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 (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 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)
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 (glossary) 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)
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)