TIME IN PETRI NETS - Modelling with Generalised Stochastic Petri Nets - page 61

p 1 p 2

T 1 t 2

Figure 3.10: Interrupting an activity with an immediate transition

(timed) transition to fire next is said to be used.
Conflicts comprising timed and immediate transitions have an important
use in timed PN systems: they allow the interruption (or preemption) of
ongoing activities, when some special situation occurs. Consider for example
the subnet in Fig. 3.10. A token in place p 1 starts the activity modelled by
timed transition T 1 . If a token arrives in p 2 before the firing of T 1 , immediate
transition t 2 becomes enabled and fires, thus disabling timed transition T 1 .
This behaviour again provides an example of the precedence of immediate
over timed transitions.
The presence of immediate transitions induces a distinction among markings.
Markings in which no immediate transitions are enabled are called tangible,
whereas markings enabling at least one immediate transition are said to
be vanishing. The timed PN system spends a positive amount of time in
tangible markings, and a null time in vanishing markings.

3.4 Memory

An important issue that arises at every transition firing when timed transi-
tions are used in a model is how to manage the timers of all the transitions
that do not fire.
¿From the modeling point of view, the different policies that can be adopted
link the past history of the systems to its future evolution considering various
ways of retaining memory of the time already spent on activities. The
question concerns the memory policy of transitions, and defines how to set
the transition timers when a state change occurs, possibly modifying the
enabling of transitions. Two basic mechanisms can be considered for a
timed transition at each state change.

• Continue. The timer associated with the transition holds the present