Queueing theory is the mathematical study of waiting lines, or queues.^{[1]} In queueing theory a model is constructed so that queue lengths and waiting times can be predicted.^{[1]} Queueing theory is generally considered a branch of operations research because the results are often used when making business decisions about the resources needed to provide a service.
Queueing theory has its origins in research by Agner Krarup Erlang when he created models to describe the Copenhagen telephone exchange.^{[1]} The ideas have since seen applications including telecommunication, traffic engineering, computing^{[2]} and the design of factories, shops, offices and hospitals.^{[3]}^{[4]}
Contents

Single queueing nodes 1

Service disciplines 2

Queueing networks 3

Mean field limits 4

Fluid limits 5

Heavy traffic/diffusion approximations 6

Software for simulation/analysis 7

See also 8

References 9

Further reading 10

External links 11
Single queueing nodes
Single queueing nodes are usually described using Kendall's notation in the form A/S/C where A describes the time between arrivals to the queue, S the size of jobs and C the number of servers at the node.^{[5]}^{[6]} Many theorems in queue theory can be proved by reducing queues to mathematical systems known as Markov chains, first described by Andrey Markov in his 1906 paper.^{[7]}
Agner Krarup Erlang, a Danish engineer who worked for the Copenhagen Telephone Exchange, published the first paper on what would now be called queueing theory in 1909.^{[8]}^{[9]}^{[10]} He modeled the number of telephone calls arriving at an exchange by a Poisson process and solved the M/D/1 queue in 1917 and M/D/k queueing model in 1920.^{[11]} In Kendall's notation

M stands for Markov or memoryless and means arrivals occur according to a Poisson process

D stands for deterministic and means jobs arriving at the queue require a fixed amount of service

k describes the number of servers at the queueing node (k = 1, 2,...). If there are more jobs at the node than there are servers then jobs will queue and wait for service.
The M/M/1 queue is a simple model where a single server serves jobs that arrive according to a Poisson process and have exponentially distributed service requirements. In an M/G/1 queue the G stands for general and indicates an arbitrary probability distribution. The M/G/1 model was solved by Felix Pollaczek in 1930, a solution later recast in probabilistic terms by Aleksandr Khinchin and now known as the Pollaczek–Khinchine formula.^{[11]}
After World War II queueing theory became an area of research interest to mathematicians.^{[11]}^{[12]} Work on queueing theory used in modern packet switching networks was performed in the early 1960s by Leonard Kleinrock. It was in this period that John Little gave a proof of the formula which now bears his name: Little's law.^{[13]} In 1961 John Kingman gave a formula for the mean waiting time in a G/G/1 queue: Kingman's formula.^{[14]}
The matrix geometric method and matrix analytic methods have allowed queues with phasetype distributed interarrival and service time distributions to be considered.^{[15]}
Problems such as performance metrics for the M/G/k queue remain an open problem.^{[11]}
Service disciplines
Various scheduling policies can be used at queuing nodes:

First in first out

This principle states that customers are served one at a time and that the customer that has been waiting the longest is served first.^{[16]}

Last in first out

This principle also serves customers one at a time, however the customer with the shortest waiting time will be served first.^{[16]} Also known as a stack.

Processor sharing

Service capacity is shared equally between customers.^{[16]}

Priority

Customers with high priority are served first.^{[16]} Priority queues can be of two types, nonpreemptive (where a job in service cannot be interrupted) and preemptive (where a job in service can be interrupted by a higher priority job). No work is lost in either model.^{[17]}

Shortest job first

The next job to be served is the one with the smallest size

Preemptive shortest job first

The next job to be served is the one with the original smallest size^{[18]}

Shortest remaining processing time

The next job to serve is the one with the smallest remaining processing requirement.^{[19]}
Queueing networks
Networks of queues are systems in which a number of queues are connected by customer routing. When a customer is serviced at one node it can join another node and queue for service, or leave the network. For a network of m the state of the system can be described by an m–dimensional vector (x_{1},x_{2},...,x_{m}) where x_{i} represents the number of customers at each node. The first significant results in this area were Jackson networks,^{[20]}^{[21]} for which an efficient productform stationary distribution exists and the mean value analysis^{[22]} which allows average metrics such as throughput and sojourn times to be computed.^{[23]}
If the total number of customers in the network remains constant the network is called a closed network and has also been shown to have a product–form stationary distribution in the Gordon–Newell theorem.^{[24]} This result was extended to the BCMP network^{[25]} where a network with very general service time, regimes and customer routing is shown to also exhibit a productform stationary distribution.
Networks of customers have also been investigated, Kelly networks where customers of different classes experience different priority levels at different service nodes.^{[26]}
Another type of network are Gnetworks first proposed by Erol Gelenbe in 1993:^{[27]} these networks do not assume exponential time distributions like the classic Jackson Network.
Routing algorithms
In discrete time networks where there is a constraint on which service nodes can be active at any time, the maxweight scheduling algorithm chooses a service policy to give optimal throughput in the case that each job visits only a single service node. In the more general case where jobs can visit more than one node, backpressure routing gives optimal throughput.
A network scheduler must choose a queuing algorithm, which affects the characteristics of the larger network.
Mean field limits
Mean field models consider the limiting behaviour of the empirical measure (proportion of queues in different states) as the number of queues (m above) goes to infinity. The impact of other queues on any given queue in the network is approximated by a differential equation. The deterministic model converges to the same stationary distribution as the original model.^{[28]}
Fluid limits
Fluid models are continuous deterministic analogs of queueing networks obtained by taking the limit when the process is scaled in time and space, allowing heterogeneous objects. This scaled trajectory converges to a deterministic equation which allows us stability of the system to be proven. It is known that a queueing network can be stable, but have an unstable fluid limit.^{[29]}
Heavy traffic/diffusion approximations
In a system with high occupancy rates (utilisation near 1) a heavy traffic approximation can be used to approximate the queueing length process by a reflected Brownian motion,^{[30]} Ornstein–Uhlenbeck process or more general diffusion process.^{[31]} The number of dimensions of the RBM is equal to the number of queueing nodes and the diffusion is restricted to the nonnegative orthant.
Software for simulation/analysis

Java Modelling Tools, a GPL suite of queueing theory tools written in Java^{[32]}

Queueing Package for GNU Octave^{[33]}
See also
References

^ ^{a} ^{b} ^{c} Sundarapandian, V. (2009). "7. Queueing Theory". Probability, Statistics and Queueing Theory. PHI Learning.

^ Lawrence W. Dowdy, Virgilio A.F. Almeida, Daniel A. Menasce (Thursday Janery 15, 2004). "Performance by Design: Computer Capacity Planning By Example". p. 480.

^ Schlechter, Kira (March 2, 2009). "Hershey Medical Center to open redesigned emergency room". The PatriotNews.

^ Mayhew, Les; Smith, David (December 2006). Using queuing theory to analyse completion times in accident and emergency departments in the light of the Government 4hour target.

^ Tijms, H.C, Algorithmic Analysis of Queues", Chapter 9 in A First Course in Stochastic Models, Wiley, Chichester, 2003

^

^ A.A. Markov, Extension of the law of large numbers to dependent quantities, Izvestiia Fiz.Matem. Obsch. Kazan Univ., (2nd Ser.), 15(1906), pp. 135–156 [Also [37], pp. 339–361].

^ "Agner Krarup Erlang (1878  1929)  plus.maths.org". Pass.maths.org.uk. Retrieved 20130422.

^ Asmussen, S. R.;

^ "The theory of probabilities and telephone conversations". Nyt Tidsskrift for Matematik B 20: 33–39. 1909.

^ ^{a} ^{b} ^{c} ^{d}

^

^

^

^ Ramaswami, V. (1988). "A stable recursion for the steady state vector in markov chains of m/g/1 type". Communications in Statistics. Stochastic Models 4: 183–188.

^ ^{a} ^{b} ^{c} ^{d} Penttinen A., Chapter 8 – Queueing Systems, Lecture Notes: S38.145  Introduction to Teletraffic Theory.

^ HarcholBalter, M. (2012). "Performance Modeling and Design of Computer Systems". p. 499.

^ HarcholBalter, M. (2012). "Performance Modeling and Design of Computer Systems". p. 508.

^ HarcholBalter, M. (2012). "Performance Modeling and Design of Computer Systems". p. 518.

^

^ Jackson, James R. (Oct 1963). "Jobshoplike Queueing Systems".

^ Reiser, M.; Lavenberg, S. S. (1980). "MeanValue Analysis of Closed Multichain Queuing Networks".

^ Van Dijk, N. M. (1993). "On the arrival theorem for communication networks". Computer Networks and ISDN Systems 25 (10): 1135–2013.

^ Gordon, W. J.;

^ Baskett, F.;

^

^

^ Bobbio, A.; Gribaudo, M.; Telek, M. S. (2008). "2008 Fifth International Conference on Quantitative Evaluation of Systems". p. 215.

^ Bramson, M. (1999). "A stable queueing network with unstable fluid model". The Annals of Applied Probability 9 (3): 818.

^ Chen, H.; Whitt, W. (1993). "Diffusion approximations for open queueing networks with service interruptions".

^ Yamada, K. (1995). "Diffusion Approximation for Open StateDependent Queueing Networks in the Heavy Traffic Situation". The Annals of Applied Probability 5 (4): 958.

^ Bertoli, Marco; Casale, Giuliano; Serazzi, Giuseppe (2009). "JMT: performance engineering tools for system modeling". SIGMETRICS Perform. Eval. Rev. 36: 10–15.

^ Marzolla, Moreno. "The queueing Package". Retrieved 31 July 2014.
Further reading

Gross, Donald; Carl M. Harris (1998). Fundamentals of Queueing Theory. Wiley.

Deitel, Harvey M. (1984) [1982]. An introduction to operating systems (revisited first ed.). AddisonWesley. p. 673. chap.15, pp. 380–412

Lazowska, Edward D.; John Zahorjan; G. Scott Graham; Kenneth C. Sevcik (1984). Quantitative System Performance: Computer System Analysis Using Queueing Network Models. PrenticeHall, Inc.

Zukerman, Moshe. Introduction to Queueing Theory and Stochastic Teletraffic Models.
External links

Queueing theory calculator

Teknomo's Queueing theory tutorial and calculators

Virtamo's Queueing Theory Course

Myron Hlynka's Queueing Theory Page

Queueing Theory Basics

A free online tool to solve some classical queueing systems
Queueing theory


Single queueing nodes



Arrival processes



Queueing networks



Service policies



Key concepts



Limit theorems



Extensions





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