System

A system is defined as a group of objects that are joined together in some regular interaction toward the accomplishment of some purpose. e.g. considering a hospital as a system, doctors, nurses and patients would be the objects.

A system is often affected by changes occuring outside the system: system environment. e.g. the arrival patients in a hospital

#Components of a System

• Entity: An object of interest in the system: machines, doctors or nurses in a hospital.
• Attribute: The property of an entity: speed, capacity, etc…
• Activity: A time period of specific length (can be endogenous or exogenous): surgical operation, room cleaning, etc…
• State: A collection of variables that describe the system in any time: status of machine (busy, inactive, down, etc…)
• Event: An instantaneous occurrence that might change the state of the system (can be endogenous or exogenous): e.g. breakdown.

So a system is a set of related entities, sometimes called components or elements. These elements have certain characteristics or attributes that take on logical or numerical values. e.g. in the hospital example, an attribute might be considered the number of beds in a room or the skill level of a professional.

Typically, the activities of individual components interact in time. These activities cause changes in the system’s state. e.g. the state of the hospital’s waiting room might be described by the number of patients waiting for a doctor. When a patient arrives at the hospital or leaves it, the system jumps to a new state.

#Types of Systems

Depending on how we measure change in terms of system activities, we divide our system into two types:

• Discrete Events Systems (DES): In those systems, the state variables change instantaneously through jumps at discrete points in time. e.g. a queue in a bank.

  

• Continuous systems (CS): Where state variables change continuously over time. e.g. water level in a dam.