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TABLE 1: Uptime Institute definitions of tier levels
听	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

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