Description of the problem

After replacing the SSN1GXCSA01 veneer with the old clocking scheme of the Huawei OSN3500 device, the customer found after testing that if the two clock tracking sources are disconnected at the same time, the network element enters the free oscillation mode instead of the hold mode.

Processing

The results were confirmed by field tests and R&D experts:

Clock hold mode test on OSN3500 equipment network elements, OSN3500 track two external clock sources, set clock priority first external clock higher than the second external clock. Use SDH meters on the service to monitor the impact on the service when the clock is reversed. The test results are as follows:

1, carry out the test of disconnecting the first clock followed by disconnecting the second clock, if the interval between the two disconnection operations is very short (< 30s), the network element does not enter the hold mode and directly enters the free oscillation mode. Observe the alarm situation of SDH meter, the service is normal and there is no error code. The reverse order side enters the hold mode.

2. Simulate the test of simultaneously disconnecting the two external clocks by unplugging the AUX single board. After unplugging the AUX single board, disconnect the two external clocks on the AUX single board while the AUX is powered down, and then power up the AUX again. The verification results show that the network elements using these two types of cross-single boards can enter the hold state. Observe the SDH meter alarm situation, the service is normal and there is no error code.

3, cross-single board in the clock source switching will re-save the new tracking source clock data information, while clearing the original clock save data. At that time, the first external clock failure, inverted to the second external clock after the original saved clock data information are cleared, and in the case of tracking the second external clock is not long enough, there may be disconnected from the second external clock after the single board does not keep the data, and thus directly into the free oscillation state, did not enter the hold mode. If the order is reversed, the device enters the hold state, because this time is equivalent to the second clock did not work.

4, unplug the AUX, means that the network element at the same time disconnect the 2-way external clock source, no clock source switching action, the laboratory to verify that the network element is able to enter the hold state. However, even if you pull out the board, the two clock source disconnect may still exist in sequence, due to the software every 100ms detection round, each round is in accordance with the order of priority for the detection of all the clock sources, and each detection of a clock source may take 1 to 2ms of time, there may be a very small probability of detecting the second clock source and the second clock source does not have the situation, so the clock source switching will occur. Therefore, pulling the board and disconnecting the clock at the same time may have a very small probability of not being able to enter the hold state.

Root Cause

The crossover single board will re-save the clock data information of the new tracking source after the clock source is switched, and at the same time clear the original clock saving data. At that time, the first external clock failure, reversed to the second external clock after the original saved clock data information are cleared, and track the second external clock time is not long enough, therefore, there may be disconnecting the second external clock after the veneer does not hold the data, and thus directly into the free oscillation state, did not enter the hold mode. If the tracking time of the second clock is long enough, disconnecting the second clock at this time enters the hold mode. When disconnecting the two external clocks at the same time, if by pulling the external clock line, if you first pull the first and then pull the second way to enter the free oscillation mode, on the contrary, enter the hold mode, because at this point in fact, the equivalent of the second clock does not work.

The related technical information in this chapter and the troubleshooting process of SDH equipment are collected and organized by Shenzhen Optical Transmission Network Technology Company Limited (www.opticaltrans.com), please retain! Our company specializes in Huawei SDH optical transmission equipment, SDH transmission equipment sales.