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Simulation kernel: low level operations: route lookup, packet forwarding, TCP protocol processing

Tcl: high level control, link failures, route changes

Simulation engine executes the next earliest event.

void Scheduler::schedule(Handler* h, Event* e, double delay)

Node 0

Link

Node 1

Yet it is extensible

Simulators are complement to analysis: checking the assumptions and results or even obtaining results for scenarios difficult to analyze.

Inexpensive –basic workstation is enough for both the simulation and analysis

Flexible – wide range of scenarios in short amount of time

•Different link speeds (even non-existing in real world)

•Different topologies (difficult to create or change with testbeds)

Access to ALL data generated in the simulation – useful for having insight into the competing traffic

Testbeds – real hosts on a small isolated network

Emulation – models a particular piece of the network path between two real hosts