A threshold for the sampled statistic. When the current
sampled value is less than or equal to this threshold,
and the value at the last sampling interval was greater than
this threshold, a single event will be generated.
A single event will also be generated if the first
sample after this entry becomes valid is less than or
equal to this threshold and the associated
alarmStartupAlarm is equal to fallingAlarm(2) or
risingOrFallingAlarm(3).
After a falling event is generated, another such event
will not be generated until the sampled value
rises above this threshold and reaches the
alarmRisingThreshold.
This object may not be modified if the associated
alarmStatus object is equal to valid(1).