March 29, 2002Complexity in Engineering SystemsBasic Terms related to Engineering SystemsIlities and related system issuesESD Terms and Definitions (Version 16) The ESD Symposium Committee March 29, 2002 Updated February 4, 2007 for Advanced System Architecture ESD.342 Complexity in Engineering Systems Arguably the key concept in Engineering Systems is complexity. Thus we discuss it at greater length than the other terms that will follow. There are many definitions of complex systems, but we shall concentrate on just two. A system is behaviorally complex if its behavior is difficult to predict, analyze, describe, or manage. In contrast a system is structurally complex if the number of parts is large and the interconnections between its parts is intricate or hard to describe briefly. Systems that are structurally complex are usually behaviorally complex. Systems that have complex behavior need not have complex structure, since we know of relatively simple mechanical systems whose behavior is chaotic, and hence complex. On the other hand, most behaviorally complex systems are structurally complex as well. Complex engineering systems are not simply technical in nature, but rely on people and their organizations for the design, manufacturing and operation of the system, and are influenced by and influence the societal and physical context as well. Basic Terms related to Engineering Systems Engineering – bringing to reality useful artifacts and algorithms that heretofore did not exist; in English, especially in England, the term is often associated with maintenance and operation, especially of engines, but the French root (ingénieur) is related to ingenious (ingénieux). System – A set of interacting components having well-defined (although possibly poorly understood) behavior or purpose. The purpose is understood not just in a designed but also in an evolved sense. Alternately: A system is an instance of a set of elements having relationships with one another sufficiently cohesive to distinguish the system from its environment thereby giving the system an identity. It can then also be said to have a reason for existence. The concept is subjective in that what is a system to one person may not appear to be a system to another. System Architecture –Practical definition: An architecture is the conceptualization, description, and design of a system, its components, their interfaces and relationships with internal and external entities, as they evolve over time. Fundamental definitions: 1. The architecture of a complex system is a description of the structure or regularity of the interactions of the elements of that system (inherently the non-random and longer lived aspects of the system relationships). 2. The architecture of a complex system describes the functional character of the elements and the structure of the relationships among the elements. Intermediate definition: An architecture declares the modules and defines their functions It also declares and defines the interfaces, including which modules they relate and what relations are supported Finally it declares or embraces standards that define common rules of design, structure, interfaces, or behavior not otherwise declared, including performance evaluation Engineering System – a system designed or evolved by humans having some purpose; large scale and complex engineering systems, which are of most interest to the Engineering Systems Division, will have a management or social dimension as well as a technical one. System Environment – a set of conditions external to and affecting a system; environments include both natural and man-made conditions Natural environment – natural surroundings and context (e.g., air, water); the natural environment can sometimes be considered the core of a system, with engineering systems providing interfaces to it Complex system – a system with components and interconnections, interactions, or interdependencies that are difficult to describe, understand, predict, manage, design, or change. (This implies non-random and non-simple structure.) The concept of complexity has several meanings: dynamic (in the sense of complex behavior), and static or structural (in the sense that a complex system is composed of many components interconnected in intricate ways). In addition, while the complexity of a system can be a quantifiable and even an absolutequantity, what makes a system appear complicated to people is subjective (simplicity is in the eye of the beholder); how complicated the system appears depends on the nature of the interface of the system that is presented to its users. Interdependence – the relationship between entities that cannot exist or operate without each other; independent entities exist and operate without influence from each other; interdependencies may be intended or unintended Interaction – the property of entities that exchange something material or immaterial that constitutes or contributes to their interdependence Interconnection – the relationship between entities that are physically or abstractly connected and the connection provides the pathway for the interaction; software connections are often abstract These three terms are clearly interrelated (pun intended). We distinguish at least three types of interdependence: one is the interdependence that may occur among components or subsystems in a given design of a system, a second is the interdependence created when global constraints (such as weight, volume, cost, or 2nd Law of thermodynamics) force a redesign, and a third is one that occurs as a result of subdivision of tasks or the management of the flow of materiel or information. Components – parts of a system relative to that system; a component can be a system too if it contains other components Large scale systems – systems that are large in scale and/or scope; such systems have a large number of components; as a result large scale physical systems will be distributed over a region that is large relative to its smallest components; Designing –an open-ended human process whereby plans for useful artifacts and processes are created Function(s) –broad: desired behavior(s) of a system or a component; these behaviors are presumably desired because they contribute to a stated purpose; more specifically, fundamental behaviors (not including the ilities – see below) of an engineering system that fulfill its