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Modeling and Simulation Glossary

Modeling and Simulation terms and terms related to M&S are defined in this section.

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Gas Cloud Simulation

A simulation of how a gas cloud that has been observed approaching the supermassive black hole at the centre of the galaxy.

Advanced Test Reactor Simulation

Advanced simulation capabilities can model reactors, such as INL's Advanced Test Reactor shown here, from atomic scales to full-sized reactor assemblies.

Underwater EVA Simulation

Underwater simulation of extravehicular activity (EVA).

Combat Simulation

A screen capture depicts a combat situation in a digital world during a command post exercise inside the Joint Multinational Simulations Center.

Motorcycle Simulator

Testing motorcycle simulator at the Pentagon to get a feel for how the bike would handle some situations.

Wake Turbulence Simulation

Wake turbulence simulation for an Airbus A340 on final approach, just before touchdown.

Driving Conditions Simulator

View from the cockpit of the simulator. Multiple screens provide a view similar to what one would see in a snowplow on the road.

Molecules Simulation

Dynamic computer simulations of molecular systems.

Global Aerosols Simulation

This portrait of global aerosols was produced by a Goddard Earth Observing System Model, Version 5 (GEOS-5) simulation at a 10-kilometer resolution.

Driving Simulation

Simulation for driving training.

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Special Interest Group (SIG) on SImulation and Modeling (SIM)
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Modeling and Simulation Glossary

 

Index Index
   

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Abstract (Mathematical) Model is the one in which symbols and logic constitute the model. The symbolism used can be a language or a mathematical notation. A verbal or written description in English is an abstract model. A mathematical model is described in the "language" of mathematical symbols and is an abstract model. A simulation model is built in terms of logic and mathematical equations and is an abstract model.
Accreditation is a “procedure by which an authoritative body gives formal recognition that a body or person is competent to carry out specific tasks.” [International Organization for Standardization]
Accuracy is the degree of transformational accuracy (verity) or behavioral / representational accuracy (validity).
Activity is the state of an object over an interval of time. An activity starts with an event and ends with another event.
Active Stakeholder is the one who will actively interact with the system or product (e.g., M&S application) once it is operational and in use. Examples: users of the M&S application, administrators of the M&S application, trainers of the M&S application users.
Actor is an entity engaged in a use case. An actor can be a user (human), model component, submodel, submodel of another submodel, or another model as a whole.
Adaptive System is a system which reacts to changes in its environment.
ADO ActiveX Data Objects
Agent-based M&S

is the one which uses a model representing agents and their interactions. An agent is “intelligent”, adaptive, autonomous, goal/self-directed, has the ability to learn, and can change its behaviors based on experience. Agent-based M&S draws characteristics from AI-based M&S and object-oriented M&S.

An Agent is represented by attributes, behavioral rules, memory, resources, decision making rules, and rules to modify behavioral rules. An Agent is similar to an object in object-oriented M&S. Agent-based M&S is typically used in disciplines such as Biological Sciences, Cognitive Sciences, Computational Sciences, Economics, Organizational Sciences, Physical Sciences, Social Sciences, and Sociology.

AJAX Asynchronous JavaScript and XML
Analytical Model is the one which is solved by using the deductive reasoning of mathematical theory. An M/M/1 queuing model, a Linear Programming model, a Mixed Integer Linear Programming model, a nonlinear optimization model are examples of analytical models.
Architecture

“The structure of components, their relationships, and the principles and guidelines governing their design and evolution over time.” [DoD Integrated Architecture Panel, 1995, based on IEEE STD 610.12]

“An architecture is the fundamental organization of a system embodied in its components, their relationships to each other, and to the environment, and the principles guiding its design and evolution.” [IEEE STD 1471-2000]

Architecture versus Design A design is an instantiation of an architecture similar to how an object is an instantiation from a class.
Artificial Intelligence (AI) is a kind of simulation that involves a model intended to represent human intelligence or knowledge. An AI-based simulation model typically mimics human intelligence such as reasoning, learning, perception, planning, language comprehension, problem-solving, and decision making. Rule-based knowledge representation is commonly used for building AI-based simulation models. An Expert System is also a kind of simulation of some knowledge, typically constructed using rule-based knowledge representation. Artificial Intelligence is typically used in disciplines such as Cognitive Science, Computer Science, Engineering, Language Translation, Neuroscience, Philosophy, Psychology, Game Playing, and Robotics.
ASP Active Server Pages
Assertion is a statement that should hold true as the model executes.
Attribute denotes a property of an object or describes an aspect of an object.
   

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Certification

is a “procedure by which a third party gives written assurance that a product, process or service conforms to specified characteristics.” [International Organization for Standardization]

Certification is the process of independently awarding a “Certificate”, a “Seal of Approval” or a “Mark of Conformity” formally attesting that a simulation model fulfills specific quality criteria under a set of prescribed intended uses.

Clarity of a requirement specification is the degree to which a requirement specification is unambiguous and understandable.
Class In the Object-Oriented Paradigm: A class is a grouping or categorization of objects with the same characteristics, services, and behaviors. A class may be extended into other classes. Extending a class is called subclassing and an extended class is called a subclass. A subclass inherits all the characteristics, services, and behaviors of the parent class. It customizes what it inherits and/or provides more characteristics, services, or behaviors. The parent of a subclass is called the superclass. The top class having no superclass is called the root class.
Closed System is a system which has no inputs (exogenous activities) and no outputs.
Cohesion of a submodel is the degree of interaction within a submodel.
COI Community of Interest
Completeness of a requirement specification is the degree to which all parts of the requirement specification are stated with no missing information, i.e., each requirement is self-contained.
Conceptual Model A simulation Conceptual Model is a repository of high-level conceptual constructs and knowledge specified in a variety of communicative forms intended to assist in the design of any type of large-scale complex M&S application in a COI.
Consistency of a requirement specification is the degree to which the requirement specification is stated using uniform notation, terminology, and symbology, and any one requirement does not conflict with any other.
Continuous M&S is the one which uses a model consisting of differential equations and the simulation time is represented as a continuous variable. Continuous M&S is typically used in disciplines such as aerospace engineering, computational fluid dynamics, computational solid mechanics, computational engineering, computational physics, materials science and engineering, and heat transfer.
Continuous System is a system in which the changes are predominantly smooth such as the missile system.
CORBA Common Object Request Broker Architecture
Coupling of submodels is the degree of dependency between submodels.
CSA Client-Server Architecture
CSS Cascading Style Sheets
   

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DCOM Distributed Component Object Model
Dependability is the degree to which the M&S application (a) delivers services when requested, (b) delivers services as specified, (c) operates without catastrophic failure, and (d) protects itself against accidental or deliberate intrusion.
Descriptive Model is a model which describes the behavior of a system without any value judgment on the “goodness” or “badness” of such behavior. The equation F (orce) = M (ass) x A (cceleration) is a descriptive model. All simulation models are descriptive models.
Design The Design process deals with the instantiation (creation) of a design of the M&S application from the Architecture Specification. The Design process takes the M&S Requirements Specification, Conceptual Model, and Architecture Specification as input and produces a design specification of the M&S application as the output work product.
Design Principle

Design a submodel or model component (class) in such a way that the submodel has the highest possible cohesion and the lowest possible coupling.

The maintenance effort is reduced and reusability is increased when there is maximal interaction within each submodel (cohesion) and minimal dependencies (coupling) between submodels.

Design of Experiments is the process of formulating a plan to gather the desired information from a simulation model at minimal cost and to enable the analyst to draw valid inferences.
Deterministic Activity is an activity the outcome of which can be described completely in terms of its input.
DIS Distributed Interactive Simulation
Discrete M&S

is the one which uses a model built in terms of logic and the simulation time is represented as a discrete variable. Discrete M&S is typically used in disciplines such as computer science, systems engineering, industrial engineering, operations research, business, civil engineering, and management science.

This type of M&S is commonly referred to as Discrete Event Simulation. The "event" in the name comes from the traditional use of the Event Scheduling simulation programming approach. However, other approaches exist based on activity, process, object, and agent. Therefore, the term "event" should be dropped from the name so as to include all possible programming approaches.

Discrete System is a system in which the changes are predominantly discontinuous such as a factory system.
Distributed M&S is the one which executes its model parts (e.g., federation of models, submodels, model components, subcomputations) on geographically or locally distributed computers. Distributed M&S enables geographically dispersed people to interact with the running simulation typically for training purposes.
DOA Distributed Objects Architecture
DoDAF

The U.S. Department of Defense Architecture Framework (DoDAF)

  • defines a common approach for architecture description development, architecture presentation, and architecture integration.
  • is intended to ensure that architecture descriptions can be compared and related across organizational boundaries, including joint and multinational boundaries.
  • facilitates the use of common principles, assumptions, and terminology.
DOM Document Object Model
Dynamic Binding In the Object-Oriented Paradigm: Dynamic Binding implies that the implementation code executed in responding to a particular message in the source code is not known at compile time, and is dispatched at run time depending on the form of the polymorphic object that receives the message.
Dynamic Model is the one which describes time-varying relationships. A wind tunnel which shows the aerodynamic characteristics of proposed aircraft designs is a dynamic physical model. The equations of motion of the planets around the sun constitute a dynamic mathematical model of the solar system.
Dynamic VV&T Techniques require model execution and are intended for evaluating M/S based on execution behavior. Many dynamic VV&T techniques require model instrumentation.
   

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EJB Enterprise Java Bean
Emulation

is imitation of a system where the imitation provides the exact desired functionality of the system.

The difference between simulation and emulation is that simulation uses a model, which is abstract and an approximation, but the emulation uses a representation intended to provide the exact functionality of the system emulated.

Encapsulation In the Object-Oriented Paradigm: Encapsulation means that a consumer sees only the services that are available from an object, but not how those services are implemented.
Endogenous Activity is produced within the system resulting from internal causes.
Entity denotes an element of interest in the system.
Event is a change in the state of an object at a particular instant of time.
Exogenous Activity is produced within the system environment and affects the system. Exogenous activities are inputs to a system.
   

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Feasibility of a requirement is the degree of difficulty of implementing a single requirement, and simultaneously meeting competing requirements. Sometimes requirements conflict with each other. It may be possible to achieve a requirement by itself, but it may not be possible to achieve a number of them simultaneously.
Financial Independence implies that the IV&V / Certification Agent is allocated its own budget for the M&S application V&V / Certification and does not rely on the M&S application development budget.
Formal VV&T Techniques are based on mathematical proof of correctness. If attainable, proof of correctness is the most effective means of M/S V&V.
Functional requirements Statements of services the model should provide, how the model should react to particular inputs and how the model should behave in particular situations. Functional requirements are requirements about the input-output transformations of the simulation model.
Functionality is the degree to which the M&S application completely captures all of the desired functional modules that need to be present.
   

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Gaming-based M&S is the one which uses humans as part of its model. It is typically intended to train people. For example: Management Games are performed to train managers for a business. War Gaming is performed to train military commanders. The gaming known as Video Games are simulations that are intended for entertainment, educational or training purposes. Gaming-based M&S is typically used in disciplines such as Business, Education, Management, and Training.
Garbage-in garbage-out Refers to a simulation with insufficient credibility.
   

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Hardware-in-the-loop M&S can be viewed as Simulation-based Hardware Evaluation. A hardware system can be operated under simulated input conditions for the purpose of evaluating how well the hardware functions under such input conditions. For example, a space vehicle can be operated under simulated input conditions for the purpose of evaluating the vehicle’s autopilot mode. Hardware-in-the-loop M&S is a cost-effective method for evaluating a complex, mission-critical hardware before it is used in the real world.
HLA

High Level Architecture (HLA) is a general purpose architecture that facilitates interoperability among network-centric simulations and enables reuse of simulations and their components. HLA-compliant simulations running on different hardware platforms can interoperate with each other over a network. Interoperability among the network-centric simulations is enabled by a RunTime Infrastructure (RTI).

HLA is an IEEE Standard, a DoD Standard, and a NATO Standard.

HTML HyperText Markup Language
HTTP HyperText Transfer Protocol
Human-in-the-loop M&S is also called Simulation-based Training. A simulation model of a system, e.g., airplane, air traffic control center, emergency management plan, or military operation is developed for the purpose of training people. Trainees interact with the visual simulation model for the purpose of learning, e.g., how to fly an airplane (using the flight simulator), how to control air traffic at an airport, how to manage an emergency in response to a disaster, or how to make military decisions. Human-in-the-loop M&S can also be used as a cost-effective method for evaluating human performance and behavior for a proposed system design.
   

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IDE Integrated Development Environment
IDL Interface Definition Language
Implementation is the process of programming a simulation model design specification in a simulation software product or a high-level programming language such as Java or C++.
Inheritance In the Object-Oriented Paradigm: When an object is declared as member of a class, it inherits the characteristics (instance variables) and behaviors (instance methods) of that class as well as the characteristics and behaviors of that class’s superclass, and any ancestral classes, tracing back to the root class. Inheritance significantly facilitates reusability of earlier developed classes and decreases simulation model development time.
Informal VV&T Techniques are called informal because the tools and approaches used rely heavily on human reasoning and subjectivity without stringent mathematical formalism. The “informal” label does not imply any lack of structure or formal guidelines for the use of the techniques. In fact, these techniques are applied using well structured approaches under formal guidelines and they can be very effective especially during the early stages of the M&S life cycle.
Instantiation In the Object-Oriented Paradigm: Creation of an object belonging to a class is called instantiation. The new object inherits all characteristics (instance variables) and behaviors (instance methods) specified in the class from which it is instantiated. Instance variables of a class are created for each object instantiated as a member of that class. Instance methods are inherited, but no method code is replicated.
Intended Use refers to the explicitly and clearly defined purpose for which the simulation model is intended for use.
IV&V Independent Verification and Validation
   

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Java EE Java Platform, Enterprise Edition (Java EE)
JSF Java Server Face
JSP Java Server Page
Junk-input junk-output Refers to a simulation with insufficient credibility.
   

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Life Cycle The M&S Life Cycle represents a framework for organization of the processes, work products, quality assurance activities, and project management activities required to develop, use, maintain, and reuse an M&S application from birth to retirement, and is created to modularize and structure an M&S application development and to provide guidance to an M&S developer (engineer), manager, organization, and community of interest.
Linear Model is the one which describes relationships in linear form. The equation y = 3x + 4z + 1 is a linear model.
   

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M&S Modeling and Simulation
M/S Model and/or Simulation
Ms/Ss Models and/or Simulations
Managerial Independence implies that the IV&V / Certification Agent reports to the M&S application sponsor independently of the M&S application developer organization.
Methods (Services)

In the Object-Oriented Paradigm: Services provided and behaviors exhibited by an object are specified in methods. Two types of methods exist: class methods and instance methods.

Class Methods are used to provide services specific to a class. For example, the method “new” which creates an instance of a class.

Instance Methods, given in a class, are used to specify the services provided and behavior exhibited for each object instantiated from that class. Each instance (i.e., instantiated object) created as belonging to a class provides the services and behavior specified in the instance methods of that class. The method code is specified only once in the class and is not replicated for each instantiation of an object from that class.

Model is a representation and abstraction of anything such as a real system, a proposed system, a futuristic system design, an entity, a phenomenon, or an idea.
Model Builder's Risk is the probability of committing the Type I Error.
Model Instrumentation refers to the insertion of additional code (probes, stubs or trap code) into the executable model or submodel (module) for the purpose of collecting information about model behavior during execution. Probe locations are determined manually or automatically based on static analysis of model structure.
Model User's Risk is the probability of committing the Type II Error.
Modeling is the act of constructing a model. Modeling is an artful balancing of opposites; on the one hand, a model should not contain unnecessary details and become needlessly complex and difficult to analyze, on the other hand, it should not exclude the essential details of what it represents.
Modifiability of a requirement is the degree to which the requirement can easily be changed.
Monte Carlo M&S is the one which uses a model built based on statistical random sampling. The model typically does not represent time-varying relationships. Monte Carlo M&S is typically used in disciplines such as Chemistry, Computational Engineering, Financial Probabilistic Modeling, Mathematics, Nuclear Engineering, (Computational, Nuclear, Statistical) Physics, and Reliability Engineering.
   

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Network-Centric Simulation Architecture

refers to the fundamental organization of simulation components that interoperate over a network, relationships among the simulation components, and the principles and guidelines governing the design and evolution of those simulation components.

We use the term “network” to refer to one or a combination of many types of communications networks such as Global System for Mobile communication (GSM), Internet, local area network (LAN), mobile ad-hoc network (MANET), virtual private network (VPN), or wireless network.

Nominal Score is a named score such as excellent, good, fair, or poor.
Nonadaptive System is a system which does not react to changes in its environment.
Non-functional requirements Are requirements that are unrelated to functionality of the simulation model such as requirements for interoperability, performance, usability, standards, delivery, and portability.
Nonlinear Model is the one which describes relationships in nonlinear form. The equation F = (2x + 4z—2) / (3y—x) is a nonlinear model.
Numerical Model is the one which is solved by applying computational procedures. Finding the roots of a nonlinear algebraic equation, f(x) = 0, using the method of Interval Halving or Simple Iteration involves the use of a numerical model. System Simulation is considered to be a numerical computation technique.
   

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Object

denotes an element of interest in the system.

In the Object-Oriented Paradigm: An object is an entity which has a state and a defined set of operations which operate on that state. The state is represented as a set of object attributes. The operations associated with the object provide services to other objects (clients) which request these services when needed. Objects are created according to some object class definition. An object class definition serves as a template for objects. It includes declarations of all the attributes and services which should be associated with an object of that class.

Open System is a system which has exogenous activities.
ORB Object Request Broker
   

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Parallel M&S is the one which executes its model parts (e.g., submodels, model components, subcomputations) on different processors of the same computer for the purpose of achieving faster execution time.
Passive Stakeholder

is the one who will not actively interact with the system or product (e.g., M&S application) once it is operational and in use. Examples: M&S application developers, decision makers about the use of the M&S application, logistics personnel, manufacturer, owners/sponsors if they don't use/operate the M&S application.

Performance is the degree to which the M&S application executes its work in a speedy, efficient, and productive manner.
Physical Model is the one which is usually a physical replica, often on a reduced scale, of the system it represents. A physical model "looks like" the object it represents and is also called an Iconic Model. A model of an airplane (scaled down), a model of the atom (scaled up), a map, a globe, a model car are examples of physical (iconic) models.
Polymorphism In the Object-Oriented Paradigm: Polymorphism refers to the ability of an object to assume more than one form.
Prescriptive or Normative Model is a model which describes the behavior of a system with a value judgment on the “goodness” or “badness” of such behavior. A Linear Programming model, a Mixed Integer Linear Programming model and a nonlinear optimization model are examples of prescriptive models. When solved, these models provide a description of the solution as optimal, suboptimal, feasible, infeasible, etc.
Presentation is the process of interpretation of the simulation results, documentation of the simulation results, and communication of the simulation results to the decision makers.
Problem Formulation is the process by which the initially communicated problem is transformed into a formulated problem sufficiently well defined to enable specific research action.
Process is the succession of states of an object over a span which is the contiguous succession of one or more intervals.
Pseudo-Random Number Generation Refers to the generation of random numbers in a way that is reproducible by using a starting value called seed. Pseudo implies that the numbers are not truly random, but satisfy statistical properties for randomness.
   

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Quality Assurance (QA) of M/S refers to the planned and systematic activities that are established throughout the M&S life cycle to substantiate adequate confidence that a M/S possesses a set of characteristics needed and expected by the user for a set of intended uses.
   

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Random Variable is a real-valued function that maps a sample space into the real line (numbers). Example random variable: Interarrival Time (X)
Random Variate is a particular outcome or sample value of a random variable. Example: 26 as a particular value of the random variable X.
RDBMS Relational Data Base Management System
Requirements Engineering is the process of elicitation of requirements based on the formulated problem, and specification of the requirements in an authoritative manner. This process takes the formulated problem as input and produces a M&S requirements specification document (RSD) as the output work product.
RMI Remote Method Invocation
RSD Requirements Specification Document
RVG Random Variate Generation
   

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Self-Driven Simulation is the one which uses random numbers in sampling from probability distributions so as to drive the model.
Simulation

To Simulate, according to Webster’s Dictionary, is:“To feign, to attain the essence of without the reality” To Simulate, in simple terms, implies to imitate or mimic.

Simulation is the act of executing, experimenting with or exercising a model for a specific objective such as acquisition, analysis (problem solving), education, entertainment, research, or training.

SOA Service-Oriented Architecture
SOAP Simple Object Access Protocol, an XML-based messaging protocol used to encode the information in web service request and response messages before sending them over a network.
Software-in-the-loop M&S can be viewed as Simulation-based Software Evaluation. A software system can be executed under simulated input conditions for the purpose of evaluating how well the software system functions under such input conditions. For example, the software used to display the common operating picture (COP) in a combat operation on a handheld computer can be executed under simulated input data (e.g., video, voice, images, text) received from many different sources for the purpose of evaluating how well the software satisfies its requirements. Software-in-the-loop M&S is a cost-effective method for evaluating a complex, mission-critical software system before it is used in the real world.
SQL Structured Query Language
Stability of requirements is the degree to which the requirements are changing while the M&S application is under development.
Stable Model is the one which tends to return to its initial condition after being disturbed. Like a simple pendulum that is set in motion, it may overshoot and oscillate, but the disturbances decline and die out.
State of an object is the enumeration of all attribute values of that object at a particular instant of time.
State of a system is the exhaustive enumeration of all attribute values (of all objects) at a particular instant of time.
Static Model is the one which describes relationships that do not change with respect to time. An architectural model of a house which helps us visualize floor plans and space relationships is a static physical model. An equation relating the lengths and weights on each side of a playground seesaw is a static mathematical model.
Static VV&T Techniques are concerned with accuracy assessment on the basis of characteristics of the static model design and source code. Static techniques do not require computer execution of the model, but mental execution can be used.
Steady-State Model is the one whose behavior in one time period is of the same nature as any other period.
Stochastic Activity is an activity the outcome of which varies randomly.
Storage is the process of placing a certified Simulation Model with its full documentation into an organization-wide repository for reuse.
Supportability is the degree to which the M&S application can be supported.
System is any collection of interacting elements that operate to achieve some goal.
System Boundary is the logical and physical partitioning that defines which elements are included within the system definition and which elements are excluded from the system definition.
System Dynamics M&S is the one which uses a model representing cause-and-effect relationships in terms of causal-loop diagrams, flow diagrams with levels and rates, and equations. The equations are used for simulating system behavior. System Dynamics M&S is typically used in disciplines such as Business, Decision Sciences, Economics, Management, Organizational Sciences, Policy Studies, Social Sciences, and System Sciences.
   

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TCP/IP Transmission Control Protocol / Internet Protocol
Technical Independence implies that the M&S application IV&V / Certification Agent determines, prioritizes, and schedules its own tasks and efforts.
Technology-in-the-loop M&S can be viewed as Simulation-based Technology Evaluation. A technology (e.g., IP-based wireless network, Unmanned Aircraft System, a Satellite Communication System) can be tried, demonstrated or experimented with under simulated scenarios for role players (e.g., subject matter experts, operators, war fighters) to assess how well the proposed technology fulfills its objectives and desired quality characteristics such as Interoperability. In this case, a given technology typically consists of hardware, software, and human operators. Technology-in-the-loop M&S is a cost-effective method for evaluating a potential technology to determine its readiness level and judge if it is ready for deployment for real-world use. Technology-in-the-loop M&S can also be used as a cost-effective method for evaluating human performance and behavior for a proposed technology.
Testability of a requirement is the degree to which the requirement can easily be tested. A testable requirement is the one that is specified in such a way that pass/fail or assessment criteria can be derived from its specification.
Testing is the process of designing a test, specifying test conditions and data, and determining a procedure to follow for the purpose of judging transformational accuracy (verity) and/or representational/behavioral accuracy (validity). Testing is conducted to perform either verification and/or validation.
Time Flow Mechanism is that portion of a simulation that advances time in the simulation, and provides synchronization of the various parts of the simulation.
Traceability of requirements is the degree to which the requirements related to a particular requirement can easily be found. Requirements should be specified in such a way that related requirements are cross-referenced. When it is necessary to change a requirement, those requirements affected by the changed requirement should be easily identified by using the cross-references.
Trace-Driven Simulation is the one which uses the actual input data traced during the operation of the real system so as to drive the model.
Transient Model is the one whose behavior changes with respect to time.
Turing Test is based on the subject matter expert (SME) knowledge about the system under study. The SMEs are presented with two sets of output data obtained, one from the model and one from the system, under the same input conditions. Without identifying which one is which, the SMEs are asked to differentiate between the two. If they succeed, they are asked how they were able to do it. Their response provides valuable feedback for correcting model representation. If they cannot differentiate, our confidence in model validity is increased.
Type I Error is the error of rejecting the credibility of a M/S when in fact the M/S is sufficiently credible.
Type II Error is the error of accepting the credibility of a M/S when in fact the M/S is not sufficiently credible.
Type III Error is the error of solving the wrong problem. It is committed when the formulated problem does not completely contain the actual problem.
   

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UDDI Universal Description, Discovery and Integration, a web-based distributed directory that enables listing of web services and discovering each other, similar to a traditional phone book's yellow and white pages.
UML Unified Modeling Language
Unambiguity of a requirement specification is the degree to which the requirement specification can only be interpreted one way.
Understandability of a requirement specification is the degree to which the meaning of the requirement specification is easily comprehended by all of its readers.
Unstable Model is the one which may or may not come back to its initial condition after being disturbed.
Usability is the degree to which the M&S application can easily be employed for its intended use.
Use Case specifies the behavior of a model or a part of a model and is a description of a set of sequences of actions, including variants, that a system performs to yield an observable result of value to an actor. A use case carries out some tangible amount of work.
   

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Validity is the degree of behavioral or representational accuracy.
Variables (Attributes)

In the Object-Oriented Paradigm: Characteristics (attributes) of an object are represented by variables. Typically three kinds of variables exist: class variables, instance variables, and local variables.

Class Variables are attributes of a class and are declared in the class for use by the methods of that class and its subclasses and by the objects instantiated from that class and its subclasses.

Instance Variables, declared in a class, are used by the instance methods of that class and are created for each object instantiated as belonging to that class or any of its subclasses, i.e., for each instance of a class, and hence the designator “instance”.

Local Variables are declared within a method for use only during the execution of that method. On completion of a method, all local variable values are lost.

Example variables can have the following data types: boolean, character, class reference, object reference, integer number, real number, enumeration.

Verification and Validation (V&V)

The terms Verification and Validation (V&V) are consistently defined for whatever entity they are applied to. Let X be that entity such as model, simulation, software, data, or a life cycle artifact (work product) such as requirements specification, conceptual model, design specification, or executable submodel. Then, V&V can be defined generically as follows:

  • X Verification deals with the assessment of transformational accuracy of the X and addresses the question of “Are we creating the X right?”
  • X Validation deals with the assessment of behavioral or representational accuracy of the X and addresses the question of “Are we creating the right X?”

For whatever entity to be subjected to V&V, substitute the entity name in place of X above, the definitions will hold.

Verity is the degree of transformational accuracy.
Virtual Reality is the use of modeling and simulation to enable a person to interact with a three-dimensional visual representation of a real or imaginary system in an immersive, multi-sensory, and interactive manner. The user wears goggles, headsets, gloves, or body suits to interact with the simulation. The motion sensors pick up the user’s movements and adjust his or her view and action accordingly during the interaction, usually in real-time. Virtual Reality is typically used in disciplines such as Architecture, Computer-aided Design and Manufacturing, Education, Entertainment (Movies, Video Games), Human-Computer Interaction, Medical Science, Real Estate, and Training.
VV&T Verification, Validation, and Testing
   

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WSDL Web Services Description Language, an XML-formatted language used to describe a Web service's capabilities.
   

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XHTML Extensible HTML
XML Extensible Markup Language
XSLT Extensible Stylesheet Language Transformation
   

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