# Modelling concepts

Concepts as items, locations, customers, demands, …don’t need a much of an explanation.
The modelling concepts of operations, material and capacity consumption however need some introduction.

Operation

The key modelling element is an operation, which defines an activity.

There are different operation types:

• fixed time: the duration is identical regardless of the planned quantity. Eg, a transport operation
• time-per: the duration increases linearly with the planned quantity. Eg, a manufacturing operation
• routing: this operation type represent a sequence of sub-operations.
• alternate: an alternate operation models the choice among multiple choices.
Example: Operation “Make product X” takes 10 minutes per unit.

Buffer

Items are stocked in buffers. A buffer, often called SKU -stock keeping unit-, is a (physical or logical) inventory point.

There are different buffer types:

• default: a buffer that is replenished with a producing operation
• procure: a buffer that is replenished with a procurement operation
• infinite supply: a buffer without replenishing operation
Example:
Buffer “Part A” models inventory of the item “part A” in location “my factory”.
Buffer “Part B” models inventory of the item “part B” in location “my factory”.
Buffer “Bulk product” models inventory of the item “bulk product” in location “my factory”.

Flow

Operations consume and produce material through flows.

A flow defines an association between the operation and a buffer. Flows thus represent the Bills of Material (aka BOM) and Bills of Distribution (aka BOD) in your model.

There are 2 types of flows:

• start: Flows happening at the start of the operation. These are normally consuming material and have a negative quantity.
• end: Flows happening at the end of the operation. These are normally producing material and have a postive quantity.
Example:
Operation “Make product X” has 3 flows.
A first flow to consume 1 units of a buffer “Part A” at the start of the operation.
A second flow to consume 2 units of a buffer “Part B” at the start of the operation.
And finally a third flow to model the production of 1 unit of “product X” when the operation finishes.

Operations also require capacity, which is modelled through loads. Capacity is available in resources and a load defines an association between the operation and a resource.

Example:
The resource “production line” is required to perform the operation.

Operationplan

An operation only statically defines the activity, and doesn’t specify any planned dates or quantities. Concrete activities are then instantiated in operationplans.

Example:
To satisfy a customer demand we plan to run “Assemble product X” for 12 units from tomorrow 8am till 10am.
Another instance of “Make product X” is planned the next day from 3pm to 4pm for 6 units to meet another customer demand.
Another instance of “Make product X” for 20 units is planned today to replenish a buffer storing the product X to its safety stock level.

Putting it all together

Combining all of the above modelling objects we can construct a network representing the flow of material and usage of capacity in your plant.

The picture below shows a simple network with 3 levels.

Important

Drawing this type of schematic network of your environment is extremely useful before you start entering data in frePPLe. Each shape and line in such a diagram will be defined as an input record in the frePPLe data model.