Q and dBq: a walkthrough

1. Q and dBQ : A Walkthrough
is the quality of a communication signal and is related to BER. . QQ A lower BER gives a higher Q and thus a higher Q gives better performance
is primarily used for translating relatively large BER differences into manageable values.
Pre-FEC signal fail and Pre-FEC signal degrade thresholds are provisionable in units of so that the user does not need to worry about FECdBQ
scheme when determining what value to set the thresholds to as the software will automatically convert the dBQ values to FEC corrections per
time interval based on FEC scheme and data rate.
The Q-Factor, is in fact a metric to identify the attenuation in the receiving signal and determine a potential LOS and it is an estimate of the
Optical-Signal-to-Noise-Ratio (OSNR) at the optical receiver. As attenuation in the receiving signal increases, the dBQ value drops and
vice-versa. Hence a drop in the dBQ value can mean that there is an increase in the Pre FEC BER, and a possible LOS could occur if the
problem is not corrected in time.
The Quality of an Optical Rx signal can be measured by determining the number of "bad" bits in a block of received data. The bad bits in each
block of received data are removed and replaced with "good" zero's or one's such that the network path data can still be properly switched and
passed on to its destination. This strategy is referred to as Forward Error Correction (FEC) and prevents a complete loss of traffic due to small
un-important data-loss that can be re-sent again later on. The process by which the "bad" bits are replaced with the "good" bits in an Rx data
block is known as Mapping. The Pre FEC are the FEC Counts of "bad" bits before the Mapper and the FEC Counts (or Post FEC Counts) are
those after the Mapper.
The number of Pre FEC Counts for a given period of time can represent the status of the Optical Rx network signal; An increase in the Pre FEC
count means that there is an increase in the number of "bad" bits that need to be replaced by the Mapper. Hence a change in rate of the Pre FEC
Count (Bit Erro Rate - BER) can identify a potential problem upstream in the network. At some point the Pre FEC Count will be too high as there
will be too many "bad" bits in the incoming data block for the Mapper to replace ... this will then mean a Loss of Signal (LOS).
As the normal number of Pre FEC Counts are high (i.e. 1.35E-3 to 6.11E-16) and constantly fluctuate, it can be difficult for an network operator to
determine whether there is a potential problem in the network. Hence a dBQ value, known as the Q-Factor, is used as a measure of the Quality
of the receiving optical signal. It should be consistent with the Pre FEC Count Bit Error Rate (BER).
The standards define the Q-Factor as Q = 10log[(X1 - X0)/(N1 - N0)] where Xj and Nj are the mean and standard deviation of the received
mark-bit (j=1) and space-bit (j=0) ................ In some cases Q = 20log[(X1 - X0)/(N1 - N0)]
For example, the linear Q range 3 to 8 covers the BER range of 1.35E-3 to 6.11E-16.
Nortel defines asdBQ 10xlog10(Q/Qref) where Qref is the pre-FEC raw optical Q, which gives a BER of 1E-15 post-FEC assuming a particular
aserror distribution. Some organizations define dBQ ,20xlog10(Q/Qref) so care must be taken when comparing dBQ values from different
sources.
The dBQ figure represents the dBQ of margin from the following pre-FEC BERs (which are equivalent to a post-FEC BER of 1E-15). The
equivalent linear Q value for these BERs are Qref in the above formula.
Pre-FEC signal degrade can be used the same way a car has an “oil light” in that it states that there is still margin left but you are closer to the fail
point than expected so action should be taken.
Further discussion is open for readers…
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