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Location Decision Rules Dialog Box

The Rules dialog box, opened by double-clicking in the Rules field in the Locations edit table, is used to choose the rule for ProModel to follow when making the following decisions:

• Selecting incoming entities
• Queuing for output
• Selecting a unit

Note: The queuing rules for a location take effect only when entities are "queued" at the particular location due to being in a blocked state. In other words, the entities are ready to move to the next location, but the next location is not available due to insufficient capacity or to being inoperative.

Selecting Incoming Entities

When a location becomes available and there is more than one entity waiting to enter, a decision must be made regarding which one to admit. The primary determining factor is the priority of the input routing. The entity with the highest routing priority will be admitted regardless of the incoming selection rule. However, if two or more entities have the same priority for claiming the location, then the location selects an incoming entity based on the incoming selection rules listed below.

Oldest by Priority Selects the entity waiting the longest among those having the highest routing priority.

Random Selects randomly with equal probability among all waiting entities.

Least Available Capacity Selects the entity coming from the location having the least available capacity. Ties are broken by the entity waiting the longest.

Last Selected Location Selects the entity coming from the location that was selected last. Ties are broken by the entity waiting the longest. If no entities are waiting at the last selected location, the Oldest by Priority rule takes effect.

Highest Attribute Value Selects the entity with the highest attribute value for a specified attribute. Ties are broken by the entity that has been waiting the longest. Location attributes are also valid entries.

Lowest Attribute Value Selects the entity which has the lowest attribute value for a specified attribute. Ties are broken by the entity waiting the longest. Location attributes are also valid entries.

Queuing For Output

When an entity finishes its operation at a location, other entities to finish ahead of it may not have departed. A decision must be made to allow the entity to leave or to wait according to some queuing rule. If one of the following queuing rules is not specified, “No Queuing” will be used.

No Queuing Entities that have completed their operations at the current location are free to route to other locations independent of other entities that have finished their operations. If this option is selected it is not displayed in the Rules Box.

First In, First Out (FIFO) The first entity completing operation must leave for its next location before the second entity completing its operation can leave, and so on.

Last In, First Out (LIFO) Entities that have finished operations queue for output LIFO so the last one finished is the first to leave.

By Type Entities that have finished operations queue for output FIFO by entity type so the routing for each entity type is processed independently of routings for all other types.

Highest Attribute Value Entities that have completed operations queue for output according to the highest value of a specified attribute.

Lowest Attribute Value Entities that have completed operations queue for output according to the lowest value of a specified attribute.

Selecting a Unit

If the location has multiple units, then incoming entities must select which available unit to use. One of the following rules may be selected. These decision rules apply to multi-unit locations only.

First Available Selects the first available unit.

By Turn Rotates the selection among the available units.

Most Available Capacity Selects the unit having the most available capacity. This rule has no effect with single capacity units.

Fewest Entries Selects an available unit with the fewest entries.

Random Selects an available unit randomly.

Longest Empty Selects the unit that has been empty the longest.

When specifying the decision rules for selecting incoming entities at a location, it is important to remember that the routing of an entity is also dependent on the queuing for output decision rules at the previous location. The following example will clarify this principle.

Rules Dialog Box Example

Consider a location, Loc1, which has a “Last In, First Out (LIFO)” as the queuing for output rule. Suppose that two other locations, Loc2 and Loc3, have “No Queuing” for the output rule. The three locations, Loc1, Loc2, and Loc3 feed into Loc4 which has an “Oldest by Priority” rule for selecting incoming entities.

Two parts are queued for output at Loc1. The part waiting the longest, EntA, at Loc1 has been waiting 10 minutes. The other part, EntB, which queued for output after EntA, has been waiting 5 minutes. At Loc2, the part queued for output, EntC, has been waiting 7 minutes. At Loc3, the part queued for output that has been waiting the longest, EntD, has waited 3 minutes.

The part to enter Loc4 first is EntC at Loc2 which waited 7 minutes. Even though EntA has been waiting ten minutes, it must wait until EntB has been routed, because EntB is ahead of it in the output queue according to the LIFO queuing rule. Once Loc4 finishes processing EntC, EntB at Loc1 enters Loc4. EntB enters before EntA because entities must be output before a destination selects incoming entities. Next, EntA at Loc1 enters Loc4 after which EntD at Loc3 enters Loc4.