Page 249 - ExtendSim User Guide
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Resources and Shifts 223
Modeling resources
Resource Pool Release (Item > Resources)
Releases the specified number of resource pool units, making them available for re-use and causing the count in the Resource Pool block to increase.
Other resource blocks
Resource Item (Item > Resources)
Unlike the resource pool method, this block stores resources as items for use in the
model. Resource items are usually batched with items that require them; they may or may not be unbatched at some later point in the process.
Shift (Item > Resources)
Generates a shift schedule that can be used to change the capacity or stop the activity of other blocks in the model.
Modeling resources
Resources are the means by which process activities and operations are performed. Different parts of a model can share the same resource, just not at the same time. While a particular resource is being used in one place in a model, it is not available for any other part of the model. Thus the availability or lack of availability of resources causes constraints in a model.
How to model resources
As seen in the following sections, there are many ways to simulate resources when building models – some are explicit and some are implicit. Resources can be explicitly modeled using specialized resource management blocks; this has the advantage of direct access to features like automatic costing and utilization calculations. In some situations, however, it could be simpler or provide more control to model resources just as any other item in the model or by limiting block capacity.
Explicit methods
The ExtendSim discrete event architecture supports three explicit methods for modeling resources:
• ResourcePoolmethod.Asacountoftheresourcesthatareavailableinapool.Bykeep- ing track of the available resource pool units, this method controls the flow of items that require the resources. This is accomplished using the resource pool blocks (Resource Pool, Queue [in resource pool queue mode], and Resource Pool Release), as shown in the “Resource Pool method” section of the User Guide.
• ResourceItemmethod.Asoneormoreresourceitemsthatareavailabletoanotheritem. This method involves batching resource items from the Resource Item block with the items that require them and, typically, unbatching the resource when it is no longer needed, as described in the “Resource Item method” section of the User Guide.
• AdvancedResourceManagementmethod.Asrecordsinatableinaninternaldatabase, where each record represents a resource with individual properties and statistics. The Resource Manager block provides the interface for managing advanced resources. This
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