The Petri net is a formal modelling tool applicable to distributed systems and communication protocols. Two methods of analysis are applied to formal models of the "Alternating Bit Protocol". (i) A timed Petri net model is simulated to measure protocol performance. (ii) A modular numeric Petri net model is validated by reachability analysis. The simulation and validation tools are programmed in (i) "C" language and (ii) Prolog. A specification language "Needle" is developed. It describes the model system as a hierarchy of modular state transition networks. The model is searched for all possible event sequences, and the result displayed as a reachability tree. The specification language is capable of describing models which execute backwards in simulation time. The modular numeric Petri net is the basis of a powerful computer architecture, capable of parsing its own specification language to build complex models. Attention is drawn to the similarities between Petri net theory and quantum mechanics.