In designing a good queuing system, it is necessary to have a good information about the model. The characteristics listed below would provide sufficient information.
1. The arrival pattern.
2. The service mechanism.
3. The queue discipline.
4. The number of customers allowed in the system.
5. The number of service channels.
The Arrival Pattern
The arrival pattern describes how a customer may become a part of the queuing system. The arrival time for any customer is unpredictable. Therefore, the arrival time and the number of customers arriving at any specified time intervals are usually random variables. A Poisson distribution of arrivals correspond to arrivals at random. In Poisson distribution, successive customers arrive after intervals which independently are and exponentially distributed. The Poisson distribution is important, as it is a suitable mathematical model of many practical queuing systems as described by the parameter “the average arrival rate”.
The Service Mechanism
The service mechanism is a description of resources required for service. If there are infinite number of servers, then there will be no queue. If the number of servers is finite, then the customers are served according to a specific order. The time taken to serve a particular customer is called the service time. The service time is a statistical variable and can be studied either as the number of services completed in a given period of time or the completion period of a service.
The Queue Discipline
The most common queue discipline is the “First Come First Served” (FCFS) or “First-in, First-out” (FIFO). Situations like waiting for a haircut, ticket-booking counters follow FCFS discipline. Other disciplines include “Last In First Out” (LIFO) where last customer is serviced first, “Service In Random Order” (SIRO) in which the customers are serviced randomly irrespective of their arrivals. “Priority service” is when the customers are grouped in priority classes based on urgency. “Preemptive Priority” is the highest priority given to the customer who enters into the service, immediately, even if a customer with lower priority is in service. “Non-preemptive priority” is where the customer goes ahead in the queue, but will be served only after the completion of the current service.
The Number of Customers allowed in the System
Some of the queuing processes allow the limitation to the capacity or size of the waiting room, so that the waiting line reaches a certain length, no additional customers is allowed to enter until space becomes available by a service completion. This type of situation means that there is a finite limit to the maximum queue size.