Network Synchronization
Page 162 7210 SAS M, X Basic System Configuration Guide
• Limits the need for high quality clocks at each network element and only requires that
they reliably replicate input to remain traceable to its reference.
• Uses reliable physical media to provide transport of the timing signal; it doesn't consume
any bandwidth and requires limited additional processing.
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 a holdover or free-run state) 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 in 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.
Each synchronization capable port can be independently configured to transmit data using the
node reference timing or loop timing. In addition, some TDM channels can use adaptive timing.
Transmission of a reference clock through a chain of Ethernet equipment requires that all
equipment supports Synchronous Ethernet. A single piece of equipment that is not capable of
performing Synchronous Ethernet breaks the chain. Ethernet frames will still get through but
downstream devices should not use the recovered line timing as it will not be traceable to an
acceptable stratum source.
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