This read-only 64-bit counter tracks the cumulative number of Path Unavailable Seconds (UAS) experienced by an E1 or T1 circuit at the far end during a historical measurement period (stored in a bin). A UAS is a second in which the path carries no valid traffic due to equipment failure or line degradation, and far-end measurement reflects what the remote endpoint reports back to the originating device. Telecommunications maintenance crews use this metric to measure path availability and quality for SLA compliance and troubleshooting remote circuit problems.