Software Reliability is the probability of failure-free software operation for a specified period of time in a specified environment. Software Reliability is also an important factor affecting system reliability. ... The high complexity of software is the major contributing factor of Software Reliability problems.
Software Reliability is the probability of failure-free software operation for a specified period of time in a specified environment. Software Reliability is also an important factor affecting system reliability. ... The high complexity of software is the major contributing factor of Software Reliability problems.
The Constructive Cost Model (COCOMO) is an algorithmic software cost estimation model developed by Barry Boehm. The model uses a basic regression formula, with parameters that are derived from historical project data and current project characteristics.
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This Presentation contains all the topics in design concept of software engineering. This is much more helpful in designing new product. You have to consider some of the design concepts that are given in the ppt
The Constructive Cost Model (COCOMO) is an algorithmic software cost estimation model developed by Barry Boehm. The model uses a basic regression formula, with parameters that are derived from historical project data and current project characteristics.
Basic COCOMO compute software development effort (and cost) as a function of program size. Program size is expressed in estimated thousands of source lines of code (SLOC, KLOC).
This Presentation contains all the topics in design concept of software engineering. This is much more helpful in designing new product. You have to consider some of the design concepts that are given in the ppt
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4. Reliability
• Reliability is a broad concept.
• Reliability is one of the metrics that are used to measure quality.
• It is a user-oriented quality factor relating to system operation.
• Intuitively, if the users of a system rarely experience failure, the system is
considered to be more reliable than one that fails more often.
• A system without faults is considered to be highly reliable.
5. Key concepts
• Failure
If observable outcome of a program execution is different from the
expected outcome.
• Fault
Cause of failure.
• Time
If the time gap between two successive failures is short, we say that the
system is less reliable. Two types of time models are:
• Execution time()
• Calendar time (t)
6. Key concepts cont.
• MTTF: Mean Time To Failure
• MTTR: Mean Time To Repair
• MTBF: Mean Time Between Failures (= MTTF + MTTR)
Figure: Relationship between MTTR, MTTF, and MTBF.
Reference: Software Testing and Quality Assurance- Naik,Tripathy
7. Definition of Software Reliability
• First definition
oSoftware reliability is defined as the probability of failure-free
operation of a software system for a specified time in a specified
environment.
Key elements of the above definition
oProbability of failure-free operation
oLength of time of failure-free operation
oA given execution environment
Example
oThe probability that a PC in a store is up and running for
eight hours without crash is 0.99.
Reference: Software Testing and Quality Assurance- Naik,Tripathy
8. Definition cont.
• Second definition
• Failure intensity is a measure of the reliability of a software system
operating in a given environment.
• Example: An air traffic control system fails once in two years.
• Comparing the two
• The first puts emphasis on MTTF, whereas the second on count.
Reference: Software Testing and Quality Assurance- Naik,Tripathy
9. Factors influencing software reliability
User’s perception- Reliability of a software depends upon two
categories of information
1)The number of faults present in the software
2)The ways user operate the system-Operational profile
Reference: Software Testing and Quality Assurance- Naik,Tripathy
10. Fault count is influenced by following:
• Size and complexity of code
• Characteristics of development process used
• Education, experience and training of development personnel
• Operational Environment
Reference: Software Testing and Quality Assurance- Naik,Tripathy
11. Operational Profile
• An OP describes how actual users operate a system.
• Two ways to represent operational profiles
• Tabular
• Graphical
Reference: Software Testing and Quality Assurance- Naik,Tripathy
12. Use of OP in different ways
• Use an OP as a guiding document in designing user interfaces.
• Use an OP to design early version of a software for release
• Use an OP to determine where to put more resources in a software.
Note:
For accurate estimation of reliability of a system, test the system in the
same way it will be actually used in the field.
13. Software reliability methodologies
Critical systems (spacecraft, aircraft, nuclear power plant etc. ) require
a high level of dependability in their operation.
Dependability means:
1)Fault avoidance
2)Fault tolerance
3)Fault removal
4)Fault forecasting
Reference: Software Testing and Quality Assurance- Naik,Tripathy
14. Fault avoidance
• Prevent the introduction of faults during the development of the
software.
How?
• Use standards and guidelines
-How to implement the code?
-When and where to use functions, pointers etc.,
• Use formal methods
-state m/c to verify system working
• Methods against software aging
-to prevent memory leaks-system crash
15. Fault tolerance
• Used to address the shortcoming of fault avoidance
How?
By mitigating the risks cos
-there will be some potential or hidden faults remaining in software.
16. Fault removal
• Aim at detecting and fixing faults once the code has been developed.
How?
• Testing techniques
• Using various methods and verification
• Analysis ( Dynamic , Semantic etc.,)
17. Fault forecasting
• Estimating the presence of faults.
• Occurrence and consequences of failure.
• Main aim of fault forecasting is predicting the reliability of a software
product reliability.
• They are mainly concerned with reliability models.
18. Why Software Reliability Models?
• The fault forecasting methodologies includes the
reliability models.
• Return us the reliability of the software or predict the
reliability of the software.
20. Classification of Models (contd..)
Reference: Software Testing and Quality Assurance-
Naik,Tripathy
21. Time Dependent Models
Time between failures models (TBF):
In this class of models under study is the time between failures.
Failure Count Models (FC):
The interest of this class of models is in the number of faults and failures in
specified time intervals
22. Time Independent Models
Fault Seeding Models (FS):
Program has unknown number of indigenous faults , to this a known number
of faults are seeded.
Input Domain Based Models (IDB):
An estimate of the reliability of the program is obtained from the failures
observed during execution of the sample test cases.
23. Example models
• TBF - J-M De-Eutrophication.
• FC - Goel-Okumoto NHPP Model.
• FS - Mills seeding Model.
• IDB - Ramamoorthy and Bastani Model.
24. SR Modeling Flow Chart
Reference: Software Testing and Quality Assurance-
Naik,Tripathy
25. Applications of SR Models
The applicability of these models can be assessed during
Design Phase.
Unit Testing.
Integration Testing.
Operational Phases.
26. Summary
• Software reliability is defined as the probability of failure-free
operation of a software system for a specified time in a specified
environment.
• Factors influencing SR are fault count and operational profile
• Dependability means fault avoidance, fault tolerance, fault
removal and fault forecasting.
• SR models helps in predicting the reliability of the software using
mathematical approach.