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19.11. Mixed type system with limited queue length

Lecture



In the previous   19.11.  Mixed type system with limited queue length we considered a queuing system with a time limit in the queue. Here we consider a mixed-type system with a different kind of restriction of expectations - according to the number of applications queuing. Suppose that an application that has made all channels busy is queued only if it has less than   19.11.  Mixed type system with limited queue length applications; if the number of applications in the queue is   19.11.  Mixed type system with limited queue length (more   19.11.  Mixed type system with limited queue length it can not be), then the last arrived application in the queue does not become and leaves the system unserved. The remaining assumptions about the simplest flow of applications and the exponential distribution of the service time will remain the same.

So, there is   19.11.  Mixed type system with limited queue length -channel system with the expectation in which the number of applications queuing is limited by the number   19.11.  Mixed type system with limited queue length . We construct differential equations for the probabilities of the states of the system. Note that in this case the number of system states will be finite, since the total number of applications associated with the system cannot exceed   19.11.  Mixed type system with limited queue length (   19.11.  Mixed type system with limited queue length serviced and   19.11.  Mixed type system with limited queue length standing in line). We list the system state:

  19.11.  Mixed type system with limited queue length - all channels are free, there is no queue,

  19.11.  Mixed type system with limited queue length - one channel is busy, there is no queue,

………

  19.11.  Mixed type system with limited queue length - busy   19.11.  Mixed type system with limited queue length channels, no queues,

………

  19.11.  Mixed type system with limited queue length - busy   19.11.  Mixed type system with limited queue length channels, no queues,

  19.11.  Mixed type system with limited queue length - all busy   19.11.  Mixed type system with limited queue length channels, no queues,

  19.11.  Mixed type system with limited queue length - all busy   19.11.  Mixed type system with limited queue length channels, one application is in the queue,

………

  19.11.  Mixed type system with limited queue length - all busy   19.11.  Mixed type system with limited queue length channels,   19.11.  Mixed type system with limited queue length applications standing in line.

Obviously, the first   19.11.  Mixed type system with limited queue length equations for probabilities   19.11.  Mixed type system with limited queue length will coincide with the Erlang equations (19.8.8). We derive the remaining equations. We have

  19.11.  Mixed type system with limited queue length ,

from where

  19.11.  Mixed type system with limited queue length .

Next, we derive the equation for   19.11.  Mixed type system with limited queue length   19.11.  Mixed type system with limited queue length

  19.11.  Mixed type system with limited queue length ,

from where

  19.11.  Mixed type system with limited queue length .

The last equation will be

  19.11.  Mixed type system with limited queue length .

Thus, the resulting system   19.11.  Mixed type system with limited queue length differential equations:

  19.11.  Mixed type system with limited queue length (19.11.1)

Consider the limiting case of   19.11.  Mixed type system with limited queue length . Equating all derivatives to zero, and considering all the probabilities constant, we obtain a system of algebraic equations

  19.11.  Mixed type system with limited queue length (19.11.2)

and additional condition:

  19.11.  Mixed type system with limited queue length . (19.11.3)

Equations (19.11.2) can be solved in the same way as we solved similar algebraic equations in previous   19.11.  Mixed type system with limited queue length . Without dwelling on this solution, we present only the final formulas:

  19.11.  Mixed type system with limited queue length   19.11.  Mixed type system with limited queue length , (19.11.4)

  19.11.  Mixed type system with limited queue length   19.11.  Mixed type system with limited queue length . (19.11.5)

The probability that an application will leave the system unattended is equal to   19.11.  Mixed type system with limited queue length of what is in line already   19.11.  Mixed type system with limited queue length applications.

It is easy to see that formulas (19.11.4) and (19.11.5) are obtained from formulas (19.10.11), (19.10.12), if we put in them   19.11.  Mixed type system with limited queue length and limit the summation by   19.11.  Mixed type system with limited queue length upper boundary   19.11.  Mixed type system with limited queue length .

Example. The simplest order flow arrives at the vehicle maintenance station with a density of   19.11.  Mixed type system with limited queue length (cars per hour). There is one room for repair. In the courtyard of the station can be at the same time, waiting for the queue, no more than three cars. Average time to repair one machine   19.11.  Mixed type system with limited queue length (hours) Determine: a) system bandwidth; b) average station downtime; c) determine how these characteristics will change if you equip a second room for repair.

Decision. We have:   19.11.  Mixed type system with limited queue length ,   19.11.  Mixed type system with limited queue length ,   19.11.  Mixed type system with limited queue length ,   19.11.  Mixed type system with limited queue length .

a) According to the formula (19.11.5), assuming   19.11.  Mixed type system with limited queue length , we find the probability that the incoming request will leave the system unattended:

  19.11.  Mixed type system with limited queue length .

Relative system bandwidth   19.11.  Mixed type system with limited queue length . Absolute bandwidth:   19.11.  Mixed type system with limited queue length (cars per hour).

b) The average proportion of time that the system will stand idle will be found by the formula (19.11.4):   19.11.  Mixed type system with limited queue length .

c) Believing   19.11.  Mixed type system with limited queue length , we will find:

  19.11.  Mixed type system with limited queue length ,

  19.11.  Mixed type system with limited queue length (i.e. about 98% of all applications will be satisfied).

  19.11.  Mixed type system with limited queue length (cars per hour).

Relative downtime:   19.11.  Mixed type system with limited queue length i.e. the equipment will stand idle for about 34% of the total time.

created: 2017-07-03
updated: 2021-03-13
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Queuing theory

Terms: Queuing theory