Configuration Principles

During clock configuration, comply with the following general principles:

• Networks at the backbone layer and at the convergence layer should be configured with clock protection and be set with the primary and secondary PRCs for active/standby switching of clock. In the case of networks at the access layer, generally, only one PRC is set on the central NE. Other NEs trace the clock of the central NE.
• The central node or the node with high reliability provides the clock source.
• If the building integrated timing supply system (BITS) or other external clock equipment with high precision exists, use the external timing mode for the NE. Otherwise, use the line timing mode instead. It is recommended to use the internal timing as a clock source of the lowest level.
• Make a proper planning for the clock network to prevent clock interlock and clock loop.
• Ensure that the path is the shortest when the line clock is traced.
  • In the case of a ring network that consists of less than six NEs, the PRC can be traced from one direction.
  • In the case of a ring network that consists of at least six NEs, make sure that the tracing path is the shortest. That is, when a network consists of N NEs, there should be N/2 NEs tracing the reference clock from one direction and another N/2 NEs tracing the reference clock from another direction. "N" represents the number of NEs. When N is an odd number, the intermediated NEs can trace the reference clock in either direction.
• Do not configure the clocks into rings in the network when the configuration of SSM information is not required.
• If you want to enable the SSM protocol, settings of the SSM protocol information for all NEs in the network should be consistent.
• Between stations, the clock is obtained from STM-N instead of from tributary signals.
• In order to avoid the drift of clock signals after the transmission through multiple stations, compensate clock signals after a long clock chain. According to ITU-T G.781, compensate clock signals after a long clock chain that contains 20 NEs. Considering the factors, such as the transmission distance of the fiber, clock signals are compensated after over 10 NEs usually.

The principles for enabling the SSM protocol are as follows. When the clock switching occurs:

• If you only want an NE to select a clock sources according to the preset priority without consideration of the quality of the clock source, the SSM protocol can be disabled.
• If you want an NE to select a clock source with the highest quality and priority automatically, enable the standard SSM protocol. If the clock network consists of the equipment of Huawei and other vendors, you can enable the standard SSM protocol only and the extended SSM protocol cannot be enabled.
• If you want an NE to select a clock source with the highest quality and priority automatically and the clock network consists of the equipment of Huawei only, you can enable the extended SSM protocol. The clock ID provided by this protocol can avoid the timing loops in the clock network.

If you do not enable the SSM protocol or you enable different SSM protocols, comply with the following principles:

• If you do not enable the SSM protocol, you can configure the clock network to unidirectional only and cannot configure the clocks into rings.
• If you enable the standard SSM protocol, you can configure the clock network to bidirectional but cannot configure the clocks into rings.
• If you enable the extended SSM protocol, you can configure the clock network to bidirectional or configure the clocks into rings. In this case, however, intersection and tangent rings are not permitted.