Groupings of Systems

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This article forms part of the Systems Fundamentals knowledge area (KA). It expands on groups of system classification introduced in Types of Systems.

Systems can be grouped together to create more complex systems. In some cases, considering systems as system elements in a system hierarchy is sufficient. However, there are cases where the groupings of system produce an entity that must be treated differently from a single integrated system. This article explores the different descriptions of how and why system groupings might be considered.

System of Systems

The term “ system of systems” (SoS) is commonly used, but there is no widespread agreement on its exact meaning, or on how it can be distinguished from a conventional system. An extensive history of SoS is provided in “System-of-Systems Engineering Management: A Review of Modern History and a Path Forward” (Gorod & Boardman 2008). This paper provides a historical perspective for systems engineering from Brill (Brill 1998). The authors then provide a chronological history for SoS engineering from 1990 to 2008. Their history provides an extensive set of references to all of the significant papers and textbooks on SoS. Gorod and Boardman cite Maier as one of the most influential contributors to the study of SoS.

Maier examined the meaning of SoS in detail and used a characterization approach to create a definition (Maier 1998, 267-284). His definition has been adopted by many working in the field (AFSAB 2005). Maier provides this definition:

A system-of-systems is an assemblage of components which individually may be regarded as systems, and which possess two additional properties:

  1. Operational Independence of the Components: If the system-of-systems is disassembled into its component systems, the component systems must be able to usefully operate independently. That is, the components fulfill customer-operator purposes on their own.
  2. Managerial Independence of the Components: The component systems not only can operate independently, they do operate independently. The component systems are separately acquired and integrated but maintain a continuing operational existence independent of the system-of-systems. (Maier 1998, 271)

Maier goes on further, saying that “the commonly cited characteristics of systems-of-systems (complexity of the component systems and geographic distribution) are not the appropriate taxonomic classifiers” (Maier 1998, 268). According to the Defense Acquisition Guide: "A SoS is defined as a set or arrangement of systems that results from independent systems integrated into a larger system that delivers unique capabilities" (DAU 2010, 4.1.4. System of Systems (SoS) Engineering). For further details on SoS, see the Systems Engineering Guide for SoS developed by the US Department of Defense (DoD) (DUS(AT) 2008).

Four kinds of SoS have been defined (Maier 1998; Dahmann and Baldwin 2008; DUS(AT) 2008; Dahmann, Lane, and Rebovich 2008):

  1. 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 upon relatively invisible mechanisms to maintain it.
  2. Collaborative. In collaborative SoS the component systems interact more or less voluntarily to fulfill agreed upon central purposes. The Internet is a collaborative system. The Internet Engineering Task Force works out standards but has no power to enforce them. The central players collectively decide how to provide or deny service, thereby providing some means of enforcing and maintaining standards.
  3. 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 collaboration between the SoS and the system.
  4. Directed. Directed SoS are those in which the integrated system-of-systems is built and managed to fulfill specific purposes. It is centrally managed during long-term operation to continue to fulfill those purposes, as well as any new ones the system owners might wish to address. The component systems maintain an ability to operate independently, but their normal operational mode is subordinated to the central managed purpose (DUS(AT) 2008, 4-5; and, Dahmann, Lane, and Rebovich 2008, 4; in reference to (Maier 1998; Dahmann and Baldwin 2008).)

The terms emergence and emergent behavior are increasingly being used in SoS contexts, fueled, in part, by the movement to apply systems science and complexity theory to problems of large-scale, heterogeneous information technology based systems. In this context, a working definition of emergent behavior of a system is behavior which is unexpected or cannot be predicted by knowledge of the system’s constituent parts.

One of the leading authors in the area of SoS is Mo Jamshidi, who is the editor of a leading textbook (Jamshidi 2009) and articles such as “System of Systems Engineering – New Challenges for the 21st Century” (Jamshidi 2008). This article provides numerous references to papers that have examined the definition of SoS. The author selects six of the many potential definitions. His lead definition is

Systems of systems exist when there is a presence of a majority of the following five characteristics: operational and managerial independence; geographic distribution; emergent behavior; and evolutionary development. (Jamshidi 2008, 5; adapted from Sage and Cuppan 2001, 326)

Federation of Systems

Different from the SoS concept, but related to it in several ways, is the concept called federation of systems (FoS). This concept might apply when there is a very limited amount of centralized control and authority (Sage and Cuppan 2001). Each system in an FoS is very strongly in control of its own destiny, but “chooses” to participate in the FoS for its own good and the good of the “country,” so to speak. It is a coalition of the willing.

An FoS is generally characterized by significant autonomy, heterogeneity, and geographic distribution or dispersion (Krygiel 1999). Krygiel (1999) defined a taxonomy of systems showing the relationships among conventional systems, SoSs, and FoSs. This taxonomy has three dimensions: autonomy; heterogeneity; and dispersion. An FoS would have a larger value on each of these three dimensions than a non-federated SoS. An enterprise system, as described in Types of Systems, could be considered to be an FoS if it rates highly on these three dimensions. However, it is possible for an enterprise to have components that are not highly autonomous, that are relatively homogenous, and are geographically close together. Therefore, it would be a mistake to say that an enterprise is necessarily the same as an FoS.

Handy (1992) describes a federalist approach called “New Federalism”, which identifies the need for structuring of loosely coupled organizations to help them adapt to the rapid changes inherent in the Information Age. This leads to the need for virtual organizations where alliances can be quickly formed to handle the challenges of newly identified threats and a rapidly changing marketplace (Handy 1995). Handy sets out to define a number of federalist political principles that could be applicable to an FoS. Handy’s principles have been tailored to the domain of systems engineering and management by Sage and Cuppan (2001).

Families of Systems

The Defense Acquisition University (DAU 2010, 4.1.4. System of Systems (SoS) Engineering) defines families of systems as:

A grouping of systems having some common characteristic(s). For example, each system in a family of systems may belong to a domain or product line (e.g., a family of missiles, aircraft, or situation awareness systems). In general, a family of systems is not considered to be a system per se because it does not necessarily create capability beyond the additive sum of the individual capabilities of its member systems. A family of systems lacks the synergy of a SoS. The family of systems does not acquire qualitatively new properties as a result of the grouping. In fact, the member systems may not be connected into a whole. (DAU 2010)

Very few papers have been written that address families of systems or compare them to systems of systems.

James Clark provides a view that a family of systems is equivalent to a product line:

By family, we mean a product-line or domain, wherein some assets are re-used un-modified; some assets are modified, used, and re-used later; and some assets are developed new, used, and re-used later. Product-lines are the result. (Clark 2008)


Works Cited

AFSAB. 2005. Report on Domain Integration. Washington, D.C.: U.S. Air Force Scientific Advisory Board/U.S. Air Force. SAB-TR-05-03.

Brill, J. H. 1998. "Systems Engineering – A Retrospective View." Systems Engineering. 1(4): 258-266.

Clark, J. 2008. "System of Systems Engineering and Family of Systems Engineering From a Standards, V-Model, and Dual-V Model Perspective." Proceedings of the 18th Annual International Council on Systems Engineering International Symposium, 15-19 June, 2008, Utrecht, The Netherlands.

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.

Dahmann, J.S., J.A. Lane, and G. Rebovich. 2008. "Systems Engineering for Capabilities." CROSSTALK: The Journal of Defense Software Engineering. (November 2008): 4-9.

DAU. February 19, 2010. Defense acquisition guidebook (DAG). Ft. Belvoir, VA, USA: Defense Acquisition University (DAU)/U.S. Department of Defense.

DUS(AT). 2008. Systems Engineering Guide for Systems of Systems," version 1.0. Washington, DC, USA: Deputy Under Secretary of Defense for Acquisition and Technology (DUS(AT))/U.S. Department of Defense (DoD).

Gorod, A., B. Sauser, and J. Boardman. 2008. "System-of-Systems Engineering Management: A Review of Modern History and a Path Forward". IEEE Systems Journal, 2(4): 484-499.

Handy, C. 1995. "How Do You Manage People Whom You Do Not See? Trust and the Virtual Organization." Harvard Business Review.' 73(3) (May-June): 40-50.

Handy, C. 1992. "Balancing Corporate Power: A New Federalist Paper". Harvard Business Review. 70(6) (November/December): 59-72.

Jain, P. and Dickerson, C. 2005. "Family-of-Systems Architecture Analysis Technologies." Proceedings of the 15th Annual International Council on Systems Engineering International Symposium, 10-15, July 2005, Rochester, NY, USA.

Jamshidi, M. "Theme of the IEEE SMC 2005" in IEEE SMC 2005 - International Conference on Systems, Man, and Cybernetics. Accessed 11 September 2011. Available at:

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

Jamshidi, M. 2008. "System of Systems Engineering – New Challenges for the 21st Century". IEEE Aerospace and Electronic Systems Magazine. 23(5) (May 2008): 4-19.

Krygiel, A.J. 1999. Behind the Wizard's Curtain: An Integration Environment for a System of Systems. Arlington, VA, USA: C4ISR Cooperative Research Program (CCRP).

Maier, M.W. 1998. "Architecting Principles for Systems-of-Systems". Systems Engineering, 1(4): 267-84.

Sage, A., and C. Cuppan. 2001. "On the Systems Engineering and Management of Systems of Systems and Federations of Systems". Information-Knowledge-Systems Management Journal. 2(4) (December 2001): 325-45.

Primary References

Gorod, A., B. Sauser, and J. Boardman. 2008. "System-of-Systems Engineering Management: A Review of Modern History and a Path Forward." IEEE Systems Journal. 2(4): 484-499.

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

Jamshidi, M. 2008. "System of Systems Engineering – New Challenges for the 21st Century." IEEE Aerospace and Electronic Systems Magazine. 23(5) (May 2008): 4-19.

Maier, M.W. 1998. "Architecting Principles for Systems-of-Systems". Systems Engineering. 1(4): 267-84.

Sage, A. and C. Cuppan. 2001. "On the Systems Engineering and Management of Systems of Systems and Federations of Systems." Information-Knowledge-Systems Management Journal. 2(4) (December 2001): 325-45.

Additional References


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