Types of Systems
This article forms part of the Systems Fundamentals knowledge area (KA). It provides various perspectives on system classifications and types of systems, expanded from the definitions presented in What is a System?.
The modern world has numerous kinds of systems that influence daily life. Some examples include transport systems; solar systems; telephone systems; the Dewey Decimal System; weapons systems; ecological systems; space systems; etc. Indeed, it seems there is almost no end to the use of the word “system” in today’s society.
This article considers the different classification systems which some systems science authors have proposed in an attempt to extract some general principles from these multiple occurrences. These classification schemes look at either the kinds of elements from which the system is composed or its reason for existing.
The idea of an engineered system is expanded. Four specific types of engineered system context are generally recognized in systems engineering: product system, service system, enterprise system and system of systems capability.
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. No single standard classification system exists, though several attempts have been made to produce a useful classification taxonomy. Kenneth Boulding (Boulding 1956), 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. He classifies systems into nine types:
- Structures (Bridges)
- Clock works (Solar system)
- Controls (Thermostat)
- Open (Biological cells)
- Lower organisms (Plants)
- Animals (Birds)
- Man (Humans)
- Social (Families)
- Transcendental (God)
Bertalanffy (1968) divided systems into nine types, including control mechanisms, socio-cultural systems, open systems, and static structures. Miller (Miller 1986) offered cells, organization, and society among his eight nested hierarchical living systems levels, with twenty critical subsystems at each level.
Peter Checkland (Checkland 1999, 111) divides 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 – 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 are systems that go beyond the aforementioned four systems classes, and are considered to be systems beyond knowledge.
Checkland refers to these five systems as comprising a “systems map of the universe”. Other, similar categorizations of system types can be found in (Aslaksen 1996), (Blanchard 2005) and (Giachetti 2009).
These approaches also highlight some of the subsequent issues with these kinds of classification. Boulding implies that physical structures are closed and natural while social ones are open. However, a bridge can only be understood by considering how it reacts to traffic crossing it, and it must be sustained or repaired over time (Hitchins 2007). Boulding also separates humans from animals, which would not fit into more modern thinking.
Magee and de Weck (Magee and de Weck 2004) provide a comprehensive overview of sources on system classification such as (Maier and Rechtin 2009), (Paul 1998) and (Wasson 2006). They cover some methods for classifying natural systems, but their primary emphasis and value to the practice of systems engineer is in their classification method for human-designed, or man-made, systems. They 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.
Systems of Systems
However, there are cases where the groupings of system produce an entity that must be treated differently from an 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).
Maier examined the meaning of System of Systems in detail and used a characterization approach which emphasizes the independent nature of the system element, rather than “the commonly cited characteristics of systems-of-systems (complexity of the component systems and geographic distribution) which are not the appropriate taxonomic classifiers” (Maier 1998, 268).
Wherever independent systems are combined into groups the interaction between the systems adds a further complexity; specifically, by constraining how the resulting system can be changed or controlled. This dimension of complexity affects the management and control aspects of the systems approach.
A more detailed discussion of the different system grouping taxonomies developed by systems science can be found in Groupings of Systems.
Engineered Systems Classifications
The classification approaches discussed above have either been applied to all possible types of systems or have looked at how man-made systems differ from natural systems. The idea of an engineered system is to provide a focus on systems containing both technology and social or natural elements, developed for a defined purpose by an engineering life cycle. Engineered Systems:
- are created, used and sustained to achieve a purpose, goal or mission that is of interest to an enterprise, team, or an individual.
- require a commitment of resources for development and support.
- are driven by stakeholders with multiple views on the use or creation of the system, or with some other stake in the system, its properties or existence.
- contain engineered hardware, software, people, services, or a combination of these.
- exist within an environment that impacts the characteristics, use, sustainment and creation of the system.
Engineered systems typically
- are defined by their purpose, goal or mission.
- have a life cycle and evolution dynamics.
- may include human operators (interacting with the systems via processes) as well as other natural components that must be considered in the design and development of the system.
- are part of a system-of-interest hierarchy.
Economists divide all economic activity into two broad categories, goods and services. Goods-producing industries are agriculture, mining, manufacturing, and construction; each of them creates some kind of tangible object. Service industries include everything else: banking, communications, wholesale and retail trade, all professional services such as engineering, computer software development, and medicine, nonprofit economic activity, all consumer services, and all government services, including defense and administration of justice.... (Encyclopedia Britannica 2011).
A product or service is developed and supported by an individual, team, or enterprise. For example, express package delivery is a service offered worldwide by many enterprises, public and private, small and large. These services might use vehicles, communications or software products, or a combination of the three as needed.
The nature of engineered systems has changed dramatically over the past several decades from systems dominated by hardware (mechanical and electrical) to systems dominated by software. In addition, systems that provide services, without delivering hardware or software, have become common as the need to obtain and use information has become greater. Recently, organizations have become sufficiently complex that the techniques that were demonstrated to work on hardware and software have been applied to the engineering of enterprises.
Products and Product Systems
The word product is defined as "a thing produced by labor or effort; or anything produced" (Oxford English Dictionary). In a commercial sense a product is anything which is acquired, owned and used by an enterprise (hardware, software, information, personnel, an agreement or contract to provide something, etc.).
Product systems are systems in which products are developed and delivered to the acquirer for the use of internal or external user. For product systems, the ability to provide the necessary capability must be defined in the specifications for the hardware and software or the integrated system that will be provided to the acquiring enterprise.
Services and Service Systems
A service can be simply defined as an act of help or assistance, or as any outcome required by one or more users which can be defined in terms of outcomes and quality of service without detail to how it is provided (e.g., transport, communications, protection, data processing, etc.) Services are processes, performances, or experiences that one person or organization does for the benefit of another, such as custom tailoring a suit; cooking a dinner to order; driving a limousine; mounting a legal defense; setting a broken bone; teaching a class; or running a business’s information technology infrastructure and applications. In all cases, service involves deployment of knowledge and skills (competencies) that one person or organization has for the benefit of another (Lusch and Vargo 2006), often done as a single, customized job. In all cases, service requires substantial input from the customer or client (Sampson 2001). For example, how can a steak be customized unless the customer tells the waiter how the customer wants the steak prepared?
A service system is one that provides outcomes for a user without necessarily delivering hardware or software products to the service supplier. The hardware and software systems may be owned by a third party who is not responsible for the service. The use of service systems reduces or eliminates the need for acquirers to obtain capital equipment and software in order to obtain the capabilities needed to satisfy users.
...unique features that characterize services – namely, services, especially emerging services, are information-driven, customer-centric, e-oriented, and productivity-focused. (Tien and Berg 2003, 13)
A more detailed discussion of the system theory associated with service systems can be found in History of Systems Science.
Enterprises and Enterprise Systems
An enterprise is one or more organizations or individuals sharing a definite mission, goals, and objectives to offer an output such as a product or service.
An enterprise system consists of a purposeful combination (network) of interdependent resources (e.g., people; processes; organizations; supporting technologies; and funding) that interact with 1.) each other (e.g., to coordinate functions; share information; allocate funding; create workflows; and make decisions), and 2) their environment(s), to achieve business and operational goals through a complex web of interactions distributed across geography and time (Rebovich and White 2011, 4, 10, 34-35).
Both product and service systems require an enterprise system to create them and an enterprise to use the product system to deliver services either internally to the enterprise or externally to a broader community.
According to Maier’s definition, an enterprise would not necessarily be called a system of systems (SoS) since the systems within the enterprise do not usually meet the criteria of operational and managerial independence. In fact, the whole purpose of an enterprise is to explicitly establish operational dependence between systems that the enterprise owns and/or operates in order to maximize the efficiency and effectiveness of the enterprise as a whole. Therefore, it is more proper to treat an enterprise system and an SoS as different types of things, with different properties and characteristics (DeRosa 2005).
Enterprise systems are unique, compared to product and service systems, in that they are constantly evolving; they rarely have detailed configuration controlled requirements; they typically have the goal of providing shareholder value and customer satisfaction, which are constantly changing and are difficult to verify; and they exist in a context (or environment) that is ill-defined and constantly changing.
The notion of enterprises and enterprise systems permeates Part 5 Enabling Systems Engineering.
System of Systems Capability
As discussed above, "system of systems" is a classification used for any system which contains elements which in some way can be considered as independent (Maier 1998). Any of the other three engineered system contexts described above may have some aspects of SoS to be considered across their life cycle. Similarly, capability is a concept relevant to all system contexts, relating to the real world outcomes which system users can achieve when the system is fully deployed in its operational environment.
The term System of Systems Capability is used here to describe an engineering context in which a number of enterprise, service and product systems are brought together dynamically to provide a capability which is beyond the scope of any individual enterprise.
Understanding the need for system of systems capability is a way of setting a broader problem context for the engineering of other systems. Both product and service systems may be engineered to both satisfy immediate stakeholder needs and to have the potential to be used for the composition of SoS capabilities. Engineering at the Enterprise level can include an Enterprise Capability Management activity, in which possible SoS problems are explored and used to identify gaps in the enterprise's current product and service portfolio. (See the SEBoK, Part 4 Applications of Systems Engineering)
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