WSC 2008 Final Abstracts

Real-Time Delay Estimation in Call Centers
Rouba Ibrahim and Ward Whitt (Columbia University)

Abstract:
We use computer simulation to study the performance of alternative real-time delay estimators in heavily loaded multiserver queueing models. These delay estimates may be used to make delay announcements in call centers and related service systems. We consider the classical delay estimator based on the queue length, QL_s, which multiplies the queue length plus one times the mean interval between successive service completions, ignoring customer abandonment. We show that QL_s has a superior performance in the GI/M/s model, but that there is a need to go beyond it in the GI/GI/s+GI model, allowing abandonment. To this end, we propose new, simple and effective, delay estimators based on the queue length. We also consider a delay estimator based on recent customer delay history in the system: the delay of the last customer to enter service, LES.

A Simulation Based Scheduling Model for Call Centers with Uncertain Arrival Rates
Thomas R. Robbins (East Carolina University) and Terry P. Harrison (Penn State)

Abstract:
In this paper we develop a two stage algorithm for scheduling call centers with strict SLAs and arrival rate uncertainty. The first cut schedule can be developed in less than a minute using a constructive heuristic. The schedule is then refined via a simulation based optimization approach. We find that when allowed to run for five minutes or less this two stage process can create a schedule with a total expected cost within a few percentage points of schedules generated using much more computationally intensive methods. This rapid scheduling process is designed to support front line managers who wish to evaluate multiple scheduling options in a what if analysis mode.

Enhanced Bandwidth-Delay Based Routing Algorithm for a Packet-Switched Virtual Call Centre Environment
Akinbola Adetunji and Hadi Larijani (Glasgow Caledonian University)

Abstract:
Traditionally call centres were based on circuit-switched systems. But with the advancement of communication technologies, call centres have shifted to packet-switched systems. This packet-switched system aids the creation of virtual Call Centre Environments. The current dynamic routing algorithms used for circuit-switched systems do not fully support packet-switched virtual call centre environments. We addressed this issue in this paper by developing a new call routing algorithm capable of supporting this type of virtual environments. Our new routing algorithm was compared with a commonly used call routing algorithm known as Minimum Expected Delay. We used both analytical and simulation methods to achieve our goal of comparison study. Call centre data collected from a real call centre was utilised to aid our model development, validation and scenario generation. The results from this study concluded that under high traffic arrival rates, systems running EBDRA outperforms MED by possessing a lower probability of delay.

Simulating the Performance of a Class-Based Weighted Fair Queueing System
Martin John Fischer and Denise Masi (Noblis) and John Shortle (George Mason University)

Abstract:
Class Based Weighted Fair Queueing (CBWFQ) is a very important router discipline that allows different types of Internet Protocol (IP) traffic like voice, video, and best ef-fort data to receive the required quality of service measures they individually need. CBWFQ dynamically allocates the available bandwidth to each traffic class based on the class’s weight. This discipline is playing a vital role as IP brings these traffic classes together in a truly converged network. Under stress and in extreme emergencies, it is critical to be able to determine how the CBWFQ discipline will perform. In this paper, we present and discuss the critical role simulation has played in our development of performance analysis tools for the CBWFQ discipline.

Designing Simulation Experiments with Controllable and Uncontrollable Factors
Christian Dehlendorff, Murat Kulahci, and Klaus Kaae Andersen (Technical University of Denmark)

Abstract:
In this study we propose a new method for designing computer experiments inspired by the split plot designs used in physical experimentation. The basic layout is that each set of controllable factor settings corresponds to a whole plot for which a number of subplots each corresponding to one combination of settings of the uncontrollable factors is employed. The caveat is that the subplots within each whole plot are desired to cover the design space uniformly. Furthermore, it is also desired that in the combined design where all experimental runs are considered at once, the uniformity of the design space coverage should be guaranteed. Our proposed method allows for a large number of uncontrollable and controllable settings to be run in a limited number of runs while uniformly covering the design space for the uncontrollable factors.

Automated Execution of Simulation Studies Demonstrated Via a Simulation of a Car
Sven Dominka (The University of Melbourne) and Eduard Bröcker (Technical University Munich)

Abstract:
In the automotive area, a huge number of different technologies were developed in the last couples of years. The car man-ufacturers have to meet the challenge of selecting the most suitable components for their purpose. One way of doing so is to evaluate these components by the help of a car simulation model. Because of the huge number of different technologies, also a lot of simulation models are needed to be able to compare all of those innovative technologies. In this paper, a system is described, with which it is possible to automatically generate simulation models. This system is used to generate varied simulation models which represent different car configurations. After the generation of a model for each car configuration, a parameter optimization is automatically executed. The result is a chosen car simulation model with its set of parameters, which presents an optimal solution for the regarded aim, e.g. efficiency.