The line reflection leads to the inaccurate BER detection of the SSC6LWX board. The board reports the B1_EXC alarm.
Symptom
As shown in Figure 1, when an intensive reflection exists on the line, the SDH instrument reports bit errors. When the performance event reporting of the LWX board along the signal flow is queried, however, the first bit error reporting point is found on the RX port of the downstream board 2 instead of on the IN port of board 1. Alternatively, the IN port of board 1 has only a few bit errors, whereas plenty of bit errors appear on the RX port of board 2. Such a phenomenon in which the first bit error reporting point is on the second receiving side in the downstream instead of on the first receiving side is called "beating an ox on the other side of a mountain". The phenomenon, caused by line reflection, directly impacts the fault analysis and troubleshooting and misleads the fault locating on site. By now, the problem has occurred in two sites.
The LWX board reports the B1_EXC alarm.
Cause Analysis
As shown in Figure 2, the LWX is a wavelength conversion board of any rate ranging from 32 Mbit/s to 2.5 Gbit/s. The board implements performance monitoring through bypass detection. That is, it divides the received signals into two parts. One part is sent to the performance monitoring module for bit error statistics, and the other part is sent to the downstream chip for output after being processed by the board. Because the performance monitoring path is independent of the signal output path, it is possible that the receiving side of the board is normal but bit errors occur after board processing. Because the LWX is an access board of any rate ranging from 32 Mbit/s to 2.5 Gbit/s, no phase-locked loop circuit exists inside the board. Nevertheless, the performance monitoring module is implemented by the FPGA. Depending on the service type monitored, the board is loaded with a specific logic file. A phase-locked loop (that is, access at a fixed rate) exists inside every implemented circuit. Hence, the two parts of signals differ in performance. Generally, this difference is ignorable. If no bit error occurs in performance monitoring, the signals received by the board on this point can be regarded normal; however, when the multipath interference (MPI) effect is triggered by an intensive reflection point on the line, the BER detection of the board may be inaccurate (inaccurate BER reporting of the board is the only reflection-caused problem found during the three years of application of the LWX board).
To locate the preceding LWX bit error problem, the normal location method is preferred to check the optical power, OSNR, and dispersion of the system, which are found normal. Then, the software and hardware of the board are checked to ensure that the board is normal (these are the items that should be inspected first, and are the most easily discernible causes for the problem). After the foregoing factors are precluded, if the LWX still reports bit errors, the upstream line might be faulty. An OTDR is used to measure the line and check whether any intensive reflection event occurs, that is, whether intensive reflection point exists on the line (according to the China national standard, the reflection must be less than -27 dB). After the reflection point is found, the problem can be cleared by cleaning the fiber, replacing the connector, or splicing the fiber connector.
Because the interference light and signal light caused by reflection are in the same direction and at the same frequency, and impose little impact on the power of the main signal light, the problem is unable to be discerned by measuring the optical power of signals or the OSNR with the optical power meter or spectrum analyzer.
Procedure
- Use the OTDR instrument to measure the line and observe whether intensive reflection events occur on the instrument, and detect the reflection problem on the line.
- A research shows that the MIP effect greatly impacts the probability distribution of the "1" codes in the system. Based on the special optical power distribution generated by secondary reflection interference on the "1" codes of the signal light, the statistic function of the eye pattern tester can be used to test the optical power distribution diagram (histogram) of the "1" codes in the eye pattern. In this way, the existence of the secondary reflection interference can be discerned effectively.
Reference Information
Figure 3 shows the main signal eye patterns. Figure (a) shows the eye pattern when the secondary reflection interference does not exist; figure (b) shows the eye pattern when the secondary reflection interference exists.
Figure 3 Main signal eye patterns when the secondary reflection interference does not exist and exists
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