System Management
270
7210 SAS-M, T, R6, R12, Mxp, Sx, S Basic System
Configuration Guide
© 2021 Nokia.
Use subject to Terms available at: www.nokia.com
3HE 17358 AAAB TQZZA
Figure 23 Conventional Network Timing Architecture (North American Nomenclature)
The architecture shown in Figure 23 provides the following benefits.
• It 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.
• It uses reliable physical media to provide transport of the timing signal. It does
not 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.
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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