Ch1: Introduction to:

SIMULATION
 What is Simulation
• A method to model (represent), observe, and analyze real-
world systems on a computer. Why? Because systems are:
– Complex
– Stochastic
– Dynamic

• Mimic (imitate) system behavior over time

• The Goal is understanding system behavior, answering

what-if questions, designing new systems, and improving
performance of existing systems.

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 System vs. Its Model

• Simplification
Real System • Abstraction Model
• Assumptions

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 Model Classification

• Physical (prototypes) Simulation types:

• Analytical
(mathematical) • Discrete (DES)
• Computer
(Monte Carlo • Continuous
Simulation)

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 Typical Uses of Simulation
• Manufacturing Applications
• Material Handling Systems (AGVs, Forklifts,
Conveyors, Storage systems, etc.)

• Project Management (Construction Engineering)

• Logistics and Supply Chain applications
• Transportation (Traffic, Airports, Ports, etc.)
• Healthcare clinics and systems
• Banks and financial operations
• Computer Systems and Communication Networks
• Restaurants and catering
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 System Elements
• Entities (Attributes)

• Activities (tasks and operations)

• Resources (Manpower, machines, etc.)

• Logic (Events, Flow, State Variables)

• Data (specifications, time, cost, etc.)

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 Model Building Process
• Problem formulation and objectives
• Conceptual modeling
• Data collection
• Model translation (construction)
• Model verification
• Model validation
• Setup of simulation experiments
• Output analysis
• Recommendations/reporting
• Implementation

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 Simulating Queuing Systems
Example 1: A Simple Workstation
• Consider a single workstation, known in queueing theory as the
M/M/1 queue, where
• there is a machine with an infinite buffer in front of it
• jobs arrive randomly and wait in the buffer while the machine is busy
• jobs are processed by the machine and then leave the system
• job inter-arrival times are exponentially distributed with mean 30 minutes
• job processing times are exponentially distributed with mean 24 minutes

ARENA model:

Create 1 Process 1 Dispose 1

0 0
0

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 Simulation Objects and Actions
• Simulating the above workstation calls for the following actions:
• jobs are created, one at a time, according to their arrival distribution
• if the machine is busy processing another job, then the arriving job is
queued in the buffer
• when a job advances to the head of the buffer, it seizes the machine for
processing once it becomes available, and holds it for a time period,
sampled from its processing-time distribution
• on process completion, the job departs the machine and is removed from
the system

• Simulation objects and their actions and interactions

• are modeled by Arena modules
• parameterized by associated dialog boxes

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 Create Module
• The Create module generates a stream of arrivals of Arena
entities (jobs, people, messages, etc.)

Dialog box for a Create module

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 Create Module (Cont.)
• The Type pull-down menu for the Time Between Arrivals field
offers the following options:
• Random (exponential inter-arrival times with mean given in the Value
field)
• Schedule (allows the user to create arrival schedules using the Schedule
module from the Basic Process template panel
• Constant (specifies fixed inter arrival times)
• Expression (any type of inter-arrival time pattern specified by an Arena
expression, including Arena distributions)

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 Process Module
• The Process module processes (serves) Arena entities

Dialog box for a Process module

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 Process Module (Cont.)
• The Action field option, selected from the pull-down menu,
is Seize Delay Release, which stands for a sequence of
SEIZE, DELAY and RELEASE SIMAN blocks
• SEIZE and RELEASE blocks are used to model contention for a resource
possessing a capacity (e.g., machines)
• when resource capacity is exhausted, the entities contending for the
resource must wait until the resource is released
• thus, the SEIZE block operates like a gate between entities and a resource
• the processing (holding) time of a resource (called Machine in the example)
by an entity is specified via a DELAY block within the Process module

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 Dispose Module
• The Dispose module implements an entity “sunset” mechanism
• entities that enter it are simply discarded

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 Arena Simulation Results
• The end-result of a simulation run is a set of requisite statistics,
referred to as run results, such as
• mean waiting times
• buffer content probabilities
• resource utilization
• Arena provides a considerable number of default statistics
in a report that is automatically generated at the end of a
simulation run in Arena reports
• Additional statistics can be obtained by adding statistics
collection modules in the model, such as
• Record (Basic Process template panel)
• Statistic (Advanced Process template panel)

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Example: a Resources Report

Resources statistics from a single replication

of the simple workstation model
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Example: a Queues Report

Queues statistics from a single replication

of the simple workstation model
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 Example: Two Processes in Series
• Consider a manufacturing network of two workstations in series,
consisting of an assembly workstation followed by a painting
workstation, where
• jobs arrive at the assembly station with exponentially distributed inter-arrival
times of mean 8 hours
• the assembly process always has all the raw materials necessary to carry out
the assembly operation
• the assembly time is uniformly distributed between 2 and 6 hours
• after the process is completed, a quality control test is performed, and past
data reveal that 15% of the jobs fail the test and go back to the assembly
operation for rework
• jobs that pass the test proceed to the painting operation that takes 3 hours
for each unit
• We are interested in
• simulating the system for 100,000 hours
• estimating process utilizations, average job waiting times and average
job flow times (the elapsed time for a job from start to finish)
Altiok / Melamed Simulation
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Modeling and Analysis with
Arena
 The Arena Model
0 True
Quality Check
Assembly
Arrival Time
0 0 False

Painting

Job Arrivals 0

Completed
Flow Time
0

Arena manufacturing model of assembly and painting processes in series

Altiok / Melamed Simulation
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Modeling and Analysis with
Arena
 Assign Module
• The Assign module is used by entities to assign values to attributes
• the dialog box below assigns an arrival time to a job attribute

Dialog boxes for an Assign module

Altiok / Melamed Simulation
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Modeling and Analysis with
Arena
 Decide Module
• The Decide module is used by entities to make branching decisions,
based on chance or the truth/falsity of prescribed conditions
• the dialog box below makes a two-way probabilistic branching decision

Dialog box for a Decide module

Altiok / Melamed Simulation
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Modeling and Analysis with
Arena
 Record Module
• The Record module is used by entities to collect statistics
• the dialog box below tallies job flow times

Dialog box for a Record module

Altiok / Melamed Simulation

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Modeling and Analysis with
Arena
Resources Report

Altiok / Melamed Simulation

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Modeling and Analysis with
Arena
Queues Report

Altiok / Melamed Simulation

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Modeling and Analysis with
Arena
User Specified Report

Altiok / Melamed Simulation

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Modeling and Analysis with
Arena