Types of Systems

Introduction

A taxonomy is "a classification into ordered categories" (Dictionary.com 2011). Taxonomies are useful ways of organizing large numbers of individual items so their similarities and differences are apparent. (Magee and de Weck 2004) provided a taxonomy for complex systems, and in doing so, did a short review of other classification approaches for systems. They identified the pioneering work of (Bertalanffy 1968) and the later work of (Miller 1986). Bertalanffy divided systems into 9 types, including control mechanisms, socio-cultural systems, open systems, and static structures. Miller offered cells, organization, and society among his 7 system types. Classifications of Systems offers a more detailed look into system classification approaches.

One simple categorization of systems is to divide it into: natural, social, and engineered. The SEBoK focuses on engineered systems .

• A natural system is one whose elements , boundary, and relationships exist independently of human control. Examples: the real number system, the solar system, planetary atmosphere circulation systems.

• A social system includes humans as elements.

• An engineered system is a man-made aggregation which may contain physical, informational, human, natural and social elements; normally created for the benefit of people.

As discussed in What is a System? these three types overlap to cover the full scope of real-world systems.

systems science offers a number of ways of further classifying systems, related to a range of perspectives and attributes. There are also a number of system grouping approaches, which specify ways to define combinations of similar systems, including system of systems (sos) .

Classification methods for systems have been proposed over the past forty years, yet no standard classification system exists. Various methods that have been proposed are summarized in this article.

Classification Methods

Kenneth Boulding, one of the founding fathers of general system theory , developed a systems classification which has been the starting point for much of the subsequent work. (Boulding 1956). He classifies systems into 9 types: 1. Structures (Bridges); 2. Clock works (Solar system); Controls (Thermostat); 4. Open (Biological cells); 5. Lower organisms (Plants); 6. Animals (Birds); 7. Man (Humans); 8. Social (Families); and 9. Transcendental (God). This approach also highlights some of the subsequent issues of classification. Boulding implies that physical structures are closed and natural or social ones are open. He also separates humans from animals. (Hitchins 2007).

Peter Checkland proposed a classification system described below. (Checkland 1999) Arthur Paul surveyed the work to date and proposed methods for classifying systems. (Paul 1998) One of the most recent work was performed by Magee and de Weck, who developed a classification approach for complex systems and focused on engineered systems. (Magee and de Weck 2004) All of these classification approaches separate human-designed from non-human-designed systems or natural from man-made systems. While they provide some methods for classifying natural systems, their primary emphasis and value to the practicing systems engineer is in their classification method for human-designed or manmade systems. Peter Checkland divided systems into five classes: natural systems, designed physical systems, designed abstract systems, human activity systems and transcendental systems. The first two classes are self explanatory.

• Designed abstract systems – These systems do not contain any physical artifacts but are designed by humans to serve some explanatory purpose.

• Human activity systems (glossary) – These systems are observable in the world of innumerable sets of human activities that are more or less consciously ordered in wholes as a result of some underlying purpose or mission. At one extreme is a system consisting of a human wielding a hammer. At the other extreme lies international political systems.

• Transcendental systems – These systems that go beyond the aforementioned four systems classes, systems beyond knowledge.

Checkland refers to these five systems as comprising a “systems map of the universe”. (Checkland 1999, p.111)

Checkland, himself starting from a systems engineering perspective, successively observed the problems in applying a systems engineering approach to the more fuzzy, ill-defined problems found in the social and political arenas. (Checkland 1999, p. A9) Thus he introduced a distinction between hard systems and soft systems:

• hard systems of the world are characterized by the ability to define purpose, goals, and missions that can be addressed via engineering methodologies in attempting to, in some sense, “optimize” a solution.

• soft systems of the world are characterized by extremely complex, problematical, and often mysterious phenomena for which concrete goals cannot be established and which require learning in order to make improvement. Such systems are not limited to the social and political arenas and also exist within and amongst enterprises where complex, often ill-defined patterns of behavior are observed that are limiting the enterprise's ability to improve. Historically, the systems engineering discipline was primarily aimed at developing, modifying or supporting hard systems. More recently, the systems engineering discipline has expanded to address software systems as well.

Arthur Paul surveys previously defined classification methods and arrives at five definitions of system types based on function and usage of the systems. (Paul 1998) He defines: personal/household, military, civil, industrial and infrastructure systems as the five types of operating systems.

Magee and de Weck examine many possible methods that include: degree of complexity, branch of the economy that produced the system, realm of existence (physical or in thought), boundary, origin, time dependence, system states, human involvement / system control, human wants, ownership and functional type. They conclude by proposing a functional classification method that sorts systems by their process: transform, transport, store, exchange, or control and by the entity that they operate on: matter, energy, information and value.

Other categorizations of system types can be found throughout the literature. The varieties of suggested types that relate to specific presentations of various authors include:

• Eric Aslaksen describes three main classes of systems, according to the actions they perform: transport systems (translations in space), storage systems (translations in time), and production systems (time and space independent transformations). (Aslaksen 1996)
• Ben Blanchard describes several types including human-made systems, physical systems, conceptual systems, static systems, closed systems and open systems. (Blanchard 2005)
• Ronald Giachetti describes enterprise systems. (Giachetti 2009)
• Scott Jackson describes technological (or product) systems, product-centered infrastructure systems, technological system with human interface, human-intensive systems, process systems, socio-ecological systems, complex adaptive systems and infrastructure systems. (Jackson 2010)
• Mark Maier describes builder-architected systems, form-first systems, politico-technical systems and socio-technical systems. (Maier 2009)
• Charles Wasson describes cultural systems, business systems, educational systems, financial systems, government systems, medical systems and transportation systems. (Wasson 2006)

Groupings of Systems

Systems can be grouped together to create more complex systems. In some cases systems become subsystems in a higher level system. However, there are cases where the groupings of system produce an entity that must be treated differently from a single integrated system. The most common groupings of systems that have characteristics beyond a single integrated system are systems of systems (sos) and federations of systems (fos) . Wherever systems are combined into groups and interaction between the systems is present the complexity will be increased. It is this increase in complexity that creates the greatest challenge to the systems engineer. This article provides a definition of and fundamental information about the various groupings of systems. Other articles within the Systems Engineering Body of Knowledge (SEBoK) provide methods for dealing with the additional complexity that grouping systems produces.

System of Systems (SoS)

The phrase “system of systems” 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, et. al. 2008). This paper provides a historical perspective for systems engineering from (Brill 1998). The authors then provide a chronological history for System-of-Systems (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 et. al. cite Maier as one of the most influencial 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):

• 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.
• 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.
• 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.
• 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. While the concept of emergence and its derivative terms has a long history in science and technology, to this day there is no single, universal definition of emergence.

In SoS contexts, the recent interest in emergence has been 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). His article, that addresses the challenges, also 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 (FOS)

Different from the SoS concept, but related to it in several ways, is the concept called “federation of systems” or 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 The Enterprise View of Engineered 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 family of systems is 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 (2008) 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)

Engineered Systems Classifications

There is no agree terminology for classifying the types of engineered systems to which a systems approach can be applied.

In the SEBoK we classify Engineered systems into product systems , service systems , and enterprise systems .

References

Works Cited

Aslaksen, E.W. 1996. The Changing Nature of Engineering. New York, NY, USA: McGraw-Hill.

Blanchard, B.S., and W.J. Fabrycky. 2005. Systems Engineering and Analysis, 4th ed. Prentice-Hall International Series in Industrial and Systems Engineering. Englewood Cliffs, NJ, USA: Prentice-Hall.

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

Hitchins, D. 2007. Systems Engineering: A 21st Century Systems Methodology. Hoboken, NJ, USA: Wiley.

Giachetti, R.E. 2009. Design of Enterprise Systems: Theory, Architectures, and Methods. Boca Raton, FL, USA: CRC Press.

Magee, C. L., O.L. de Weck. 2004. "Complex System Classification." Proceedings of the 14th Annual International Council on Systems Engineering International Symposium, 20-24 June 2004, Toulouse, France.

Maier, M., and E. Rechtin. 2009. The Art of Systems Architecting, 3rd Ed.. Boca Raton, FL, USA: CRC Press.

Paul, A. S. 1998. "Classifying Systems." Proceedings of The Eighth Annual International Council on Systems Engineering International Symposium, 26-30 July, 1998, Vancouver, BC, Canada.

Wasson, C. S. 2006. System Analysis, Design and Development. Hoboken, NJ, USA: John Wiley and Sons.

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.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: http://ieeesmc2005.unm.edu.

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.

Bertalanffy, L. von. 1968. General System Theory. New York, NY, USA: Brazillier.

Dictionary.com, s.v. "Taxonomy," accessed September 7, 2011. Available at http://dictionary.reference.com/browse/taxonomy.

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Magee, C.L. and O.L. de Weck. 2004. "Complex System Classification". Proceedings of the Fourteenth Annual International Symposium of the International Council on Systems Engineering. June 20-24, 2004, Toulouse, France.

Miller, J.G. 1986. "Can Systems Theory Generate Testable Hypothesis?: From Talcott Parsons to Living Systems Theory." Systems Research. 3: 73-84.

Primary References

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

Magee, C. L., O.L. de Weck. 2004. "Complex System Classification." Proceedings of the 14th Annual International Council on Systems Engineering International Symposium, 20-24 June 2004, Toulouse, France.

Paul, A. S. 1998. "Classifying Systems." Proceedings of the Eighth Annual International Council on Systems Engineering International Symposium, 26-30 July 1998, Vancouver, BC, Canada.

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. editor. 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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