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Table 43: Expected range of typical availability
expected of communications systems:
Table 43: Expected range of typical availability
Availability
99.95
99.99
99.999+
High availability – general design considerations
High availability requires dedicated design diligence at multiple layers and with several
overlapping objectives
Table 44: Measurements used for assessing availability expectations
Design element
Failures of each component and subsystem must
be infrequent
System outages must be infrequent
When there is a failure or outage:
The system collects its own performance
statistics
Not only are failures at the device or subassembly level minimal, but also when there is a
failure, the design itself helps in many ways to alleviate the impact of the failure. For example,
the design tests itself frequently to detect problems before they become customer affecting. The
design isolates subassemblies that are not functioning properly, and runs verification tests on
Downtime
Reliability
per year
level
4 ½ hours
"Standard"
53 minutes
"High"
5 minutes
"Critical"
or less
(Table 44: Measurements used for assessing availability
The impact must be minimized and
isolated.
Recovery must be speedy.
shows the range of availability that is typically
Who might need this
Generally accepted as the minimum
standard of acceptable downtime for
business
Businesses or organizations that
highly depend on their
communication system
Hospitals, emergency services,
high-performance call centers,
critical government agencies,
financial institutions
Measurement
Mean Time between
Failures (MTBF)
Mean Time between
Outages (MTBO)
Mean Time to
Recovery (MTTR)
Various
Issue 3.4.1 June 2005
Reliability
expectations).
231
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