Any co-simulation framework must manage the exchange of information as well as the synchronization of the clocks between simulations. The Fenix (Trademark) framework for network co-simulation addresses the drawbacks of other known co-simulation frameworks. Specifically, Fenix (Trademark) provides a capability allowing all simulations to speculate whether they can forego time synchronization for a certain period without loss of simulation accuracy. Further, Fenix (Trademark) allows speculative multithreading at runtime, without compiler assistance. This technique has not yet been applied as a means of accelerating co-simulations. Besides speculation, Fenix (Trademark) also provides novel time synchronization algorithms which take into account network traffic delays if a network simulator is part of the co-simulation.