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Decomposition: Interfaces & Alternatives

Mark E. Sampson

UPDATED 9/20/04

EMIS 8390

Systems Engineering Tool—applying tools to engineering systems

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Remember External Interfaces…

• Captured during project scopeing…What do you have to interface with in the outside world?

…these are the stimulus & recipient of our system.

Applications Study: Henry Ford’s first car and Explorer Tire Recall Upon completion of his first automobile in his workshop, Henry Ford was

amazed to discover it didn't fit through the door (Henry Ford, p.30, A. Knopf, 1966)

 …or a more recent example from the news… May 20, 2001--Ford Motor Co. is recalling 50,000 brand new Explorers

because an assembly line conveyor belt that was too narrow for the wider 2002 model may have cut the tire tread (http://abcnews.go.com/sections/us/DailyNews/tires_recall_wire010520.html)

[SE Handbook 4.1] [Hooks 2001]

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External Interfaces…

[Lacy 1992] [Hooks 2001]

Software

Test Equipment

People

Printer

Power

EMI

Networks

Storage

Be sure to ask the question, “What is the worst thing otherelements could do to you across this interface?” [Kuchta, 1989]

…and defend against it! [Hooks 1999]

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Internal Interfaces…

• Developing functional interfaces…

• Each function requires an input to operate (goes’intos)• Each function produces an output (goes’outofs)

• Identify & Document what & how each function will obtain its input and where it will send its outputs

Tools help capture & keep up with these interfaces—SE tools like:Popkin SA, Cradle-SEE, CORE, SLATE,… SW modeling tools like: Tau, Rational, Artemus, Describe…

[DSMC 1986]

Func 1

Func 2

Func 3

Func 4

[SE Handbook 9.2]

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Internal Interfaces…Continued

• Decomposing functions, decomposes interfaces

• They eventually end up in the product(hopefully). If not, you have test &integration problems (or worse yet,field problems)

• An interface represents an agreement between membersof your product. Agreementsneed to be documented.

• Don’t “lose” an interface…

[DSMC 1986]

Func 1

Func 2

Func 3

Func 4

[SE Handbook 9.2]

Func 2.1

Func 2.2Func 2.3…all the bad things happen at the interfaces

…as documented by many of your SE application studies…Dr. Stephen Wheelwright predicting next model year problems

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Documenting Interfaces…

…as you drive down through functions, interfaces are defined.

• Interfaces show up in product, & need to be managed

• Some tools for describinginterfaces:• NxN (aka ‘N-squared’) charts• Design Structure Matrix • Cross-Correlation Matrix• Functional Thread

[SE Handbook 9.5 Lacy 1992]

…functions placed on diagonal. Information flowsclockwise between functions. Outputs are horizontal, Inputs vertical. Interface name at the intersection.

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Documenting Interfaces…ICD’s

…interface agreements can also be documented in Interface Control Documents (ICD’s), Interface Block Diagrams, Schematic Block Diagrams,…

• Start with where you are (add details as available…) Possibly Includes: • Interface function—what the

interface must do, clarifies responsibilities of each end of the interface

• Specify how well it must be performed

• Define any constraints• Physical characteristics• …

Tools help you generate these documents…[SE Handbook 9.5] [DOE 2003]

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Documenting Interfaces…Data Dictionaries

…Data Dictionaries normally associated with documenting database schema, field names, etc.

…also applies to systems to document information exchange agreements

Possibly Includes: • Name• Information type• Default values• Brief Description

…delivered in referencable form…common crib sheet…usually under ICB control

Tools help you generate these documents…

[SE Handbook 9.2]

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Developing Alternatives…

• Now that we have functions captured, there’s a variety of ways they can be accomplished. • Systems Engineering needs to do an unbiased review of as many alternatives/ “What if’s” as feasible. Why? Some possible solutions are better than others.

• arrive at best possible overall solution (vs. sub-optimized)• creates robust designs (robust to change, etc.)• ensures you’re not backing into pre-selected solution

…a discussion about superballs…

Use your brainstorming techniques (described earlier) to build an alternative’s list

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Developing Alternatives…Morphological Charts

• A structured method for widening search area for possible solutions…

• Start with functions• List means for accomplishing

functions (see also brainstorming)

• Chart the functions & explore combinations

[betterproductdesign.net]

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Developing/Evaluating Alternatives…Converging

• Pugh’s Controlled Convergence……an iterative method of converging to the best possible solution among

alternatives.

• “sketch” the concepts • Develop the matrix—with criteria• Pick one potential solution as the datum• Compare each concept

against the datum• The best becomes the

new datum• Combine, eliminate,…• Repeat until design

converges to acceptablesolution

[Lacy 1992] [see also various web sites—like betterproductdesign.net]

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Evaluating Alternatives…Criteria

• How are you going to decide what is the best alternative?1. Does it meet the constraints…pass/fail--does it meet the musts?2. Does the best in scoring criteria & utility functions…

• What criteria to use?1. Should be tied to objectives (scope statement)2. Ask the customer, stakeholders,…3. Cost, schedule, performance,…4. Competition…5. Policies, vision/mission, values,…

…should probably be done before developing alternatives to ensure an unbiased evaluation

Remember to capture your scoring criteria rationaleDocument the why/how of your scoring so in case something changes

you can go back and pick up that thread (and save someone else some time when they come to that same decision point) [Lacy 1992]

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Evaluating Alternatives…Weighting

• Not all criteria are of equal importance…weighting factors associated with criteria

• How to determine the weighting factor?1. Empirical—which criteria has the

greatest impact on meeting objectives (most expensive…)

2. Statistical—the scoring is most sensitive to which criteria (ala DFSS)

3. Subjective—SME (subject matter expert) scoring (methods include fixed scoring budgets, judging best solutions & backing out weighting, pair-wisedecision trees…)

[Lacy 1992] [DSMC 1986]

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Evaluating Alternatives…Utility Curves

• All criteria are not of equal importance…at different times, different performance levels,…

• Loss: At what time/level does the benefit improve/end/accelerate?

Some sample curves

[Lacy 1992] [DSMC 1986]