Functional Petri nets and subnets are introduced and studied for the purpose of speed-up of Petri nets analysis with algebraic methods. We show that any functional subnet may be generated by a composition of minimal functional subnets. We propose two ways to decompose a Petri net: via logical equations solution and with an ad-hoc algorithm, whose complexity is polynomial. Then properties of functional subnets are studied. We show that linear invariants of Petri net may be computed from invariants of its functional subnets; similar results also hold for the fundamental equation of Petri nets. A technique for Petri net analysis using composition of functional subnets is also introduced and studied. We show that composition-based calculation of invariants and solutions of fundamental equation provides a significant speed-up of computations. For an additional speed-up we propose a sequential composition of functional subnets. Sequential composition is formalised in the terms of graph theory and was named the optimal collapse of a weighted graph. At last, we apply the introduced technique to the analysis of Petri net models of such well-known telecommunication protocols as ECMA, TCP, BGP.
