| Description | Bitswapping attempts to maintain an acceptable margin for
each bin; equalizing the margin across all bins through bit
reallocation, thereby maximizing error performance.
The ATU-C performs bitswapping by monitoring the margin
measured for each bin by the transmitter. If a bin's margin
falls below adslAtucConfTargetSnrMgn and the difference
between the margin and adslAtucConfTargetSnrMgn exceeds
the value of this object, then the ATU-C marks a bin as
a 'from' bin.
The ATU-C attempts to equalize the margin across the bins by
swapping bits out of bins marked 'from' into bins marked 'to'.
When the ATU-C swaps a bit out of bin, it decreases its b(i).
Thus, the bins constellation is smaller. which increases its
margin. When the ATU-C swaps a bit into a bin, it increases
its b(i). Thus, the bins constellation is larger, which
decreases its margin.
If this value is too small, then the ATU-C may frequently
toggle the bit allocation on bins, and if impulse noise is
present, this can cause errors. If this value is too large,
then the ATU-C may not identify an adequate number of bins
that it can swap bits 'from', thereby decreasing the chances
for the ATU-C to equalize the margin across all the bins.
Unequalized margin across all the bins translates into poor
error performance. |