Abstract
In a simulation, the system being emulated is called the physical system. The behavior of the system is modeled in terms of state, objects and their attributes, events and time. In a simulation, state is defined by a collection of variables that describe the physical system at any point in time. Changes in the physical system are realized in the simulation by updating one or more of the variables. An object is any component in the physical system that requires explicit representation. The properties of a given object are called attributes. An event is an instantaneous occurrence that changes the state of the system. Each event has a time associated with it indicating when the event occurred. Time in the simulated system is represented as a totally ordered set of values, where each value represents an instant of time in the physical system being modeled. From this brief description of a simulation, it is clear that time is an integral part of how simulations represent the real world. This chapter will start by defining a temporal framework - how time is represented in simulations. The framework includes five dimensions: Time, Clocks, Time Flow, State Updates and Interactions. Each of these dimensions will then be described in greater detail.
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Notes
- 1.
If more than one simulation is interacting, there may be a need for a global clock to maintain a notion of time for the system. A global clock’s value of time is computed from the individual local clocks of each simulation.
- 2.
Logical time and simulation time are not the same. Logical time refers to instants in wall-clock time, whereas simulation time refers to a model of time in the physical system.
- 3.
Constructive simulations attempt to capture detailed quantitative data concerning the system being simulated. They require the model to reproduce actual system behaviors to the extent that the generated statistical results are valid. This type of simulation has many advantages, for example, testing new hardware designs or transportation systems without committing resources for their acquisition.
- 4.
Virtual simulations give the user the “look and feel” of being embedded in the system being modeled. It is not necessary for this type of simulation to exactly emulate the actual system, so long as the objectives of the simulation are not compromised. An example is a video game where the player is represented by a character on the game’s display and is placed in a computer-generated world such as a racetrack or spaceship.
- 5.
That is, except in the special case of events containing exactly the same time stamp.
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Additional Reading
Ceranowicz A (2014) Metasimulation. In: Proceedings of the 14th Interservice/Industry Training, Simulation and Education Conference (I/ITSEC). I/ITSEC Fellows Talk. Orlando, Florida
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Loper, M. (2015). Modeling Time. In: Loper, M. (eds) Modeling and Simulation in the Systems Engineering Life Cycle. Simulation Foundations, Methods and Applications. Springer, London. https://doi.org/10.1007/978-1-4471-5634-5_9
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