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Quick note on Pre-FEC , Post-FEC ,BER and Q relation.

Pre-FEC BER corresponding to Q.

  • BER and Q can be calculated from one another according to the following relationship:

  • Or, in Excel: dBQ = 20*LOG10(-NORMSINV(BER))
  • dBQ = 20Log(Q)


  • Post-FEC BER of <1e-15 essentially means no post-FEC errors
  • This is equivalent to ~18dBQ, which is about as high as can be measured


FEC Limit

  • This is the lowest Q (or highest BER) that can be corrected by the FEC
  • Beyond this post-FEC errors will occur


 FEC Limit: 8.53dBQ or a BER of 3.8e-3
 FEC Limit: 5.23dBQ or a BER of 3.4e-2 (<97% of bits have to be correct!)

Pre-FEC Calculation eg.




219499456 : Bit Errors

0 : Uncorrectable words

6.4577198E-6 : PRE-FEC BER

Assume the Time at this instant of performance was 12:05:04 which means : 304 Seconds since the last time interval.

Assume The FEC settings was : STANDARD FEC so the Rate used for 100 G transponder is : 1.1181 * 10^11
General Formula to calculate PRE_FEC: 
PRE_FEC BER =    TotalErrors 

                (secsFromLast * (rate)) 

TotalErrors =((bitErrorCorrected count + (9 * (uncorrected Words count))

So Substituting the Values     
219499456 / (304*1.1181 * 10^11) = 6.4577198E-6   



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