Selected Examples |
When a customer arrives at a facility that includes queues and service, they may choose to enter a queue, if there is room, or leave the facility. Once in a queue, they may choose to leave it if they have waited too long. Not entering a queue and leaving a queue are two types of reneging. This example shows how to model some typical queues with reneging.
The model in Figure 10.1 shows a single queue for three
servers. It models the M/M/c/K system where and .
This system has Poisson arrivals to a single queue
with a capacity of transactions for service by
parallel servers.
Another way to model this system is with the MServer,
as shown in Figure 10.1.
Figure 10.1: An M/M/c/K Model
This model is often compared to one with parallel queues
and servers, as shown in Figure 10.2.
Figure 10.2: A 3-Queue 3-Server Model
In this model, the Switch component directs the transaction to
one of the three queues.
In this case, the transaction is routed
to the shortest length queue.
This is accomplished with two formulas tied to the switch.
A model for this is shown in Figure 3.11.
Another variant on the parallel server models in Figure 10.1
and Figure 10.2 has customers entering a queue and, if
they have waited for too long, deciding to switch to another
queue.
This decision-making and queue-switching policy is more complex,
but it can be modeled as shown in Figure 10.3.
Figure 10.3: A 2-Queue 2-Server Model with Reneging
In this model, upon arrival, the transaction is assigned an attribute
named "priority," whose value is the current simulation time.
This is done in the Modifier component
labeled "priority."
Next, the transaction goes to a Switch,
which compares the two
queues and sends the transaction
down the path leading to the shorter of the two queues.
Next, the transaction encounters a Trigger, which schedules
the transaction to balk when it has spent a given amount
of time in the queue.
The default is a random variable with exponential distribution with mean 1.
When a transaction balks, it goes into another Switch, which
checks whether the other queue is shorter.
If it is, the transaction is routed to the Connector
a
and goes to the end of the other queue.
Otherwise, the transaction goes back into the queue
after scheduling, in the Trigger, another future check.
Copyright © 2008 by SAS Institute Inc., Cary, NC, USA. All rights reserved.