Currently, many engineers do not understand the indications of performance events related to the OTU boards that support the FEC function. Besides, no documents can provide specific description on the indications. Here, we conclude the FEC-related performance events and the algorithm for computing the bit error rate (BER) before the FEC.
In the history performance events, fec_bef_cor_er indicates an instantaneous value. It indicates the value at the last second in each 15-minute or 24-hour period rather than the mean value of each period.
1. BER before the FEC: fec_bef_cor_er.
If value of fec_bef_cor_er is 0, it indicates that the BER is of a magnitude less than 1E-11. If the value is 11, it indicates that the BER is of a magnitude at 1E-11. If the value is 10, it indicates that the BER is of a magnitude at 1E-10. If the value is 8, it indicates that the BER is of a magnitude at 1E-8. The magnitude of BER can be obtained based on the value of fec_bef_cor_er by following the same rule. If the value of fec_bef_cor_er is 0, it indicates that there are critical alarms such as OUT_LOF. In this case, the services are completely interrupted.
Note that fec_bef_cor_er indicates an instantaneous value. It indicates the value at the last second in each 15-minute or 24-hour period rather than the mean value of each period. Currently, the maximum and minimum values in the statistical period are not indicated. The system bit errors, however, are evenly distributed. If a channel does not have burst bit errors, fec_bef_cor_er can reflect the mean BER in a period. In addition, fec_bef_cor_er indicates the magnitude of BER. If the actual BER is a random value between 1E-9 and 9.9E-9, the value of fec_bef_cor_er is 9. When the BER before the FEC is beyond the threshold, the beffecsd or beffecexc alarm is reported. The default alarm threshold is 6.
2. Bit error rate after the FEC: fec_aft_cor_er.
If the BER before the FEC is not beyond the limit of the FEC, the value of fec_aft_cor_er is 0, that is, not bit errors occur after the FEC.
3. Count of frames that cannot be corrected: fec_uncor_block_cnt. Usually, frames that cannot be corrected are present when bit errors in the line exceed the correction limit of the FEC or burst bit errors occur in the case of a fiber cut or large signal fluctuation. Normally, the value of fec_aft_cor_er is 0. In the case of 15-minute and 24-hour performance monitoring, this performance event does not exist.
4. The count of corrected "0" bits plus the count of correct "1" bits equal the count of corrected bytes.
5. BER before the correction = The count of corrected bytes / Signal rate * Statistical period. The unit of the signal rate is bits/s and the time for statistics is second (s). For example, if the number of corrected bytes within the 15-minute period of LWF is 123456, BER before the FEC = 123456 / (1E10*900) = 1.37E-8. That is, the BER before the FEC is of a magnitude at 8.
5. The previous computation is about the mean BER within a certain period of time. The result may be different from the BER at a certain point of time. Bit errors in the lines of the WDM equipment are evenly distributed in a stable manner. Hence, in a statistical period of time, the BER at a certain point of time is almost the same as the mean BER.