3. DEVS As discussed in the previous section, we use formal M&S methodology to model LTE networks. Discrete Event
System Specification (DEVS) formalism has been chosen for this purpose, since the M&S aspects of a DEVS sys- tem are separated in order to modularize and formulate the design of a model, based on the requirements of the source system. To this end, DEVS has been proposed as a sound formal framework for modeling generic dynamic systems and includes hierarchical, modular and component-oriented structure and formal specifications for defining structure and behavior of a discrete event model. A DEVS model is comprised of structural (Coupled) and behavioral (Atomic) components, in which the coupled component maintains the hierarchical structure of the system, while each atomic component represents a behavior of a part of the system. The atomic component is the basic building block of the system which is composed of I/O ports and a finite state timed automaton representing the behavior of the model. An input to the atomic component via an input port triggers a state transition(referredtoas“externaltransition”),and incontrast the state transition(referredtoas“internaltransition”)atthe end of the time-delay of each state leads to an output generation through an output port. Figure 1 illustrates the state transition of an atomic component. An atomic component is in state s for a specified time ta(s). If the atomic component passes this time without interruption it will produce an output y at the end of thistimeandchangestatebasedonitsδ int function (internal transition) and continues the same behavior. However, if it receives an input x during its ta(s) time, it changes its state whichisdeterminedbyitsδext function and does not pro- duce an output (external transition).
A coupled model connects the basic models together in order to form a new model. This model can itself be employed as a component in a larger coupled model, thereby allowing the hierarchical construction of complex models.