For the ease of analysis and administration, systems are organized into subsystems, components and parts. Multiple systems are combined together to form a System of Systems. Let’s check’em out! Here I will be safely assuming that you already are well aware of what a system is, how it functions and how it is organized.
What is a Subsystem?
A subsystem is a set of interacting components that perform a function but aren’t relevant to the mission on their own, i.e., they don’t accomplish the mission at hand on their own. Strictly speaking, subsystems perform sub-functions which when concocted together satisfy the mission statement the system yearns to achieve. The definition is quite similar and can be confusing, however you gotta know that a thin boundary separates the two.
Let’s consider a car. A car is a system. It performs the function of mobility and transcports goods or passengers from one place to another. It is assembled with components like the engine, powertrain, braking mechanism, fuel tank, carburetors, and compressors, onboard electronics, passenger experience systems which interact with each other and accomplish the grand objective.
On the other hand, the engine powers the automobile in a four stroke cycle, but itself has several interacting components like the piston assembly, fuel injection mechanisms, igniters, and compressors. However, mechanical power it is of little use on its own. Goods can’t be transported with raw power; we need external mechanisms like the powertrain, wheels, gears, brakes to convert the rotating shaft into linear motion for mobility. In this sense, the engine performs a subfunction of ‘power generation’ and in conjuction external components realises mobility. Hence the engine is but a subsystem.
Your approach matters
Observe carefully that it is you who decides what function will the system perform. It is you who decomposes the primary function into multiple sub-functions and decides how they will co-ordinate and interact amongst. Every designer has complete freedom to choose the system of interest. For me the ‘car’ might be the system of interest, if yes, then everything else, the engines, fuel tanks, chassis become subsystems, merely existing to support and facilitate the system. For someone else, say a mechanical engineer, the engine might be the system of interest. The car automatically, as one terms, becomes the system of systems and the rest fuel injectors, pistons become subsystems in this particular case.
Note that it is all about the reference frame and attitude you look at it from. However once the reference is defined you must ensure it is strcitly followed throughout the system life cycle. In some sense, Systems Engineering provides some fluidity over conventions by not mandating strict regulations; however one must understand that is the underlying decomposition and classification activity is important, and the rest is just jargon.
What is a System of Systems?
A System of Systems is again a system, but a special one, since some (in some cases, all) of its components are systems themselves, meaning each has its own independent function/purpose and existence.
For instance, consider the Air Transportation System. Mind you; I don’t mean an aircraft here. An Air transportation system comprises aircraft, airports, air traffic control centers, maintenance centers, equipment, personnel, and many others. If you think closely, some of them are systems in themselves; the airport is a system since it has many interacting components like ATC, cargo units, and maintenance units and performs the function of smooth air transportation and control.
As a rule of thumb, it is recommended that you decompose a system into, at most, 5 to 7 subsystems. However, this rule can be overruled if the system organization needs more subsystems based on the functions they all perform.
All in all, hierarchy and organization in a system are just a way to ease the administration and analysis of a system. There aren’t strict rules on demarcating systems, but some general guidelines and thumb rules can ease the process.
You now know the basic barebones of a system and how subsystems or Systems of Systems are identified. We will next look at the various processes which go on before the successful deployment of a system and, not only that, the maintenance and retirement phase of any system.
Stay tuned!