1 welcome to coe 755: queuing theory instructor: dr. wissam f. fawaz office 103, bassil bldg. email:...
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Welcome to COE 755: Queuing Theory Instructor:
Dr. Wissam F. Fawaz
Office 103, Bassil Bldg. Email: [email protected]
Required text book: Chee Hock Ng, Queuing Modelling Fundamentals, second
edition, John Wiley & Sons, 2008.
Course website: http://www.wissamfawaz.com/queueing_theory.htm
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Course Description
Queuing Analysis is a vital tool used in evaluating system performances
its application covers A wide spectrum
from Bank automated teller machines
to transportation and communications data networks
This queuing course focuses on queuing modeling techniques
and its application in data networks
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Course synopsis The course covers
Discrete/continuous random variables
Birth and death process
Machine Interference Problem
Hypo/Hyper exponential distributions
Markovian/semi-Markovian Queueing Systems
Discrete and Continuous Markov Processes
Queueing Networks
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Course learning objectives Students are expected to be able:
gain a mastery of Markovian queuing systems
construct queuing models for engineering problems and solve queuing problems
become familiar with the derivation techniques used to measure system performance
work with an emerging class of arrival processes Markov-modulated arrival process
use QNAP (Queuing Network Analysis Package) software To solve real world network related problems
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Assessment & grading
Topic presentation/Homeworks 15%
Simulation projects 15%
Exam I 20%
Exam II 20%
Final 30%
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Queuing definitions Kleinrock
“We study the phenomena of standing, waiting, and serving, and we call this study Queueing Theory.”
Bose “The basic phenomenon of queuing arises whenever a shared facility
needs to be accessed for service by a large number of jobs or
customers.”
Takagi “A queue is formed when service requests arrive at a service facility
and are forced to wait while the server is busy working on other requests”
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Wrap up
The study of queuing is the study of waiting
Customers may wait on a line
Planes may wait in a holding pattern
Jobs may wait for the attention of the CPU in an computer
Packets may wait in the buffer of a node in a computer network
Telephone calls may wait to get through an exchange
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Real life queuing examples
Call center service level Service statement (Regie (public service) du Quebec)
If you call the Regie
The waiting time to speak with an information clerk is 30 seconds in 75% of cases
and you will never wait more than 3 minutes
This is one of the commitments that the Regie respect 95% of the time
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History lesson History goes back to primitive man
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History of queuing theory
Markov Analysis
Andrey A. Markov (born 1856). Created the Markov models and produced first results for these processes in 1906
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History of queuing theory
Erlang Analysis
Agner Erlang (in 1909): A Danish engineer in the Copenhagen Telephone Exchange,published his first paper on queuing theory
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Course prerequisite
I assume That you are familiar with the pre-requisite knowledge
Probability theory
Transform theory
And matrices
Still, I will review some essential background knowledge
of certain related disciplines
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Introduction to probability theory
Example 1 Suppose that we toss 2 coins
What is the probability that the first coin falls heads?
What is the probability that the second coin falls heads?
What is the probability that either the first or the second coin falls heads?
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Solution Sample space (Ω)
collection of all possible outcomes
Ω = {(H, H), (H,T), (T, H), (T, T)}
Possible Events The first coin falls heads
E = {(H, H), (H, T)} => Pr (E) = 1/2
The second coin falls heads F = {(H, H), (T, H)} => Pr (F) = 1/2
: the event that either the first or second coin falls heads
FE
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Probability: definition
Probability function Pr ( ) Define a mapping between Ω and set of real numbers
Assigning a number to each event E in Ω
Furthermore, Pr must satisfy, for all A, B in Ω Pr(A) ≥ 0 - probability is a positive measure
Pr(A) = (cardinality of A)/(cardinality of Ω)
Pr(Ω) = 1 – probability is a finite measure
A, B disjoint events => )Pr()Pr()Pr( BABA
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Properties
Set of properties Property 1
Pr (impossible event) = 0
Property 2 If A B => Pr(A) <= Pr(B)
Property 3 Pr(Ac) = 1 – Pr(A)
Property 4: )Pr()Pr()Pr()Pr( BABABA
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Combinatorics
Permutations K-permutation of a set of n elements
Combinations K-combination of a set of n elements
=> k-permutation / k! (where k! is the number of possible ways to permute that combination)
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Combinatorics (cont’d)
Binomial coefficients
Binomial expansion
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Conditional probability
It enables us to determine whether 2 events A, and B
are related in the sense That knowledge about the occurrence of one
alters the likelihood of occurrence of the other
Probability of A given B has occurred =>
)Pr(
)Pr()|Pr(
B
BABA
)Pr().|Pr()Pr( BBABA
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Conditional probability: Example
Example 2 A family has two children
What is the conditional probability that both are boys Given that at least one of them is a boy
Assumptions: Sample space S = {(b,b), (b,g), (g,b), (g,g)}
All outcomes are equally likely For instance, (b,g) means the older child is a boy and the
younger child is a girl
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Solution
Let B denote the event that both children are boys
Let A denote the event that at least one of them is a boy
=>
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1
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)}),(),,(),,Pr({(
)}),Pr({(
)Pr(
)Pr()|Pr(
bggbbb
bb
A
ABAB
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Independent events
Two events A and B are independent if Pr(A|B) = Pr(A)
Example 3 Suppose we roll two fair dice. Let E1 denote the event
that the sum of the dice is six and F denote the event that the first die equals four.
)Pr().Pr()Pr( BAAB
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Analysis of the example 3
P(E1∩F) = Pr({(4,2)}) = 1/36
P(E1)P(F) = 5/36 x 1/6 = 5/216
=> E1 and F are not independent
Reason Our chance of getting a total of six
Depends on the outcome of the first die
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Example of independent events
Let E2 be the event that the sum of dice = 7
Is E2 independent of F?
The answer is YES since: P(E2∩F) = Pr({(4,3)}) = 1/36
P(E2)P(F) = 1/6 x 1/6 = 1/36
Equivalent
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Conditional probabilities and Independence
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Total probability
Let B1, B2, …, and Bn be mutually exclusive events
Disjoint events whose union equals Ω (partition of sample space)
Then, probability of any given event A in Ω )Pr().|Pr(...)Pr().|Pr()Pr().|Pr()Pr( 2211 nn BBABBABBAA
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Baye’s formula
Let {B1, B2, …, Bn } be a partition of a sample space The problem is to calculate P(Bi) given A has occurred
it enables us to calculate a conditional probability When we know the reverse conditional probabilities
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Baye’s formula: example Example 4
Three cards with different colours on different sides
The upper side of a randomly drawn card is red What is the probability that the other side is blue?
Solution (using Baye’s formula)