Unavailability for IP Extensions
For IP extensions, the following situations will make a specific user unavailable.
Contributions from the IP infrastructure are not included.
Failure of the phone, MTBF varies between 10 and 125 years for the different models.
MTTR 2 hours assuming spares held locally.
Unavailability (worst case, MTBF 10 years), 1/10 * 2 hours /8760 = 0.0000225
Unavailability (best case, MTBF 125 years), 1/125 * 2 hours /8760 = 0.00000183
Change over to standby server, MTBF is 10 years for MX-ONE ServiceNode and change over is approximately 5 minutes. Using HLR redundancy gives the same result for the average extension.
Unavailability, worst case, with 0.2 Erlang/extension, 1/10 * 0.2 * 5 minutes/525600 = negligible.
Double server failure which may affect a specific user, with an MTBF of 10 years, 4 hours MTTR and 10 active servers in the redundancy cluster.
MTB double failure. MTBF*MTBF/MTTR/8760hours / 10 (=number of servers) = 21900 years.
Unavailability, 1/21900 * 4/8760 = negligible.
Availability for IP extensions varies between 99.9978% and 99.9999%