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4.2Availability management (availability rate, redundancy, single point of failure)
Covers the availability rate, computed as MTBF / (MTBF + MTTR); the distinction between the mean time between failures (reliability, MTBF) and the mean time to repair (maintainability, MTTR); estimating overall availability as the product of availabilities for a series configuration and as 1 - (1 - a)^n for parallel redundancy; and eliminating a single point of failure (SPOF) through redundancy.
When a service manager wants to raise availability, blindly lining up expensive equipment yields nothing. Availability is determined by two factors—"resistance to breaking (reliability)" and "ease of being fixed (maintainability)"—and the availability of the whole system varies greatly with how the components are connected (series versus parallel redundancy). This section quantifies the availability rate as MTBF / (MTBF + MTTR) and, using that value as a clue, builds judgment to diagnose and design where the weak point (single point of failure) is and whether redundancy or shorter repair time is the more effective lever. The calculations center not on "the value itself" but on "what the value tells you to improve."
4.2.1Availability rate, MTBF (reliability), and MTTR (maintainability)
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