The line unavailable second (UAS-L) is a count of 1-second intervals for which the SONET line was unavailable during a previous 1-day interval. The SONET line becomes unavailable at the onset of 10 contiguous SES-Ls. The 10 SES-Ls are included in unavailable time. Once unavailable, the SONET line becomes available at the onset of 10 contiguous seconds with no SES-Ls. The 10 seconds with no SES-Ls are excluded from unavailable time.