the typical methodology is that real packets from a live application are sent to the emulation server (where the virtual network is simulated). 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.