In order to analyze, design, and evaluate a system, the description of its components and their interconnections must be put into a suitable form. A mathematical or graphical representation of a system is called the model. The model resembles the system in its salient features but is easier to study. Some authors refer to models of physical systems as systems. Therefore, a physical system is a device or a collection of devices existing in the real world, and a system is a model of a physical system [12].