Understanding Modelers, Emulators, and Simulators FAQ
In network impairment testing, there are modelers, emulators, and simulators. They all have different roles to play.
Network modelers represent phenomenon as a set of mathematical equations. A network modeler lets you define traffic volumes, flows, network architectures, etc. So you can visualize the application performance, and do "what-if" analysis. Modelers do not deal with real traffic; no packets flow through a modeler.
OpNet is an example of a modeler.
Network Simulators generate test conditions approximating actual or operational conditions. Simulators rely on mathematical formulas to determine behavior.
For example, an SNMP agent simulator running on an inexpensive PC, would simulate the behavior of the SNMP agent inside an expensive router. You could query this simulated SNMP agent for the values of MIB objects. Or the simulated agent could send an alarm that a link was down. However, the values would not be real and there is no real link that is down. So, what is simulated is the behavior of an agent, but not a real link down condition, or the real value of a MIB object in the router.
A network simulator uses mathematical models to simulate, for example, a frame relay connection. It appears to the client-server application that it is operating over a frame relay "cloud", however, it is really running over a mathematical model that has made several assumptions about how frame relay connections operate. Shunra is an example of a simulator.
Network Emulators imitate the function of (another system), as by modifications to hardware, software, or network activity that allow the imitating system (the emulator) to accept the same data, execute the same programs, and achieve the same results as the imitated system.