For most engineering professions, the term “specification” refers to the assignment of numerical values or limits to a product’s design goals.
Requirement Specification
In software engineering, “software requirements specification” typically refers to the production of a document that can be systematically reviewed, evaluated, and approved. For complex systems, particularly those involving substantial nonsoftware components, as many as three different types of documents are produced: system definition, system requirements, and software requirements. For simple software products, only the third of these is required. All three documents are described here, with the understanding that they may be combined as appropriate. A description of systems engineering can be found in the Related Disciplines of Software Engineering chapter of this Guide.
System Definition Document
This document (sometimes known as the user requirements document or concept of operations document) records the system requirements. It defines the high-level system requirements from the domain perspective. Its readership includes representatives of the system users/customers (marketing may play these roles for marketdriven software), so its content must be couched in terms of the domain. The document lists the system requirements along with background information about the overall objectives for the system, its target environment, and a statement of the constraints, assumptions, and nonfunctional requirements. It may include conceptual models designed to illustrate the system context, usage scenarios, and the principal domain entities, as well as workflows.
System Requirements Specification
Developers of systems with substantial software and nonsoftware components—a modern airliner, for example—often separate the description of system requirements from the description of software requirements. In this view, system requirements are specified, the software requirements are derived from the system requirements, and then the requirements for the software components are specified. Strictly speaking, system requirements specification is a systems engineering activity and falls outside the scope of this Guide.
Software Requirements Specification
Software requirements specification establishes the basis for agreement between customers and contractors or suppliers (in market-driven projects, these roles may be played by the marketing and development divisions) on what the software product is to do as well as what it is not expected to do.
Software requirements specification permits a rigorous assessment of requirements before design can begin and reduces later redesign. It should also provide a realistic basis for estimating product costs, risks, and schedules.
Organizations can also use a software requirements specification document as the basis for developing effective verification and validation plans.
Software requirements specification provides an informed basis for transferring a software product to new users or software platforms. Finally, it can provide a basis for software enhancement.
Software requirements are often written in natural language, but, in software requirements specification, this may be supplemented by formal or semiformal descriptions. Selection of appropriate notations permits particular requirements and aspects of the software architecture to be described more precisely and concisely than natural language. The general rule is that notations should be used that allow the requirements to be described as precisely as possible. This is particularly crucial for safety-critical, regulatory, and certain other types of dependable software. However, the choice of notation is often constrained by the training, skills, and preferences of the document’s authors and readers.
A number of quality indicators have been developed that can be used to relate the quality of software requirements specification to other project variables such as cost, acceptance, performance, schedule, and reproducibility. Quality indicators for individual software requirements specification statements include imperatives, directives, weak phrases, options, and continuances. Indicators for the entire software requirements specification document include size, readability, specification, depth, and text structure.
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Published on : 30-May-2018
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