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Continuity Check Messages (CCM) are sent periodically from the service source to the destination node at regular intervals. If neither end receives a CCM within a specified duration, then a fault is detected against the service with an alarm.
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Normally continually transmits the CCM (Continuity Check Message) signal between A and F to guarantees the linked condition.
When E and F link chain fail causes CCM can't transmits to A and B.
When A and f do not received three consecutive CCM signals, A and F will issue alarm thereafter.
A and F sends FNG (Fault Notification Generator) trap to EMS
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