Petri Nets were first introduced by C. A. Petri [17] to model and analyze communication systems. Since then, Petri Nets have been studied extensively (e.g. [16], [23], [12]) because they are able to describe many of the common characteristics of DES's. As such modeling tools, Petri Nets possess many advantages, including: their well-developed mathematical and practical foundation; compact representation of system behavior; available tools for both qualitative and quantitative analyses; and their graphical nature that aids in the understanding of complex systems.