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Prescriptive Requirements are usually specified before designing and implementing the system. They provide a set of constraints that the design and implementation needs to comply with.
These requirements specify what the system behaviors should be from a high level point of view. They are sometimes called "Product Requirements".
They also answer other basic, high level questions:
- Are there external standards to which the system should comply with? Examples:
- The system shall comply with IEC-60601-2
- What are the system level parameters? Examples:
- The pump shall be able to provide 0.1ml/hr to 1000 ml/hr delivery rates for all therapies.
- There shall be 2 types of therapies: Basic and Intermittent
- The pump shall communicate to the Server via 802.11abgn and a cellular modem
Most User Requirements can be traced to Prescriptive Requirements, i.e. it should be possible to confirm that a User Requirement is being met using a set of one or more Prescriptive Requirements. Some simply "bleed through" into Prescriptive Requirements. Example:
- The battery capacity shall be sufficient to provide at least 8 hours of operation
Note that there will be Prescriptive Requirements that may not tie back to a User Requirement, e.g. the system shall comply with IEC-60601-2.
Prescriptive Requirements rarely change since they describe what the final product's overall characteristics are. As the design and implementation is completed, there may be changes to clarify them or to mark them as for future development.