This failure can occur for two reasons. The first reason is that the received K2 byte indicates that 1:N protection switching is being used by the far end of the OC-N line, rather than 1+1 protection switching. The second reason is that the received K2 byte indicates that unidirectional mode is being used by the far end while the near end is using bidirectional mode. This defect is detected within 100 ms of receiving a K2 byte that indicates either of these conditions.
If the failure persists for 2.5 s, a Mode Mismatch Failure alarm is raised. When this failure is declared, if the defect indicates that the far end is configured for unidirectional mode, then the OC-N port reverts from its current bidirectional mode to unidirectional mode. However, the port continues to monitor the received K2 byte, and if the K2 byte indicates that the far end has switched to bidirectional mode, the OC-N port then reverts to bidirectional mode as well. The monitoring stops if the user explicitly reconfigures the local port to operate in unidirectional mode.
When the failure is absent for 10 s, the alarm is cleared, and the configured mode, which is 1+1 bidirectional, is used.
This alarm can only be raised by the active port operating in bidirectional mode.