What does the “N+1” redundancy model signify?

The "N+1" redundancy model provides one additional component, module, or path beyond the minimum 'N' required to support the critical load. This ensures that if one component fails or is taken offline for maintenance, a backup component is available to take its place, thus preventing a service interruption. This model is a common approach to increasing availability for power and cooling systems in a data center. It is a foundational concept for achieving higher levels of reliability beyond a basic, non-redundant (N) design.

B. This is imprecise. N+1 is a specific level of redundancy; total backup capacity could be N+1, 2N, or other configurations.
C. N+1 is a general redundancy principle applicable to power and cooling systems, not exclusively to network links.
D. This is the opposite of N+1. An 'N' system is one with no backup provision.

1. Uptime Institute. (2018). Tier Standard: Topology. Section: Tier II: Redundant Capacity Components, p. 10. "Tier II solutions add redundant capacity components to the Tier I single path... The simplest Tier II configuration is N+1..."

2. Turner, W. P., Seader, J. H., & Brill, K. G. (2005). Tier Classifications Define Site Infrastructure Performance. The Uptime Institute. White Paper. p. 3. "N+1 provides one extra unit for redundancy."

3. ANSI/TIA-942-B. (2017). Telecommunications Infrastructure Standard for Data Centers. Annex G, Section G.3.2, Redundancy. Describes N+1 as having "at least one more module than the minimum number of modules required."