Software Engineering

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4.3 
Complexity control mechanisms
The complete definition of the PBS is extracted from the software product 
physical architecture and reflects the work effort to design, code, and test software 
units and the integration of these units into software components and configuration 
items. Therefore, the initial PBS that supports early project and technical planning 
is based on abstract software components that represent large wedges of anticipated 
work. These initial planning approximations are known as ballpark estimates and 
provide a placeholder within the work plan that must be enhanced in precision as 
more knowledge of the software product architecture is made available. 
Figure 4.5
 
depicts a conceptual evolution of the PBS as the software product physical architec-
ture is refined and characterized. This occurs when software requirements are ana-
lyzed, design alternatives are investigated, and a design approach is chosen. The 
figure only shows a partial decomposition for one configuration item. The definition 
of the PBS is expanded throughout the early software definition activity, but should 
be finalized by critical design review (CDR). By this milestone, the software prod-
uct physical architecture should be complete with a full set of identified structural 
units and software integration strategy.