This attribute indicates the total number of seconds that the
interface has been in an unavailable state. An unavailable state
occurs at the onset of 10 consecutive Severely Errored Seconds.
When the unavailable state is entered, the 10 Severely Errored
Seconds are immediately transfered to the count of
unavailableSeconds by incrementing the count of unavailSec by 10
and decrementing the count of sevErroredSec by 10. Whilst in the
unavailable state, an unavailableSecond is counted for each SES or
SEFS that occurs, and the count of SES and SEFS are both
inhibited. The unavailable state is cleared at the onset of 10
consecutive seconds which are not Severely Errored Seconds.