This object specifies a bit rate threshold. When the
following condition is met, the agent generate an
adslAturRateChangeTrap:
( I + F ) < cAdslAturDmtThreshRatefallback
where I = adslAturChanCurrTxRate for ATU-R's interleave channel
F = adslAturChanCurrTxRate for ATU-R's fast channel
Setting this value to '0' prevents the agent from generating an
adslAtucRateChangeTrap due to rate fallback.