1. Decomposition: Functions
EMIS 8340
Systems Engineering Tool—applying tools to engineering systems
Decomposition: Functions
Mark E. Sampson

2. Setting Expectations for Modeling
Modeling is not Systems Engineering
Systems Engineering is not Modeling
…modeling is one aspect of SE
…according to analysis at TI & DSMC only 15% of product development tasks require systems modeling
Be careful that 15% of tasks don’t take 100% of time & money
                                                            
[Sampson 1994]

3. New modeling methodologies/techniques are added all the time…
Methodology Wars…
New modeling methodologies/techniques are added all the time…
We won’t be able to visit them all…just of few of the most widely used ones
Methodology Cults…
…one particular methodology does not fit all problems
“To a person with a hammer…
Everything looks like a nail”
…find the tool that matches the
Problem.
                                                            
[Armstrong 1993]

4. Start with functions/features…
Why? Allows you to…
Avoid bias that leads to pre-selection errors
Captures behaviors which lead to requirements
You need to understand functions:
drives out sequence
understand inter-relationships
timing
before deciding on partitions
optimize inputs/outputs
…essentially your requirements elicitation process has also extracted high level functions/features your product should have.
…on going chicken/egg argument about what comes first—requirements or functions-RFAT vs. FRAT
                                                            
[SE Handbook 9.1]
[Lacy 1992]

5. Functional Decomposition…
Start with higher-level requirements, behaviors, or functions…
Expressed as two words— a verb, noun pair that…
is measurable/verifiable (…stop vehicle)
is not a part or activity
keeps the viewpoint of the user
is expressed as what the user desires
is active-tense (power vehicle better than provide power)
is not once thru, it looks at alternative functions, decomposes to lower levels, and cycles…
[SE Handbook 9.2]
[DMSC 1986]

6. Functional Analysis: Tools
Functional Flow Block Diagrams (Cradle-SEE, CORE, TcSE, Visio, Rhapsody, Simulink, Smartdraw,…)
Requirement tools (Functions as indented lists… problem is that indented lists lose a lot of information)
…allocate requirements to functions (that’s how you find holes)
…drive out additional functions/requirements
How far down do you go? Until it’s realizable.
                                                            
[DSMC 1986]

7. Functional Analysis: Analysis/Simulation
Once captured, you may want to simulate the system
Some diagramming tools include built-in simulators like Cradle-SEE, CORE, Rhapsody, Simulink
Others only capture diagram & logic (Powerpoint, Visio,…)
Simulation engines are available…
                                                            

8. Functional Analysis: Time Lines
…used to depict concurrency, overlaps, & sequential relationships of functions
Identifies time critical functions (those that affect reaction, down time, or availability)
Basis of performance trace studies
                                                            
NASA:
Mission to Planet Earth
Earth Observing System
[DSMC 1986]

9. Functional Analysis: Time lines cont…
…shows function interaction which drives out additional requirements
Ground station location
Bandwidth
Storage
Power
Critical Paths—aka project mgmt.
Race conditions
Earth Observing System
                                                            
[SE Handbook 9.5]
[DSMC 1986]