Decomposition: Functions

1
EMIS 8340
Systems Engineering Toolapplying tools to
engineering systems
Decomposition Functions
UPDATED 2/20/2006
Mark E. Sampson
2

• Setting Expectations for Modeling
• Modeling is not SystemsEngineering
• Systems Engineering is not Modeling
• modeling is one aspect of SE
• according to analysis at TI DSMConly 15 of
  product development tasksrequire systems
  modeling
• Be careful that 15 of tasks dont take 100 of
  time money

Sampson 1994
3

• Methodology Wars
• New modeling methodologies/techniques are added
  all the time
• We wont be able to visit them alljust of few
  of the most widely used ones
• Methodology Cults
• one particular methodology doesnot 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
  firstrequirements or functions-RFAT vs. FRAT

Lacy 1992
SE Handbook 9.1
5

• Functional Decomposition
• Start with higher-level requirements,
  behaviors, or functions
• Expressed as two wordsa 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

DMSC 1986
SE Handbook 9.2
6

• Functional Analysis Tools
• Functional Flow Block Diagrams (Cradle-SEE,
  CORE, TcSE, Visio,Rhapsody, Simulink,
  Smartdraw,)
• Requirement tools(Functions as indented
  listsproblem is that indented listslose a lot
  of information)
• allocate requirements to functions (thats how
  you find holes)
• drive out additional functions/requirements
• How far down do you go? Until its realizable.

DSMC 1986
7

• Functional Analysis Analysis/Simulation
• Once captured, you maywant to simulate the
  system
• Some diagramming tools include built-in
  simulatorslike Cradle-SEE,CORE, Rhapsody,
  Simulink
• Others only capture diagram logic
  (Powerpoint, Visio,)
• Simulation enginesare available

8

• Functional Analysis Time Lines
• used to depict concurrency,overlaps,
  sequential relationships of functions
• Identifies time critical functions (those that
  affect reaction, downtime, 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 Pathsaka project mgmt.
• Race conditions

Earth Observing System
DSMC 1986
SE Handbook 9.5