CALL CENTERS (CENTRES) - Faculty of Industrial Engineering and ...

the International Teletraffic Congress, ITC-16., Elsevier Science, Amsterdam, The Netherlands,
1999, 355–364.
Abstract. In this paper we consider a multiserver queueing model where waiting customers
may abandon and subsequently retry. This model is of particular interest for analyzing performance
and setting staffing levels in call centers. All of the parameters (arrival rate, service rate,
etc.) are allowed to be time-dependent. We propose a simple fluid approximation for the queue
length process arising in this model. The fluid approximation, which is obtained as the solution
of an intuitively appealing ordinary differential equation, is in fact asymptotically exact as the
size of the system (arrival rate and number of servers) grows large. The fluid approximation is
compared with simulations for several sets of parameters and performs extremely well.
Keywords: Time varying queues, Abandonment, Retrials, Multiserver queueing model, Performance,
Staffing levels, Call centers, Arrival rate, Service-rate, Fluid approximation, Queue
length, Ordinary differential equation, Simulations
74. Mandelbaum, A., W.A. Massey, M.I. Reiman and A. Stolyar. Waiting time asymptotics for time
varying multiserver queues with abandonment and retrials. Allerton Conference Proceedings,
1999.
Abstract. The queue length results for the abandonment/retrial model in Theorem 5.1 (A.
Mandelbaum, W.A. Massey, R.I. Reiman, “Strong approximations for Markovian service networks”,
Queueing Systems (1998), Section 5) are extended to include the fluid and diffusion
limits for the waiting time in nonstationary, many server Jackson networks with abandonment.
Keywords: Strong approximations, Fluid approximations, Diffusion approximations, Multiserver
queues, Queues with abandonment, Queues with retrials, Priority queues, Queueing networks,
Jackson networks, Nonstationary queues.
75. Samuelson, Douglas A. Predictive dialing for outbound telephone call centers, Interfaces, 29 (5),
1999, 66–81.
Abstract. In the late 1980s, I used queueing and simulation to invent predictive dialing, a
method to determine when computer-directed outbound telephone dialing systems should dial.
I included a real-time estimation updating feature that was highly robust against sudden changes
in the system’s operation environment; thorough validation to ensure that the models tracked
all important features of the real systems; and a modular software design that allowed “plug-in”
replacement of the control software, eliminating debugging of field upgrades. The improved systems
kept operators busier and drastically reduced the number of calls the systems abandoned
because no operator was available to talk to the answering party. This invention was critical to
the success, in the late 1980s, of International Telesystems Corporation (ITC), a small company
founded in 1984, which a competitor, EIS International, bought in 1993 for approximately $12
million.
Keywords: Predictive dialing, Outbound telephone call centers, Real-time estimation updating
feature, Modular software design
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