Re-entrant lines are manufacturing system where parts may
return more than once to the same machine,for repeated stages of
processing.Example of such system are semiconductor
manufacturing plants.Scheduling problem arise to improve
performance measures such as mean waiting time,which is also
known as the mean "cycle time". In this paper, we use a
technique to obtain lower bound on the performance of re-entrant
lines, and improve Kumar's lower bound. We also analyze upper
bound of some typical policies.Especially for the FCFS policy,
we regard the system as a Kelly network. And takesome study, we
get a more easy method (tandem queues) to realize the system on
the mean waiting time. Moreover, we try to prove LBFS is better
than FCFS on the mean waiting time. We also present simulations
which confirm our intuition that W(LBFS) < W(FCFS) < W (FBFS).
Finally, some open problems are suggested.