Variations on queueing models not covered in this project

• Balking - This is a phenomenon whereby a customer decides not to join a queue if the queue length is too long. It can be allowed for in queueing analysis.
• Other Queue Disciplines - First-come-first-served does not have to be the queue discipline used to select customers for service. Others are also sometimes considered in queue modelling, and some of the more popular variations are listed below:
• Last-come-first-served (LCFS) - the customers are processed such that the customer at the back of the queue is always processed next.
• Random selection - customers are selected from the queue randomly when another customer is required for processing.
• Priority selection - customers are selected from the queue based on some priority selection process. This can vary according to the model used.
• Pre-emption selection - customers can be pre-empted during service if a higher priority customers joins the queue, and their service suspended or terminated.
• Intermittent Service - Sometimes, servers are not available to customers for certain periods of time. These periods of time are sometimes known in advance, sometimes not. Both of these problems, as well as the situation were a server is not available to a particular queue or queues, can be allowed for in queueing theory.
• Lack of Steady-State - This can occur when  (the arrival rate) is greater than  (the service rate) multiplied by the number of servers. This means that the state of the system (the total number of customers in the system) tends to increase as time goes on and a steady state is never reached. Generalised results for non-steady-state models are hard to derive, but some do exist.