Software and Hardware Reliability
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Software and hardware reliability are defined differently. Software reliability is the probability that software will operate as required for a specified time in a specified environment without failing, while hardware reliability tends towards a constant value over time and usually follows the "bathtub curve". Ensuring reliability involves testing like fault tree analysis, failure mode effects analysis, and environmental testing for hardware, and techniques like defensive programming, fault detection and diagnosis, and error detecting codes for software. Reliability is measured through metrics like time to failure and failure rates over time.
Software and Hardware Reliability
- 1. Software and Hardware Reliability 1
- 2. Software Reliability Definition The probability that the software will; operate as required (i.e., without fail), for a specified time, in a specified environment. 2
- 3. Software Reliability - features • failures in software are design faults, • reliability during test changes continually (new problems are found as old ones are fixed / new code is never perfect) • phenomenon of software reliability growth • environment is important (platform/inputs) - new envt. may require s/w retest 3
- 4. Hardware Reliability - features • failure is usually due to physical deterioration • hardware reliability tends, more than software, towards a constant value, • hardware reliability usually follows the ‘bathtub’ principle, • again, environment is important; a proportion of hardware faults are design faults 4
- 5. Fault, Error, Failure (PERSON MAKES) ENVIRONMENT INPUT MISTAKES OPERATOR CAN BE ATTRIBUTED TO POTENTIALLY ONE OR MANY LEADS TO ZERO OR MANY OR FAULTS REVEALING CAN BE ATTRIBUTED TO AND MECHANISM ONE OR MANY LEADS TO ZERO OR MANY ERRORS CAN BE ATTRIBUTED TO LEADS TO ZERO OR MANY ONE OR MANY FAILURE 5
- 6. Reliability Measures When we talk of reliability measures the irony is that we invariably talk about failure measures. There are four general ways of measuring failures against time; • time of failure, • interval between failures, • cumulative failures experienced up to a given time, • failures experienced in a time interval. 6
- 7. Ensuring Hardware Reliability Hardware Reliability is ensured by conducting the following tests: • Fault Tree Analysis • Failure Modes Effects and Criticality Analysis • Failsafe Tests • Fault Injection Tests • PCB Trace Anaysis and Circuit Simulation • Environmental Tests 7
- 8. Ensuring Software Reliability Software Reliability is ensured by following the following Techniques: • Defensive Programming To produce programs which detect anomalous control flow, data flow or data values during their execution and react to these in a redetermined and acceptable manner. • Fault Detection & Diagnosis To detect faults in a system, which might lead to a failure, thus providing the basisfor countermeasures in order to minimise the consequences of failures. 8
- 9. • Error Detecting and Correcting Codes To detect and correct errors in sensitive information. • Diverse Programming Detect and mask residual software design faults during execution of a program, in order to prevent Safety critical failures of the system, and to continue operation for high reliability. • Software Error Effect Analysis To identify software modules, their criticality; to propose means for detecting software errors and enhancing software robustness; to evaluate the amount of validation needed on the various software components. 9
- 10. Software Reliability Tests • Software Quality Audit • Software Rule Checking • Unit Testing • Software Integration Tests • Software/Hardware Integration Tests • Fault Injection Tests • System Validation 10
Editor's Notes
- #4: an elaboration of the pdca