Systems of Systems (SoS)

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System of systems engineering (SoSE) is not a new discipline; however, this is an opportunity for the systems engineering community to define the complex systems of the twenty-first century (Jamshidi 2009). While systems engineering is a fairly established field, SoSE represents a challenge for the present systems engineers on a global level. In general, SoSE requires considerations beyond those usually associated with engineering to include socio-technical and sometimes socio-economic phenomena.


Each part of the SEBoK is divided into knowledge areas (KAs), which are groupings of information with a related theme. The KAs in turn are divided into topics. This KA contains the following topics:

Definition and Characteristics of Systems of Systems

There are several definitions of system(s) of systems (SoS), 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 has reviewed several potential definitions of SoS; 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. (Jamshidi 2009)

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 coordinated way for specific goals such as robustness, cost, efficiency, etc.

DeLaurentis and Crossley (2005) have 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 characterized 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 that have the sole purpose of enabling 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, there are four types of SoS (Maier 1998; Dahmann and Baldwin 2008):

  • Directed - The SoS is created and managed to fulfill specific purposes and the constituent systems are subordinated to the SoS. The component systems maintain an ability to operate independently; however, their normal operational mode is subordinated to the central managed purpose;
  • Acknowledged - The SoS has 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 - The component systems interact more or less voluntarily to fulfill agreed upon central purposes. The central players collectively decide how to provide or deny service, thereby providing some means of enforcing and maintaining standards; and
  • Virtual - The SoS lacks a central management authority and a centrally agreed upon purpose for the SoS. Large-scale behavior emerges—and may be desirable—but this type of SoS must rely on relatively invisible mechanisms to maintain it.

Further, in most actual cases, an SoS will reflect a combination of SoS types.

It is important to note that in many cases, a system of systems will not be identified as such because it is the result of unplanned interactions between constituent systems. These are not explicitly the subject of systems engineering. This added ‘ad hoc’ type of SoS, has also been termed ‘accidental’ (Kemp).

This taxonomy is based on the aforementioned aspect of degree of independence of constituents. It offers a framework for understanding SoS based on the origin of the SoS objectives and the relationships among the stakeholders for both the SoS and its constituent systems. Other taxonomies may focus on nature/type of components, their heterogeneity, etc.


Emergence is key to SoS, since in effect multiple, independent systems are brought together in an SoS explicitly to create new capability based on the interaction among the constituent systems.

As defined by Checkland, emergence is “the principle that entities exhibit properties which are meaningful only when attributed to the whole, not to its parts.” (Checkland 1999, 314). Emergent system behavior can be viewed as a consequence of the interactions and relationships between system elements rather than the behavior of individual elements. It emerges from a combination of the behavior and properties of the system elements and the systems structure or allowable interactions between the elements, and may be triggered or influenced by a stimulus from the systems environment. [SEBoK EMERGENCE]

However, 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 US Department of Defense Systems Engineering Guide for Systems of Systems (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 the military environment, SoSE application is now much broader and still expanding. The early work in the defense sector has provided the initial basis for SoSE, including its intellectual foundation, technical approaches, and practical experience. Now, SoSE concepts and principles apply across other governmental, civil and commercial domains. Some examples include:

  • Transportation - air traffic management, the European rail network, integrated ground transportation, cargo transport, highway management, and space systems,
  • Energy - smart grid, smart houses, and integrated production/consumption,
  • Health Care - regional facilities management, emergency services, and personal health management,
  • Natural Resource Management - global environment, regional water resources, forestry, and recreational resources,
  • Disaster Response - responses to disaster events including forest fires, floods, and terrorist attacks,
  • Consumer Products - integrated entertainment and household product integration,
  • Business- banking and finance, and

* Media - film, radio, and television.

Observations regarding differences between individual or constituent systems and SoS are listed in Table 1. These differences are not as black and white as the table might suggest. 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. (SEBoK Original), adapted from Dahmann and Baldwin (2008) and 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 Focus (Goals)
Operational Focus 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
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


Works Cited

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

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

DoD. 2008. Systems Engineering Guide for Systems of Systems. Arlington, VA: US Department of Defense, Director, Systems and Software Engineering, Deputy Under Secretary of Defense (Acquisition and Technology), Office of the Under Secretary of Defense (Acquisition, Technology and Logistics). Accessed November 12, 2013. Available:

Neaga, E.I., M.J.d. 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, April 20-23, 2009, Loughborough University, Loughborough, England, 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." Presented at IEEE Systems Conference, April 7-10, 2008, Montreal, Canada.

Jamshidi, M. (ed). 2009a. Systems of Systems Engineering – Innovations for the 21st Century. Hoboken, NJ, USA: Wiley.

Jamshidi, M. (ed). 2009b. Systems of Systems Engineering - Principles and Applications. Boca Raton, FL, USA: CRC Press.

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

DoD. 2008. Systems Engineering Guide for Systems of Systems, version 1.0. Washington, DC, USA: US Department of Defense (DoD). Available:

Additional References

Barot, V., S. Henson, M. Henshaw, C. Siemieniuch, M. Sinclair, S.L. Lim, M. Jamshidi, and D. DeLaurentis. 2012. Trans-Atlantic Research and Education Agenda in Systems of Systems (T-AREA-SoS) SOA Report. Longborough, England, UK: Longborough University. Ref. TAREA-RE-WP2-R-LU-7.

Boardman, J., and B. Sauser. 2006. "System of Systems - the Meaning of Of." IEEE Conference on Systems of Systems Engineering, April 24-26, 2006, Los Angeles, CA.

Carlock, P., and J.A. Lane. 2006. System of Systems Enterprise Systems Engineering, the Enterprise Architecture Management Framework, and System of Systems Cost Estimation. Los Angeles, CA, USA: Center for Systems and Software Engineering (CSSE), University of Southern California (USC). USC-CSE-2006-618.

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

Dahmann, J., Rebovich, G., Lane, J., Lowry, R. & Baldwin, K. 2011. "An Implementer's View of Systems Engineering for Systems of Systems." IEEE Systems Conference, April 4-7, 2011, Montreal, Canada. p. 212-217.

Keating C.B., J.J. Padilla, and K. Adams. 2008. "System of systems engineering requirements: Challenges and guidelines". EMJ - Engineering Management Journal. 20 (4): 24-31.

Luzeaux, D., and J.R. Ruault. 2010. Systems of Systems. London, UK: ISTE.

Poza, A.S., S. Kovacic, and C. Keating. 2008. "System of Systems Engineering: An Emerging Multidiscipline". International Journal of System of Systems Engineering. 1 (1/2).

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

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

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