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Telephone call centers: Tutorial, review, and research prospects
 Mgmt
, 2003
"... Telephone call centers are an integral part of many businesses, and their economic role is significant and growing. They are also fascinating sociotechnical systems in which the behavior of customers and employees is closely intertwined with physical performance measures. In these environments trad ..."
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Cited by 285 (13 self)
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Telephone call centers are an integral part of many businesses, and their economic role is significant and growing. They are also fascinating sociotechnical systems in which the behavior of customers and employees is closely intertwined with physical performance measures. In these environments traditional operational models are of great value – and at the same time fundamentally limited – in their ability to characterize system performance. We review the state of research on telephone call centers. We begin with a tutorial on how call centers function and proceed to survey academic research devoted to the management of their operations. We then outline important problems that have not been addressed and identify promising directions for future research. Acknowledgments The authors thank Lee Schwarz, Wallace Hopp and the editorial board of M&SOM for initiating this project, as well as the referees for their valuable comments. Thanks are also due to L. Brown, A. Sakov, H. Shen, S. Zeltyn and L. Zhao for their approval of importing pieces of [36, 112].
Fair Resource Allocation in Wireless Networks using Queuelengthbased Scheduling and Congestion Control
"... We consider the problem of allocating resources (time slots, frequency, power, etc.) at a base station to many competing flows, where each flow is intended for a different receiver. The channel conditions may be timevarying and different for different receivers. It is wellknown that appropriate ..."
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Cited by 200 (49 self)
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We consider the problem of allocating resources (time slots, frequency, power, etc.) at a base station to many competing flows, where each flow is intended for a different receiver. The channel conditions may be timevarying and different for different receivers. It is wellknown that appropriately chosen queuelength based policies are throughputoptimal while other policies based on the estimation of channel statistics can be used to allocate resources fairly (such as proportional fairness) among competing users. In this paper, we show that a combination of queuelengthbased scheduling at the base station and congestion control implemented either at the base station or at the end users can lead to fair resource allocation and queuelength stability.
Stable scheduling policies for fading wireless channels
 IEEE/ACM Trans. Networking
, 2005
"... We study the problem of stable scheduling for a class of wireless networks. The goal is to stabilize the queues holding information to be transmitted over a fading channel. Few assumptions are made on the arrival process statistics other than the assumption that their mean values lie within the capa ..."
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Cited by 134 (39 self)
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We study the problem of stable scheduling for a class of wireless networks. The goal is to stabilize the queues holding information to be transmitted over a fading channel. Few assumptions are made on the arrival process statistics other than the assumption that their mean values lie within the capacity region and that they satisfy a version of the law of large numbers. We prove that, for any mean arrival rate that lies in the capacity region, the queues will be stable under our policy. Moreover, we show that it is easy to incorporate imperfect queue length information and other approximations that can simplify the implementation of our policy. 1
A queueing analysis of maxmin fairness, proportional fairness and balanced fairness
, 2006
"... We compare the performance of three usual allocations, namely maxmin fairness, proportional fairness and balanced fairness, in a communication network whose resources are shared by a random number of data flows. The model consists of a network of processorsharing queues. The vector of service rates ..."
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Cited by 64 (13 self)
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We compare the performance of three usual allocations, namely maxmin fairness, proportional fairness and balanced fairness, in a communication network whose resources are shared by a random number of data flows. The model consists of a network of processorsharing queues. The vector of service rates, which is constrained by some compact, convex capacity set representing the network resources, is a function of the number of customers in each queue. This function determines the way network resources are allocated. We show that this model is representative of a rich class of wired and wireless networks. We give in this general framework the stability condition of maxmin fairness, proportional fairness and balanced fairness and compare their performance on a number of toy networks.
Dynamic routing in largescale service systems with heterogeneous servers, Queueing Systems
, 2005
"... Abstract. Motivated by modern call centers, we consider largescale service systems with multiple server pools and a single customer class. For such systems, we propose a simple routing rule which asymptotically minimizes the steadystate queue length and virtual waiting time. The proposed routing s ..."
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Cited by 62 (17 self)
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Abstract. Motivated by modern call centers, we consider largescale service systems with multiple server pools and a single customer class. For such systems, we propose a simple routing rule which asymptotically minimizes the steadystate queue length and virtual waiting time. The proposed routing scheme is FSF which assigns customers to the Fastest Servers First. The asymptotic regime considered is the HalfinWhitt manyserver heavytraffic regime, which we refer to as the Quality and Efficiency Driven (QED) regime; it achieves high levels of both service quality and system efficiency by carefully balancing between the two. Additionally, expressions are provided for system limiting performance measures based on diffusion approximations. Our analysis shows that in the QED regime this heterogeneous server system outperforms its homogeneous server counterpart.
Optimal Transmission Scheduling in Symmetric Communication Models with Intermittent Connectivity
, 2002
"... We consider a slotted system with N queues, and i.i.d. Bernoulli arrivals at each queue during each slot. Each queue is associated with a channel that changes between "on" and "off" states according to i.i.d. Bernoulli processes. We assume that the system has K identical transmit ..."
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Cited by 33 (2 self)
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We consider a slotted system with N queues, and i.i.d. Bernoulli arrivals at each queue during each slot. Each queue is associated with a channel that changes between "on" and "off" states according to i.i.d. Bernoulli processes. We assume that the system has K identical transmitters ("servers"). Each server, during each slot, can transmit up to C packets from each queue associated with an "on" channel. We show that a policy that assigns the servers to the longest queues whose channel is "on" minimizes the total queue size, as well as a broad class of other performance criteria. We provide several extensions, as well as some qualitative results for the limiting case where N is very large. Finally, we consider a "fluid" model under which fractional packets can be served, and subject to a constraint that at most C packets can be served in total from all of the N queues. We show that when K = N , there is an optimal policy which serves the queues so that the resulting vector of queue lengths is "Most Balanced."
Throughputoptimal Scheduling in Multichannel Access Point Networks under TimeVarying Channel Rates
, 2005
"... We consider the problem of uplink/downlink scheduling in a multichannel wireless access point network where channel states differ across channels as well as users, vary with time, and can be measured only infrequently. We demonstrate that, unlike infrequent measurement of queue lengths, infrequent m ..."
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Cited by 29 (1 self)
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We consider the problem of uplink/downlink scheduling in a multichannel wireless access point network where channel states differ across channels as well as users, vary with time, and can be measured only infrequently. We demonstrate that, unlike infrequent measurement of queue lengths, infrequent measurement of channel states reduce the maximum attainable throughput. We then prove that in frequency division multiplexed systems, a dynamic scheduling policy that depends on both the channel rates (averaged over the measurement interval) and the queue lengths, attains the maximum possible throughput. We also generalize the scheduling policy to solve the joint power allocation and scheduling problem in orthogonal frequency division multiplexed systems. In addition, we provide simulation studies that demonstrate the impact of the frequency of channel and queue state measurements on the average delay and attained throughput. Index Terms Throughputoptimal scheduling, Multichannel access point networks, Infrequent channel measurements.
Stable scheduling policies for maximizing throughput in generalized constrained queueing systems
 IN PROCEEDINGS OF IEEE INFOCOM
, 2006
"... We consider a class of queueing networks referred to as “generalized constrained queueing networks” which form the basis of several different communication networks and information systems. These networks consist of a collection of queues such that only certain sets of queues can be concurrently ser ..."
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Cited by 23 (11 self)
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We consider a class of queueing networks referred to as “generalized constrained queueing networks” which form the basis of several different communication networks and information systems. These networks consist of a collection of queues such that only certain sets of queues can be concurrently served. Whenever a queue is served, the system receives a certain reward. Different rewards are obtained for serving different queues, and furthermore, the reward obtained for serving a queue depends on the set of concurrently served queues. We demonstrate that the dependence of the rewards on the schedules alter fundamental relations between performance metrics like throughput and stability. Specifically, maximizing the throughput is no longer equivalent to maximizing the stability region; we therefore need to maximize one subject to certain constraints on the other. Since stability is critical for bounding packet delays and buffer overflow, we focus on maximizing the throughput subject to stabilizing the system. We design provably optimal scheduling strategies that attain this goal by scheduling the queues for service based on the queue lengths and the rewards provided by different selections. The proposed scheduling strategies are however computationally complex. We subsequently develop techniques to reduce the complexity and yet attain the same throughput and stability region. We demonstrate that our framework is general enough to accommodate random rewards and random scheduling constraints.
Local Search Scheduling Algorithms for Maximal Throughput In . . .
, 2004
"... We consider the (generalized) packet switch scheduling problem, where the switch service configuration has to be dynamically chosen based on observed queue backlogs, so as to maximize the throughput. A class of recently developed `projective' scheduling algorithms, which substantially generaliz ..."
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Cited by 21 (10 self)
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We consider the (generalized) packet switch scheduling problem, where the switch service configuration has to be dynamically chosen based on observed queue backlogs, so as to maximize the throughput. A class of recently developed `projective' scheduling algorithms, which substantially generalize the wellknown maximum weight matching (MWM) algorithms for crossbar switches, are explored from the perspective of complexity. The typically huge number of possible switch configurations that the scheduler has to consider in each timeslot has been previously observed to lead to an impractical computational requirement. We introduce
Flowlevel stability of data networks with nonconvex and timevarying rate regions
 In Proceedings of ACM Sigmetrics
, 2007
"... In this paper we characterize flowlevel stochastic stability for networks with nonconvex or timevarying rate regions under resource allocation based on utility maximization. Similar to prior works on flowlevel stability, we consider exogenous data arrivals with finite workloads. However, to mode ..."
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Cited by 20 (4 self)
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In this paper we characterize flowlevel stochastic stability for networks with nonconvex or timevarying rate regions under resource allocation based on utility maximization. Similar to prior works on flowlevel stability, we consider exogenous data arrivals with finite workloads. However, to model many realistic situations, the rate region, which constrains the feasibility of resource allocation, may be either nonconvex or timevarying. When the rate region is fixed but nonconvex, we derive sufficient and necessary conditions for stability, which coincide when the set of allocated rate vectors has continuous contours. When the rate region is timevarying according to some stationary, ergodic process, we derive the precise stability region. In both cases, the size of the stability region depends on the resource allocation policy, in particular, on the fairness parameter α in αfair utility maximization. This is in sharp contrast with the substantial existing literature on stability under fixed and convex rate regions, in which the stability region coincides with the rate region for many utilitybased resource allocation schemes, independently of the value of the fairness parameter. We further investigate the tradeoff between fairness and stability when rate region is nonconvex or timevarying. Numerical examples of both wired and wireless networks are provided to illustrate the new stability regions and tradeoffs proved in the paper.