Service-Level Differentiation in Many-Server Service Systems: A Solution Based on Fixed-Queue-Ratio Routing

Motivated by telephone call centers, we study large-scale service systems with multiple customer classes and multiple agent pools, each with many agents. For the purpose of delicately balancing service levels of the different customer classes, we propose a family of routing controls called Fixed-QueueRatio (FQR) rules. A newly available agent next serves the customer from the head of the queue of the class (from among those he is eligible to serve) whose queue length most exceeds a specified proportion of the total queue length. We show that the proportions can be set to achieve desired service-level targets for all classes; these targets are achieved asymptotically as the total arrival rate increases. The FQR rule is a special case of the Queue-and-Idleness-Ratio (QIR) family of controls which in a previous paper where shown to produce an important state-space collapse (SSC) as the total arrival rate increases. This SSC facilitates establishing asymptotic results. In simplified settings, SSC allows us to solve a combined design-staffing-and-routing problem in a nearly optimal way. Our analysis also establishes a diminishing-returns property of flexibility: Under FQR, very moderate cross-training is sufficient to make the call center as efficient as a single-pool system, again in the limit as the total arrival rate increases. ∗Columbia Business School, 4I Uris Hall, 3022 Broadway, New York, NY 10027. ([email protected]) †IEOR Department, Columbia University, 304 S. W. Mudd Building, 500 West 120th Street, New York, NY 10027-6699. ([email protected])