Ch 6 part 2

Question 1

M/M/1 Queuing system

Answer 1

Poisson - arrival process, exponential- service time.
-It is a good model for many service systems
where customers arrive without appointments
 λ < μ: system is stable (customers do not arrive
to the system faster than they can be served)
 First Come First Service, no balking, no
reneging
 The system has reached steady state

Question 2

Decision making in Queues

Answer 2

A. How Busy Should a Server Be?
Tradeoff betw. the benefits of high server utilization and the costs of long customer waits.
B. Should we set up multiple single-server queues or one multiple-server queue?
C. Should we add workers?
Case Study of tradeoff between the costs of speeding up service and the benefits of shorter customer waits.
D. Should we double the service speed or the number of servers?

Question 3

is high utilization a good thing

Answer 3

It avoids wastage of resources, but too high causes system congestion

Question 4

can we have high utilization in emergency serv.

Answer 4

Queuing analysis reveals that up to about 70% utilization there is no significant deterioration in wait times/queue length.
-high utilzation is good for high volume mfg. as it is itmes not people.

Question 5

Why does the multi server system perform better

Answer 5

-In the multiple single-server system, one server could be idle while another server has a queue – but customers are stuck in the line.
-In a single-server system, a single “long” processing time may result in many long queue times for other customers

Question 6

But why do grocery stores continue to use multiple single server system

Answer 6

-Space issues: more space is required so less product can be displayed; store traffic concerns
-Impulse items at checkout are profitable
-Preparation time: put items on conveyor belt

Question 7

what is the case study about

Answer 7

trade off between cost of speeding up service and benefits of shorter cust. waits

Question 8

CRT Reverse supply chain

Answer 8

Product acquistion
Reverse logistics
Inspection & sorting
Reconditioning
Remarketing

Question 9

Other Process Improvement

Answer 9

-Unload faster
Pallets at drop off locations
Conveyor belts / rollers extended into truck
-Rent a second loading dock (2nd server)
-Get their own trucks and/or trailers
Unloading time costs more than $10K per month
Leasing / buying may be cheaper
-Set delivery times
Appointments could eliminate arrival variability
-Flexible crew size
Have disassembly team members help unload

Question 10

How do you decide between the 2 options

Answer 10

Depends on cust. perception
-If they dislike the wait times more, then add a server.
-If they dislike the service time more, then speed up service.

Question 11

Analyzing line up Queuing theory vs simulation

Answer 11

QT ~ S
-Accuracy: excellent if assump. is ok, good approx. if assump. are not too far off ~ excellent
-Perf. measures: ltd choices(avg val only)~ extensive
-Flexibility: ltd (useless for many sit) ~ useful 4 many situation
-Convenience: very conv. , ~ programming excel; using @ RISK, Arena
-Resources: Quick , not comp. intensive ~ time consuming: comp. intensive