System “state”: The variables maintained by the simulation program is defined as a system “state.”. Simulated Time: The simulated time is kept by the internal (to simulation program) variable. OPNET (commercial), NS-2 (open source), OMNeT++ The number of packages is used in the Ns2 simulation. Continuously, State changes can occur across time. Using random number generators are used in discrete points in time. Implement these algorithms and let them interact. Mathematically, we know possible for some bits. Algorithmically, we know how bits and pieces of the system work. Programmatic representation of system entities. Ns2 simulation can give more detail than analytical modeling. Usually, only steady-state can be modelled in Ns2 simulation. Most systems are complex and require highly complex mathematical models. The model is available and is computationally efficient when it useful. 
The simulation can be done only on completed running systems. The behavior/characteristics are affected when the user/system is disturbed. Detail: At an arbitrary level, system details also can be simulated. Generality: over analytic/numerical techniques, it gives more generality. Sometimes it finds bugs (in design) in an advanced manner. The complex functions for which closed-form formulas or numerical techniques not available are also evaluated. The design alternatives (e.g., different system configurations) were also evaluated quickly. The real-systems are not available which is complex/costly or dangerous (e.g., space simulations, and also flight simulations).
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