Calculate network uptime percentage and allowed downtime from SLA availability targets.
Combining component availabilities depends entirely on topology. In series (no redundancy — every component must be up for the whole system to be up), overall availability is the product of each component's availability, so reliability always degrades as more components are chained without redundancy. In parallel (redundant paths — the system is up if at least one component is up), overall availability is 1 minus the product of each component's failure probability, so redundancy dramatically improves availability even with individually less-reliable components.
Series availability
A_series = A1 × A2 × A3 × ...
Parallel (redundant) availability
A_parallel = 1 − (1−A1) × (1−A2) × ...
Every additional link in a series chain adds another point of failure that can take down the entire path, and since each component's availability fraction is less than 1, multiplying more of them together always produces a smaller overall product.
With parallel paths, the whole system only fails if every redundant path fails simultaneously — an event whose probability is the product of each path's (typically small) failure probability, making total failure exponentially less likely as you add more independent paths.
They refer to the count of consecutive 9s in the availability percentage: 99.9% ('three nines') allows about 8.76 hours of downtime per year, while 99.999% ('five nines') allows only about 5.26 minutes per year — each additional nine is roughly a 10x improvement.
Yes — both formulas assume each component fails independently of the others. Correlated failures (e.g. a single power outage taking down two 'redundant' devices in the same rack) violate this assumption and can make real-world availability worse than the calculated parallel figure suggests.