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

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]

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]

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]

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]

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]

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…                                                             

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]

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]