Simulation chapter 4

Chapter - 4
Queueing Models
System Simulation and Modeling
Lecture Notes
From Banks, Carson, Nelson & Nicol
Discrete-Event System Simulation

2
Introduction
 Each one of us has spent a great deal of time waiting
in lines.
 One example in the Cafeteria.
 Other examples of queues are
 Printer queue
 Customers in front of a cashier
 Calls waiting for answer by a technical support

3
Purpose
 Simulation is often used in the analysis of queueing models.
 A simple but typical queueing model:
 Queueing models provide the analyst with a powerful tool for
designing and evaluating the performance of queueing systems.
 Typical measures of system performance:
 Server utilization, length of waiting lines, and delays of
customers
 For relatively simple systems, compute mathematically
 For realistic models of complex systems, simulation is
usually required.

4
Outline
 Discuss some well-known models (not development
of queueing theories):
General characteristics of queues,
Meanings and relationships of important
performance measures,
Estimation of mean measures of performance.
Effect of varying input parameters,
Mathematical solution of some basic queueing
models.

5
Characteristics of Queueing Systems
 Key elements of queueing systems:
Customer: refers to anything that arrives at a
facility and requires service, e.g., people, machines,
trucks, emails.
Server: refers to any resource that provides the
requested service, e.g., repairpersons, retrieval
machines, runways at airport.

6
Calling Population
[Characteristics of Queueing System]
 Calling population: the population of potential
customers, may be assumed to be finite or infinite.
Finite population model: if arrival rate depends on
the number of customers being served and waiting.
 e.g., model of one corporate jet, if it is being
repaired, the repair arrival rate becomes zero.
Infinite population model: if arrival rate is not
affected by the number of customers being served and
waiting.
 e.g., systems with large population of potential
customers.

7
System Capacity
 System Capacity: a limit on the number of customers
that may be in the waiting line or system.
Limited capacity, e.g., an automatic car wash only
has room for 10 cars to wait in line to enter the
mechanism.
Unlimited capacity, e.g., concert ticket sales with
no limit on the number of people allowed to wait to
purchase tickets.

8
Arrival Process
 For infinite-population models:
 In terms of interarrival times of successive customers.
 Random arrivals: interarrival times usually characterized by a probability
distribution.
 Most important model: Poisson arrival process (with rate λ), where An
represents the interarrival time between customer n-1 and customer n, and
is exponentially distributed (with mean 1/λ).
 Scheduled arrivals: interarrival times can be constant or constant plus or
minus a small random amount to represent early or late arrivals.
 e.g., patients to a physician or scheduled airline flight arrivals to an airport.
 At least one customer is assumed to always be present, so the server is
never idle, e.g., sufficient raw material for a machine

9
Arrival Process
 For finite-population models:
 Customer is pending when the customer is outside the queueing system,
e.g., machine-repair problem: a machine is “pending” when it is
operating, it becomes “not pending” the instant it demands service form
the repairman.
 Runtime of a customer is the length of time from departure from the
queueing system until that customer’s next arrival to the queue, e.g.,
machine-repair problem, machines are customers and a runtime is time
to failure.
 Let A1
(i), A2
(i), … be the successive runtimes of customer i, and S1
(i), S2
(i)
be the corresponding successive system times:

10
Queue Behavior and Queue Discipline
 Queue behavior: the actions of customers while in a queue
waiting for service to begin, for example:
 Balk: leave when they see that the line is too long,
 Renege: leave after being in the line when its moving too slowly,
 Jockey: move from one line to a shorter line.
 Queue discipline: the logical ordering of customers in a queue
that determines which customer is chosen for service when a
server becomes free, for example:
 First-in-first-out (FIFO)
 Last-in-first-out (LIFO)
 Service in random order (SIRO)
 Shortest processing time first (SPT)
 Service according to priority (PR).

11
Service Times and Service Mechanism
 Service times of successive arrivals are denoted by S1, S2, S3.
 May be constant or random.
 {S1, S2, S3, …} is usually characterized as a sequence of
independent and identically distributed random variables,
e.g., exponential, Weibull, gamma, lognormal, and truncated
normal distribution.
 A queueing system consists of a number of service centers and
interconnected queues.
 Each service center consists of some number of servers, c,
working in parallel, upon getting to the head of the line, a
customer takes the 1st available server.

12
 Example: consider a discount warehouse where customers
may:
 Serve themselves before paying at the cashier:

13
 Wait for one of the three clerks:
 Batch service (a server serving several customers
simultaneously), or customer requires several servers
simultaneously.

14
Queueing Notation
 A notation system for parallel server queues: A/B/c/N/K
 A represents the interarrival-time distribution,
 B represents the service-time distribution,
 c represents the number of parallel servers,
 N represents the system capacity,
 K represents the size of the calling population.
 Common symbols for A and B include M (exponential or Markov), D (constant
or deterministic), G (arbitrary or general).
 For example, M /M /1/ ∞ / ∞ indicates a single-server system that has unlimited
queue capacity and an infinite population of potential arrivals. The interarrival
times and service times are exponentially distributed.
 When N and K are infinite, they may be dropped from the notation. For
example, M/M/1 / ∞ / ∞ is often shortened to M/M/1 .

15
Queueing Notation
 Primary performance measures of queueing systems:
 Pn: steady-state probability of having n customers in system,
 Pn(t): probability of n customers in system at time t,
 λ : arrival rate,
 λe: effective arrival rate,
 μ : service rate of one server,
 ρ server utilization,
 An: interarrival time between customers n-1 and n,
 Sn: service time of the nth arriving customer,
 Wn: total time spent in system by the nth arriving customer,
 Wn
Q: total time spent in the waiting line by customer n,
 L(t): the number of customers in system at time t,
 LQ(t): the number of customers in queue at time t,
 L: long-run time-average number of customers in system,
 LQ: long-run time-average number of customers in queue,
 w: long-run average time spent in system per customer,
 wQ: long-run average time spent in queue per customer.

16
Time-Average Number in System L
 There are two types of estimators: an ordinary sample average,
and a time-integrated (or time-weighted) sample average
 Consider a queueing system over a period of time T,
 Let Ti denote the total time during [0,T] in which the system
contained exactly i customers, the time-weighted-average
number in a system is defined by:
 Consider the total area under the function is L(t), then,
 The long-run time-average # in system, with probability 1:












00
1ˆ
i
i
i
i
T
T
iiT
T
L
 


T
i
i dttL
T
iT
T
L
0
0
)(
11ˆ
  TLdttLL
T
as)(
1ˆ

 The time-weighted-average number in queue is:
 G/G/1/N/K example: consider the results from the queueing system.
17
 


TLdttL
T
iT
T
L Q
T
Q
i
Q
iQ as)(
11ˆ
0
0

18






1if,1)(
0if,0
)(
L(t)tL
L(t)
tLQ
customers3.0
20
)1(2)4(1)15(0ˆ 

QL
cusomters15.120/23
20/)]1(3)4(2)12(1)3(0[ˆ

L

19
Average Time Spent in System Per
Customer w [Characteristics of Queueing System]
 The average time spent in system per customer, called the
average system time, is:
where W1, W2, …, WN are the individual times that each of the N
customers spend in the system during [0,T].
 For stable systems:
 If the system under consideration is the queue alone:
 G/G/1/N/K example (cont.): the average system time is
 Nww asˆ


N
i
iW
N
w
1
1
ˆ
1
1
ˆ as
N
Q
Q i Q
i
w W w N
N 
   
unitstime6.4
5
)1620(...)38(2
5
...
ˆ 521





WWW
w

Average Time Spent in System Per
Customer w [Characteristics of Queueing System]
20

21
The Conservation Equation
 Conservation equation (a.k.a. Little’s law)
 It says that the average number of customers in the system at an
arbitrary point in time is equal to the average number of arrivals
per time unit, times the average time spent in the system.
 Holds for almost all queueing systems or subsystems (regardless of the
number of servers, the queue discipline, or other special circumstances).
 G/G/1/N/K example (cont.): On average, one arrival every 4 time units
and each arrival spends 4.6 time units in the system. Hence, at an
arbitrary point in time, there is (1/4)(4.6) = 1.15 customers present on
average.
wL ˆˆˆ 
 NTwL andas
Arrival rate
Average
System time
Average # in
system

22
Server Utilization
 Definition: the proportion of time that a server is busy.
 Observed server utilization, , is defined over a specified
time interval [0,T].
 Long-run server utilization is ρ .
 For systems with long-run stability:
 assuming that the system has a single server, it can be seen that
the server utilization is = (total busy time)/T
 Tasˆ 
ˆ

23
Server Utilization
 For G/G/1/∞/∞ queues:
 Any single-server queueing system with average arrival rate λ
customers per time unit, where average service time E(S) = 1/ μ
time units, infinite queue capacity and calling population.
 Conservation equation, L = λw, can be applied.
 For a stable system, the average arrival rate to the server, λs, must
be identical to λ.
 The average number of customers in the server is:
  T
TT
dttLtL
T
L
T
Qs
0
0
)()(
1ˆ 
 

24
Server Utilization
 In general, for a single-server queue:
 For a single-server stable queue:
 For an unstable queue , long-run server utilization
is 1.






)(and
asˆˆ
sE
TLL ss
1




25
Server Utilization
 For G/G/c/∞/∞ queues:
 A system with c identical servers in parallel.
 If an arriving customer finds more than one server idle, the
customer chooses a server without favoring any particular
server.
 For systems in statistical equilibrium, the average number of
busy servers, Ls, is: Ls, = E(s) = /.
 The long-run average server utilization is:
systemsstableforwhere, 


 c
cc
Ls


Server Utilization [Characteristics of Queueing System]
 Example: Customers arrive at random to a license bureau at a rate of 50
customers per hour. Currently, there are 20-clerks, each serving 5
customers per hour on the average. The long-run; or steady-state, average
utilization of a server, given by
26

27
Server Utilization and System Performance
 System performance varies widely for a given utilization .
 For example, a D/D/1 queue where E(A) = 1/ and E(S) =
1/, where:
L =  = /, w = E(S) = 1/, LQ = WQ = 0.
 By varying  and , server utilization can assume any value
between 0 and 1.
 Yet there is never any line.
 In general, variability of interarrival and service times
causes lines to fluctuate in length.

28
Server Utilization and System Performance
 Example: A physician who schedules patients every 10 minutes and
spends Si minutes with the ith patient:
 Arrivals are deterministic, A1 = A2 = … = -1 = 10.
 Services are stochastic, E(Si) = 9.3 min and V(S0) = 0.81 min2.
 On average, the physician's utilization =   / = 0.93 < 1.
 Consider the system is simulated with service times: S1 = 9, S2 = 12,
S3 = 9, S4 = 9, S5 = 9, …. The system becomes:
 The occurrence of a relatively long service time (S2 = 12) causes a
waiting line to form temporarily.




1.0yprobabilitwithminutes12
9.0yprobabilitwithminutes9
iS

29
Costs in Queueing Problems
 Costs can be associated with various aspects of the
waiting line or servers:
 System incurs a cost for each customer in the queue, say at a rate
of $10 per hour per customer.
 The average cost per customer is:
 If customers per hour arrive (on average), the average cost
per hour is:
 Server may also impose costs on the system, if a group of c
parallel servers (1  c  ∞) have utilization r, each server imposes
a cost of $5 per hour while busy.
 The total server cost is: $5*c.
Q
N
j
Q
j
w
N
W
ˆ*10$
*10$
1

Wj
Q is the time
customer j spends
in queue
ˆ
hour/ˆ*10$ˆˆ*10$
customer
ˆ*10$
hour
customerˆ
QQ
Q
Lw
w
















30
Steady-State Behavior of Infinite-Population
Markovian Models
 Markovian models: exponential-distribution arrival process
(mean arrival rate = ).
 Service times may be exponentially distributed as well (M) or
arbitrary (G).
 A queueing system is in statistical equilibrium if the probability
that the system is in a given state is not time dependent:
P( L(t) = n ) = Pn(t) = Pn.
 Mathematical models in this chapter can be used to obtain
approximate results even when the model assumptions do not
strictly hold (as a rough guide).
 Simulation can be used for more refined analysis (more faithful
representation for complex systems).

31
Steady-State Behavior of Infinite-Population
Markovian Models
 For the simple model studied in this chapter, the steady-state
parameter, L, the time-average number of customers in the
system is:
 Apply Little’s equation to the whole system and to the queue alone:
 G/G/c/∞/∞ example: to have a statistical equilibrium, a
necessary and sufficient condition is /(c) < 1.




0n
nnPL
QQ
Q
wL
ww
L
w




1
,

32
M/G/1 Queues [Steady-State of Markovian Model]
 Single-server queues with Poisson arrivals & unlimited capacity.
 Suppose service times have mean 1/ and variance s2 and  = /
< 1, the steady-state parameters of M/G/1 queue:
)1(2
)/1(
,
)1(2
)/1(1
)1(2
)1(
,
)1(2
)1(
1,/
2222
222222
0

s

s


s

s















Q
Q
ww
LL
P

33
 No simple expression for the steady-state probabilities P0, P1, …
 L – LQ =  is the time-average number of customers being
served.
 Average length of queue, LQ, can be rewritten as:
 If  and  are held constant, LQ depends on the variability, s2, of the
service times.
)1(2)1(2
222

s





QL

Example:
There are two workers competing for a job. Able claims an average
service time that is faster than Baker's, but Baker claims to be more
consistent, even if not as fast. The arrivals occur according to a
Poisson process at the rate A, = 2 per hour (l/30 per minute). Able's
service statistics are an average service time of 24 minutes with a
standard deviation of 20 minutes. Baker's service statistics are an
average service time of 25 minutes, but a standard deviation of only
2 minutes. If the average length of the queue is the criterion for
hiring, which worker should be hired?
34

35
 Poisson arrivals at rate  = 2 per hour (1/30 per minute).
 Able: 1/ = 24 minutes and s2 = 202 = 400 minutes2:
 The proportion of arrivals who find Able idle and thus experience no delay is P0
= 1- = 1/5 = 20%.
 Baker: 1/ = 25 minutes and s2 = 22 = 4 minutes2:
 The proportion of arrivals who find Baker idle and thus experience no delay is
P0 = 1- = 1/6 = 16.7%.
 Although working faster on average, Able’s greater service variability
results in an average queue length greater than Baker’s.
customers711.2
)5/41(2
]40024[)30/1( 22



QL
customers097.2
)6/51(2
]425[)30/1( 22



QL

36
M/M/1 Queues [Steady-State of Markovian Model]
 Suppose the service times in an M/G/1 queue are
exponentially distributed with mean 1/, then the variance
is s2 = 1/2.
 M/M/1 queue is a useful approximate model when service
times have standard deviation approximately equal to their
means.
 The steady-state parameters:
 
 
  )1(
,
)1(
11
1
,
1
1,/
22































Q
Q
n
n
ww
LL
P

37
M/M/1 Queues [Steady-State of Markovian Model]
 Example: M/M/1 queue with service rate 10 customers
per hour.
 Consider how L and w increase as arrival rate, , increases from 5
to 8.64 by increments of 20%:
 If /  1, waiting lines tend to continually grow in length.
 Increase in average system time (w) and average number in
system (L) is highly nonlinear as a function of .
 5.0 6.0 7.2 8.64 10.0
 0.500 0.600 0.720 0.864 1.000
L 1.00 1.50 2.57 6.35 ∞
w 0.20 0.25 0.36 0.73 ∞

38
Effect of Utilization and Service Variability
[Steady-State of Markovian Model]
 For almost all queues, if lines are too long, they can be reduced
by decreasing server utilization () or by decreasing the service
time variability (s2).
 A measure of the variability of a distribution, coefficient of
variation (cv):
 The larger cv is, the more variable is the distribution relative to its
expected value
 2
2
)(
)(
)(
XE
XV
cv 

39
Effect of Utilization and Service Variability
[Steady-State of Markovian Model]
 Consider LQ for any M/G/1
queue:







 













2
)(1
1
)1(2
)1(
22
222
cv
LQ



s
LQ for M/M/1
queue
Corrects the M/M/1
formula to account
for a non-exponential
service time dist’n

40
Multiserver Queue [Steady-State of Markovian Model]
 M/M/c/∞/∞ queue: c channels operating in parallel.
 Each channel has an independent and identical exponential
service-time distribution, with mean 1/.
 To achieve statistical equilibrium, the offered load (/) must satisfy
/ < c, where /(c) =  is the server utilization.
 Some of the steady-state probabilities:
 












L
w
cLP
c
cc
Pc
cL
c
c
cn
P
c
c
cc
n
n

























































1
)(
)1)(!(
)(
!
1
!
)/(
/
2
0
1
1
1
0
0

41
Multiserver Queue [Steady-State of Markovian Model]
 Other common multiserver queueing models:
 M/G/c/∞: general service times and c parallel server. The
parameters can be approximated from those of the M/M/c/∞/∞
model.
 M/G/∞: general service times and infinite number of servers, e.g.,
customer is its own system, service capacity far exceeds service
demand.
 M/M/C/N/∞: service times are exponentially distributed at rate m
and c servers where the total system capacity is N  c customer
(when an arrival occurs and the system is full, that arrival is turned
away).

42
Steady-State Behavior of Finite-Population
Models
 When the calling population is small, the presence of one or
more customers in the system has a strong effect on the
distribution of future arrivals.
 Consider a finite-calling population model with K customers
(M/M/c/K/K):
 The time between the end of one service visit and the next call for
service is exponentially distributed, (mean = 1/).
 Service times are also exponentially distributed.
 c parallel servers and system capacity is K.

43
Models
 Some of the steady-state probabilities:









cLwnPL
Kccn
ccnK
K
cnP
n
K
P
ccnK
K
n
K
P
ee
K
n
n
n
cn
n
n
K
cn
n
cn
c
n
n
/,/,
,...1,,
!)!(
!
1,...,1,0,
!)!(
!
0
0
1
1
0
0





















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


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
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

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
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

















K
n
ne
e
PnK
0
)(
service)xitingentering/e(orqueuetocustomersofratearrivaleffectiverunlongtheiswhere



44
Models
 Example: two workers who are responsible for10 milling
machines.
 Machines run on the average for 20 minutes, then require an
average 5-minute service period, both times exponentially
distributed:  = 1/20 and  = 1/5.
 All of the performance measures depend on P0:
 Then, we can obtain the other Pn.
 Expected number of machines in system:
 The average number of running machines:
065.0
20
5
2!2)!10(
!10
20
510
1
10
2
2
12
0
0 
























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







 n
n
n
n
n
nn
P
machines17.3
10
0
 n
nnPL
machines83.617.310  LK

45
Networks of Queues
 Many systems are naturally modeled as networks of single
queues: customers departing from one queue may be routed
to another.
 The following results assume a stable system with infinite
calling population and no limit on system capacity:
 Provided that no customers are created or destroyed in the
queue, then the departure rate out of a queue is the same as the
arrival rate into the queue (over the long run).
 If customers arrive to queue i at rate i, and a fraction 0  pij  1 of
them are routed to queue j upon departure, then the arrival rate
form queue i to queue j is ipij (over the long run).

46
Networks of Queues
 The overall arrival rate into queue j:
 If queue j has cj < ∞ parallel servers, each working at rate j, then
the long-run utilization of each server is j=j/(cj) (where j < 1
for stable queue).
 If arrivals from outside the network form a Poisson process with
rate aj for each queue j, and if there are cj identical servers
delivering exponentially distributed service times with mean 1/j,
then, in steady state, queue j behaves likes an M/M/cj queue with
arrival rate

i
ijijj pa
all

Arrival rate
from outside
the network
Sum of arrival rates
from other queues in
network

i
ijijj pa
all


47
Network of Queues
 Discount store example:
 Suppose customers arrive at the rate 80 per hour and 40%
choose self-service. Hence:
 Arrival rate to service center 1 is 1 = 80(0.4) = 32 per hour
 Arrival rate to service center 2 is 2 = 80(0.6) = 48 per hour.
 c2 = 3 clerks and 2 = 20 customers per hour.
 The long-run utilization of the clerks is:
2 = 48/(3*20) = 0.8
 All customers must see the cashier at service center 3, the
overall rate to service center 3 is 3 = 1 + 2 = 80 per hour.
 If 3 = 90 per hour, then the utilization of the cashier is:
3 = 80/90 = 0.89

48
Summary
 Introduced basic concepts of queueing models.
 Show how simulation, and some times mathematical analysis, can
be used to estimate the performance measures of a system.
 Commonly used performance measures: L, LQ, w, wQ, , and e.
 When simulating any system that evolves over time, analyst must
decide whether to study transient behavior or steady-state behavior.
 Simple formulas exist for the steady-state behavior of some queues.
 Simple models can be solved mathematically, and can be useful in
providing a rough estimate of a performance measure.

Simulation chapter 4

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