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Role of Gates in SimEvents Models

Overview of Gate Behavior

By design, certain blocks change their availability to arriving entities depending on the circumstances. For example,

  • A queue or server accepts arriving entities as long as it is not already full to capacity.

  • An input switch accepts an arriving entity through a single selected entity input port but forbids arrivals through other entity input ports.

Some applications require more control over whether and when entities advance from one block to the next. A gate provides flexible control via its changing status as either open or closed: by definition, an open gate permits entity arrivals as long as the entities would be able to advance immediately to the next block, while a closed gate forbids entity arrivals. You configure the gate so that it opens and closes under circumstances that are meaningful in your model.

For example, you might use a gate

  • To create periods of unavailability of a server. For example, you might be simulating a manufacturing scenario over a monthlong period, where a server represents a machine that runs only 10 hours per day. An enabled gate can precede the server, to make the server's availability contingent upon the time.

  • To make departures from one queue contingent upon departures from a second queue. A release gate can follow the first queue. The gate's control input determines when the gate opens, based on decreases in the number of entities in the second queue.

  • With the First port that is not blocked mode of the Entity Output Switch block. Suppose each entity output port of the switch block is followed by a gate block. An entity attempts to advance via the first gate; if it is closed, then the entity attempts to advance via the second gate, and so on.

Gate Behavior

The Entity Gate block offers these fundamentally different kinds of gate behavior:

  • The enabled gate, which uses a control signal to determine time intervals over which the gate is open or closed

  • The release gate, which uses a control message to determine a discrete set of times at which the gate is instantaneously open. The gate is closed at all other times during the simulation.

    Tip   Many models follow a gate with a storage block, such as a queue or server.

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