Figure 3 is a Petri Net model which combines elements from the previous examples to define a first come, first served priority queue. When they do so, a priority number y is assigned and the next available y value is incremented. A floor control model incorporating a first come, first served protocol to be employed when the controller releases control and the queue of those wishing control is non-empty. A set of CSCW protocol models For computer supported collaborations there are, among others, computer resources to share. There is a computer which each may control via a personal keyboard and pointing device.
All participants can see the screen of the shared computer.
These are the protocols subjected to an empirical study described in the next section. This protocol is described by Figure 3 applied to control of the shared computer. If someone is in control, then others wishing control must request control by entering a first come, first served queue and wait there until the person in control, voluntarily releases control in order to move forward in the queue. The queue is not visible to participants. There are many, many other possibilities, of course, but these are among the simplest and serve to illustrate the roles of Petri Net modeling in assessing protocols.