When the quality of the clock tracking source of the OSN3500 transmission equipment in the network deteriorates, clock reversal may not have occurred at each downstream network element. At this time, clock source inversion must be performed manually at the downstream network elements to avoid service disruption due to clock degradation. Before performing clock inversion, you must make sure that other clock sources have been created in the clock source priority table, and that the clock source is in a normal state and is not locked.

Problem Description

Detailed networking diagram, please see the attached description, ring networking, -----NE1-12(slot11)-------(Slot8)NE1-23(Slot11)-----(Slot8)NE1-13-------, all the fibers are not directly connected through the optical fiber, in the middle of all through the Friends of the wavelength division equipment hopping, the whole network clock are extended SSM protocol, NE1-23 began to track the clock in the direction of NE1-12. Field engineers feedback that when disconnecting the fiber between NE1-12 and NE1-23, the NE1-23 clock is first inverted to the internal clock source, and then after a period of time, it is inverted to the line board in the direction of NE1-13 network elements.

Alarm Message

LTI, S1_SYN_CHANGE

Processing

1, first suspect that there are problems with the clock configuration, after the recovery of MO, analysis, the entire ring on the network elements have been opened to expand the SSM protocol, and the center of the clock ID of the network elements have been set up, and from the phenomenon of the problem, the clock source is inverted to the internal clock after 30 seconds or can be inverted to the other direction, so initially ruled out for the problem of clock configuration;

2, suspected that the middle through the Friends of the wavelength division equipment on the optical signal processing delays, but a line in the ring of other sites for testing, the clock can be immediately inverted, initially ruled out the doubt;

3, suspect that NE1-23 site cross board above the existence of faults, but after the cross inversion, the failure phenomenon remains;

4, a line again for testing, and then let a line to provide part of the clock data, through the data analysis, finally found the cause of the problem, see the description of the cause analysis section.

Root Cause

Please refer to the attached document for a detailed description

From the returned data, it can be judged that the clock situation is normal. But there are some irrationalities in the reporting of LTI alarms: LTI alarms are reported after S1SYNCCHANGE alarms; in fact, there is a fixed delay of 7-8 seconds for s1syncchange alarms. In this case, it is more accurate to look at the clknotracemode alarm.

The other important point about the cause of this problem is that: suppose a network element has two line clock sources A&B, when clock source A has a failure to invert to clock source B, and clock source A is troubleshooting, then by default there is a 5-minute recovery time, then in these 5 minutes, if clock source B has a failure, then before the end of the inversion recovery time, the clock source will not track the clock in B but will instead be inverted to the internal clock source. I heard from R&D that this mechanism is done as suggested. The problem should be caused by RLOF being reported above both clock sources for a short period of time at both ends.

Suggestion and summary

You need to understand the difference between clock reversal and business reversal above, when business reversal, as long as the standby channel, regardless of whether the reversal recovery time has arrived, will be reversed to the standby channel; while the clock reversal, you need to be in the standby channel after the expiration of the recovery time of the reversal of the standby channel, will be reversed to the standby channel above.

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