INCOSE’s Systems Engineering Handbook includes many terms starting with the words ‘Life Cycle’: life cycle stages, life cycle processes, life cycle characteristics, life cycle approaches, life cycle concepts, life cycle aspects, life cycle constraints, and life cycle models. Confused? You are likely to be, especially if you are seeing these terms for the first time. However, it is not a surprise to see so many ‘life cycle’ terms because systems engineering (SE) focuses on the entire life of the system of interest (SoI) being engineered.
In our previous article, we talked about lifecycle stages, where the entire life of an SoI may be divided into six generic stages. We also talked about activities that will have to be performed repeatedly. The term ‘lifecycle model’ is used to denote the activities and the manner (or sequence) in which they are performed, as the SoI journeys through its entire life. Lifecyle models tend to focus on the concept and development stages of the SoI, typically shown with an illustration. In this article, we will look at some lifecycle models related to Systems Engineering (SE).
The Traditional Vee
The lifecycle model that is very commonly associated with SE is the Vee model, called so because it looks like the alphabet ‘V’. if you search for “Vee model” in Google images, you are likely to find many variations of this model. The only commonality you will find is the basic shape! But even there, you will find a few variants with a flange on the top left and top right, and many without; in some, you will find a flat portion at the bottom. In addition, you will also find some Vee models associated with ‘software engineering’ and ‘software development life cycle (SDLC)’. In a future article, we will discuss the overlap between systems engineering and software engineering.
In its basic form, the left half of the Vee (going down to the bottom starting from the top left) is used to denote activities related to requirements, architecture, and design of a SoI (i.e. decomposition), terminating with sufficient detail (i.e. definition) to build the SoI. These activities are usually performed on ‘paper’ or digitally. The right half of the Vee (going up from the bottom to the top right, including the bottom) is used to denote activities related to building (or realizing) the components of the SoI (i.e. implementation) and then putting them together (i.e. integration) in a progressive manner to arrive at the final SoI in its final form (i.e hardware, software, etc.).
The variants of the Vee model will primarily differ in the activities that are labelled along the left and right of the Vee. Visual representation of the Vee model captures the idea of a top-down approach to design and development (left side of V), and a bottom-up approach to integration and testing (right side of V). But then, it hides the iterative nature of activities and instead conveys a linear progression through them. In reality, there are multiple iterations and loops before the final definition of the SoI emerges. However, the major advantage of the Vee model is the intuitive horizontal connection between the left and right sides of the Vee. It encourages one to think across and think ahead, beyond the current activities being performed on the SoI (i.e. verification and validation). This aligns well with a systems engineering approach, where one needs to take a holistic lifecycle approach to the engineering of an SoI.
Extensions of the Vee
Despite the fact that the Vee model inadvertently masks many iterations and loops, it still falls short of reflecting the reality of engineering a complex system. It is very rare that a single Vee is transcended just once in the development of an SoI. It is common in the development of complex SoIs, to build and test multiple prototypes (physical and/or digital) before the SoI is ready for mass production (as required). A couple of extensions of the basic Vee model reflect this reality in a better way.
In an incremental lifecycle model, the development activities on the left of the Vee, lead to multiple versions (or prototypes) of the SoI to be built. These versions could be identical or could differ in terms of functionalities or features. In an iterative life cycle model, the Vee model is executed multiple times in succession, most often with an overlap.
When a complex SoI is being engineered, the life cycle model that actually gets followed is some combination of the iterative and incremental models.
Beyond the Vee
In addition to the Vee model and its extensions, there are a few other lifecycle models that are considered.
The Waterfall model is commonly used when indeed a linear progression of activities across multiple stages is indicated, with limited repetition or feedback. This is helpful when the design and development of an SoI is known with reasonable clarity and when no major surprises are anticipated.
The Spiral model is another lifecycle model that is common in SE, though one could argue that this is similar to the iterative Vee model, just that it is illustrated differently. Every phase of the spiral model progresses towards the final SoI. This model is common in the development of software systems as well.
The Agile model has its origins in software development, but is increasingly being used in software-intensive hardware systems, though the ‘sprints’ are much longer.
With so many different lifecycle models in existence and the fact that there is no single lifecycle model that is strictly followed in the development of an SoI, one might wonder what is the value of these life cycle models! Further, the development of the entire SoI could follow one life cycle model, while the development of some sub-systems of components of the SoI could follow a different life cycle model!
The value of using a lifecycle model to guide the development of an SoI is purely to help in the management of activities, resources, reviews, interactions, teams, and organizations; and the orderly progress of the SoI across its lifecycle stages in a way that can minimize surprises and maximize success!
- KS & DH
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