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Decomposition: Functions

EMIS 8390. Systems Engineering Tool—applying tools to engineering systems. Decomposition: Functions. UPDATED 9/02/04. Mark E. Sampson. Setting Expectations for Modeling… Modeling is not Systems Engineering. Systems Engineering is not Modeling. …modeling is one aspect of SE

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Decomposition: Functions

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  1. EMIS 8390 Systems Engineering Tool—applying tools to engineering systems Decomposition: Functions UPDATED 9/02/04 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. Methodology Wars… • New modeling methodologies/techniques are added all the time… • We won’t be able to visit them all…just a few of the most widely used ones. • Methodology Cults… • …one particular methodology does not fit all problems… • “To a person with only 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,TcR, Visio,Rhapsody, Simulink, Smartdraw,…) • Requirements 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 maywant to simulate the system… • Some diagramming toolsinclude 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 relationship of functions • Identifies time critical functions (those that affect reaction, down time, or availability • Basis for performance trade studies NASA: Mission to Planet Earth; Earth Observing System [DSMC 1986]

  9. Functional Analysis: Time lines cont… • …shows function interaction whichdrives out additional requirements: • Ground station location • Bandwidth • Storage • Power • Critical Paths—aka project mgmt • Race Conditions Earth Observing System …download over Hawaii, retransmit to mainland processing center. Processing center was located in West Virginia (Sen. Byrd’s home state—chair of NASA over-sitecommittee) [DSMC 1986] [SE Handbook 9.5]

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