Short Answer:
Contrary to MBSE and tool-vendor Muddle-Driven Marketecture hype, the differences between the SysML and UML modeling languages are more lightweight and dialectical in nature than heavyweight and substantive. This should be expected since SysML was originally designed to be used by Systems Engineers collaborating with Software Engineers applying UML for software analysis and design, and SysML is defined as a modestly extended pragmatic subset of UML 2. (See SysML FAQ: Can SysML and UML model elements be combined in the same model?) Indeed, although SysML adds two useful diagram usages to UML (Requirements diagrams extend UML Class diagrams; Parametric diagrams extend UML Class & Composite Structure diagrams), the other diagrams that SysML borrows from UML are either largely reused without modification (e.g., Use Case, Sequence, State Machine diagrams) or are modestly tweaked with lightweight customizations called stereotypes that lack substantive semantics: e.g., renaming Classes as Blocks and adding lightweight syntax and semantics for physical item flows; adding stereotypes to Activity diagrams without bona fide executable semantics.


Longer Answer:
SysML is defined as a lightweight dialect (Profile) of UML 2.x, the industry standard modeling language for software-intensive applications. (The SysML Profile is lightweight in the sense that the changes that it makes to the underlying language are relatively modest in scope and extent, using a small number of simple stereotypes, tagged values and constraints. Compare and contrast with a heavyweight Profile, which could significantly impact how the underlying language is used.) The advantage of defining SysML as a UML Profile is that it can reuse the relatively mature notation and semantics of UML 2.x, which many modeling tool vendors have already implemented. The disadvantage of specifying SysML as a UML Profile is that SysML inherits many of the problems associated with UML 2.x, such as gratuitously complex notation, imprecise semantics, and a dysfunctional diagram interoperability standard (XMI).

Relationship between SysML & UML

Relationship between SysML & UML

Reproduced by Permission © 2003-2017 PivotPoint Technology Corp.



SysML offers systems engineers the following advantages over UML for specifying systems and systems-of-systems:
• SysML expresses systems engineering semantics (interpretations of notations) better than than UML. It reduces UML's software bias and adds two new diagram types for requirements management and performance analysis: Requirement diagrams and Parametric diagrams, respectively.
• SysML is smaller and easier to learn than UML. Since SysML removes many software-centric and gratuitous constructs, the overall language is smaller as measured in diagram types (9 vs. 13) and total constructs.
• SysML model management constructs support the specification of models, views, and viewpoints that are architecturally aligned with IEEE-Std-1471-2000 (IEEE Recommended Practice for Architectural Description of Software-Intensive Systems).
The following SysML-UML 2 Comparison table compares SysML diagrams with their UML counterparts where one exists. Where no UML diagram counterpart exists for a SysML diagram (e.g., Parametric and Requirement diagrams), it is marked N/A; similarly, where no SysML diagram counterpart exists for UML diagram it is marked N/A (e.g., UML 2 Communication diagram).

  • SYSML DIAGRAM

  • PURPOSE

  • UML DIAGRAM ANALOG


  • Activity diagram (ACT or act)
  • [Behavioral diagram] An Activity diagram shows system behavior as control and data flows.
    Useful for functional analysis.
    Compare Flow Block Diagrams (FBDs) and Extended Functional Flow Block diagrams (EFFBDs), already commonly used among systems engineers. 
  • UML::
    Activity diagram
  • Block Definition diagram (BDD or bdd)
  • [Structural diagram] A Block Definition diagram shows system structure as components along with their Properties, Operations and Relationships.
    Useful for system analysis and design.
  • UML::
    Class diagram
  • Internal Block diagram (IBD or ibd)
  • [Structural diagram] An Internal Block diagram shows the internal structures of system components, including their Parts and Connectors.
    Useful for system analysis and design.
  • UML::
    Composite Structure diagram
  • Package diagram (PKG or pkg)
  • [Structural diagram] A Package diagram shows how a model is organized into Packages, Views and Viewpoints.
    Useful for model management.
  • UML::
    Package diagram
  • Parametric diagram (PAR or par)
  • [Structural diagram] A Package diagram shows parametric constraints between structural elements. Useful for performance and quantitative analysis.
  • [No analogous diagram in UML 2]
  • Requirement diagram (REQ or req)
  • [Requirement diagram] A Requirement diagram shows system requirements and their relationships with other elements.
    Useful for requirements engineering, including requirements verification and validation (V&V).
  • [No analogous diagram in UML 2]
  • Sequence diagram (SD or sd)
  • [Behavioral diagram] An Sequence diagram shows system behavior as interactions between system components.
    Useful for system analysis and design.
  • UML::
    Sequence diagram
  • State Machine diagram (STM or stm)
  • [Behavioral diagram] A State Machine diagram shows system behavior as sequences of states that a component or interaction experience in response to events.
    Useful for system design and simulation/code generation.
  • UML::
    State Machine diagram
  • Use Case diagram (UC or uc)
  • [Behavioral diagram] A Use Case diagram shows system functional requirements as transactions that are meaningful to system users.
    Useful for specifying functional requirements. (Note potential semantic overlap with functional Requirements specified in Requirement diagrams.)
  • UML::
    Use Case diagram
  • Allocation Table
  • [Mapping table; not a diagram] An Allocation Table shows various kinds of assignment relationships (e.g., requirement allocation, functional allocation, structural allocation) between model elements.
    Useful for  facilitating automated verification and validation (V&V) and gap analysis.
  • [No analogous table in UML 2]
  • Instances (but no Object diagram)
  • As per the OMG SysML 1.2 minor revision, Instance Specifications, but not Object diagrams are allowed.
  • UML::
    Object diagram
  • [No analogous diagram in SysML]
  • UML::
    Communication diagram
  • [No analogous diagram in SysML]
  • UML::
    Component diagram
  • [No analogous diagram in SysML]
  • UML::
    Deployment diagram
  • [No analogous diagram in SysML]
  • UML::
    Interaction Overview diagram
  • [No analogous diagram in SysML]
  • UML::
    Profile diagram
  • [No analogous diagram in SysML]
  • UML::
    Timing diagram


SysML-UML 2 Comparison

Reproduced by Permission © 2003-2017 PivotPoint Technology Corp.