Systems Thinking and Systems Engineering

Systems Engineering: Requirements model

07 February 2013

François Christophe

Galina Medyna

Eric Coatanéa

Objectives of the lecture

1. Know different types of representations for requirements

2. Understand the importance of modeling requirements

3. Apply techniques for refining requirements

Definition of requirement structure

Structure of requirement

1- Identified: Qualitatively defined, this is a need,

2- Specified: Quantitatively defined, this is a specified requirement,

3- Allocated: Assigned to a product, process, subsystem or a combination,

4- Covered: Satisfied by the element to which it has been allocated

A requirement is:

Structure of requirement

Structure of requirement

Types of representations of requirements

Representation types

• Document• Classification of requirements under categories• List of requirements• Tree of Requirements• Detailed requirements with performance values

Requirements document

http://www.c-jump.com/CIS75/Week06/samples/requirements.html

http://www.swisseurobot.ch/images/2012/e2012_rules_en_final.pdf

Classification of requirements

Classification of requirements

User requirements

Technical requirements

Adapted from: Hull, E., Jackson, K., Dick, Jeremy., 2005, Requirement Engineering, 2nd ed., Springer, London.

Classification of requirements• Business requirements:

– High level requirements related to company organisation

• User (stakeholder) requirements– Requirements related to user wishes or needs from a project

• Functional requirements– Functionalities required from a system derived from analysis of user

requirements

• Non-functional requirements– Requirements related to constraints such as cost, performance,

dimension, maintenance...

List of requirements

Tree of requirements

Detailed requirements

Detailed requirements

Detailed requirements

• A detailed requirement should be quantitatively defined (specified)– Can be defined with its utility

Detailed requirements:Trade-offs between requirements• Example:

– Power source should provide enough energy for playing 2 matches consecutively: (2*90s.)

BUT– Constraints for the robot dimensions

Allocated requirements

• Requirements should be allocated to specific parts of the system

• Examples of allocation:

Why is it important to model requirements?

Model of requirements? Why?

• Siemens PLM requirements• Dassault RFLP• Providia• IBM Rational Doors

Why so many vendors give importance to requirement modeling?

Because...

• Requirements descriptions are similar to a contract between customers and suppliers

• It is important to capture the interactions and possible contradictions between requirements (contradictions can lead to design constraints)

• It is important to keep track of initial objectives– Allocation and satisfaction mechanisms used for traceability

Refinement techniques

Functional or non-functional?

• Analysis of types of verbs and objects used in requirements sentences

• Helps classifying requirements into different categories

Functional or non-functional?

A requirement is a sentence in natural language.

A function is always related to transitive and intransitive verbs.

Examples:

The seat must prevent injury (seat=subject, prevent= transitive verb, injury=object)

The airplane seat must float. (seat =subject, float =intransitive verb)

Functional or non-functional?Third type of requirement sentences exist: sentences

where the verb is a linking verb

Example:

The seat must be easy to adjust.

(seat=subject, be=linking verb, easy to adjust=subject compliment)

In this type of sentence, the verb is not representing an action, this structure of sentence is representing

a non-functional requirement.

Classifying rules from grammatical structures

Tool example

• This tool defines specific grammatical structures for expressing requirements:

http://freespace.virgin.net/gbjedi/books/re/boilerplates/repository.htm

Common keywords

• Finding common keywords in different requirements sentences

• Helps finding links/interactions/contradictions between 2 requirements from different categories

• Example: file:///C:/Users/Francois/Desktop/MadeByGraph/simAnalysis.htm

Searching for required performance

From Qualitative to Quantitative requirements:• For requirements containing comparative or superlative

such as: easier, better, simpler, faster

• Ask about:– What does the comparative or superlative refer to?

– How to measure?

– Which unit?

Searching for required performance

• Words like never, always, ever, everybody, nobody, all: – often generalize a fact

• Ask about:– Really always?– Really everybody?– Really never?

Thank for your attention

Exercises:

- Analyse EU-robot rules document.

- Extract requirements related to the objectives of your own project