This failure occurs when a K1 byte is received in three consecutive frames that indicates a signal fail (SF) at the far end of the protection line. This failure forces the received signal to be selected from the working line.
If the failure persists for 2.5 s, a Far-End Protection Line Failure alarm is raised. This alarm can only be raised by the active port operating in bidirectional mode. When the failure is absent for 10 s, the alarm is cleared.