Network Synchronization
252 Basic System Configuration Guide
Figure 13: Conventional Network Timing Architecture (North American
Nomenclature)
The architecture shown in Figure 13 provides the following benefits:
• 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 time-frame. 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.
OSSG287
ST 2
ST 3 ST 3 ST 3 ST 3 ST 3
ST 2
Primary Reference Clock
Stratum 1
Gateway
Class 1 or 2 CO
ST 4 ST 4 ST 4ST 4 ST 4 ST 4 ST 4
Stratum 2
Class 2 or 3
Central Office
Stratum 3
Class 4 or 5
Toll/End Office
Stratum 4
Customer
Prem
Primary Reference
Secondary Reference
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