This object specifies the number of symbols per codeword being
used by the ATU-R for upstream messages on an interleaved
channel during the training sequence. The ATU-C can force the
ATU-R to fallback from this value based on the aggregate data
rate achieved during training.
It is desirable to choose a large codeword size to minimize
overhead. However, increasing the codeword size increases the
possibility of not being able to correct errors in the data
delivered by a frame.