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Lecture 11 – Stochastic Processes. Topics Definitions Review of probability Realization of a stochastic process Continuous vs. discrete systems Examples Classification scheme. Basic Definitions. Stochastic process : System that changes over time in an uncertain manner

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lecture 11 stochastic processes
Lecture 11 – Stochastic Processes
  • Topics
  • Definitions
  • Review of probability
  • Realization of a stochastic process
  • Continuous vs. discrete systems
  • Examples
  • Classification scheme

J. Bard and J. W. Barnes

Operations Research Models and Methods

Copyright 2004 - All rights reserved

basic definitions
Basic Definitions

Stochastic process: System that changes over time in an uncertain manner

State: Snapshot of the system at some fixed point in time

Transition: Movement from one state to another

  • Examples
  • Automated teller machine (ATM)
  • Printed circuit board assembly operation
  • Runway activity at airport
elements of probability theory
Elements of Probability Theory

Experiment: Any situation where the outcome is uncertain.

Sample Space,S:All possible outcomes of an experiment (we will call it “state space”).

Event:Any collection of outcomes (points) in the sample space. A collection of events E1, E2,…,En is said to be mutually exclusive if EiEj =  for all i ≠ j = 1,…,n.

Random Variable: Function or procedure that assigns a real number to each outcome in the sample space.

Cumulative Distribution Function (CDF),F(·): Probability distribution function for the random variable X such that

F(a) = Pr{X ≤ a}.

model components continued

Time: Either continuous or discrete parameter.

Model Components (continued)

State: Describes the attributes of a system at some point in time.

s = (s1, s2, . . . , sv); for ATM example s = (n)

Convenient to assign a unique nonnegative integer index to each possible value of the state vector. We call this X and require that for each sX.

For ATM example, X = n.

In general, Xt is a random variable.

slide5

Transition: Caused by an event and results in movement from one state to another. For ATM example,

Activity: Takes some amount of time – duration. Culminates in an event.

For ATM example  service completion.

Stochastic Process: A collection of random variables {Xt}, where t T = {0, 1, 2, . . .}.

markovian property
Markovian Property

Given that the present state is known, the conditional probability of the next state is independent of the states prior to the present state.

Present state at time t is i: Xt = i

Next state at time t + 1 is j: Xt+1 = j

Conditional Probability Statement of Markovian Property:

Pr{Xt+1= j | X0 = k0, X1 = k1,…,Xt = i} = Pr{Xt+1= j | Xt = i}

  for t = 0, 1,…, and all possible sequences i, j, k0, k1, . . . , kt–1.

realization of the process

Number in system, n

(no transient response)

Realization of the Process

Deterministic Process

birth and death processes

Pure Death Process; e.g., Delivery of a truckload of parcels

Birth-Death Process; e.g., Repair shop for taxi company

Birth and Death Processes

Pure Birth Process; e.g., Hurricanes

queueing systems
Queueing Systems

Queue Discipline: Order in which customers are served; FIFO, LIFO, Random, Priority

Five Field Notation:

Arrival distribution / Service distribution / Number of servers /

Maximum number in the system / Number in the calling population

queueing notation
Queueing Notation

Distributions (interarrival and service times)

M = Exponential

D = Constant time

Ek = Erlang

GI = General independent (arrivals only)

G = General

Parameters

s = number of servers

K = Maximum number in system

N = Size of calling population

characteristics of queues

Finite queue: e.g., Airline reservation system (M/M/s/K)

a. Customer arrives but then leaves b. No more arrivals after K

Characteristics of Queues

Infinite queue: e.g., Mail order company (GI/G/s)

characteristics of queues continued
Characteristics of Queues (continued)

Finite input source: e.g., Repair shop for trucking firm (N vehicles) with s service bays and limited capacity parking lot (K – s spaces). Each repair takes 1 day (GI/D/s/K/N).

In this diagram N = K so we have GI/D/s/K/K system.

examples of stochastic processes
Examples of Stochastic Processes

Service Completion Triggers an Arrival: e.g., multistage assembly process with single worker, no queue.

state = 0, worker is idle

state = k, worker is performing operation k = 1, . . . , 5

examples continued

s1 = number of parts in system

s2 = current operation being performed

s = (s1, s2) where {

d

d

3

d

3

3

d

d

d

2

2

2

d

d

d

1

1

1

Examples (continued)

Multistage assembly process with single worker with queue.

(Assume 3 stages only)

1,3

2,3

3,3

a

a

Assume

k = 1, 2, 3

1,2

2,2

3,2

a

a

0,0

1,1

2,1

3,1

a

a

a

queueing model with two servers one operation

0 if server i is idle i = 1, 2

1 if server i is busy

s = (s1, s2 , s3) where si = {

s3 =number in queue

State-transition network

Queueing Model with Two Servers, One Operation
series system with no queues

si = {

0 if server i is idle

1 if server i is busy

for i = 1, 2, 3

Series System with No Queues
transitions for markov processes

State-transition matrix

P =

Transitions for Markov Processes

Exponential interarrival and service times (M/M/s)

State space: S = {1, 2, . . .}

Probability of going from state i to state j in one move: pij

Theoretical requirements: 0 pij 1, jpij = 1, i = 1,…,m

single channel queue two kinds of service

State-transition network

Single Channel Queue – Two Kinds of Service

Bank teller: normal service (d), travelers checks (c), idle (i)

Let p = portion of customers who buy checks after normal service

s1 = number in system

s2 = status of teller, where s2Î {i, d, c}

part processing with rework

State-transition network

a = arrival

s1 = service completion from state 1

s2 = service completion from state 2

Part Processing with Rework

Consider a machining operation in which there is a 0.4 probability that upon completion, a processed part will not be within tolerance.

Machine is in one of three states:

0 = idle, 1 = working on part for first time, 2 = reworking part.

markov chains
Markov Chains
  • A discrete state space
  • Markovian property for transitions
  • One-step transition probabilities, pij, remain constant over time (stationary)

Example: Game of Craps

Roll 2 dice: Win = 7 or 11; Loose = 2, 3, 12; otherwise 4, 5, 6, 8, 9, 10

(called point) and roll again  win if point  loose if 7

otherwise roll again, and so on.

(There are other possible bets not include here.)

classification of states
Classification of States

Accessible: Possible to go from state i to state j (path exists in the network from i to j).

Two states communicate if both are accessible from each other. A system is irreducible if all states communicate.

State i is recurrent if the system will return to it after leaving some time in the future.

If a state is not recurrent, it is transient.

classification of states continued

a. Each state visited every 3 iterations

b. Each state visited in multiples of 3 iterations

Classification of States (continued)

A state is periodic if it can only return to itself after a fixed number of transitions greater than 1 (or multiple of a fixed number).

A state that is not periodic is aperiodic.

classification of states continued1
Classification of States (continued)

An absorbingstate is one that locks in the system once it enters.

This diagram might represent the wealth of a gambler who begins with $2 and makes a series of wagers for $1 each.

Let ai be the event of winning in state i and dithe event of losing in state i.

There are two absorbing states: 0 and 4.

classification of states continued2
Classification of States (continued)

Class: set of states that communicate with each other.

A class is either all recurrent or all transient and may be either all periodic or aperiodic.

States in a transient class communicate only with each other so no arcs enter any of the corresponding nodes in the network diagram from outside the class. Arcs may leave, though, passing from a node in the class to one outside.

illustration of concepts
Illustration of Concepts

Example 1

Every pair of states communicates, forming a single recurrent class; however, the states are not periodic.

Thus the stochastic process is aperiodic and irreducible.

illustration of concepts1
Illustration of Concepts

Example 2

States 0 and 1 communicate and for a recurrent class.

States 3 and 4 form separate transient classes.

State 2 is an absorbing state and forms a recurrent class.

illustration of concepts2
Illustration of Concepts

Example 3

Every state communicates with every other state, so we have irreducible stochastic process.

Periodic?

Yes, so Markov chain is irreducible and periodic.

what you should know about stochastic processes
What you Should know about Stochastic Processes
  • What a state is
  • What a realization is (stationary vs. transient)
  • What the difference is between a continuous and discrete-time system
  • What the common applications are
  • What a state-transition matrix is
  • How systems are classified