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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/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

e.g

 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.

 

 

Assume:

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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