Redundancy is particularly critical for centralized VoIP servers because they support
large numbers of subscribers. Newer ones are capable of supporting up to 600,000
VoIP lines. If any one of these servers fails, entire VoIP networks can be incapacitated.
The most common causes of failure are power losses and fiber-optic cabling cuts. To
protect networks from these outages, providers often build backup Network Operations
Centers (NOCs) in different geographic regions. The main centers have the ability
to automatically fail over to the backup center in the event of a malfunction. Once
the main center is operational again, service automatically reverts to it
