What does ODUk_PM_BDI indicate?

ODUkPMBDI is the acronym for Optical Data Unit k Path Monitoring Backward Defect Indication. In the Optical Transport Network (OTN), this is a maintenance signal sent from a destination (sink) network element back to the source (originating) element along the ODUk path. It serves to inform the source that the destination has detected a failure or a severe defect in the signal it is receiving. This mechanism is crucial for rapid fault localization and for triggering protection switching actions at the source of the connection.

A. ODUk PM signal degradation is a condition (e.g., an excessive bit error rate) that can trigger a BDI, but it is not the BDI signal itself.
B. A BIP-8 bit error threshold crossing is a specific event that indicates signal degradation (ODUk-PM-DEG), which is a potential cause for asserting BDI, not the definition of BDI.
D. ODUk PM open connection indication (OCI) is a different, specific maintenance signal used to indicate that a connection is not properly terminated.

1. ITU-T Recommendation G.709/Y.1331 (06/2021)

"Interfaces for the Optical Transport Network":

Section 15.8.2.2.1

"Backward defect indication (BDI)": This section explicitly defines BDI. It states

"The BDI signal is used to convey to the upstream ODUk path terminating equipment the detection of a server signal failure (e.g.

ODUk-AIS)

or a consequential ODUk-LCK or ODUk-OCI defect at the downstream ODUk path terminating equipment." This confirms BDI is a "Backward Defect Indication".

2. Huawei OptiX OSN Product Documentation

"Alarms and Performance Events Reference":

Alarm Description for ODUkPMBDI: The documentation for Huawei OTN equipment describes the ODUkPMBDI alarm as being generated when the path monitoring (PM) overhead of an ODUk signal contains a backward defect indication. It clarifies that this means a fault has been detected by the downstream equipment in the receive direction. (e.g.

in the documentation for OptiX OSN 8800/9800 series).