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

Abstract. In modern telephone exchanges, subscriber lines are usually connected to the socalled
subscriber line modules. These modules serve both incoming and outgoing traffic. An
important difference between these two types of calls lies in the fact that in the case of blocking
due to all channels busy in the module, outgoing calls can be queued whereas incoming calls get
busy signal and must be re-initiated in order to establish the required connection. The authors
study the corresponding queueing model which takes into account subscriber retrials and investigate
some of its properties such as existence of stationary regime, derive explicit formulas for
the system characteristics, limit theorems for systems under high repetition intensity of blocked
calls and limit theorems for systems under heavy traffic.
Keywords: Telephone traffic, Queueing model, Retrial queue, Queueing theory, Congestion, Subscriber
line modules, Telephone exchanges, Limit theorems
34. Perry, M. and A. Nilsson. Performance modeling of automatic call distributors: assignable grade
of service staffing. International Switching Symposium 1992. ‘Diversification and Integration of
Networks and Switching Technologies towards the 21st Century’ Proceedings. Inst. Electron.
Inf. Commun. Eng, Tokyo, Japan, 1992, 294–298.
Abstract. A new generation of automatic call distributors (ACDs) has been developed that can
handle multiple cell queues and associate priorities with these queues. The priority of a call is
determined through an aging factor, which is a real number used to age the time a call has been
queued. For example, if the oldest call in call queue 1 has waited 10 seconds, and the oldest
call in call queue 2 has waited 7 seconds, and the aging factors for queue 1 and queue 2 are 3
and 5, then the second call would have a higher priority, since the calculated priority factors
are (10*3) and (7*5). The paper gives an analytical model for computing the expected waiting
time for call types that have different aging factors. The results can be used to determine both
the number of agents and the aging values needed to yield specified expected waiting times.
Numerical examples focus on the ACD application operator services, in which both directory
assistance and toll/assist calls are served from the same positions.
Keywords: Performance modeling, Automatic call distributors, Operator services staffing, Heterogeneous
positions, Telephony industry, Multi-purpose operator positions, Automatic call distributor,
ACD, Toll and assist calls, Directory-assistance calls, Classical Erlang-type queueing
models, Expected waiting-time, Average operator occupancy, Average occupancies, Simulation
results
35. Whitt, W. Understanding the efficiency of multi-server service systems, Management Science,
38 (5), 1992, 708–723.
Abstract. In the design and operation of service systems, it is important to determine an
appropriate level of server utilization (the proportion of time each server should be working).
In a multiserver queue with unlimited waiting space, the appropriate server utilization typically
increases as the number of servers (and the arrival rate) increases. They explain this economy of
scale and give a rough quantitative characterization. The authors also show how increased variability
in the arrival and service processes tends to reduce server utilization with a given grade
of service. As part of this analysis, the author develops simple approximations for the mean
steady-state waiting time and the fully steady-state waiting-time distribution. These approxi-
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