The rate at the time of the last
adslAturRateChangeTrap event. It is also set at
initialization to prevent a trap being sent.
Rate changes less than adslAturThresh(*)RateDown
or less than adslAturThresh(*)RateUp will not
cause a trap or cause this object to change.
(*) == Fast or Interleave.
See AdslLineAlarmConfProfileEntry.