In computer network research, network simulation is a technique whereby a software program models the behavior of a network by calculating the interaction between the different network entities. Most simulators use discrete event simulation - the modeling of systems in which state variables change at discrete points in time. The behavior of the network and the various applications and services it supports can then be observed in a test lab; various attributes of the environment can also be modified in a controlled manner to assess how the network / protocols would behave under different conditions.
Most of the commercial simulators are GUI driven, while some network simulators are CLI driven. The network model / configuration describes the network and the events. Output results would include network level metrics, link metrics, device metrics etc. Further, drill down in terms of simulations trace files would also be available. Trace files log every packet, every event that occurred in the simulation and are used for analysis. Most network simulators use discrete event simulation, in which a list of pending "events" is stored, and those events are processed in order, with some events triggering future events—such as the event of the arrival of a packet at one node triggering the event of the arrival of that packet at a downstream node.
Network emulation
allows users to introduce real devices and applications into a test network that alters packet flow in such a way as to mimic the behavior of a live network. Live traffic can pass through the simulator and be affected by objects within the simulation. The typical methodology is that real packets from a live application are sent to the emulation server. The real packet gets 'modulated' into a simulation packet. The simulation packet gets demodulated into a real packet after experiencing effects of loss, errors, delay, jitter etc., thereby transferring these network effects into the real packet. Thus it is as-if the real packet flowed through a real network but in reality it flowed through the simulated network. Emulation is widely used in the design stage for validating communication networks prior to deployment.
Education - Online courses, Lab experimentation and R & D. Most universities use a network simulator for teaching / R & D since its too expensive to buy hardware equipment
There are a wide variety of network simulators, ranging from the very simple to the very complex. Minimally, a network simulator must enable a user to
Model the network topology specifying the nodes on the network and the links between those nodes
