Reliability Indexes
Reliability indexes include the mean time to repair (MTTR), mean time between failures (MTBF), and availability.
Generally, product or system reliability is assessed based on the MTTR and MTBF.
MTTR
The MTTR indicates the fault rectification capability in terms of maintainability. This index refers to the average time that a component or a device takes to recover from a failure. The MTTR involves spare parts management and customer service and plays an important role in evaluating device maintainability.
The MTTR is calculated using the following formula:
MTTR = Fault detection time + Board replacement time + System initialization time + Link recovery time + Route convergence time + Forwarding recovery time
A smaller addend indicates a shorter MTTR and higher device availability.
MTBF
The MTBF indicates fault probability. This index refers to the average time (usually expressed in hours) when a component or a device is working properly.
Availability
Availability indicates system utility. Availability can be improved when the MTBF increases or the MTTR decreases.
Availability is calculated using the following formula:
Availability = MTBF/(MTBF + MTTR)
In the telecom industry, 99.999% availability means that service interruptions caused by device failures are less than 5 minutes each year.
On live networks, network faults and service interruptions are inevitable due to various causes. Availability can be improved by decreasing the MTTR.
