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280 Chapter 10 Sociotechnical systems

Requirements
Domain and Problem Elicitation and
Understanding
Analysis

Start

Review and Architectural
Assessment Design

Figure 10.8 System Requirements and
Requirements and Design Documentation
design spiral

designs and requirements may be accepted or rejected. The process ends when a
review decides that the requirements and high-level design are sufficiently detailed
for subsystems to be specified and designed.
In the subsystem engineering phase, the hardware and software components of
the system are implemented. For some types of system, such as spacecraft, all hard-
ware and software components may be designed and built during the development
process. However, in most systems, some components are commercial off-the-shelf
(COTS) systems. It is usually much cheaper to buy existing products than to develop
special-purpose components.
Subsystems are usually developed in parallel. When problems that cut across sub-
system boundaries are encountered, a system modification request must be made.
Where systems involve extensive hardware engineering, making modifications after
manufacturing has started is usually very expensive. Often ‘work-arounds’ that com-
pensate for the problem must be found. These ‘work-arounds’ usually involve soft-
ware changes because of the software’s inherent flexibility.
During systems integration, you take the independently developed subsystems
and put them together to make up a complete system. This integration can be done
using a ‘big-bang’ approach, where all the subsystems are integrated at the same
time. However, for technical and managerial reasons, an incremental integration
process where subsystems are integrated one at a time is the best approach:

1. It is usually impossible to schedule the development of the subsystems so that
they are all finished at the same time.
2. Incremental integration reduces the cost of error location. If many subsystems
are simultaneously integrated, an error that arises during testing may be in any of