6.3 Suspended Ceiling Design Process - Bilkent University

interpreted as resources. The idea is that resources (tokens) are used
by activities (transitions), and then moved from one place to another.
Input arcs connect places with transitions, while output arcs start at a
transition and end at a place. A marking in a Petri net refers to a change
in the state of the system. An initial state of a Petri net is called the
initial marking. Transitions are only allowed to occur if they are enabled
(all the preconditions for the activity are fulfilled). The concept of time
can also be associated with a transition. Moreover, pre- and post-
conditions can be added to a Petri net and additional attributes (colors)
can be added to tokens (Karhu, 2001).
Figure 3.3 shows a process that consists of only one activity. To
execute the activity, two resources have to be used. The net in Figure
3.3(a) models this process. Transition in t1 in Figure 3.3(b) is enabled
because each of its input places, p1 and p2, contains a token. After the
transition t1 occurs, p3 (the output place of t1) gets a token; the new
marking is shown in Figure 3.3(c) (Zakarian and Kusiak, 2000).
Figure 3.3: A Petri Net of an Activity that Uses Two Resources: (a) Petri
Net Model (b) Initial Marking, and (c) Marking after t1 has been
Occurred.
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