In my daily projects in the automotive and industrial automation industry I’m continually confronted with the following question: How many levels of software requirements have to be written? That’s an interesting question, especially if we take the aerospace industry also into account. Software requirement level are a key topic if you want to improve your requirement engineering process. Therefore, I want to highlight in this blog post this topic a bit closer. I will compare the specifications of functional safety standards IEC 61508, ISO 26262 and DO-178B / C. In the final conclusion I will provide project best practices based on my more than 15 years of experience.

In my view, a good software specification is divided into two major parts: architecture / design and textual requirements.
The architecture describes, most predominantly in graphical form, the structure and design of the software. In particular, the data and control flows are shown. The focus of textual requirements is on the description of the functionality, and the time demands on the system.
The initial question of this blog refers to the number of levels of textual requirements. Not included is the level of system requirements, which must always be present. Read more