Introduction to Life Cycle Processes
In this article we introduce key principles of life cycle, life cycle model and life cycle processes. A generic SE paradigm is described; this forms a starting point for discussions of more detailed life cycle knowledge.
Life Cycle Terminology
The term life cycle is one that engineering has borrowed from the natural sciences, where it is used to describe both the changes a single organism goes through over it life and how the lives of multiple organisms interact to sustain or evolve a population. We use it in the same way to describe the complete life of an instance of a system-of-interest (SoI); and the managed combination of multiple such instances to provide capabilities which provide stakeholder satisfaction.
A life cycle model identifies the major stages that a specific SoI goes through, from its inception to its retirement. The stages are terminated by decision gates where the key stakeholders decide whether to proceed into the next stage, to remain in the current stage, or to terminate or re-scope related projects. Systems Engineering life cycle process define technical and management activities performed across one or more stages to provide the information needed to make these life cycle decisions; and to enable to realization, use and sustainment of systems across the life cycle as necessary. This relationship between life cycle model and process activities describes how SE is applied to different system contexts.
Generic Systems Engineering Paradigm
Figure 1 identifies the overall goals of any SE effort, which are: the understanding of stakeholder value; the selection of a specific need to be addressed; the transformation of that need into a system (the product or service that provides for the need); and the use of that product or service to provide the stakeholder value. This paradigm has been developed according to the principles of the systems approach discussed in Part 2 and is used to establish a basis for the KAs in Part 3 and Part 4 of the SEBoK.
On the left side of Figure 1, there are SoI's identified in the formation of a system breakdown structure. SoI 1 is broken down into its basic elements, which in this case are systems as well (SoI 2 and SoI 3). These two systems are composed of system elements that are not refined any further.
On the right side of Figure 1, each SoI has a corresponding life cycle model which is composed of the stages that are populated with processes. The function of these processes is to define the work that is to be performed. Note that some of the requirements defined to meet the need are distributed in the early stages of the life cycle for SoI 1, while others are designated to the life cycles of SoI 2 or SoI 3. The decomposition of the system illustrates the fundamental concept of recursion as defined in the ISO/IEC 15288 standard; with the standard being reapplied for each SoI (ISO/IEC 15288). It is important to point out that the requirements may be allocated to different system elements, which may be integrated in different life cycle stages of any of the three SoI's; however, together they form a cohesive system. For example, SoI 1 may be a simple vehicle composed of a chassis, motor and controls, SoI 2 an embedded hardware system, and Sol 3 a software intensive interface and control system.
When performing SE processes in stages, iteration between stages is often required (e.g. in successive refinement of the definition of the system or in providing an update or upgrade of an existing system). The work performed in the processes and stages can be performed in a concurrent manner within the life cycle of any of the systems of interest and also among the multiple life cycles.
This paradigm provides a fundamental framework for understanding generic SE (seen in Part 3), as well as for the application of SE to the various types of systems described in Part 4.
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