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4.9 Discrete Event (DE)

The DE domain is a relatively mature domain using an event-driven model of computation. In this domain, particles carry time stamps, and represent events that occur at arbitrary points in simulated time. Events are processed in chronological order. Two schedulers are available. The default scheduler is based on the "calendar queue" mechanism developed by Randy Brown and was written by Anindo Banerjea and Ed Knightly. Since this scheduler is relatively new, the older and simpler but less efficient scheduler is also provided.

DE schedulers maintain an event queue, which is a list of events sorted chronologically by time stamp. The scheduler selects the next event on the list, and determines which star should be fired to process the event. The difference between the efficient calendar queue scheduler and the naive simple scheduler is in the efficiency with which this queue is updated and accessed. Considerable effort was put into consistent and predictable handling of simultaneous events.

The DE domain is suitable for high-level modeling of communications networks, queueing systems, hardware systems, and transportation networks. The demos included with the domain include a variety of queueing systems, shared resource management, communication network protocols, packet-switched networks, wireless networks, and multimedia systems. The latter class of applications take advantage of the ability that Ptolemy has to mix domains by modeling speech and video encoding algorithms using the SDF domain and a packet switched network using the DE domain. There are also some more specialized uses of the DE domain, such as modeling shot noise and synchronizing a simulation to a real-time clock.

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