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System modelling elements

Defining system modelling elements: entities and attributes.
© Coventry University. CC BY-NC 4.0

We’ll start this week by defining system modelling elements.

Element Definition
Temporary entity An object of interest in the system, such as a vehicle waiting for container collection at one of the seaports, while permanent entities are the resources used for serving temporary entities.
Attribute A property of an entity, such as a vehicle number or a collection type.
Activity Represents a time period of specified length. Refers to a collection of operations that transform the state of an entity, such as check out of containers operations.
Event Refers to a change in the system state, such as vehicle arrival/departure and the busy/idle status of serving forklifts.
State variables A collection of variables used for describing the performance of a system over time, such as the number of trucks (queue length) waiting for a materials collection, and the average waiting time of these trucks (queue time).

Basics of selecting entities

This involves relevant entities being identified from the start. Examples include vehicles in a terminal, aeroplanes in an airport, or customers and clerks in a post office model.

In this method, you also need to distinguish between permanent entities (eg a machine or lorry) and temporary entities (eg a customer) that can be created and removed as appropriate.

Basics of selecting attributes

Entities can have attributes, such as customer ID, time spent waiting in a queue, type of service requested by a customer and time spent being served.

Your task

  1. Have a go at answering the following questions based on a temporary vehicles container:
    • What are the attributes?
    • What are the events?
    • What are the state variables?
  2. Have a go at answering the following questions based on a permanent overhead crane and forklift resources:
    • What are the attributes?
    • What are the activities?
    • What are the events?
    • What are the state variables?
Share your answers in the comments section below.

Once you have completed the task, check your answers by accessing the document in the downloads section at the end of this step.

© Coventry University. CC BY-NC 4.0
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Simulation for Logistics: An Introduction

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