Availability:A Telecom Network Performance Parameter.

• Circuit un-availability is a measure of the average down time of a traffic demand / service.
  • A circuit is un-available only if traffic affecting components that help transport the demand / service have failed.
  • Circuit unavailability is calculated by considering the unavailabilities of components which are traffic affecting and by taking into consideration those components that are hardware protected.
  • For example, the failure of both 10G line cards on an NE can cause a traffic outage.
• Nodal un-availability is a measure of the average down time of a node.
  • Each time there is a failure in a node, regardless if it is traffic affecting or not, an engineer is required to visit the node to fix the failure.
  • Therefore nodal un-availability is based on calculated failure rates, it is still a direct measure of an operational expenditure.
  • Nodal unavailability is calculated by adding all components of a network element regardless of hardware protection, i.e. in series.
  • For example, failure of a protected switch card is non-traffic affecting but still requires a site visit to be replaced.

3.Terms & Definitions

Failure rate

•  Failure rate is usually measured as Failures in Time (FIT), where one FIT equals a single failure in one billion (10⁹) hours of operation.
•  FITs are calculated according to industry standard (Telcordia SR 332).

MTBF- (Mean time between failure)

•  Average time between failures for a given component.
•  Measured either in hours or years.
• MTBF is inversely proportional to FITs.

MTTR-(Mean time to repair)

•  Average time to repair a given failure.
•  Measured in hours.
•  Availability is always quoted in terms of number of nines
•  For example, carrier grade is 5 9’s, which is 99.999%
• Availability is better understood in terms of unavailability in minutes per year
• Therefore for an availability of 99.999%, the unavailability or downtime is 5.3 minutes per year