Availability:A Telecom Network Performance Parameter.

1.OverView

Availability is a probabilistic measure of the length of time a system or network is functioning.

• Generally calculated as a percentage, e.g. 99.999% (referred to as 5 nines up time) is carrier grade availability.
• A network has a high availability when downtime / repair times are minimal.
• For example, high availability networks are down for minutes, where low availability networks are down for hours.
• Unavailability is the percentage of time a system is not functioning or downtime and is generally expressed in minutes.
• Unavailability = (1 – Availability)*365*24*60
• Unavailability(U)=MTTR/MTBF
• The unavailability of a 99.999% available system is 5.3 minutes per year.
• Availability is generally measured as either failure rates  or mean time before failure (MTBF).
• Availability calculations always assume a bi-directional system.

2.Circuit vs. Nodal Availability

Circuit and nodal availability measure different quantities.  To help explain this clearly un-availability (Unavailability=1-Availablity) will be used in this section.

• 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.