Nokia 7210 Configuration Manual page 270

System Management
Conventional Network Timing Architecture (North American Nomenclature)
Figure 23
Stratum 1
Gateway
Class 1 or 2 CO
Stratum 2
Class 2 or 3
Central Office
Stratum 3
Class 4 or 5
Toll/End Office
Stratum 4
Customer
Prem
The architecture shown in
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.
270
ST 2
ST 3 ST 3
ST 4 ST 4
Primary Reference
Secondary Reference
• 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.
Use subject to Terms available at: www.nokia.com
7210 SAS-M, T, R6, R12, Mxp, Sx, S Basic System
Primary Reference Clock
ST 3
ST 4 ST 4 ST 4
Figure 23 provides the following benefits.
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Configuration Guide
ST 2
ST 3 ST 3
ST 4 ST 4
OSSG287
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