On First Come, First Served Queues with Two Classes of Impatient Customers

TL;DR

This paper analyzes multi-class impatient customer queues with FCFS discipline, introducing a virtual waiting time process to derive performance measures and comparing theoretical results with real call center data.

Contribution

It develops a novel virtual waiting time analysis for multi-class impatient queues, providing practical performance metrics and validating them with real-world data.

Findings

Performance measures accurately predicted system behavior.

Virtual waiting time approach simplifies analysis of complex queues.

System performance closely matches real call center data.

Abstract

We study systems with two classes of impatient customers who differ across the classes in their distribution of service times and patience times. The customers are served on a first-come, first served basis (FCFS) regardless of their class. Such systems are common in customer call centers, which often segment their arrivals into classes of callers whose requests differ in their complexity and criticality. We first consider an $M$/$G$/1 + $M$ queue and then analyze the $M$/$M$/$k$ + $M$ case. Analyzing these systems using a queue length process proves intractable as it would require us to keep track of the class of each customer at each position in queue. Consequently, we introduce a virtual waiting time process where the service times of customers who will eventually abandon the system are not considered. We analyze this process to obtain performance measures such as the percentage of customers receiving service in each class, the expected waiting times of customers in each class, and the average number of customers waiting in queue. We use our characterization to perform a numerical analysis of the $M$/$M$/$k$ + $M$ system, and find several managerial implications of administering a FCFS system with multiple classes of impatient customers. Finally, we compare the performance a system based on data from a call center with the steady-state performance measures of a comparable $M$/$M$/$k$ + $M$ system. We find that the performance measures of the $M$/$M$/$k$ + $M$ system serve as good approximations of the system based on real data.