Software Engineering

1.6 
Software breakdown structure
The technical inputs for the WBS are dependent on the software architecture, which 
is defined and evolved until the critical design review (CDR). The software break-
down structure (SBS) represents the hierarchy of software elements that must have 

---

16
CHAPTER 1 
Introduction to Software Engineering
work allocated for their specification, implementation, and testing. The challenge 
here is that the software architecture and resulting SBS morph over time as archi-
tectural design decisions are made. Architectural decisions may affect the software 
requirements and functional and/or physical definitions. 
Figure 1.2
 shows the evo-
lution of the SBS as the software architecture is defined and refined. With each 
refinement of the software architecture, the work definitions and resource estimates 
should become increasingly accurate and maintained within the scope of initial pro-
ject speculation.
The flow of software development stages follows a typical project sequence 
of time-phased, incremental steps. The initial phase of development addresses the 
product requirements. This is followed by a stage of design that establishes a func-
tional architecture that establishes 
what
 the software product must accomplish to 
enable each operational data processing transaction. The third stage, detailed archi-
tecture definition, transforms the functional representation into a structural con-
figuration (physical architecture) that provides the elemental specifications and 
guidance for software implementation. These three stages provide the detailed 
architectural information, specifications, diagrams, and drawings to permit the 
product to be implemented (fabricated, assembled, integrated, and tested). During 
these first three stages of development, the software breakdown structure evolves in 
depth (number of layers of decomposition) and in detail (technical accuracy). The