiley & Sons Ltd.

DeLaurentis, D., and W. Crossley. "A Taxonomy-Based Perspective for System of Systems Design Methods," Paper 925, IEEE 2005 Conference on Systems, Man, and Cybernetics, Waikoba, HI, Oct. 10-12, 2005.

Neaga, E.I., M. Henshaw, and Y. Yue. 2009. The Influence of the Concept of Capability-based Management on the Development of the Systems Engineering Discipline. Presented at 7th Ann. Conf. on Systems Eng. Research, 20th - 23rd April. Loughborough, UK.

Rebovich, Jr., G. 2009. "Chapter 6: Enterprise System of Systems", Systems of Systems Engineering - Principles and Applications. Boca Raton, FL, USA: CRC Press, pg. 169.

Ring J. 2002. "Toward an ontology of systems engineering." INSIGHT, 5(1): 19-22.

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