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Topic Title: Requirements vs Constraints
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Created On: 2-Jun-2006 10:23
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 2-Jun-2006 10:23
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Matthew Thomas

Posts: 35
Joined: 18-Jun-2007

The debate we are having is whether to use constraints at all.

Consider the requirement and constraint pair:
[list]
[*]"The TV shall display the last viewed service when it is repowered from Standby mode".
[*]"The maximum time for the TV to display a service from power up is 10ms". (It is a Panasonic TV ).
[/list]

This can also be expressed as a single requirement:
[list]"The TV shall display the last viewed service within 10ms when it is repowered from Standby mode".
[/list]

So, superficially, it looks like constraints aren't necessary:

REASONS FOR USING CONSTRAINTS:
[list]
[*] you can write something formal in their specifications that could help with executable validation, e.g. startupToDisplay<=10ms
[*] they parcel up the quality of service / performance items so that they can be specifically addressed, e.g. for a tester seeking to verify performance
[*] they encourage engineers to consider performance
[*] multiplicity - a constraint may attach to more than one requirement
[/list]

REASONS FOR NOT USING CONSTRAINTS:
[list]
[*] they confuse people
[*] they need to be managed e.g. should they be uniquely ID'd and should those IDs be on a different series to the IDs given to requirements
[/list]

(Regarding IDs, we use unique numeric IDs [now supported in 6.1 but previously we used tags] which are assigned indelibly using a VBA macro. What do others do?)

Please feel free to add comment and to supplement the list of fors/againsts?

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Matthew Thomas
Panasonic Manufacturing UK Ltd
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 7-Nov-2006 11:47
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Alan Miller

Posts: 1
Joined: 16-May-2006

Everything is a requirement of some sort. I don't worry overmuch.
I usually reserve 'Constraints' for non-behavioural/physical elements - weight, volume, finish etc. Many can be hierarchically aggregated; weight for instance.
In a narrower sense what I refer to as 'Specifying Requirements' specify required functionality (behaviour) - "shall work out the square root of something".
Quality of Service (QoS) Requirements or Performance Indices (PI) I reserve for specifying the behavioural performance - time taken, accuracy etc. and are often associated with a specifying requirement. In some respects they are similar to Constraints in that they may sometimes be hierarchically aggregated, time allocation for instance.
In essence the 'specifying requirement' describes what a function has to do and a QoS defines how well. QoS requirements tend to be more tradeable than specifying requirements.
Any help?
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