RTN 950 supports SSM protection. After SSM protection is enabled on an NE, automatic protection switching of clock sources follows these rules:
l According to the clock source priority list, the NE selects the clock source of the highest quality as the synchronization source.
l If multiple clock sources have the same highest quality, the NE selects the source with the highest priority as the synchronization source.
l The NE broadcasts quality information about the synchronization clock source to its downstream NEs and also notifies its upstream NE that its own clock source cannot be used for synchronization.
Figure 7-12 is a radio ring where SSM protection is enabled.
When the network operates normally, the NEs on the ring select the clock source as follows:
1. NE1 selects the external clock source as the synchronization source and notifies NE2 and NE4 of the external clock quality.
2. NE2 and NE4 select the clock source from NE1 as the synchronization source and notify NE1 that the clock sources from NE2 and NE4 are unavailable.
3. After determining that the clock sources from NE2 and NE4 have the same quality, NE3 selects the clock source with a higher priority (the clock source from NE2) as the synchronization source. In addition, NE3 transmits quality information about the synchronization source to NE4 and notifies NE2 that the clock source from NE3 is unavailable.
4. After determining that the clock sources from NE1 and NE3 have the same quality, NE4 selects the clock source with a higher priority (the clock source from NE3) as the synchronization source. In addition, NE4 transmits quality information about the synchronization source to NE1 and notifies NE3 that the clock source from NE4 is unavailable.
5. According to the clock quality in the west and east directions and configured clock source priorities, NE2, NE3, and NE4 determine that the synchronization source does not need to be modified. The clock source selection is completed.
When the radio links between NE1 and NE2 become faulty, the NEs on the ring select the clock source as follows:
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1. NE2 selects the internal source as the synchronization source and transmits quality information about the synchronization source to NE1 and NE3.
2. NE3 selects NE2 as the clock source and notifies NE4 of the clock quality.
3. After determining that the quality of the clock from NE1 is higher than the quality of the clock from NE3, NE4 selects the clock source from NE1 as the synchronization source. In addition, NE4 transmits quality information about the synchronization source to NE3 and notifies NE1 that the clock from NE4 is unavailable.
4. After determining that the quality of the clock from NE4 is higher than the quality of the clock from NE2, NE3 selects the clock source from NE4 as the synchronization source. In addition, NE3 transmits quality information about the synchronization source to NE2 and notifies NE4 that the clock source from NE3 is unavailable.
5. After determining that the quality of the clock from NE3 is higher than the quality of the internal clock source, NE2 selects the clock source from NE3 as the synchronization source.
In addition, NE2 transmits quality information about the synchronization source to NE1 and notifies NE3 that the clock source from NE2 is unavailable.
6. According to the clock quality in the west and east directions and the configured clock source priorities, NE2, NE3, and NE4 determine that the synchronization source does not need to be modified. The clock source selection is completed.
NOTE
Even though SSM protection is enabled, clocks cannot constitute a ring during clock source configuration. For example, configuration of clock sources on NE1 prevents a clock ring, as shown in Figure 7-12.
Figure 7-12 SSM protection
NE1
After extended SSM protection is enabled on an NE, automatic clock switching follows these rules:
l According to the clock source priority list, the NE selects the clock source of the highest quality as the synchronization source.
l If the clock ID of a clock source indicates that the clock source is from the local NE, the clock source is not processed.
l If multiple clock sources have the same highest quality, the NE selects the source with the highest priority as the synchronization source.
l The NE broadcasts quality information and the clock ID of the synchronization clock source to its downstream NEs, and also notifies its upstream NE that its own clock source cannot be used for synchronization.
The clock ID takes a value in the range of 0 to 15. 0 is the default value, indicating that the clock ID is invalid. After the extended SSM protocol is enabled on the NE, the NE does not select any clock source with an ID of 0 as its current clock source.
Follow these guidelines when you allocate clock IDs:
l When the extended SSM is used, the clock ID of an external clock source cannot be automatically extracted. Therefore, allocate clock IDs to all external clock sources.
l At all the NEs that are connected to external clock sources, allocate clock IDs to the internal clock sources.
l At all the intersecting nodes of a ring/chain and a ring, allocate clock IDs to the internal clock sources.
l At all the intersecting nodes of a ring/chain and a ring, allocate clock IDs to the clock sources that are transmitted to the ring.
l Do not allocate clock IDs to clock sources different from the preceding four types. Their clock IDs are 0 by default.
l Clock IDs do not determine clock source priorities.
Figure 7-13 is a radio ring where the extended SSM protection is enabled.
On the ring, the following clock sources require clock IDs:
l Ethernet interface 1 on NE1 l Ethernet interface 2 on NE3 l Internal clock source on NE1 l Internal clock source on NE3 7 Configuring Networkwide Service Data
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Figure 7-13 Extended SSM protection Ethernet interface 2 /
Internal
Ethernet interface 1 / West /
l When extended SSM protection is enabled, clocks can constitute a ring during clock source configuration.
l Extended SSM protection is advantageous in complex clock protection network topologies, for example, in a network with dual external clocks. Therefore, extended SSM protection is used in only a few scenarios.
7.1.17 Clock Synchronization Policy
Users plan an appropriate clock synchronization policy based on the network topology.