Requirements

A requirement is a specification for an individual system function. Requirements that must be supported by the system are specified in detail and must be achievable and testable.

They are the specification of the essential subject matter of the system. Specifying a requirement clarifies the system's subject matter and in doing so, may trigger requirements that have not yet been thought of.

Often requirements are written in incomplete form as "The system shall do some described task." To be complete a requirement needs to include:

• The name of the external system causing the excitation Basics of Behaviour.What the external system does.The outputs from the external system to the subject system.All pertinent conditions.What the subject system does in response.All quantitative aspects of the response, how fast or how much.The outputs from the subject system.All pertinent conditions.

• The names of the external systems which receive those outputs.

Requirements are the foundation of a system and form the basis for the design, manufacture, test, and operation of a system. Consequently, each requirement has a cost impact on the system. It is therefore essential that a complete, but minimum set of requirements be established early in development. Changes in requirements later in the development cycle can have a significant cost impact on the system, possibly resulting in project cancellation.

Requirements come from a variety of sources, some come from the customer/user, some come from regulations/codes, and some come from the corporate entity.

Requirements definition is a complex process that employs performance analysis, trade studies, constraint evaluation and cost/benefit analysis.

Capturing Requirements

Requirements are extracted, clarified, and prioritised from all of the written directives embodied in source documents. Examples of source documents are:

• New or updated customer needs, requirements, and/or objectives.Technology base data including identification of key technologies, performance, maturity, cost, and risks.The outputs from the preceding acquisition phase.Requirements from contractually cited documents for the system and its configuration items.Technical objectives.

• Records of meetings and conversations with the customer.

These source or originating requirements should be reviewed for consistency. Inconsistent or questionable items should be identified for resolution with the customer or by internal trade studies.

The source requirements gained by carrying out this function are only a portion of the total system requirements. They should be expanded and clarified.

The system engineering process analyses project objectives and requirements to ensure that they are feasible and cost effective, and adds functional detail and design requirements with the goal of achieving the proper balance among operational, economic, and logistic factors. It employs a sequential and iterative methodology to reach cost-effective design alternatives. System engineering is directly concerned with the transition from customer-specified requirements to a fully defined set of system requirements.

The requirements are very useful for contractual purposes because they provide a check list of what the implementer must deliver.