Page 169 - ExtendSim User Guide
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Queueing 143
Queueing considerations
Run the model with animation turned on to watch the items with a priority of 1 (red circles) bypass items with a priority of 2 (green circles) while waiting in the Queue.
Queueing considerations
Once items are generated for the model, it is common that they will be held in a queue, typi- cally a Queue block. In addition to the queueing disciplines discussed above, queues and the items in them can exhibit other behaviors.
Blocking occurs when an item is prevented from leaving a block because there is a downstream capacity constraint. Blocking is common in serial operations where there are several activities in a row without queues in between; each activity has the potential for blocking arriving items. It also occurs when activities are preceded by queues with finite capacity, causing backups in the preceding activity. Blocking increases the waiting time for items in queues and is added to the calculation of their utilization.
The examples in the sections “Processing in series” on page 179, “Sequential ordering” on page 166, and “Machines that can only process certain types of items” on page 174 illustrate potential blocking situations.
Sometimes customers enter a facility, look at the long line, and immediately leave. This is an example of balking. In the Balking model. balking is easily represented by setting a maximum queue length and then using a Select Item Out block to prioritize sending items to that queue. If the queue is full, the Select will send the items to another part of the model; in this case, an Exit block..
Balking model using Select Item Out to prioritize the Queue
Balking can also be represented by having a Decision block (Value library) look at a queue’s length or wait time. If the line meets certain conditions (is too long, takes too long to move, etc.), a Select Item Out block routes the item out of the model before it enters the queue.
Reneging occurs when an item, having entered a queue, leaves before it reaches the output. An example of this is telephone callers who, after being put on hold, will hang up without getting help if they feel they have waited too long for assistance.
To simulate reneging, select an option in a Queue block’s Options tab. The choices are:
• Renegeitemsafteraspecifiednumberoftimeunits.Thenumberoftimeunitscanbesetin the block’s dialog or through its R (renege) connector.
Discrete Event

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