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Q: What is the relationship between SysML and UML?

A: SysML is defined as a dialect (Profile) of UML 2.x, the industry standard modeling language for software-intensive applications. 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).

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 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., Communication diagram).

SYSML DIAGRAM	PURPOSE	UML DIAGRAM ANALOG
Activity diagram	[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	[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	[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	[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	[Structural diagram] A Package diagram shows parametric constraints between structural elements. Useful for performance and quantitative analysis.	[No analogous diagram in UML]
Requirement diagram	[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]
Sequence diagram	[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	[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	[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]
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

Keep in mind that you can request additional questions to be answered in the SysML FAQ by sending email to FAQ@SysMLforum.com.

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