MBSE and SysML

Model-based systems engineering (MBSE) is a systems engineering methodology that focuses on creating and exploiting domain models as the primary means of information exchange between engineers, rather than on document-based information exchange.

Model-Based Systems Engineering (MBSE) is the practice of developing a set of related system models that help define, design, and document a system under development. These models provide an efficient way to explore, update, and communicate system aspects to stakeholders, while significantly reducing or eliminating dependence on traditional documents. In practice, engineers use models to gain knowledge (e.g. performance) and to serve as a guide for system implementation (e.g. SysML, UML).

A recommended best practice for any Model-Based Systems Engineering (MBSE) approach is the synergistic application of Model-Based Languages, Model-Based Tools, Model-Based Processes, and Model-Based Architecture Frameworks, as shown in the System Architecture Tetrad figure below. After a decade of pragmatic experience applying SysML to tough Systems Engineering problems, SysML has emerged as the de facto Model-Based Language choice for MBSE projects.

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What is SySML?

The term Model-Based Systems Engineering and its acronym MBSE are popular among Systems Engineers who advocate the use of SysML as a standard architecture modeling language for Systems Engineering applications, and who want to distinguish their approach from Model-Driven Development and its variants, which tend to be software centric.

SysML (Systems Modeling Language) is based on UML and replaces the modeling of classes and objects by block modeling for a vocabulary more suited to System Engineering. A block encompasses all software, hardware, data, process, and even people management concepts.

SysML reuses part of UML2 and it also provides its own definitions (SysML extensions) as shown in the Figure below:

UML4SysML:

• Sequence diagram
• State diagram
• Use case diagram
• Diagram of activities
• Package diagram
• Class diagrams and composite structure (used for block definition and internal block diagrams – BDD & IDB)

SysML Extensions:

• Definitions for Block Definition and Internal Block Diagrams – BDD & IDB
• Changes in the activity diagram
• Requirements diagram – New
• Parametric diagram – New
• Allocations (traceability) – New

The diagrams defined under UML2, and those that make up SysML are shown below. Apart from the new features, the majority of the changes made by SysML can be found in the structural diagrams.

UML2: 13 diagrams – 6 structural, and 7 dynamic

SysML: 9 diagrams – 4 structural, 4 dynamic, and the requirements diagram:

• structural
  • The Block Definition Diagram (BDD) replaces the Class Diagram
  • The Internal Block Diagram (IBD) replaces the composite structure diagram
  • The package diagram remains unchanged
  • The parametric diagram is a SysML extension for the analysis of critical system parameters
• dynamic
  • The activity diagram is slightly modified for SysML
  • Sequence, state, and use case diagrams remain unchanged
• The requirements diagram is a SysML extension

Why SysML for MBSE?

The advantages of SysML over UML for systems engineering become obvious when considering a concrete example, like modeling an automotive system. With SysML one can use Requirement diagrams to efficiently capture functional, performance, and interface requirements, whereas with UML one is subject to the limitations of use case diagrams to define high-level functional requirements. Likewise, with SysML one can use Parametric diagrams to precisely define performance and quantitative constraints like maximum acceleration, minimum curb weight, and total air conditioning capacity. UML provides no straightforward mechanism to capture this sort of essential performance and quantitative information.

SysML offers systems engineers several noteworthy improvements over UML, which tends to be software-centric. These improvements include the following:

• SysML’s semantics are more flexible and expressive. SysML reduces UML’s software-centric restrictions and adds two new diagram types, requirement and parametric diagrams.
  • The requirement diagram can be used for requirements engineering;
  • The parametric diagram can be used for performance analysis and quantitative analysis.

Consequent to these enhancements, SysML is able to model a wide range of systems, which may include hardware, software, information, processes, personnel, and facilities.

• SysML is a comparatively smaller language that is easier to learn and apply. Since SysML removes many of UML’s software-centric constructs, the overall language measures smaller both in diagram types and total constructs.
• SysML allocation tables support common kinds of allocations. Whereas UML provides only limited support for tabular notations, SysML furnishes flexible allocation tables that support requirements allocation, functional allocation, and structural allocation. This capability facilitates automated verification and validation (V&V) and gap analysis.
• SysML model management constructs support models, views, and viewpoints. These constructs extend UML’s capabilities and are architecturally aligned with IEEE-Std-1471-2000 (IEEE Recommended Practice for Architectural Description of Software Intensive Systems).

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