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FAQ-The LT1 Alarm Is Reported During Clock Switching

Publication Date:  2012-07-25 Views:  152 Downloads:  3

Issue Description

As shown in the attachment Clock, NE 1-12, NE 1-23, and NE 1-13 are interconnected into a ring through Siemens WDM equipment. Slot 11 on NE 1-12 and slot 8 on NE 1-23 are interconnected. Slot 11 on NE 1-23 and slot 8 on NE 1-13 are interconnected. The extended SSM protocol is enabled on all the NEs. NE 1-23 tracks the clock from NE 1-12. When NE1-12 and NE1-23 are disconnected from each other, NE 1-23 switches to the internal clock and then to the clock from NE 1-13. 

Alarm Information


Handling Process

1. The engineers suspect that there is an error in the clock configurations. Restore and analyze the MO. It is found that the extended SSM protocol is enabled on all the NEs on the ring and that both the clock IDs are set on the central NE. In addition, the NE can switch to the other clock source about 30 seconds after switching to the internal clock. Therefore, the fault is unassociated with the clock configurations.
2. The engineers suspect that the WDM equipment of Siemens has a delay in processing optical signals. Conduct the test on another NE on the ring. It is found that the NE can switch to the other clock source immediately. Therefore, the fault is unassociated with the WDM equipment of Siemens.
3. The engineers suspect that the cross-connect board on NE 1-23 fails. Switch the active and standby cross-connect boards on the NE. The fault persists.
4. Conduct the test again and collect certain clock data. Analyze the data and find out the cause. For details, see the cause analysis. 

Root Cause

For details on the cause analysis, see the attachment Cause Analysis.
According to the returned data, the clock runs properly except that the LTI alarm is reported. The LTI alarm is reported or cleared after the S1_SYN_CHANGE alarm is reported or cleared. The S1_SYN_CHANGE alarm shall be reported actually after a fixed delay of seven to eight seconds. In this case, the CLK_NO_TRACE_MODE alarm can reflect the situations more properly.
In addition, assume that an NE maintains two line clock sources: clock source A and clock source B. When clock source A fails, the NE switches to clock source B. After clock source A is recovered, the NE waits for five minutes by default and then switches back to clock source A. If clock source B fails within the five minutes, the NE switches to the internal clock source instead of tracking clock source B. This mechanism is developed as recommended. Therefore, the cause is that the two clock sources at the two ends both report the R_LOF alarm within a short time. 


Clock switching is different from service switching.
? In the case of service switching, if the protection channel is available, the services are switched to the protection channel regardless whether the WTR time expires.
? In the case of clock switching, the clock is switched to the standby clock source only after the WTR time expires.