An APS mode mismatch failure (APS-MM) can occur for two reasons. The first is if the received K2 byte indicates that 1:N protection switching is being used by the far-end of the OC-N line, while the near end uses 1+1 protection switching. The second is if the received K2 byte indicates that unidirectional mode is being used by the far-end while the near-end uses bidirectional mode.
This defect is detected within 100 ms of receiving a K2 byte that indicates either of these conditions. If the failure detected persists for 2.5 seconds, a mode mismatch failure alarm is raised. However, it continues to monitor the received K2 byte, and should it ever indicate that the far-end has switched to a bidirectional mode the mode mismatch failure clearing process starts. When the failure is absent for 10 seconds, the alarm is cleared, and the configured mode of 1+1 bidirectional is used.