Service availability or "Uptime" percentages depend on availability design, standards, service management, system management, and resource availability.

Availability or "Uptime" %

Availability %	Downtime per year	Downtime per month	Downtime per week
99% ("two nines")	3.65 days	7.20 hours	1.68 hours
99.50%	1.83 days	3.60 hours	50.4 minutes
99.80%	17.52 hours	86.23 minutes	20.16 minutes
99.9% ("three nines")	8.76 hours	43.8 minutes	10.1 minutes
99.99% ("four nines")	52.56 minutes	4.32 minutes	1.01 minutes
99.999% ("five nines")	5.26 minutes	25.9 seconds	6.05 seconds

Availability Designs
As the graph below demonstrates the higher the percentage of overall availability the greater the cost. The five milestones on the curve represent strategies to achieve greater availability. 

• Base Product Components (Standards) - The use of products with high MTBF rate will, over a large statistical sample of devices, translate into higher availability.NSS infrastructure services all start with base product and component standards (at times we have been "flexible" on these standards based on customer budget constraints).
• Effective Service Management - Effective processes such as ITIL best practices can have a significant impact on improving overall availability. Single point of accountability, IT staffing, proactive approach, and "add value" to the business.
• System Management - System management involves toolsets, policies, and procedures to keep mission-critical systems up and running. It includes the monitoring, diagnostic, and automated error recovery to enable fast detection and resolution of potential and actual infrastructure failure.
• HA Design - A design for high availability will consider the elimination of single points of failure and/or the provision of alternative components to provide minimal disruption to the business operation should an infrastructure component failure occur.
• Full redundancy - To approach continuous availability in the range of 100% (greater than 99.999% - ie., five 9s) requires expensive solutions that incorporate full redundancy.

Availability - Recoverability trade-off considerations
IT infrastructure engineers must ensure that faults happen as rarely as possible. Question is, which precautions are necessary? To answer that, the designer must first determine which services in a system actually require HA and what degree of HA each of those services needs.


Understanding Incident Lifecycle:
Every failure Incident passes through a number of stages. Below is a diagram outlining the Incident lifecycle with Service Availability Metrics.