# 11 - Model-Based Systems Engineering (MBSE)

The best way to understand the MBSE approach is to begin by understanding the alternative: what modeling partitioners call the document-based approach to engineering (and what nonpractitioners call "the way we have always done things!").

With the document-based approach, engineers manually generate some subset of the following artifacts: concept of operations (ConOps) documents, requirements specifications, requirements traceability and verification matrices, interface definition documents, systems design specifications and speciality engineering analysis such as analyses of reliability, FMEA, etc. Document-based systems engineers produce these artifacts in a form of a disjoint set of text documents, spreadsheet, diagrams, presentations (and configuration-manage them in a disjoint set of repositories).

The document-based approach is expensive. More precisely, it's more expensive than it needs to be; you incur a significant percentage of total life cycle cost maintaining that disjoint set of artifacts. And if you don't pay that cost, the artifacts become inconsistent and obsolete.

This approach is also time consuming and error prone. This scenario is common place in organization that practice the traditional document-based approach. Inconsistency is the problem and MBSE, when practiced correctly, is the solution. With MBSE approach, engineers perform the same life cycle activities and produce the same set of deliverables. But the deliverables are not the immediate outputs of the life cycle activities; they are not the primary artifacts. With the MBSE approach, the primary artifact of those activities is an integrated, coherent, and consistent system model. All other artifacts are secondary, automatically generated from the system model using that same modeling tool.

The system model serves as a central repository for design decisions; each design decision is captured as a model element, or a relationship between elements, in a single place within the system model. With the MBSE approach, all diagrams and autogenerated text artifacts are merely views of the underlying system model; they are not the model itself. And that distinction is the root of the Return On Investment (ROI) that MBSE offers over the traditional approach.


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