Approximate Expressions for Queueing Systems with Scheduled Arrivals and Established Service Order

This paper studies single server queueing systems where customers arrive according to a schedule, but not punctually, and where service might be provided in the scheduled order; thus, customers may leave the system in a sequence different to that of their arrivals. The situation arises in connection with maritime container terminals. The steady state solution to the problem follows an integral equation that may be solved numerically. When congestion is light (as is usual in well managed ports) approximate analytic solutions to the integral equation can be found. As an illustration, formulas are given that apply if the deviations from the schedule and the service times have some specific distributions. These expressions accurately predict the expected delay for systems with fairly unpunctual arrivals and occasional congestion. The paper also contains an exact analytical solution for the special case in which service times are constant and the deviations from the arrival schedule are independent Gumbel variables.