For an IMA group whose protocol is atmForum10, this attribute
counts the number of unavailable seconds. Unavailability begins at
the start of 10 contiguous severely errored seconds (SES) and ends
at the start of 10 contiguous seconds with no SES.
For an IMA group whose protocol is proprietary, this attribute
counts the number of one second intervals during which the Link
component has been out of service.