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8/2/2019 3.Requirements Categories
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System Requirements
Fundamentals
Introduction
Requirements DevelopmentRequirements CategoriesWriting Better Requirements
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RequirementsFundamentals
- Requirements Categories
(This work is based on material from many sources, including www.ppi-int.com, Systems Engineering Course)
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Importance of Requirement Types
Why different Types
Requirements are responsible formost project failures*:
45% of failures due torequirements problems
2nd reason: 11%*Source: Standish Group,1996
AllAll requirements problems arerelated to clarityclarity of information
Ambiguity, transcriptions, different
points-of-view
Making distinctions brings clarity
Distinct requirement types bringsclarity to specs
Different types exist. Recognizingthis fact implies:
Different fields of knowledge,
Different analysis skills,
Different spec techniques,
Different solution techniques,
Ability to change one thing withoutchanging another
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What is the categorization criteria?
This categorization is based on the nature of the information,not in its origin.
This helps each requirement to have only one place in a spec,
thus avoiding: Delays in searching for requirements
Surprises: finding out that a requirement where it is not supposed to be
Duplicity: having requirements in different places that:
Are the same, Have overlaps,
Are conflicting!
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Types of Requirements(Organize your specification using one section for each requirement type)(
Directs the objects designDesign
Alternatives in behavior and their transitionsStates and modes
How well a given function is to be performedPerformance
How safelySafety
How reliableReliability
How maintainableMaintainability
Directs the design to allow economical production and consistent qualityManufacturability
Other qualitative characteristicsOther Qualities
How usable for humans
Required characteristics to enable verification
Physical properties mass, dimensions, etc
Which resources consumes/uses to do its job
External environment conditions
Cross-boundary items inputs and outputs
What the item is to do
Description: required characteristics about...Description: required characteristics about...CategoryCategory
Human Factors
Verification
Physical
Resources
Environmental
External Interface
Functional
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Functional Requirements
Functional requirements statewhat the item is to do
Functional Requirement = aRequired Function and itscharacteristics
Function: A characteristic actionto be performed by the item.
Functions usually can be viewedas transformations of somesort, from inputs into outputs.
A function may be performed inone or more mode
Required Functions usually
include:
normal operation functions,
fault detection functions,
fault recovery functions, and
functions which deal withanomalous inputs
Examples:
change gear up (and itscharacteristics)
keep cruise speed (and itscharacteristics)
display instant fuel consumption
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Performance Requirements
State how well a function is to be performed
Relate only to functions
Stated in quantitative terms
Include:
Precision, response time, frequency, delays,...
Good practice: specify together with its related functional requirement
Example:
The landing gear shall retract in 101s
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Design Requirements
Define characteristics of the solution for the item Directs the internal design
Expressed in terms of components or implementation aspects
Are commonly dangerous, limiting too much the design Some projects were killed by them
Possible justifications include greater requirer expertise than that of the contractor
standardisation as a strategy to reduce life cycle cost across multiple systems
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Safety Requirements
Requirements to ensure availability, integrity and continuity ofsystem functions
Come from analyses such as
Functional Hazard Analysis,
Preliminary System Safety Assessment (PSSA),
Zonal Analysis,
Particular Risks Analysis
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External Interface Requirements
External Interface Requirements specify:
All items that enter or leave the system (information, material, energy)
All interconnection aspects
External systems/users that originate or receive I/O
Both normal and anomalous I/O
As a consequence, they exactly define the system boundary
Examples:
Power connection shall be according standard xxx ...
Windshear_Caution_Indication output to PFD is Boolean. Values= {0,1}
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Human Factors Requirements
State required usabilitycharacteristics for human-systeminterfaces
Include characteristics such as:
Shapes, colors, ergonomic
position, etc; for controls, displays and
displayed information
Symbols, symbology dynamics(time, movements, etc)
Information layout
Example: Windshear Caution Indication on
the PFD panel shall be the blackuppercase string WSHEAR insidean ambar background rectangle, asin fig. xxx
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Physical Requirements
Requirements such as:
Mass, dimensions, shape, volume, colour, etc
State physical attributes of the system or item
Ex.1: The xxx system mass shall not exceed 170 kg
Ex.2: The yyy equipment mass shall not exceed 35,0 kg
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Environmental Requirements
Limit the effects of:
the enveloping environment on the system, the system on its enveloping environment
Include parameters such as (also includes their rate of change):
Temperature, humidity, pressure, radiation, shock, vibration, wind speed, dust, ice,gasses
Noise Requirements Echological Requirements
EMI
Related to interface requirements, but are different
Standards:
MIL-STD-810 Military Standard Environmental Test Methods and EngineeringGuidelines
DO-160 Environmental Conditions and Test Procedures for Airborne Equipment
1613-2003 - IEEE Standard Environmental and Testing Requirements forCommunications Networking Devices in Electric Power Substations
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Resource ConsumptionRequirements
States the limits of consumption of externally provided resourcessuch as: Power electrical, hydraulic, pneumatic
Cooling air, cooling water
Consumables (paper, magnectic tape, disk cartridges, etc)
For software items: Computer memory
Computer processing capability
Interface bandwidth
Related to interface requirements, but are different
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Verification Requirements
State requirements for the item to enable providing evidence that allother requirements have been met
For qualification
For production
For acceptance purposes
or other purposes
Example:
Probing points,
Inspection windows
May also state the methods to be used (test, demonstration,analysis, inspection), if not in another document
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Reliability Requirements
Requirements for the item to perform its required functions understated conditions for a specified period of time (IEEE) specified using reliability parameters
Its a specialty field ...90% Reliability for 10,000 miles with 90% confidence
Examples: Aircraft Schedule Reliability, Dispatchability
Equipment MTTF, MTBF, failure rate
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Maintainability Requirements
Required characteristics to enable or ease maintenance
Examples:
Maintenance skill level requirements.
Need for special tools or test equipment.
Parts standardization. Visual inspection access.
Colour coding and labels as appropriate.
Use of handles on replaceable units.
Scope and range of technical manuals.
Human factor limitations in the design of the item
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Manufacturability Requirement
Definition:
A required characteristic of a specificitem to allow it to be
produced economically and with consistent high quality.
Example:
The Power Unit shall have from 3 to 4 hoisting points
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What is not a manufacturabilityrequirement:
Best Practices for design, to be always applied when doing thedesign.
In other words, general design rules and criteria.
They also allow it to be produced economically and with
consistently high quality... But... are NOT specific for an item.
Examples: The design activity for the System shall apply ergonomic concepts.
The project must provide standard bolts (existing in stock).
Adopt holes for nut plates according to the Design Manual (Section 5.2.4)
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Other Quality Requirements
Transportability Requirements
Survivability Requirements
Required characteristics to enable operation after some specified damage has occurred.
Flexibility Requirements
Portability
Required characteristics to enable or ease carrying by humans
Reusability Requirements
Interoperability Requirements
Required capabilities to interoperate with other systems
Expandability Requirements Security Requirements
Requirements regarding unauthorized access to the system
System DMC Direct Maintenance Costs
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End
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