Lecture 11 - Lecture 11 - Variability and Waiting Time o...

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Lecture 11 - Variability and Waiting Time o Learning objectives Understand different types of waiting time Understand four sources of variability in a queuing system Analyze stationary arrival processes o Waiting times A "queue" is made of a server and a queue in front Arrival time = average demand rate (# flow units/time Capacity (#flow units/time) We are interested in the waiting times in the queue and the queue length (inventory) Flow time = waiting time (in queue) + activity time Physical queues but also on the phone Two Causes of Waiting Lines Supply constrained process: capacity is lower than demand rate even in the absence of variability (peak hours @ Subway) Waiting time is predicable in this case, e.g., seasonal demand Increasing capacity Demand constrained process: capacity is higher than demand rate but there is variability In the arrival process: the times between the arrival of two flow units is not constant In the service process: the time to process flow units differ from one to the other In the presence of variability , queues can also arise if the implied utilization is bellow 100% We will now focus on the second case From now on we assumer that process capacity > demand tare Where is variability? Arrival:
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Lecture 11 - Lecture 11 - Variability and Waiting Time o...

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