Aerospace Systems Engineer


The Utility of Scenario Sequence Diagrams (SSDs)

Scenario Sequence Diagrams

The Scenario Sequence Diagrams are multi-purpose diagrams which capture the essential aspects of physical and functional architecture, and system behavior, in one diagram.  These diagrams supplant several of the diagrams commonly used in Systems Engineering design derivation and documentation, and provide utility beyond the combination of all of them.

Context Diagrams - The Scenario Sequence Diagram, by listing the involved actors across the top of the diagram, illustrates the system’s physical, and functional, context and shows which actors interact in the scenario.

Interface Diagrams - The Scenario Sequence Diagram, through depiction of message traffic graphically, illustrates physical and logical interfaces between actors.

Data Flow Diagram - The Scenario Sequence Diagram data messages illustrates data flow between actors.

Data Dictionary - The Scenario Sequence Diagram, and especially the companion Event Flow Table, through the capture of each specific message provides the equivalent of a data dictionary for the scenario.

Message Sequence Chart- The Scenario Sequence Diagram illustrates sequential message exchanges between actors accomplishing the scenario.

Use Case Model Diagram – The activities populating the vertical “life lines” of the actors are the Use Cases executed by each actor for the scenario.

Activity Diagram - The Scenario Sequence Diagram, through depiction of sequential Use Case execution is analogous, but superior, to the UML “Activity Diagram” in that it identifies the behaviors required of the actor in the execution of the scenario.  The UML “activity diagram” merely shows state without exposing or elaborating behavior.

Event Flow Table

In use, the Scenario Sequence Diagram or SysML Activity Diagram will typically be combined with a textual capture of the system design in an Analysis Package for the scenario.  We use the Event Flow Table as the textual mechanism.  The Event Flow Table provides significant additional aspects which enable the Analysis Package’s utility as a design and requirements validation tool. As CASE tools evolve we can see products such as Telelogic’s TAU integrating the graphic modeling tools with the traditional requirements repositories creating powerful tool sets for the practicing systems engineer and replacing the manual text approach.

Structurally the Event Flow Table is a multi column table capturing the Source, Destination, Message and Activity of each “transaction” in the Scenario Sequence Diagram or SysML Activity Diagram. Because the diagrams represent a sequence the first 2 columns of the EFT can be used to capture the sequential numbering of the transactions in the Scenario and relative times (typically t -0 times) for sequences of transactions. Additionally the Requirements and assumptions columns are used to allocate requirements to design (or derive requirements from functions) and to capture assumptions associated with either.  It is here where the integration of graphic model and requirements repositories are indicated. A comments column is often useful to capture the extraneous bits of information exposed during the analysis of the behavior.  Additional columns are often added for specific attribute capture such as constraints and fault mode identification.