On Multi-enabledness in Time Petri Nets

We consider time Petri nets with multiple-server semantics. We first prove that this setting is strictly more expressive, in terms of timed bisimulation, than its single-server counterpart. We then focus on two choices for the firing of multiple instances of the same transition: the more conservative safety-wise non deterministic choice, where all firable instances may fire in any order, and a simpler alternative, First Enabled First Fired (FEFF), where only the oldest instance may fire, obviously leading to a much more compact state-space. We prove that both semantics are not bisimilar but actually simulate each other with strong timed simulations, which in particular implies that they generate the same timed traces. FEFF is then very appropriate to deal with linear timed properties of time Petri nets.