Symptom
During expansion of WDM equipment on the network L, the pre-FEC BER in three 10G channels for expansion changes drastically. Within a day, the pre-FEC BER changes from 10–6 to 10–11.
The newly deployed 10G services report a transient alarm indicating that the pre-FEC BER crosses the threshold. The existing 2.5G services are normal without any bit error or alarm.
The network topology of network L is shown in Figure 1.
Cause analysis
Compared with low-rate services such as 2.5G services, high-rate services such as 10G or 40G services have higher requirements for specifications of cables. For example, if the accumulated PMD is higher than the tolerance value, a large non-linear cost will be introduced. As a result, system performance deteriorates.
The possible causes of the fault are as follows:
- The line optical power fluctuates, with a deviation greater than 3 dB.
- The board at the transmit end is faulty.
- The board at the receive end is faulty.
- The quality of line cables deteriorates (for example, PMD is excessively high).
Procedure
- The collected data indicates that the line optical power does not change drastically.
- Replace the relevant board, but the BER still changes drastically. Therefore, the fault is irrelevant to the board.
- The data on the line collected by using the OTDR in a test indicates that reflection is lower than or equal to -27 dB.
- Therefore, it is suspected that the line PMD exceeds the tolerance of a 10G system. The result of an on-site test indicates that the accumulated PMD over a 10G line reaches 12 ps, exceeding the design value. After the cables are replaced, the fault is rectified.
