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| TABLE 1: Uptime Institute definitions of tier levels | ||||
| Tier I | Tier II | Tier III | TierIV | |
| Number of delivery paths | Only 1 | Only 1 | 1 Active, 1 Passive | 2 Active |
| Redundancy | N | N+1 | N+1 | S+S or 2(N+1) |
| Compartmentalisation | No | No | No | Yes |
| Concurrently maintainable | No | No | Yes | Yes |
| Fault tolerance to worst event | None | None | None | Yes |