Page 146 - DCAP405_SOFTWARE_ENGINEERING

Unit 9: System Engineering

It must be emphasised, furthermore, that there is an intimate connection between the model that Notes
is ‘appropriate’ for the study and the nature of the problem that is to be solved. The important
corollary here is that there rarely exists a ‘universal’ model that will support all modelling and
simulation projects that have a common system context. This is especially true when the system
has some reasonable level of complexity. Consider, for example, the difference in the nature of
a model for an airliner, first in the case where the model is intended for use in evaluating
aerodynamic properties versus the case where it is simply a revenue-generating object within a
business model. Identification of the most appropriate model for the project is possibly the most
challenging aspect of the modelling and simulation approach to problem solving.
Although the word ‘modelling’ has a meaning that is reasonably confined in its general usage,
the same cannot be said for the word ‘simulation’. Nevertheless, the phrase ‘modelling and
simulation’ does have a generally accepted meaning and implies two distinct activities. The
modelling activity creates an object (i.e., a model) that is subsequently used as a vehicle for
experimentation. This experimentation with the model is the simulation activity.
The word ‘simulation’ is frequently used alone in a variety of contexts. For example, it is
sometimes used as a noun to imply a specialized computer program (as in, ‘A simulation has
been developed for the proposed system.’). It is also used frequently as an adjective (as in, ‘The
simulation results indicate that the risk of failure is minimal,’ or ‘Several extensions have been
introduced into the language to increase its effectiveness for simulation programming’). These
wide-ranging and essentially inconsistent usages of the word ‘simulation’ can cause regrettable
confusion for neophytes to the discipline.

!

Caution As a rule, we avoid such multiplicity of uses of this word but, as will become
apparent, we do use the word as an adjective in two specific contexts where the implication
is particularly suggestive and natural.

9.1 System Engineering Hierarchy

Good system engineering begins with a clear understanding of context – the world view – and
then progressively narrows focus until technical details are understood. System engineering
encompasses a collection of top-down and bottom-up methods to navigate the hierarchy.

9.1.1 System Modeling

System engineering process begins with a world of view which is refined to focus more fully on
a specific domain of interest. Within a specific domain, the need for targeted system elements is
analyzed. Finally, the analysis, design, and construction of targeted system element is initiated.
Broad context is established at the top of the hierarchy and at the bottom, detailed technical
activities are conducted. It is important for a system engineer narrows the focus of work as one
moves downward in the hierarchy.
System modeling is an important element of system engineering process. System engineering
model accomplishes the following:

define processes.
represent behavior of the process.

define both exogenous and endogenous input to model.
represent all linkages.

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