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Synchronization and Redundancy
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The synchronization network is designed so a clock always receives timing from a clock of equal
or higher stratum or quality level. This ensures that if an upstream clock has a fault condition (for
example, loses its reference and enters holdover or free-run) and begins to drift in frequency, the
downstream clock will be able to follow it. For greater reliability and robustness, most offices and
nodes have at least two synchronization references that can be selected in priority order (such as
primary and secondary).
Further levels of resiliency can be provided by designing a capability into the node-clock that will
operate within prescribed network performance specifications without any reference for a
specified timeframe. A clock operating in this mode is said to hold the last known state over (or
holdover) until the reference lock is once again achieved. Each level in the timing hierarchy is
associated with minimum levels of network performance.
Present and future applications require access to a high quality frequency source or time stamp,
thus, it is acknowledged that the network is an efficient way to get this information where it is
needed. It is not necessary for the network to provide these high quality sources, only transfer it
reliably, since the source can not transfer time by itself. This significantly enhances the quality of
the services transferred.
Page 192
Uses reliable physical media to provide transport derive the timing signal; it doesn't
consume any bandwidth and requires limited additional processing.
7710 SR OS Basic System Configuration Guide
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