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7/27/2019 CHAPTER 3 Single-Server Queue
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Single-Server queueing System
Simulation is often used in the analysis of queueing models. In a simple typical queueingmodel shown in figure 1, customers arrive from time to time and join a queue or waiting
line, are eventually served, and finally leave the system.
Figure-1: Simple queueing Model
The term "customer" refers to any type of entity that can be viewed as requesting
"service" from a system.
The key elements, of a queueing system are the customers and servers.
The term "customer" can refer to people, machines, trucks, mechanics, patients—
anything that arrives at a facility and requires service.
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• The term "server" might refer to receptionists, repairpersons, CPUs in a computer, or
washing machines….any resource (person, machine, etc. which provides the requested
service.
• Table 1 lists a number of different queueing systems.
The Calling Population:-
The population of potential customers, referred to as the calling population, may be
assumed to be finite or infinite.
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For example, consider a bank of 5 machines that are curing tires. After an interval of time, a
machine automatically opens and must be attended by a worker who removes the tire and
puts an uncured tire into the machine.
The machines are the "customers", who "arrive" at the instant they automatically open. Theworker is the "server", who "serves" an open machine as soon as possible. The calling
population is finite, and consists of the five machines.
In systems with a large population of potential customers, the calling population is usually
assumed to be finite or infinite. Examples of infinite populations include the potential
customers of a restaurant, bank, etc.
The main difference between finite and infinite population models is how the arrival rate is
defined.
In an infinite-population model, the arrival rate is not affected by the number of customers
who have left the calling population and joined the queueing system.
On the other hand, for finite calling population models, the arrival rate to the queueing
system does depend on the number of customers being served and waiting.
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System Capacity:-
In many queueing systems there is a limit to the number of customers that may be in
the waiting line or system. For example, an automatic car wash may have room for only
10 cars to wait in line to enter the mechanism.
An arriving customer who finds the system full does not enter but returns immediately
to the calling population.
Some systems, such as concert ticket sales for students, may be considered as havingunlimited capacity. There are no limits on the number of students allowed to wait to
purchase tickets.
When a system has limited capacity, a distinction is made between the arrival rate (i.e.,
the number of arrivals per time unit) and the effective arrival rate (i.e., the number who
arrive and enter the system per time unit).
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A queueing system is described by its calling population, the nature of the arrivals, the
service mechanism, the system capacity, and the queueing discipline. A single-channel
queueing system is represent in figure2.
Simulation of queueing systems
Figure-2: queueing system
In the single-channel queue, the calling population is infinite; that is, if a unit leaves the
calling population and joins the waiting line or enters service, there is no change in the
arrival rate of other units that may need service.
Arrivals for service occur one at a time in a random fashion; once they join the waiting
line, they are eventually served. In addition, service times are of some random length
according to a probability distribution which does not change over time.
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The system capacity; has no limit, meaning that any number of units can wait in line.
Finally, units are served in the order of their arrival by a single server or channel.
Arrivals and services are defined by the distributions of the time between arrivals and thedistribution of service times, respectively.
For any simple single or multi-channel queue, the overall effective arrival rate must be less
than the total service rate, or the waiting line will grow without bound. When queues grow
without bound, they are termed “explosive” or unstable.
The state of the system is the number of units in the system and the status of the server,
busy or idle.
In a single –channel queueing system there are only two possible events that can affect
the state of the system. They are
the entry of a unit into the system.
the completion of service on a unit.
The queueing system includes the server, the unit being serviced, and units in the queue.
The simulation clock is used to track simulated time.
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The arrival event occurs when a unit enters the system.
If the server is busy, the unit enters the queue. If the server is idle and the queue is empty,
the unit begins service. It is not possible for the server to be idle and the queue to be
nonempty.
After the completion of a service the service may become idle or remain
busy with the next unit.
If the queue is not empty, another unit will enter the server and it will be busy.
If the queue is empty, the server will be idle after a service is completed.
It is impossible for the server to become busy if the queue is empty when a service is
completed.
Similarly, it is impossible for the server to be idle after a service is completed when the
queue is not Empty.
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