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OAM: Fault Notification (AIS, RDI)
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ITU-T Y.1731 supports fault notification through Ethernet Alarm Indication Signals (AIS) based on the standard originally defined in the ATM protocol. The failure between two nodes triggers AIS packets in both directions towards the service end points. To ensure that a failure state is maintained, AIS messages are sent periodically until the service is restored.
For bidirectional network connections, Reverse Defect Indication (RDI) reports on a unidirectional defect in the reverse direction, such as a destination node could not receive traffic. RDI is generally employed when only portions of the network are directly managed. Unmanaged remote sites cannot directly trigger an alarm, so unidirectional loss of connectivity to a managed site is indicated using RDI.
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