The clock source synchronizes different parts of an NE or the upstream and downstream NEs. In this way, all function modules and chips of the NE can work in a stable and precise frequency, so that services are correctly and orderly transmitted. Synchronizing the network equipment is important.
Clock Synchronization
In the network, it is essential to maintain synchronization of NEs. All NEs trace the same PRC through a specific clock synchronization path, realizing the network-wide synchronization. Generally, an
NE can obtain the PRC through more than one path. In Figure 1, NE4 can trace both the east clock and west clock. These two clock sources are from the same PRC.
Working Mode and Timing Method of the Clock
Currently, the communication network adopts the master-slave synchronization mode. In the master/slave synchronization mode, the slave clock of the node usually has the following
three working modes:
- Tracing mode: Namely, normal working mode. Under this mode, the local clock is synchronized with the input PRC signals.
- Holdover mode: When all timing references are lost, the slave clock enters into the holdover mode. Under this mode, the slave clock takes timing reference from the last frequency information saved before the loss of timing reference signals. This mode can be used to cope with an external timing interruption lasting for many days.
- Free-run mode: When the loss time of all traceable PRCs exceeds 24 hours, or all the control data saved in the tracing mode is extracted, the clock module enters into the free-run mode from the holdover mode.
The clock source has the following two timing modes:
- External source: 2M timing signals that are extracted from the supervisory channels for NEs.
- Internal timing source: The internal timing source is available for all NEs. When the external source is lost, the NE can use the internal timing source.
Priority Table
When the SSM protocol is not enabled, the selection and switching of the clock source are performed mainly based on the priority table. Each clock source in the table is assigned with a unique priority level.
Among all the existing clock sources, the NE selects the clock source with the highest priority as the tracing source. Figure 2 simply shows the diagram of the priority table. The external source has the highest priority,
and the internal source has the lowest priority.
