HDSL 15 minute performance table. This table provides the
accumulated counts of errored seconds (ES) and unavailable
seconds (UAS) for the HDSL line including the local, remote
units and any doublers in between during a 15 minutes period
for the last 24 hours.
The table is indexed by the non-empty slot in the device and
the 15-minute history slot within the last 24 hours period.