System Model

Chapter1: System Models (Page1-20)

1-1:: The Concepts of a System::

Question: What is system? (OR) Define system.

Answer:

System: A system is defined as an aggregation or assemblages of objects/ subsystems joined in some regular interaction or interdependence. In the case of dynamic system where the interactions cause changes over time.

Example, an aircraft flying under the control of an auto-pilot (figure-1). A gyroscope in the auto-pilot detects the difference between the actual heading and the desired heading. It sends a signal to move the control surfaces. In response to the control surface movement, the airframe steers toward the desired heading.

 

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Figure-1: An aircraft under auto-pilot control.

Example, a factory that makes and assembles parts into a product (figure-2). Two major components of the system are the fabrication department making the parts and the assembly department producing the products. A purchasing department maintains a supply of raw materials and a shipping department dispatches the finished products. A production control department receives orders and assigns work to the other departments.

FINISHED PRODUCTS

RAW MATERIALS

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Figure-2: A factory system.

Question: Explain how the terms “Entity”, “Attribute”, and “Activity” is related with “SYSTEM”? (’97)

Answer: The terms entities attributes and activities are related with systems and we can defined them as follows:

Entity: It denotes an object of interest in a system.

Attributes: It denotes the property of an entity.

Activity: Any process that causes changes in the system will be called an activity.

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State of the system: It means a description of all the entities, attributes and activities as they exist at one point in time.

System progress: Changes in the state of the system.

1-2:: System Environment::

Question: Define system environment. Describe endogenous and exogenous activities. (‘01)

Answer:

System environment: A system is often affected by changes occurring outside the system. Such changes occurring outside the system are said to occur in the system environment. The environment is the “supersystem” within which an organization operates. It is the source of external elements that impinge on the system. In fact, it often determines how a system must function.

Types of activity with respect to system environment:

Endogenous activity: An activity that occurs within the system in called endogenous activity.

Exogenous activity: An activity that occurs within the system environment is called exogenous activity.

Question: Explain the difference between exogenous and endogenous activity. (’99)

Answer: The difference between exogenous and endogenous activity are given below:

Exogenous activity:

- An activity that occurs within the system environment is called exogenous activity.

- It is the regular activity

- It affects the system directly

Endogenous activity:

- An activity that occurs within the system in called endogenous activity.

- It is not regular activity

- It affects the system directly

q  Question: Define environment of a system. Can endogenous and exogenous activities turn a deterministic system into a stochastic one? (’98)

Answer:

System environment: A system is often affected by changes occurring outside the system. Such changes occurring outside the system are said to occur in the system environment. The environment is the “super-system” within which an organization operates. It is the source of external elements that impinge on the system. In fact, it often determines how a system must function.

Yes, endogenous and exogenous activities can turn a deterministic system into stochastic system. Stochastic activities such as arrival activities, random power failure activities, random telephone call generation such endogenous/exogenous activities can turn a deterministic system into stochastic system.

Question: Explain the difference between closed and open system. (’97)

Answer: The difference between closed and open system are given below:

Open system: A system, which has exogenous activity. An open system has many interfaces with its environments. It permits interactions across its boundary; it receives inputs and delivers outputs to the outside.

Closed system: A system for which there is no exogenous activity is said to be a closed system. A closed system is isolated from its environmental influences. In reality, a completely closed system is rare.

Types of system with respect to system environment:

Open system: A system which have exogenous activity.

Closed system: A system for which there is no exogenous activity is said to be a closed system.

1-3:: Stochastic Activities::

Question: Explain the difference between deterministic and stochastic activity.

Answer: The difference between deterministic and stochastic activity are given below:

Deterministic activity: If the outcome of an activity can be described completely in terms of its input, the activity is said to be deterministic.

Stochastic/ Random activity: If the effects of the activity vary randomly over various possible outcomes, the activity is said to be stochastic. The randomness of a stochastic activity would seem to imply that the activity is part of the system environment since the exact outcome at any time is not known. If an activity is truly stochastic, there is no known explanation for its randomness. Sometimes, it requires too much detail or too much trouble to describe an activity fully then the activity is represented as stochastic. If, however, the occurrence of the activity is random, it will constitute part of the environment. Sometimes the random output can often be measured in the form of a probability distribution.

Types of activities depends upon the manner in which they can be described.

1. Deterministic activity: If the outcome of an activity can be described completely in terms of its input, the activity is said to be deterministic.
2. Stochastic/ Random activity: If the effects of the activity vary randomly over various possible outcomes, the activity is said to be stochastic. The randomness of a stochastic activity would seem to imply that the activity is part of the system environment since the exact outcome at any time is not known. If an activity is truly stochastic, there is no known explanation for its randomness. Sometimes, it requires too much detail or too much trouble to describe an activity fully then the activity is represented as stochastic.

1-4:: Continuous and Discrete Systems::

Question: Explain the difference between discrete and continuous system. (’97)

Answer: The difference between discrete and continuous system are given below:

Discrete system: Systems, in which the changes are predominantly discontinuous, are called discrete system.

Continuous system: Systems, in which the changes are predominantly smooth, are called continuous system.

N.B.: Give proper examples and explain

# Sample-data systems: Systems that are intrinsically continuous but information about them is only available at discrete point of time.

1-5:: System Modeling::

q  Question: Discuss the reason for employing simulation and modeling. (’01)

Answer:                                             Reasons for simulation:

Simulation can be used for the following purposes:

1.      Simulation enables the study of, and experimentation with, the internal interactions of a complex system, or of a subsystem within a complex system.

2.      Informational, organizational and environmental changes can be simulated and the effect of these alterations on the model’s behavior can be observed.

3.      The knowledge gained in designing a simulation model may be great value toward suggesting improvement in the system under investigation.

4.      By changing simulation inputs and observing the resulting outputs, valuable insight may be obtained into which variables are most important and how variables interact.

5.      Simulation can be used to experiment with new designs or policies prior to implementation, so as to prepare for what may happen.

6.      Simulation can be used to verify analytic solutions.

Reasons for modeling:

To study a system, we need to experiment with the system. But it is likely to be impossible or impractical to experiment with the actual system. Also it is not possible to experiment with a system while it is in hypothetical form. Consequently, system studies are generally conducted with a model of the system.

Question: Why do we need to model a system?

(OR)

Question: What are the necessities of modeling a system? (’01)

Answer: To study a system, we need to experiment with the system. But it is likely to be impossible or impractical to experiment with the actual system. Also it is not possible to experiment with a system while it is in hypothetical form. Consequently, system studies are generally conducted with a model of the system.

Question: In system modeling what is meant by a model? (’97)

(OR)

Question: What is system model?

Answer:

System model: We define a model as a body of information about a system gathered for the purpose of studying the system. In the case of physical model, the information is embodied in the properties of the model, in contrast to the symbolic representation in a mathematical model. Model is also a simplification of the system, because it is not necessary to consider all the details of a system in most system studies. There is no unique model of a system. Different models of the system will be produced foe different aspects of the system.

q  Question: Describe how models can be classified in system simulation. (’99)

Answer: (Book: By Geoffrey Gordon, (Chapter:1, Topics:1-6, Page:8-9, Fig:1-5)

1-6:: Types of Models::

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Figure: Types of Models.

1-9:: Static Mathematical Models::

Static Mathematical Models: A static mathematical model gives the relationships between the system attributes when the system is in equilibrium. If the point of equilibrium is changed by altering any of the attribute values, the model enables the new values for all the attributes to be derived but does not show the way in which they changed to their new values.

Example: A market model.

1-10:: Dynamic Mathematical Models::

Dynamic Mathematical Models: A dynamic mathematical model allows the changes of system attributes to be derived as a function of time. The derivation may be made with an analytical solution or with a numerical computation, depending upon the complexity of the model.

Question: Distinguish between static and dynamic model. (’01)

Answer: distinguish between static and dynamic model are given below:

Static Mathematical Models: A static mathematical model gives the relationships between the system attributes when the system is in equilibrium. If the point of equilibrium is changed by altering any of the attribute values, the model enables the new values for all the attributes to be derived but does not show the way in which they changed to their new values.

Example: A market model.

Dynamic Mathematical Models: A dynamic mathematical model allows the changes of system attributes to be derived as a function of time. The derivation may be made with an analytical solution or with a numerical computation, depending upon the complexity of the model.

1-11:: Principles Used of Modeling::

Question: Describe some principles needed to be followed to model a system.

Answer: Some principles needed to be followed to model a system are given below:

a)      Block-building: The description of the system should be organized in a series of blocks.

b)      Relevance: The model should only include those aspects of the system that are relevant to the study objectives.

c)      Accuracy: The accuracy of the information gathered for the model should be considered.

d)     Aggregation: The number of individual entities can be grouped into larger entities.

Question: Discuss the reason for employing simulation and modeling. (’01)

Answer:

Question: Define environment of a system. Can endogenous and exogenous activities turn a deterministic system into a stochastic one? (’98)

Answer:

Question: Describe how models can be classified in system simulation. (’99)

Answer:

Exercises::

Problem-1: Extract from the following description the entities, attributes, and activities of the system. Ships arrive at a port. They dock at a berth if one is available; otherwise, they wait until one becomes available. They are unloaded by one of several work gangs whose size depends upon the ship’s tonnage. A warehouse contains a new cargo for the ship. The ship is loaded and then departs. Suggest two exogenous events (other than arrivals) that may need to be taken into account.

Solution:

Entity:

            Port

            Ship

            Dock

            Work gangs

            Ware house

Attributes:

            No of dock

            Tonnage

            Availability

            Size

            No of cargo

Activity:

            Arrive

            Availability check for dock

            Unloaded

            Loaded

            Departs

Exogenous events:

1.      Ship arrivals

2.      Ship depart

3.      Work gangs arrival/ Hair

4.      Cargo arrival

Problem-2: Name three or four of the principal entities, attributes, and activities to be considered if you were to simulate the operation of a gasoline filling station.

Solution:

Entity:

            Service man

            Pump machine

            Tank lorry

            Autos

            Parking lot

Attributes:

            Service man à id, service time;

            Pump machine à no of pump machine, fuel type, pumping rate;

            Tank lorry à capacity, petroleum type;

            Autos à type of autos, tank size;

            Parking lot à capacity;

Activity:

            Filling

            Car arrival

            Car departure