Fault–tolerant System Reliability in the Presence of Imperfect Diagnostic Coverage

The deployment of computer systems for the control of mission critical processes has become the norm in many industrial and commercial markets. The analysis of the reliability of these systems is usually understood in terms of the Mean Time to Failure. The design and analysis of high reliability systems is now a mature science. Starting with fault–tolerant central office switches (ESS4), dual redundant and n– way redundant systems are now available for variety of application domains. The technologies of microprocessor based industrial controls and redundant central processor systems creates the opportunity to build fault–tolerant computing systems on a much smaller scale than previously found in the commercial market place. The diagnostic facilities utilized in a modern Fault–Tolerant Computer System attempts to detect fault conditions present in the hardware and embedded software. Coverage is the figure of merit describing the effectiveness of the diagnostic system. This paper examines the effects of less than perfect diagnostics coverage on system reliability. The mathematical background for analyzing the coverage factor of fault–tolerant systems is presented in detail as well as specific examples of practical systems and their relative reliability measures. In a complex system, malfunction and even total nonfunction may not be detected for long periods, if ever. Number Page Figure 1 – Evaluation Criteria defining System Reliability. These criteria will be used to develop a set of time dependent metrics used to evaluate various configurations.. 1 represents a single fault with a return path to the fault free mode by a repair operation, and State { } 2 represents the fault free (TMR) operation mode, State { } 1 represents a single fault (Duplex) operation mode with a return path to the fault free mode, and State { } 1 represents a single fault with a return path to the fault free mode by a repair operation, and State { } 0 represents the system failure mode.