System Definition

Systems Definition encompasses the activities in the systems engineering process that precede systems realization.

Topics

The topics contained within this knowledge area include:

• fundamentals of system definition;
• mission analysis and stakeholder requirements;
• system requirements;
• architectural design; and
• system analysis.

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System Definition Activities

System definition is the set of technical creative activities of systems engineering (SE). The activities are grouped and described as generic processes that are performed iteratively and/or concurrently depending on the selected development cycle model life cycle. The processes involved with system definition include mission analysis, stakeholder requirements, system requirements, architectural design, and system analysis topics. The system definition processes and activities are also applied recursively at each successive level of the system hierarchy. See the discussion of iteration and recursion in the Part 3 Introduction: Systems Engineering and Management.

Top-Down Approach: from the Problem to the Solution

In a top-down approach, the system definition activities are focused primarily on understanding the problem, the conditions that constrain the system, and the design of solutions. The outcomes of the system definition are used for the system realization, system deployment and use, and product and service life management. In this approach, system definition includes the activities that are completed primarily in the front-end portion of the system design and the design itself. These consist of mission analysis, stakeholders’ requirements, system requirements, architectural design, and system analysis. Top-down activities can be sequential, iterative, or evolutionary.

• Mission analysis and stakeholder requirements focus on the identification and definition of stakeholders' needs, the development of operational and environmental conditions, of operational concepts, and the definition of applicable constraints.
• These elements are then used for the development of system requirements that consist of the refinement and translation of the stakeholders’ requirements into system (technical) requirements.
• These system requirements are then used as inputs for the architectural design, which includes functional architecture , dynamic behavior, and physical architecture .
• System analysis studies are performed to evaluate and select the potential system elements that compose the system and are the most suitable. System analysis provides a best value, balanced solution involving all the relevant engineering elements (stakeholder requirements, system requirements, and architectural Design Properties).

Bottom-Up Approach and Evolution of the Solution

Engineers are led to reconsider the system definition in order to modify or adapt some structural, functional, or temporal properties during the product and service because of the evolution of the context of use or for the purpose of improving the existing solution. Because of the existence of the system of interest (SoI) a reverse engineering of the system is often necessary to recharacterize its properties or those of its systems' or system elements before attempting any modification.

A bottom-up approach is necessary for analysis purposes, or for (re)using existing elements in the design architecture . Changes in the context of use or a need for improvement can prompt this. In contrast, a top-down approach is generally used to define an initial design solution corresponding to a problem or a set of needs.

Bottom-up and top-down approaches can be, and often are, mixed.

Separation and Iteration Between the Problem Area and the Solution Area

Problem definition and solution design depend on each other. Solutions should respond to well-defined problems. Problem definitions should be constrained to what is feasible in the solution space. System analysis activities are used to perform the link between problems and solutions.

As systems generally integrate existing and new system elements (system elements), a bottom-up approach is used with a top-down approach to take into account legacy elements, as well as to identify the services and capabilities they provide in order to define applicable interface requirements and constraints. As discussed in System Life Cycle Process Models: Iterative, this is iterative for these evolutionary systems.

For more details about systems approaches, read (Jackson, Hitchins, and Eisner, 2010), and (Hitchins, 2007).

Ontologies

System definition depends on good ontological structure. See the discussion in Systems Engineering and Management.

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References

Citations

Faisandier, A. Engineering and Architecting Multidisciplinary Systems (forthcoming).

Hitchins, D. Systems Engineering: A 21st Century Systems Methodology (New York: Wiley and Sons, Inc., 2007).

Jackson, S., D. Hitchins, and H. Eisner. “What Is the Systems Approach?” Insight, April 2010, 41-43.

Primary References

ANSI/EIA, “Processes for Engineering a System,” document ANSI/EIA-632-1998, American National Standards Institute (ANSI)/Electronic Industries Association (EIA).

INCOSE, “INCOSE Systems Engineering Handbook,” version 3.2, document INCOSE-TP-2003-002-03.2, International Council on Systems Engineering.

ISO/IEC, “Systems Engineering—A Guide for the Application of ISO/IEC 15288 System Life Cycle Processes,” document ISO/IEC 19760:2003 (E), International Organization for Standardization (ISO)/International Electronical Commission (IEC).

ISO/IEC, “Systems Engineering—Application and Management of the Systems Engineering Process,” document ISO/IEC 26702:2007, International Organization for Standardization (ISO)/International Electronical Commission (IEC).

ISO/IEC, “Systems and Software Engineering—System Life Cycle Processes,” document ISO/IEC 15288:2008 (E), International Organization for Standardization (ISO)/International Electronical Commission (IEC).

NASA, “Systems Engineering Handbook,” document NASA/SP-2007-6105, National Aeronautics and Space Administration (NASA).

Additional References

Faisandier, A. Engineering and Architecting Multidisciplinary Systems (forthcoming).

Hitchins, D. Systems Engineering: A 21st Century Systems Methodology (New York: Wiley and Sons, Inc., 2007).

ISO, “Systems Engineering and Design,” document ISO 10303-AP233, International Organization for Standardization (ISO).

Jackson, S., D. Hitchins, and H. Eisner, “What Is the Systems Approach?” Insight, April 2010, 41-43.

Oliver, D., T. Kelliher, and J. Keegan. Engineering Complex Systems with Models and Objects (New York: McGraw-Hill Book Company, 1997).

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