Closed Lines

In a closed asynchronous line, the entry of a new job is regulated by the amount of WIP in the line.  Specifically, there is a fixed number of cards or pallets circulating in the system with each job associated with a unique pallet or card. When a job finishes processing in the final station, the job or part leaves the line and the pallet/card returns to the beginning of the first station. A new job may enter the line, only when such pallet/card is available. The mechanics of the closed line ensure that total WIP is constant and hence this type of line is termed a CONWIP (CONstant Work-In-Process) line.  Because CONWIP lines authorize the release of work into the system based on downstream line status information (i.e., WIP level), they are an example of a pull system..

Depending on the capacity of the buffers closed asynchronous lines may be classified as:

• Infinite Buffer: if the interstation buffers never become full (i.e., they are infinite in size or simply large enough to prevent filling them up).. The key feature of an infinite buffer line is that no station ever gets blocked (although stations certainly may become starved). 
  
  
• Finite Buffer: if any of the buffers in the line has the possibility of getting filled up.  A finite buffer line can have stations become both blocked and starved periodically.