TABLE OF CONTENTS Let us make the following definitions.
The availability of a network element is the probability that the element can deliver some specified Quality of Service (QoS) at some point of time.
The failure of a network element is the moment when the element stops delivering the specified QoS.
The repair of a network element is the moment when the element regains its ability to deliver the specified QoS.
A fault (also known as an outage) is the period of time when a network element is not functional that is, the period of time between a failure and subsequent repair.
Note that not every defect in a network element can be categorized as a failure. Some defects degrade the quality or level of services using the affected element, but do not fully disrupt them, which leaves their QoS parameters within an acceptable (agreed upon) range. Some defects do not have a notable impact on existing services, but might prevent further services from being established. In this chapter we will only consider those defects that fully disrupt services that is, stop delivering the specified QoS as failures.
Network element availability can be calculated according to the formula:
where
| A | is the network element's availability, |
| Tmr | is the element's mean time to repair, |
| Tmbf | is the element's mean time between failures. |
For hardware elements the mean time to repair may typically be quite large because it will involve the dispatch of...
