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David Chan avatar image
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David Chan asked Phil BoBo answered

Changes in breakdown time

We need the amount of down time to be same no matter what is the queue size is. The repeat random stream is checked, but the down time for the 4 machine stringer varies with the changes of the queue size.

I have found that when I separate the backend with a sink and connect the queue to the sink, the downtimes are the same when I repeat the run. When I connect the sink with the Pre-Soldering, the downtime varies from my first finding. And I believe when I increase the queue size, the downtime will change again.

Can you explain why the downtime varies?

FlexSim 16.0.1
breakdownrandom streamdown time
5 |100000

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Phil BoBo avatar image
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Phil BoBo answered

Pass different stream values for each distribution in your MTBF and MTTR tables.

You are using the same random number stream for all of your distributions. By doing this, if the timing of events changes the order that various stochastic distributions are called, then the values returned by subsequent calls will be different.

You can avoid this type of unintended relation between seemingly unrelated elements of your system by using different values for the stream parameter on each random distribution you use. That is what the stream parameter is for.

Here's a simple example to illustrate what is happening:

Imagine that random stream 0 returns the following random values: 3, 2, 7, 5, 4, 5, 6, 3, 9, 8, 1. Then imagine that these events happen in a particular order and each sample random numbers using stream 0.

event : random number sampled

  • A source interarrival time : 3
  • B processor process time : 2
  • C processor process time : 7
  • D mtbf : 5
  • E source interarrival time : 4
  • F processor process time : 5
  • G mttr : 6
  • H processor process time : 3

Then imagine that you changed something about your model (such as the queue's size) so that those events changed order:

  • A source interarrival time : 3
  • B processor process time : 2
  • D mtbf : 7
  • C processor process time : 5
  • E source interarrival time : 4
  • G mttr : 5
  • F processor process time : 6
  • H processor process time : 3

Now the values of the mtbf and mttr stochastic functions are based on different random numbers, even though you only changed a seemingly unrelated element in the system.

By specifying different stream values for each distribution, the order of events in the other parts of the model will not change what random values are being returned for that distribution.

For more information, see Averill M. Law's book "Simulation Modeling and Analysis" 4th Edition Chapter 11 "Variance-Reduction Techniques" pages 584-586.

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