3.requirements categories

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)

Andr Mayoral, 2011 | helping systems happen... | all rights reserved
http://www.ppi-int.com/http://www.ppi-int.com/


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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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End



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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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