In school, systems engineering is taught as a top-down process, but in actual practice it involves bottom-up techniques. In the former, the desired system is broken down or partitioned into smaller subsystem parts in order for requirements, functions and architectures to be decomposed to a point where engineers can begin to build hardware, software, networks, etc.

Conversely, the bottom-up approach begins with the integration of lower level hardware, software, network and other components. These subsystems are tested and built-up until the original desired systems is created.  Almost all of the traditional engineering disciplines (like electronic, mechanical, software and network engineering) follow a subsystem or component bottom-up approach to design and test.

Most engineers and managers in the real world follow a middle-out or inside-out approach. As the name implies, the “middle-out” systems engineering method consists of concurrent bottom-up and top-down systems engineering activities. The bottom-up tasks are built on a detailed knowledge of component parts and subsystems.  The concurrent top-down activities will preserve the customer-focused, requirements-driven emphasis that keeps the system development in a functional domain.

One of the key benefits of the middle-out approach is the traceability afforded by combining the top-level requirements-function-synthesis process with the known requirements and functions from bottom-level implemented system elements. Both executive level and component/subsystem engineers are brought together in this activity to ensure the traceability of requirements. Critical members from both groups will then be involved in the design and integration decisions.

Image Source: Wiley – JB Systems

Several experts and practitioners agree – to varying degrees – that most real world systems engineering projects follow a middle-out approach.

“I agree that many projects “should” take middle-out approaches since so few projects today are creating new systems from complete scratch,” observed Cary Bryczek, Principle Solutions Architect for aerospace and defense for Jama Software. “Things like modernization efforts, developing product variants, and the Internet of Things are all requiring a consideration where the future environment itself is uncertain. But I also still see many projects in safety critical spaces – like defense and automotive – are taking traditional top down systems engineering approaches. I suspect a lot of this is driven by contract vehicles.”

Mark Sampson, product manager at Siemens, agrees that a majority of projects involve changes to existing products. However, he prefers the phrase inside-out over middle-out as the former focuses on understanding the impact of a change (e.g., add, remove, or update).

“Today that development process relies on knowledge, talking with experienced people, etc. rather than models to understand the impacts,” explains Samspon. “Of course it all gets much easier if you’ve designed your product for evolving changes by considering up front what the architecture of the product would be and where the possible areas of change are over time.

Regardless of the name, most systems engineers must meet both top-down, corporate objectives and bottom-up, product requirements. Fortunately, the growth of the Model-Based Systems Engineering (MBSE) paradigm supports a middle-out approach. Models can be used in both the top-down, multiple domain architectural and requirements design as well as the bottom-up simulation and prototyping of preliminary system, subsystems and component evaluation and verification. Together, these models provide a platform that combines high-level system models with specific component and subsystem oriented executable models.

The middle-out approach is familiar to the electronics space. Consider the PCB design tool space where vendors are now being driven both from the top-down and also from the middle-out, notes Paul Dempsey, co-founder of the Tech Design Forum. “For example, Altium community beta members have explicitly reached out to the maker community for many middle-out activates.”

John Blyler is a Design News senior editor, covering the electronics and advanced manufacturing spaces. With a BS in Engineering Physics and an MS in Electrical Engineering, he has years of hardware-software-network systems experience as an editor and engineer within the advanced manufacturing, IoT and semiconductor industries. John has co-authored books related to system engineering and electronics for IEEE, Wiley, and Elsevier