# Queuing Discipline | Elements of Queuing System | BSc.CSIT | Fifth Semester

## Queuing Discipline, a elements of queuing system Simulation and Modeling Reference Notes Fifth Semester | Third year BSc.CSIT | Tribhuvan University (TU)

Queuing Discipline
The third required component of a queuing system is a queue, in which entities wait for service. The number of customer can wait in a line is called system capacity. The simplest case is an unlimited queue which can accommodate any number of customers. It is called system with unlimited capacity. But many systems (e.g., phone exchanges, web servers, call centers), have limits on the number of entities that can be in queue at any given time.

Arrivals that come when the queue is full are rejected (e.g., customers get a busy signal when trying to dial into a call center) even if the system doesn’t have a strict limit on the queue size.

The logical ordering of customer in a waiting line is called Queuing discipline and it determines which customer will be chosen for service. We may say that queuing discipline is a rule to choose the customer for service from the waiting line.

The queuing discipline includes:

1. First in First out (FIFO): According to this rule, Service is offered on the basis of arrival time of customer. The customer who comes first will get the service first. So in other word the customer who get the service next will be determine on the basis of longest waiting time.
2. Last in First out (LIFO): It is usually abbreviated as LIFO, occurs when service is next offered to the customer that arrived recently or which have waiting time least. In the crowded train the passenger getting in or out from the train is an example of LIFO.
3. Service in Random order (SIRO): it means that a random choice is made between all waiting customers at the time service is offered. I.e. a customer is picked up randomly form the waiting queue for the service.
4. Shortest processing time First (SPT): it means that the customer with shortest service time will be chosen first for the service. I.e. the shortest service time customer will get the priority in the selection process.
5. Priority: a special number is assigned to each customer in the waiting line and it is called priority. Then according to this number, the customer is chosen for service.
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