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Table 1. Essential simulation concepts
An entity represents real-world objects. Entities that move through the system (e.g.
products, customers) are dynamic and entities that serve other entities (e.g. conveyors,
machines, warehouse) are static.
Entities
An event is an occurrence that changes the state of the simulation system and is the
beginning and ending of an activity or delay (e.g. freight is loaded).
Events
An attribute describes the characteristics of an entity (e.g. time of arrival, due date
priority, color). A set of entities may have the same attribute slots but different values
for different entities, so the attribute value is tied to a specific entity.
Attributes
An activity represents a specific period of time. The duration of this time period is
known prior and can be a constant, a random value from a statistical distribution or
input from a file, etc. (e.g. processing time of a machine).
Delays A delay is an indefinite period of time. The duration is caused by some combination of
system conditions (e.g. the freight is waiting for loading).
The fundamental goal of simulation in logistics is the study of transport volumes
and capacities of the partial logistics services over time to ensure that customers'
demand can be met. So it is possible to analyze the flows of goods through the
logistics system with regard to the capacity in order to identify bottlenecks early on.
To create simulation models of a specific domain (e.g. logistics) primarily
application-oriented modeling concepts are used [14]. Typical in logistics is the use of
"modular concepts". These provide topological, organizational and / or informational
elements - appropriately aggregated, predefined and parameterized from an
application perspective - for a specific application domain [29]. Two simulation tools
which are widely used in the logistics domain, which realize more or less the
application-oriented modeling concept (Enterprise Dynamics (ED) 2 and Arena 3 ), have
been used to create different examples of simulation models to study transport
volumes and capacities [30]. These tool-dependent models have been analyzed and
compared in terms of the used modeling concepts and the required data. The common
basic concepts were consolidated and used to create the metamodel shown in Fig. 6.
A simulation model basically consists of the following concepts. A source
generates goods at predefined time periods. A sink is the concept where goods leave a
model. The purpose of an activity is to handle goods. Therefore, goods enter an
activity and remain there for a certain time. Moreover, an activity is assigned to a
service type. All time periods can be described by a set of distribution functions.
Regarding the service type, a capacity is an additional characteristic of an activity.
For instance, an activity with the service type “warehouse service” is restricted by a
maximum capacity and has a certain queuing strategy. The connecting elements
between the activities are represented by two different relations . On the one hand,
relations can be simple, i.e., without specific characteristics. On the other hand, a
Activities
2 http://www.incontrolsim.com/
3 http://www.arenasimulation.com
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