What is Model-Based Systems Engineering

The below captures a definition for MBSE and some considerations and questions around MBSE that may be considered at the start of a project or when already doing a system development endeavour.

"Model-based systems engineering (MBSE) is the formalized application of modelling to support system requirements, design, analysis, verification and validation activities beginning in the conceptual design phase and continuing throughout development and later life cycle phases", INCOSE.

Modelling activities and effort should focus on developing an abstraction model of a product or a service. Hence, a model is a simplified abstraction of the System under development (System of Interest):

• If it was not a simplification of the System, it would be the System.
• The model is the total recorded knowledge of the project i.e. a single point of reference; ideally it will be in one place but in practice may be distributed across multiple tools or repositories.

The International Council on Systems Engineering (INCOSE) UK has produced a series of accessible guides to aspects of systems engineering, each in the form of a one page, double sided, guide designed to be folded up into three panels giving a "Z" cross section, to be known as "Z-Guides". The aim is to provide focussed, accessible, information which can be presented to individuals who are not directly involved in systems engineering on a day to day basis.

A reference to the INCOSE Z9: What is Model-Based Systems Engineering .

Why MBSE and the creation of a model?

Understand growing systems complexity

Systems are growing in complexity. Modelling, helps to break systems down into simpler constructs, this process helps complex systems to be understood.

Tackle the lack of understanding

Not always the system designed or developed meets the stakeholders needs. One of the main reasons is that the initial stakeholder needs were not fully understood. An approach to understand user needs, is to put stakeholder needs in context (i.e., use cases) and explore different validation scenarios. Exploring different scenarios may unfold needs not previously identified, promoting a better understanding of the stakeholder needs and design, implement and build the stakeholder desired system.

Remove communication problems

In a single project, it is quite common to work with people with different backgrounds and origins, where a natural language (e.g. English) may not be familiar to all. This may lead to misunderstandings and errors.

Modelling, uses of a common language to address communication problems, and overcome the language barriers and backgrounds.

A common language encourages and improves interaction with the different project stakeholders and engineering disciplines.

Improve quality

Modelling promotes and enhances quality by adding:

• Rigorous requirements traceability. Textual requirements can be traced to model elements (i.e., system itself or its constituent parts). Improved requirements specification and allocation. When a textual requirement is traced and translated to model elements, consistency is enhanced and ensured; it facilitates early identification of requirements issues. Textual requirements specifications may contain inconsistencies not always detected or only at a later development phase; normally there is an increase in the cost and time. A model supported by an ontology at the heart of the architecture, promotes consistency and early requirements issues detection, this may help to reduce costs and risk.
• To help the system design integrity: a model and all the individual system parts, can be integrated in one single source of truth, this promotes design and interfaces consistency. Having an integrated model, promotes communication, analysis and to detect design defects. An integrated model propagates and reflects any system parts design changes across the different areas. This not happens or not an easy task to detect in a document-centric approach.

Detect defects earlier in the system development cycle

Having textual requirements linked to model elements in a single model, promotes:

• On-going requirements validation through design verification and the use of simulation and automatic verification.
• Increases confidence and reduce risk.
• Reduces costs.
• Helps to verify the system correctness (e.g. interfaces captured and defined, data flow type is correct).
• Helps to verify system completeness (e.g. all modes and states identified and defined).
• The definition of a Domain Specific Language (DSL), i.e. ontology, helps the system consistency and verification with fewer changes.

Improves productivity

• Reuse of existing models to support design and technology evolution.
• Consistent documentation can be generated and automated from the model.

What can be modelled?

When a MBSE model activity is undertaking two aspects of the System can be captured:

• Structural aspect: the ‘what’ of the System; it captures what elements a system is made of (e.g. it helps to define the product breakdown structure – PBS) and the interfaces between the elements.
• Behavioural aspect: the ‘how’ of the System; how the system elements behave, how elements interact with one another and under what conditions.

A good modelling approach must always consider both the ‘what’ and the ‘how’ regardless of the System, even if only considering a system such as a database, there is the need to consider how the various stakeholder behave towards it.

When do we model?

There are several as the ones captured below (not an extensive list):

• Modelling can be applied whenever there is the need to understand a system or something.
• Modelling can be applied at any point in the System Life Cycle.
• The extent of the modelling will depend on what the stakeholder are trying to do or understand.
• Can only apply modelling successfully when we are adding value.

How do we model?

There are three fundamental aspects that needs to be taken into account when doing modelling activities: tools, language or notation and people.

Tools

As a model is not merely a collection of representations, but an integrated repository of project knowledge, one or more tools are typically required. These tools must not only provide the necessary capability in terms of language support, but should also integrate with each other to ensure completeness, correctness and consistency across the project.

Language or notation

Using a common language or notation; MBSE is most effective when standard techniques and representations are employed. The use of standards improves consistency and facilitates interoperability of tools, people and process.

People

Ensure the appropriate capabilities in modelling languages and tools does not have to be limited to project personnel. MBSE provides the maximum benefit when no translation of the model is required for the stakeholders. A minimum period of learning may be needed by the different stakeholders depending on the stakeholder involvement in the model.

What MBSE is not

Quite common there are myths around MBSE. Hereafter, some aspects of what MBSE is not.

Arcadia / SysML

A representation should use the most appropriate notation for the project. Arcadia and SysML are very powerful modelling languages, however, other languages (e.g. textual, mathematical) also exist and should be chosen on their individual merits.

Simulation

A representation can be either static or dynamic (simulation). Simulation enhances and provides automated and early defects detection.

Part of Systems Engineering activities required on a project

MBSE is an integrated approach to Systems Engineering in which all activities should reference a single point of truth, the model, in order to realise the maximum benefits.

Drawing

A representation (whether graphical, textual or other) should be connected to the underlying model in order to realise the maximum benefits of MBSE (correctness, completeness and consistency).

Modelling is MBSE

Modelling forms the core of MBSE, but not all Models are MBSE Models. Modelling and Model-based Engineering (MBE) is extensively in engineering. MBE can be applied to other engineering areas, such as design, and use of bespoke modelling and notations (circuit diagrams, 3-D modelling, CAD, etc.). MBSE, is applied at a higher level and will often be used to harmonise the MBE activities to ensure that the overall System satisfies its original need.

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