Symmetricom Time Server - Manual (Page 36)

2-22 TimeVault™ User’s Manual 6000-100Ch2.fm Rev. D
Chapter 2: Installation and Start-Up Power-Up
2.3.5 Rolling Redundancy
Rolling redundancy ensures secure, uninterrupted time delivery.
During normal operation, TimeVault receives the time through GPS, its designated
primary synchronization source. If GPS loses time synchronization, there is a brief
holdover period before TimeVault’s modem calls ACTS, the designated secondary
synchronization source.
During this period, TimeVault uses its internal oscillator to deliver precise time. If, after
three minutes, GPS is still not synchronized, ACTS takes over and provides the time until
GPS is once more available. At this point, TimeVault rolls back over to GPS.
If both GPS and ACTS were to fail, TimeVault rolls over to NTP, its designated tertiary
synchronization source. TimeVault uses NTP until either GPS or ACTS is once again
available.
This rolling redundancy sequence is summarized in the following illustration:
Start-Up GPS
ACTS
NTP
Primary Good
Secondary Good
Tertiary Good
Primary Good Primary Bad
Secondary Good Secondary Bad
Primary Good
Although TimeVault is not designed to resist a denial of service attack from a hostile source, an
excessive number of NTP requests in a short period of time will not cause the system to crash or
lock up.