A number of 15-minute intervals for which data was collected.
The value of this object will be 96 or the maximum number of
15-minute history intervals collected by the implementation
unless the measurement was (re-)started recently, in which case
the value will be the number of complete 15 minutes intervals
for which there are at least some data.
In certain cases it is possible that some intervals are
unavailable. In this case, this object reports the maximum
interval number for which data is available.
This object maps to the TR-159 attribute
aGroupPerf15MinValidIntervals.