GSPN BASICS - Modelling with Generalised Stochastic Petri Nets - page 102

5.3 Some Extensions

Some additional features can be included in a GSPN model, with an ad-
vantage in the power of the modelling paradigm and little increase in the
analysis complexity. These extensions are the possibility of using non-free-
choice conflicts of immediate transitions, the availability of multiple priority
levels for immediate transitions, and the marking-dependency of transition
annotations (rates for timed transitions and weights for immediate transi-
tions).
The availability of multiple priority levels for immediate transitions, and the
marking-dependency of transition annotations has quite a beneficial impact
on the modelling power, and hardly any impact on the complexity of the
model specification and analysis. In particular, the availability of multiple
priority levels for immediate transitions permits the simple specification of
complex sequential selection algorithms, while the marking-dependency of
transition annotations allows the development of compact models in which
the behaviours of a number of different entities are synthetically described.
Employing non-free-choice conflicts of immediate transitions, the user has
the possibility of describing a much wider range of dynamic behaviours in
vanishing markings, but he must be able to correctly associate the immedi-
ate transitions with the metrics that define the probabilistic conflict resolu-
tion. This requires the knowledge of the sets of simultaneously enabled non-
concurrent immediate transitions in any vanishing marking. This knowledge
may not be easy to obtain without the generation of the reachability set,
which is however very costly in most cases. The definition of extended con-
flict sets (ECSs) was introduced in Chapter 4 to provide the user with the
information on the sets of transitions that may be in effective conflict (either
direct or indirect) in a marking, thus helping in the definition of weights.
One extension that is not possible within the GSPN framework is the in-
troduction of more general forms of pdf for the firing delays associated with
transitions. Nevertheless, also in this respect, the availability of immediate
and exponential transitions in one modelling paradigm can be exploited for
the construction of somewhat more general pdfs in the description of the
duration of the real system activities. How this can be done will be shown
in Chapter 7.

5.4 The Definition of a GSPN Model

GSPNs were originally defined in [ 4] . The definition was later improved to
better exploit the structural properties of the modelling paradigm [ 3] . The
definition we present here is based on the version contained in this second
proposal.