This failure indicates that the received K1 byte is either invalid or inconsistent. An invalid code defect occurs if the same K1 value is received for three consecutive frames and is either an unused code or irrelevant for the specific switching operation. An inconsistent code defect occurs when no 3 consecutive received K1 bytes of the last 12 frames are the same.
If the failure persists for 2.5 s, a Protection Switching Byte alarm is raised. When this failure is declared, the protection line is treated as if it were in the SF state. The received signal is then selected from the working line.
When the failure is absent for 10 s, the alarm is cleared and the SF state of the protection line is removed.
This alarm can only be raised by the active port operating in bidirectional mode.