The discrete-time event scheduler applies a static calendar queue and unusual event memory management which results in good simulation speed.
All network nodes are objects which communicate over standardized messages.
It's only possible to choose speeds for which the cell transfer time is an integer multiple of a basic time used for the whole model. The discrete-time nature excludes some useful source models like the Poisonian types. In order to compose background traffic you therefore always have to multiplex multiple traffic streams in a multiplexer. This restriction is, however, not imposed by the simulation kernel, but only by some network object classes like multiplexer, shaper, and delay line. While the language based model description yields a high flexibility, the input may become a bit irritating in case of larger networks.