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Software Requirements
Specification
2
Software Requirements Specification: A Contract
Document
Requirements document is a reference
document.
SRS document is a contract between the
development team and the customer.
– Once the SRS document is approved by the
customer,
• any subsequent controversies are settled by
referring the SRS document.
3
SW Requirements Specification
Purpose of SRS
– communication between the Customer, Analyst,
system developers, maintainers, ...
– contract between Purchaser and Supplier
– firm foundation for the design phase
– support system testing activities
– support project management and control
– controlling the evolution of the system
4
SRS Document (CONT.)
 The SRS document is known as black-box
specification:
– the system is considered as a black box whose
internal details are not known.
– only its visible external (i.e. input/output) behavior
is documented.
S
Input Data Output Data
5
SRS Document (CONT.)
SRS document concentrates on:
– what needs to be done
– carefully avoids the solution (“how to do”)
aspects.
The SRS document serves as a contract
– between development team and the customer.
– Should be carefully written
6
SRS Document (CONT.)
The requirements at this stage:
– written using end-user terminology.
If necessary:
– later a formal requirement specification
may be developed from it.
7
Software Requirements
Specification (SRS)
 Defines the customer’s requirements in terms of :
– Function
– Performance
– External interfaces
– Design constraints
 The SRS is the basis of contract between the
purchaser and supplier
8
Specification Principles
Separate functionality from implementation
Develop model of desired behavior of the system
Establish the context in which s/w operates
Define the environment in which system operates
Create a cognitive model
Specifications must be tolerant of incompleteness
& augmentable
Content & structure of a specifications should be
amenable to change
9
What is not included in an SRS ?
 Project requirements
– cost, delivery schedules, staffing, reporting
procedures
 Design solutions
– partitioning of SW into modules, choosing data
structures
 Product assurance plans
– Quality Assurance procedures, Configuration
Management procedures, Verification &
Validation procedures
10
Benefits of SRS
 Forces the users to consider their specific
requirements carefully
 Enhances communication between the Purchaser
and System developers
 Provides a firm foundation for the system design
phase
 Enables planning of validation, verification, and
acceptance procedures
 Enables project planning eg. estimates of cost and
time, resource scheduling
 Usable during maintenance phase
11
Types of Requirements
Functional requirements
Non functional requirements
– Performance requirements
– Interface requirements
– Design constraints
– Other requirements
12
Functional Requirements
Transformations (inputs, processing, outputs)
Requirements for sequencing and parallelism
(dynamic requirements)
Data
– Inputs and Outputs
– Stored data
– Transient data
Exception handling
Nature of function: Mandatory/ Desirable/
Optional / Volatile / Stable
13
Performance Requirements
 Capacity
– no. of simultaneous users, processing
requirements for normal and peak loads, static
storage capacity, spare capacity
 Response time
 System priorities for users and functions
 System efficiency
 Availability
 Fault recovery
 All these requirements should be stated in
measurable terms so that they can be verified.
14
Verifiable
A requirement is verifiable if and only if
there exists some finite cost effective
process with which a person or machine can
check that the SW meets the requirement.
15
External Interface Requirements
User interfaces
– eg. if display terminal used, specify required
screen formats, menus, report layouts, function
keys
Hardware interfaces
– characteristics of the interface between the SW
product and HW components of the system
Software interfaces
– specify the use of other SW products eg. OS,
DBMS, other SW packages
16
Design Constraints
SW design constraints
– standards for design, coding, naming, etc.
– SW interfaces (to OS, DBMS, other SW)
– use a specific application package
– constraints on program size, data size etc.
HW design constraints
– specific type of HW, reliability requirements
– HW interfaces
– requirements for spare capacity or spare
performance
17
Design Constraints (contd)
User-interface design constraints
– features of operator/user with details of
working environment
– any special features required
18
Other Requirements
Security
Safety
Environmental
Reusability
Training
...
19
SRS Standards
ANSI/IEEE SRS Standard 830-1984
BS 6719: 1986
European Space Agency Standards
(ESA PSS-05-0, Jan 1987)
US DoD-Std-7935A
...
20
SRS Prototype Outline
[ IEEE SRS Standard ]
1. Introduction
1.1 Purpose
1.2 Scope
1.3 Definitions, Acronyms and Abbreviations
1.4 References
1.5 Overview
21
Purpose
– delineate the purpose of the particular SRS
– specify the intended audience for the SRS
Scope
– identify the SW products to be produced by name
– explain what the SW product will do, and if
necessary, what it will not do
– describe the application of the SW being specified. ie.
benefits, objectives, goals as precisely as possible
Overview
– describe what the rest of the SRS contains
– how the SRS is organized
SRS - Introduction Section
22
SRS Prototype Outline
[ IEEE SRS Standard ]
2. General description
2.1 Product perspective
2.2 Product function summary
2.3 User characteristics
2.4 General constraints
2.5 Assumptions and dependencies
23
Product Perspective
 State whether the product is independent and totally
self contained
 If the product is component of a larger system then:
– describe the functions of each component of the
larger system and identify interfaces
– overview of the principal external interfaces of this
product
– overview of HW and peripheral equipment to be
used
 Give a block diagram showing the major components
of the product, interconnections, and external
interfaces.
24
Product Functions
 Provide a summary of functions the SW will
perform
 The functions should be organized in such a way
that they are understandable by the user
User Characteristics
 Describe the general characteristics of the eventual
users of the product. (such as educational level,
experience and technical expertise )
25
General Constraints
 Regulatory policies
 HW limitations
 Interfaces to other applications
 Parallel operation
 Audit functions
 Control functions
 Criticality of the application
 Safety and security considerations
26
SRS Prototype Outline
[ IEEE SRS Standard ]
3. Specific Requirements
- Functional requirements
- External interface requirements
- Performance requirements
- Design constraints
- Attributes eg. security, availability,
maintainability, transferability/conversion
- Other requirements
Appendices
Index
27
Functional Requirements
Introduction
– describe purpose of the function and the
approaches and techniques employed
Inputs and Outputs
– sources of inputs and destination of outputs
– quantities, units of measure, ranges of valid
inputs and outputs
– timing
28
Functional Requirements
Processing
– validation of input data
– exact sequence of operations
– responses to abnormal situations
– any methods (eg. equations, algorithms) to be
used to transform inputs to outputs
29
External Interface Requirements
 User interfaces
 Hardware interfaces
 Software interfaces
 Communications interfaces
 Other requirements
– database: frequency of use, accessing capabilities,
static and dynamic organization, retention
requirements for data
– operations: periods of interactive and unattended
operations, backup, recovery operations
– site adaptation requirements
30
Appendices
Not always necessary
It may include:
– sample I/O formats
– DFD, ERD documents
– results of user surveys, cost analysis studies
– supporting documents to help readers of SRS
31
Characteristics of a Good SRS
 Unambiguous
 Complete
 Verifiable
 Consistent
 Modifiable
 Traceable
 Usable during the Operation and
Maintenance phase
32
Examples of Requirements statements
Ambiguous
Non-verifiable
Non-verifiable
 Non-verifiable
 Verifiable
 The data set will contain an end of
file character.
 The product should have a good
human interface.
 The program shall never enter an
infinite loop.
 The output of the program shall
usually be given within 10 secs.
 The output of a program shall be
given within 20secs of event X 60%
of the time.
33
Examples of Bad SRS
Documents
Unstructured Specifications:
– Narrative essay --- one of the worst types of
specification document:
• Difficult to change,
• difficult to be precise,
• difficult to be unambiguous,
• scope for contradictions, etc.
34
Examples of Bad SRS
Documents
Noise:
– Presence of text containing information irrelevant to the
problem.
Silence:
– aspects important to proper solution of the problem are
omitted.
35
Examples of Bad SRS
Documents
Overspecification:
– Addressing “how to” aspects
– For example, “Library member names should be
stored in a sorted descending order”
– Overspecification restricts the solution space for the
designer.
Contradictions:
– Contradictions might arise
• if the same thing described at several places in different
ways.
36
Examples of Bad SRS
Documents
Ambiguity:
– Literary expressions
– Unquantifiable aspects, e.g. “good user
interface”
Forward References:
– References to aspects of problem
• defined only later on in the text.
Wishful Thinking:
– Descriptions of aspects
• for which realistic solutions will be hard to find.
37
Complete
 All significant requirements are included
 Definition of responses of the SW to all realizable
classes of input data in all situations.
 Conformity to a standard
 Full labeling and referencing of all figures, tables
etc. and definition of all terms and units of measure
38
Verifiable
A requirement is verifiable if and only if
there exists some finite cost effective
process with which a person or machine can
check that the SW meets the requirement.
Consistent
No two requirements are in conflict
39
Modifiable
 Structure and style of SRS is such that changes to
requirements can be made easily, completely and
consistently.
– SRS organisation -- table of contents, index,
explicit cross-referencing
– no redundancy
40
Traceable
An SRS is traceable if the origin of each
requirement is clear and it facilitates the
referencing of each requirement in future.
Backward traceability
– requirement explicitly referencing its source in
previous documents
Foward traceability
– each requirement has a unique name or
reference number and it can be traced to design
documents, program implementation.
41
SRS Review
Formal Review done by Users, Developers,
Managers, Operations personnel
To verify that SRS confirms to the actual
user requirements
To detect defects early and correct them.
Review typically done using checklists.

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Software Requirement Specification

  • 2. 2 Software Requirements Specification: A Contract Document Requirements document is a reference document. SRS document is a contract between the development team and the customer. – Once the SRS document is approved by the customer, • any subsequent controversies are settled by referring the SRS document.
  • 3. 3 SW Requirements Specification Purpose of SRS – communication between the Customer, Analyst, system developers, maintainers, ... – contract between Purchaser and Supplier – firm foundation for the design phase – support system testing activities – support project management and control – controlling the evolution of the system
  • 4. 4 SRS Document (CONT.)  The SRS document is known as black-box specification: – the system is considered as a black box whose internal details are not known. – only its visible external (i.e. input/output) behavior is documented. S Input Data Output Data
  • 5. 5 SRS Document (CONT.) SRS document concentrates on: – what needs to be done – carefully avoids the solution (“how to do”) aspects. The SRS document serves as a contract – between development team and the customer. – Should be carefully written
  • 6. 6 SRS Document (CONT.) The requirements at this stage: – written using end-user terminology. If necessary: – later a formal requirement specification may be developed from it.
  • 7. 7 Software Requirements Specification (SRS)  Defines the customer’s requirements in terms of : – Function – Performance – External interfaces – Design constraints  The SRS is the basis of contract between the purchaser and supplier
  • 8. 8 Specification Principles Separate functionality from implementation Develop model of desired behavior of the system Establish the context in which s/w operates Define the environment in which system operates Create a cognitive model Specifications must be tolerant of incompleteness & augmentable Content & structure of a specifications should be amenable to change
  • 9. 9 What is not included in an SRS ?  Project requirements – cost, delivery schedules, staffing, reporting procedures  Design solutions – partitioning of SW into modules, choosing data structures  Product assurance plans – Quality Assurance procedures, Configuration Management procedures, Verification & Validation procedures
  • 10. 10 Benefits of SRS  Forces the users to consider their specific requirements carefully  Enhances communication between the Purchaser and System developers  Provides a firm foundation for the system design phase  Enables planning of validation, verification, and acceptance procedures  Enables project planning eg. estimates of cost and time, resource scheduling  Usable during maintenance phase
  • 11. 11 Types of Requirements Functional requirements Non functional requirements – Performance requirements – Interface requirements – Design constraints – Other requirements
  • 12. 12 Functional Requirements Transformations (inputs, processing, outputs) Requirements for sequencing and parallelism (dynamic requirements) Data – Inputs and Outputs – Stored data – Transient data Exception handling Nature of function: Mandatory/ Desirable/ Optional / Volatile / Stable
  • 13. 13 Performance Requirements  Capacity – no. of simultaneous users, processing requirements for normal and peak loads, static storage capacity, spare capacity  Response time  System priorities for users and functions  System efficiency  Availability  Fault recovery  All these requirements should be stated in measurable terms so that they can be verified.
  • 14. 14 Verifiable A requirement is verifiable if and only if there exists some finite cost effective process with which a person or machine can check that the SW meets the requirement.
  • 15. 15 External Interface Requirements User interfaces – eg. if display terminal used, specify required screen formats, menus, report layouts, function keys Hardware interfaces – characteristics of the interface between the SW product and HW components of the system Software interfaces – specify the use of other SW products eg. OS, DBMS, other SW packages
  • 16. 16 Design Constraints SW design constraints – standards for design, coding, naming, etc. – SW interfaces (to OS, DBMS, other SW) – use a specific application package – constraints on program size, data size etc. HW design constraints – specific type of HW, reliability requirements – HW interfaces – requirements for spare capacity or spare performance
  • 17. 17 Design Constraints (contd) User-interface design constraints – features of operator/user with details of working environment – any special features required
  • 19. 19 SRS Standards ANSI/IEEE SRS Standard 830-1984 BS 6719: 1986 European Space Agency Standards (ESA PSS-05-0, Jan 1987) US DoD-Std-7935A ...
  • 20. 20 SRS Prototype Outline [ IEEE SRS Standard ] 1. Introduction 1.1 Purpose 1.2 Scope 1.3 Definitions, Acronyms and Abbreviations 1.4 References 1.5 Overview
  • 21. 21 Purpose – delineate the purpose of the particular SRS – specify the intended audience for the SRS Scope – identify the SW products to be produced by name – explain what the SW product will do, and if necessary, what it will not do – describe the application of the SW being specified. ie. benefits, objectives, goals as precisely as possible Overview – describe what the rest of the SRS contains – how the SRS is organized SRS - Introduction Section
  • 22. 22 SRS Prototype Outline [ IEEE SRS Standard ] 2. General description 2.1 Product perspective 2.2 Product function summary 2.3 User characteristics 2.4 General constraints 2.5 Assumptions and dependencies
  • 23. 23 Product Perspective  State whether the product is independent and totally self contained  If the product is component of a larger system then: – describe the functions of each component of the larger system and identify interfaces – overview of the principal external interfaces of this product – overview of HW and peripheral equipment to be used  Give a block diagram showing the major components of the product, interconnections, and external interfaces.
  • 24. 24 Product Functions  Provide a summary of functions the SW will perform  The functions should be organized in such a way that they are understandable by the user User Characteristics  Describe the general characteristics of the eventual users of the product. (such as educational level, experience and technical expertise )
  • 25. 25 General Constraints  Regulatory policies  HW limitations  Interfaces to other applications  Parallel operation  Audit functions  Control functions  Criticality of the application  Safety and security considerations
  • 26. 26 SRS Prototype Outline [ IEEE SRS Standard ] 3. Specific Requirements - Functional requirements - External interface requirements - Performance requirements - Design constraints - Attributes eg. security, availability, maintainability, transferability/conversion - Other requirements Appendices Index
  • 27. 27 Functional Requirements Introduction – describe purpose of the function and the approaches and techniques employed Inputs and Outputs – sources of inputs and destination of outputs – quantities, units of measure, ranges of valid inputs and outputs – timing
  • 28. 28 Functional Requirements Processing – validation of input data – exact sequence of operations – responses to abnormal situations – any methods (eg. equations, algorithms) to be used to transform inputs to outputs
  • 29. 29 External Interface Requirements  User interfaces  Hardware interfaces  Software interfaces  Communications interfaces  Other requirements – database: frequency of use, accessing capabilities, static and dynamic organization, retention requirements for data – operations: periods of interactive and unattended operations, backup, recovery operations – site adaptation requirements
  • 30. 30 Appendices Not always necessary It may include: – sample I/O formats – DFD, ERD documents – results of user surveys, cost analysis studies – supporting documents to help readers of SRS
  • 31. 31 Characteristics of a Good SRS  Unambiguous  Complete  Verifiable  Consistent  Modifiable  Traceable  Usable during the Operation and Maintenance phase
  • 32. 32 Examples of Requirements statements Ambiguous Non-verifiable Non-verifiable  Non-verifiable  Verifiable  The data set will contain an end of file character.  The product should have a good human interface.  The program shall never enter an infinite loop.  The output of the program shall usually be given within 10 secs.  The output of a program shall be given within 20secs of event X 60% of the time.
  • 33. 33 Examples of Bad SRS Documents Unstructured Specifications: – Narrative essay --- one of the worst types of specification document: • Difficult to change, • difficult to be precise, • difficult to be unambiguous, • scope for contradictions, etc.
  • 34. 34 Examples of Bad SRS Documents Noise: – Presence of text containing information irrelevant to the problem. Silence: – aspects important to proper solution of the problem are omitted.
  • 35. 35 Examples of Bad SRS Documents Overspecification: – Addressing “how to” aspects – For example, “Library member names should be stored in a sorted descending order” – Overspecification restricts the solution space for the designer. Contradictions: – Contradictions might arise • if the same thing described at several places in different ways.
  • 36. 36 Examples of Bad SRS Documents Ambiguity: – Literary expressions – Unquantifiable aspects, e.g. “good user interface” Forward References: – References to aspects of problem • defined only later on in the text. Wishful Thinking: – Descriptions of aspects • for which realistic solutions will be hard to find.
  • 37. 37 Complete  All significant requirements are included  Definition of responses of the SW to all realizable classes of input data in all situations.  Conformity to a standard  Full labeling and referencing of all figures, tables etc. and definition of all terms and units of measure
  • 38. 38 Verifiable A requirement is verifiable if and only if there exists some finite cost effective process with which a person or machine can check that the SW meets the requirement. Consistent No two requirements are in conflict
  • 39. 39 Modifiable  Structure and style of SRS is such that changes to requirements can be made easily, completely and consistently. – SRS organisation -- table of contents, index, explicit cross-referencing – no redundancy
  • 40. 40 Traceable An SRS is traceable if the origin of each requirement is clear and it facilitates the referencing of each requirement in future. Backward traceability – requirement explicitly referencing its source in previous documents Foward traceability – each requirement has a unique name or reference number and it can be traced to design documents, program implementation.
  • 41. 41 SRS Review Formal Review done by Users, Developers, Managers, Operations personnel To verify that SRS confirms to the actual user requirements To detect defects early and correct them. Review typically done using checklists.