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Simulation Concepts 47
Modeling methodologies
empty or have any number of separate buckets of water traveling through it. Rather than a continuous flow, buckets of water would come out of the pipe at random intervals.
• Discreteratesimulationsareahybridtype,
combining aspects of continuous and discrete
event modeling. Like continuous models they
simulate the flow of stuff rather than items;
like discrete event models they recalculate
rates and values whenever events occur. Using
the pipe analogy for a discrete rate simulation, there is a constant stream of fluid passing through the pipe. But the rates of flow and the routing can change when an event occurs.
☞ In some branches of engineering, the term discrete is used to describe a system with periodic or constant time steps. Discrete, when it refers to time steps, indicates a continuous model; it does not have the same meaning as discrete event or discrete rate. Continuous models in ExtendSim are stepped using constant time intervals; discrete event and discrete rate models are not.
Comparison table
The three main modeling methodologies are summarized in the table below.
Modeling method Continuous time
Discrete event
Discrete rate
ExtendSim library Value library
Electronics library Item library
Rate library
What is modeled Processes
Individual items
Flows of stuff
Examples
Processes: chemical, biolog- ical, economic, electronics.
Things: traffic, equipment, work product, people.
Information: data, mes- sages, and network protocols at the packet level.
Rate-based flows of stuff: homogeneous products, high speed production, data feeds and streams, mining.
0 2.3 2.7 4
Time line for discrete rate simulation
Table of continuous, discrete event, and discrete rate differences
Although not definitive, the following table will help to determine which style to use when modeling a system.
Factor
What is modeled
Continuous
Values that flow through the model.
Discrete Event
Distinct entities (“items” or “things”).
Discrete Rate
Bulk flows of homoge- neous stuff. Or flows of otherwise distinct entities where sorting or separating is not necessary.
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