Nonfunctional Requirements in Systems Analysis and Design

Engineering Systems, the man-made systems that provide important functions in modern societies, are designed by engineers. The design of engineering systems is a formal process which invokes both technical and human elements to provide the blueprint for systems which simultaneously provide for the needs of a system’s stakeholders while not harming the environment or living beings. Engineering design is the term used for the formal processes and methodologies used to create and maintain man-made systems in a life cycle that runs from inception through retirement and disposal.

Engineering design is a formal discipline within the field of engineering. The study of design methodologies is a sub-discipline and requires the use of unique modes of thought and the application of a number of specific features to ensure that designs are both repeatable and result in products that are useful for a specified period of service. A methodology is purposefully positioned in a formal hierarchy of scientific approaches, supported by a specific paradigm and philosophy while acting as the framework for more detailed methods and techniques. There are a number of unique engineering design methodologies, frameworks, and models that have evolved to provide the structural framework for the applicable design processes, methods, and techniques. The Axiomatic Design Methodology provides a systems-based framework for design that permits design alternatives to be evaluated based on quantitative analysis, eliminating the need for messy qualitative and cost-based models.

One of the most easily understood tasks during any systems design endeavor is to define the systems functional requirements. The functional requirements are a direct extension of the stakeholder’s purpose for the systems and the goals and objectives that satisfy them. Less easily understood are a systems non-functional requirements, or the constraints under which the entire system must operate. Identification of non-functional requirements should happen early in the conceptual design stage of the systems life cycle, for the same reason that functional requirements are defined up-front—that is, costs sky-rocket when new requirements are added late in a systems design sequence. Approaches for addressing non-functional requirements are rarely addressed in texts on systems design. In order to provide a logical and repeatable technique for addressing over 200 existing non-functional requirements, they must be reduced parsimoniously to a manageable number. Over 200 non-functional requirements are reduced, using results reported in eight models from the extant literature. The 27 resultant non-functional requirements have been organized in a taxonomy that categorizes the 27 major non-functional requirements within four distinct categories. Utilization of this taxonomy provides a framework for addressing non-functional requirements during the early system design stages.

Effective sustainment of systems and components during the operation and maintenance stages of the system life cycle require specific purposeful actions during the design stages of the system life cycle. The reliability and maintainability of the system and its constituent components are established as part of the system design process. Reliability and maintainability are non-functional requirements that exists at both the component- and system-level and are intertwined and interrelated. Improper reliability and maintainability designs in any level of the system’s hierarchy may have far reaching affects. The ability to understand how reliability and maintainability are treated in the design process and formal metrics and measurement processes for each non-functional requirements are essential during all system design endeavors.

Effective sustainment of systems and components during the operation and maintenance stages of the system life cycle require specific purposeful actions during the design stages of the system life cycle. The availability and testability of the system and its constituent components are essential to ensure that the systems continues to provide the required functions to its stakeholders. Availability and testability are non-functional requirements that exists at both the component- and system-level and are intertwined and interrelated. Inattention to testability concerns may lead to decreased availability having far reaching affects that include the systems own viability. The ability to understand how availability and testability are implemented in the design process and formal metrics and measurement processes for each non-functional requirements ensure that they are adequately addressed during all system design endeavors.

The design of systems and components during the design stage of the systems life cycle requires specific purposeful actions to ensure effective designs and viable systems. Designers are faced with a number of design concerns that they must embed into the design in every instance of thinking and documentation. Four of these concerns are addressed by the non-functional requirements for conciseness, modularity, simplicity, and traceability. Formal understanding of each of these non-functional requirements requires definitions, terms, and equations, as well as the ability to understand how to control their effect and measure their outcomes during system design endeavors.

The design of systems and components during the design stage of the systems life cycle requires specific purposeful actions to ensure effective designs and viable systems. Designers are faced with a number of design concerns that they must embed into the design in every instance of thinking and documentation. Safety is one of these concerns and is addressed by the non-functional requirement for safety which is composed of seven attributes. The development of the seven safety attributes were developed using Leveson’s Systems-Theoretic Accident Model and Processes (STAMP). Because STAMP is a system-theoretic approach appropriate for evaluating safety in complex, systems-age engineering systems, the safety attributes provide the ability to understand how to control safety and measure its outcomes during system design endeavors.

The design of systems and components during the design stage of the systems life cycle requires specific purposeful actions to ensure effective designs and viable systems. Designers are faced with a number of core viability concerns that they must embed into the design to ensure the system remains viable. The ability for a system to remain viable is critical if it is to continue to provide required functionality for its stakeholders. Core viability concerns includes the non-functional requirements for understandability, usability, robustness, and survivability. Purposeful design requires an understanding of each of these requirements and how to measure and evaluate each as part of an integrated systems design.

The design of systems and components is a crucial element that affects both the cost and efficacy of products produced for the world economy. Design is a characteristic function of engineering. The structure of engineering education underwent a major shift after WWII. The nationwide shift toward a more science-based curricula for all levels of education led to design type courses being devalued and even omitted in engineering education. Recent efforts to re-invigorate design in both undergraduate and graduate engineering programs in the United States have re-emphasized the role of design in the engineering curricula. The current text has been developed to address a unique topic in engineering design—non-functional requirements in systems analysis and design endeavors, thereby seeking to fill a perceived void in the existing engineering literature.