1 Ardavan Asef-Vaziri Sep-09 Operations Management: Waiting Lines3 Terminology: The characteristics of a queuing system is captured by five parameters: Arrival pattern Service pattern Number of server Restriction on queue capacity The queue discipline Terminology and Classification of Waiting Lines
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1 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines3 Terminology: The characteristics of a queuing system is captured by five parameters:
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Terminology: The characteristics of a queuing system is captured by five parameters: Arrival pattern Service pattern Number of server Restriction on queue capacity The queue discipline
Example: The arrival rate to a GAP store is 6 customers per hour and has Poisson distribution. The service time is 5 min per customer and has Exponential distribution. On average how many customers are in the waiting line? How long a customer stays in the line? How long a customer stays in the processor (with the
server)? On average how many customers are with the server? On average how many customers are in the system? On average how long a customer stay in the system?
As the utilization rate increases to 1 (100%) the number of customers in line (system) and the waiting time in line (in system) is increasing exponentially.
A local GAP store on average has 10 customers per hour for the checkout line. The inter-arrival time follows the exponential distribution. The store has two cashiers. The service time for checkout follows a normal distribution with mean equal to 5 minutes and a standard deviation of 1 minute.
On average how many customers are in the waiting line? How long a customer stays in the line? How long a customer stays in the processors (with the
servers)? On average how many customers are with the servers? On average how many customers are in the system ? On average how long a customer stay in the system ?
Arrival rate: R = 10 per hourAverage interarrival time: Ta = 1/R = 1/10 hr = 6 minStandard deviation of interarrival time: SaService rate per server: 12 per hourAverage service time: Tp = 1/12 hours = 5 minStandard deviation of service time: Sp = 1 min
Coefficient of variation for interarrivals : Ci= Sa /Ta = 1
Coefficient of variation for services: Cp = Sp /Tp = 1/5 =0.2
Number of servers: c =2Rp = c/Tp = 2/(1/12) = 24 per hourρ = R/Rp = 10/24 = 0.42
A call center has 11 operators. The arrival rate of calls is 200 calls per hour. Each of the operators can serve 20 customers per hour. Assume interarrival time and processing time follow Poisson and Exponential, respectively. What is the average waiting time (time before a customer’s call is answered)?