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A Systems Engineering Model

What is systems engineering? When is it useful?

All Honourcode courses and assistance fit within the context of systems engineering. For many, however, the concepts of systems engineering are not familiar. This page provides some context and definitions for the field.


First, the concept of a system is important. The definition of a system according to the International Council on Systems Engineering is:

A system is an interacting combination of elements, viewed in relation to function.

There are two parts to this definition. First is that any system is made up of elements. Therefore, the field of systems engineering is concerned with the combination of elements into a working whole.

Second is that any system exists only in the mind of the viewer; the viewer determines what are the functions of interest and therefore the boundaries that matter. One humorous way to remember this is the acronym BOSIWIT: the Boundary Of a System Is What I Think. So systems engineering, therefore, involves also the definition of boundaries for the system in terms of its functions.

Systems Engineering

In its simplest definition, systems engineering is the engineering of systems. Any effort to apply science and technology to real-world problems (which is "engineering"), where those real-world problems involve interacting combinations of elements (which are "systems") is part of systems engineering.

Systems engineering is usually described in terms of the things that we do to engineer systems. Many people attempt to describe it by the things we "should" do, and thereby fall into traps limited by the domain in which they work. We prefer to define systems engineering by the things that people actually do in any domain, regardless of how well they do them. A good description of systems engineering applies to systems as simple as a toaster and as complex as environmental restoration. The only difference between these two extremes is the degree of formality with which each process is used.

This diagram is a model of systems engineering that includes the known processes that people do. The diagram is an amalgam of the popular systems engineering standards (ISO-15288, ANSI/EIA-632, IEEE-1220, SP-6105, ECSS-E-10A).

This basic systems engineering model has four steps, each of which encompasses several processes, and two groups of “guiding” processes that control the entire model.

  • Agreement processes define the need and create an agreement between the "customer" and the developer.
  • Requirements Definition processes enable a full understanding of the technical requirements embodied in the need.
  • Solution Definition processes develop alternative possible solutions, then evaluate the alternatives to select a baseline.
  • System Realization processes implement and deploy a system to satisfy the need.
  • Technical Evaluation processes provide analysis, verification, and validation to ensure product system quality.
  • Technical Management processes provide the structure that keeps the steps flowing.

There is a forward flow through the main diagonal, that represents the forward movement of a system development. There are also loops that interact to represent the spiral development repetition that is common in complex systems.

When the system is simple, like a toaster, many of these processes happen very quickly in the designer's mind. For example, the process Describe the Need happens simply by saying, "My customer's 'need' is to toast bread." The process Quantify the Need involves deciding what "toast bread" means to the target customers.

When the system is large and complex, however, these same processes may take months or years to complete. Describe the Need and Quantify the Need may involve writing a series of Concept of Operations documents in customer language, reviewing those concepts with target customers, modifying the concepts, and presenting the results to many domains for clarity. Even when "done," the documents will be modifed in later development stages due to new realizations that propagate upward through the model loops.

For more information on Systems Engineering, see our courses or contact the International Council on Systems Engineering.

We also offer an outline for a Systems Engineering Management Plan (SEMP) that you can download and use. [MS Word, 134 KB]

Page last modified 12 Oct 03