# Software Reliability Through Hardware Reliability by jhs20192

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```									                          NASA OSMA SAS '01

Software Reliability Through
Hardware Reliability

Dolores R. Wallace
SRS Information Services
Software Assurance Technology Center
http://satc.gsfc.nasa.gov/
1
Reliability-Sept2001
The Problem

• Critical NASA systems must execute successfully for a
specified time under specified conditions -- Reliability

• Most systems rely on software

• Hence, a means to measure software reliability is essential to
determining readiness for operation

• Software reliability modeling provides one data point for
reliability measurement

2
Reliability-Sept2001
The Issues

• Identify mathematics of hardware reliability not used in
software
• Identify differences between hardware, software affecting
reliability measurement
• Develop improvements to software reliability modeling – Dr.
Norman Schneidewind, Naval Postgraduate School
• Develop and implement scenario for typical application on
GSFC project data
• Identify how software reliability modeling can be used at
GSFC

3
Reliability-Sept2001
The Mathematics

Typical Statistical         Sample software
Distributions               reliability models
•Exponential
•Musa Basic:
•Gamma
•Weibull
•Binomial                    m(t) = ß0(1-exp(-ß1t))
•Poisson
•Normal                     •Schneidewind:
•Lognormal
•Bayes
•Markov                      D(T) = (α/β) [1-exp(-β ((T-
s+1)))] + Xs-1

4
Reliability-Sept2001
Differences between Hardware
and Software
Hardware                             Software
• Deterioration over time            • No deterioration over time
•Design faults removed before        •Faults removed after build
manufacture                          • Faults possibly entered
during fault correction
•No faults entered during life
•Initial failures during test
•Initial use, end of life failures   period, early use; then
•No need to allow time to            stable
correct faults                       •Models do not account for
• Models based on continuous         time to correct faults
use                                  •Software test may not be
continuous time

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Reliability-Sept2001

• Reliability growth occurs from fault correction
• Failure correction proportional to rate of failure detection
• Adjusted model with delay dT (based on queuing service)
but same general form as faults detected at time T
• Process: use Schneidewind model to get parameters; apply to
revised model via spreadsheet
• Results
– Show reliability growth due to fault correction
– Predict stopping rules for testing
• Optimal Selection of Failure Data
• Next: GSFC data; Fault insertion
6
Reliability-Sept2001
Applying Software Reliability
Modeling
• The modeling process
– Apply AIAA Recommended Practice
– Learn about the system
• Data collection requirements
– Dates of failure, fix
– Activities/ phase when failures occur
– Preparation of the data (interval; time between failure)
• Available software tools
– Public domain SMERFS^3
– Loglet: commercial, but for our validation
• Interpretation of results
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Reliability-Sept2001
Example of SMERFS^3 Ouput

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Reliability-Sept2001
Loglet – same data

9
Reliability-Sept2001
Options for SRM at NASA

• SATC Service
– Projects submit failure data and project information
– SATC executes models and prepares analysis
• Deployment to Project staff
– SATC provides training and public domain tool
– Project staff utilize the models
• Partnering
– SATC provides tutorials and tools
– Project staff may prepare the data and SATC the analysis
– Project staff may exercise the models but discuss results
with SATC
10
Reliability-Sept2001
Proposed Next Steps

• Continue research on improvements to existing software
reliability models

• Explore non-parametric methods

• Complete experiments with NASA data

• Provide technology transfer of knowledge

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Reliability-Sept2001

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