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Infinite Buffer Closed Lines - Principles

Two fundamental parameters that characterize the behavior of a closed line with infinite buffers are:

• Bottleneck rate: the rate of the workstation in the line having the highest utilization.
• Raw process time:  the sum of the long-term average process time of each station in the line.

However, these two parameters alone are not sufficient, since lines with the same bottleneck rate and raw processing time can behave quite differently, depending on the congestion in the line. Congestion can be measured by:

• Congestion co-efficient: a unitless quantity which is zero when congestion is minimum and increases as congestion increases.

With the bottleneck rate and the raw process time, we can compute the following fundamental quantity:

• Critical WIP:  the WIP level required by a line with no congestion to achieve maximum throughput with minimum cycle time. The critical WIP is obtained as the product of the bottleneck rate and the raw processing time.

Range of Possible Performance

Because two lines with the same bottleneck rate and  raw process time can behave so differently, it is important to understand for a given line where the current performance is relative to the range that is possible.  To articulate this range, we consider three cases:

• Best Case
• Worst Case
• Practical Worst Case

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