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