The drawbacks in using the term 'system of systems'.

Nancy Leveson, PhD, is a professor of aeronautics and astronautics and engineering systems at Massachussetts Institute of Technology, and is the author of several books on system concepts. E-mail: [email protected] A few years ago, a new term, “system of systems,” was invented and has become quite popular. I’ve puzzled over this term because it doesn’t make any sense to me with respect to systems theory and systems engineering. Let’s start by reviewing some basic definitions in systems theory. A system can be defined as a set of components that act together as a whole to achieve some common goal, objective, or end. The components are all interrelated and are either directly or indirectly connected to each other. The system state at any point in time is the set of relevant properties describing the system at that time. The system environment is a set of components (and their properties) that are not part of the system, but whose behavior can affect the system state. The existence of a boundary between the system and its environment implicitly defines inputs or outputs as anything that crosses that boundary. It is important to understand that a system is always a model—an abstraction conceived by the viewer of the system. Systems and their boundaries do not exist in reality but only in the view of the beholder. One viewer may see a very different system than another in terms of where the boundaries are drawn, the relevant system properties and components, and even the purpose of the system. Abstractions are useful in that they help humans deal with complexity. One useful abstraction in understanding complex systems is to view them as hierarchical structures. A model of a complex system can be conceived in terms of a hierarchy of levels of organization, each more complex than the one below. Each level of the hierarchy can be thought of as a system, which is made up of components at a lower level. Each of these components (or subsystems) can itself be made up of subsystems, and so on. Figure 1 shows a depiction of a system labeled A (level 1 of the hierarchy) composed of three subsystems A1, A2, and A3 at level 2 of the hierarchy, each of which is made up of other components (level 3 of the hierarchy). Note that the term “system” is recursive in that a subsystem is itself a system, which is made