Principle (Buffering): The WIP level of an open line is a non-increasing function of the buffer size.
The principle states that increasing the size of any buffer in the line cannot cause the WIP to increase. This is because in an open system releases into the line occur at a fixed rate (the release rate). Whenever a buffer gets filled up, it may cause blocking of upstream stations. This serves to slow the progress of the blocked jobs through the system and hence increases average flow time. Since throughput is fixed at the release rate, increasing the flow time increases average WIP level by Little's law.
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