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Operations Research Models and Methods
Models Section
ATM Example

To illustrate the components of a stochastic process, we use the example of a single automated teller machine (ATM) located in foyer of a bank.  The ATM performs banking operations for arriving customers.  Only one person at a time can use the machine and that person is said to be in service.  Other customers arriving when the machine is busy must wait in a single queue; these persons are said to be in the queue.  Following the rule of first-come-first-served and assuming that the average arrival rate is less than the average service rate, a person in the queue will eventually step up to the ATM, execute a series of transactions, and then leave the system.  The number of persons in the system at any point in time is the sum of the number in service and the number in the queue.




Many situations are conveniently described by a sequence of events that occur over time.  For example, the number of persons (in the queue or in service) in front of the ATM depends on the particular sequence of arrivals and departures.

Events occurring in continuous time

In the figure, time is a treated as a continuous parameter but events occur at specific instances.  We use t to represent time and use subscripts to identify the specific instances.




The state describes the attributes of a system at some point in time.  For the ATM, the state indicates the number of persons in the system (either in service or in the queue).  It may be limited to a maximum value determined by the physical size of the bank foyer or it may be unlimited for practical purposes if a line is allowed to form outside the building.  The minimum number is of course zero, occurring when there are no customers in service.  In this case, the machine is said to be idle.

Depending on the situation, the state may be very complex or very simple.  To provide for this variety, we use an m-component vector,

to describe the state.  The ATM example allows a simple state definition with only one dimension.  Here

s = (n)

where n is the number in the system.  To speak of the state at time , we attach the subscript i to the state vector to get .

It is often 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 there is a unique value of X.  For the ATM example, an obvious assignment is X = n.  For more complex state definitions there may be several possible assignments describing different characteristics of the system.  X(t) is the value of the variable at time t.  Because of the random nature of a stochastic process, the future realization of X(t) is usually not predictable with certainty.  Rather X(t) is a random variable.  To understand the behavior of the system under study, it is necessary to describe the statistical properties of this random variable.

The set of all possible states is called the state space and is denoted by S.  In general, a state space may be either continuous or discrete but here we limit our attention the discrete case only.  For the ATM example, the set of all nonnegative integers can be used to describe the number of persons in the system at any instance, so

S = {0, 1, 2,...}.

This is an infinite state space and assumes that there is no limit to the number of persons that can be in the system.  If the physical dimensions of the bank foyer restrict the number of customers to a maximum K, the state space is finite, and

S = {0, 1, 2,...,K}.


Activities and Events


The dynamic nature of a system is evidenced by its activities.  Each activity has some time interval between its initiation and completion, called the duration.  An activity culminates in an event.   In the ATM example there are two activities, one associated with service and the other with arrivals. The service activity begins when a customer first appears in front of the ATM and ends when all her banking transactions are completed and she departs.  This event is typically referred to as a service completion.  The duration of the activity that leads to this event is the time for service.

The arrival activity begins immediately after a customer arrives and ends with the arrival of the next customer.  The event is an arrival and the duration of the accompanying activity is the time between arrivals.

Symbolically, we use letters to designate activities and events.  Because each activity leads to a unique event, we use the same letter for both the activity and the subsequent event.  For the current example, a is used to represent the arrival activity as well as the event of an arrival, and d to represent the service activity as well as the event of service completion (and departure of the customer).

The duration of each activity is a random variable governed by a probability distribution.  This is the probabilistic element of the stochastic process.  The time for service, , and the time between arrivals, , may be described by their respective cumulative distribution functions

and ..

Generally speaking, the set of possible activities, and thus the set of possible events, depends on the state of the system.  For example, when there are no customers in the system, the service activity is not active and no departure can occur.  If there is some upper limit on the number of customers that can wait in the queue, no arrival can occur when the queue is full.  We use the notation Y(s) to describe the set of possible activities (events) for state s.  For the ATM example, we have

Y(0) = {a}; Y(n) = {a, d} when 0 < n < K; Y(K) = {d}.

The cardinality of Y(s) may be infinite or finite, depending on the situation.  In what follows, the term calendar is used to describe Y(s).  The set of activities that may occur while in state s determines the calendar for the state.

Given the current state s, one of the events in Y(s) will occur next.  This next event and the time it takes place have important effects on the state of the system.  Let x be the next event and be the time that it occurs.  When the activity durations are random variables, x and are random variables.  By definition,

where x is the event for which the minimum is obtained.




A transition is caused by an event and results in movement from one state to another.  Thus it always relates to two states.  For convenience we will say that a transition describes the movement from the current state to the next state.  There are cases, however, in which the current state is allowed to be the same as the next state.

A state-transition network is used to describe the components of the stochastic model.  In the diagram, each state is represented by a node and the possible transitions by directed arcs.  Each arc originates at one state and terminates at another.  The event leading to the transition is shown adjacent to the arc.  The figure depicts the network for the ATM example.  An arrival causes a transition to the next higher state, and a departure causes a transition to the next lower state.

State-transition network for ATM example

When the stochastic process is relatively simple, the transitions can be represented graphically as in the figure.  For complex situations we use a transition function. This function gives the value of the next state in terms of the current state and the next event.  Let the current state be s and the next event be x.  Let s' be the next state determined by the transition function T(s,x), such that

s' = T(s,x).

When the state vector has more than one component, the transition function is multidimensional.  It describes how each component of the state vector changes when the event occurs.

For the ATM example, the transition equation is

s' = s + 1 if x = a and

s' = s - 1 if x = d.


The Process


The state, the activity and event definitions, the probability distributions for activity durations, and the state-transition network are sufficient to describe a stochastic process.  In more formal terms, we have the following.

Definition 1: A stochastic process is a collection of random variables {X(t)}, where t is a time index that takes values from a given set T

Both X(t) and T may be continuous or discrete, giving rise to four different categories of models. We consider here only finite discrete state space ( X(t) is finite and discrete). Both continuous and discrete time models are considered.

A Markov process is a special case of a stochastic process in which all the information concerning the model is entirely determined by the current state.  It is often said to be memoryless because the future realization depends only on the current state and in no way on the past.

Definition 2: Markovian property - Given that the current state is known, the conditional probability of the next state is independent of the states traversed prior to the current state.


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Operations Research Models and Methods
by Paul A. Jensen
Copyright 2004 - All rights reserved