Generation and Detection of Alarms and Performance Events in the SDH Higher Order Signal Flow

The principle for locating fault is "line first, then tributary; higher order first, then lower
order".
Therefore, this section focuses only on the alarms and performance events generated between
the SDH interface and the cross-connect unit like Huawei GXCSA during maintenance. This section describes the signal flow and the procedure for handling each overhead byte by each module.

Alarm signals generated between the SDH interface and the cross-connect unit

Based on the positions of the various overhead byte processing in the STM-64, STM-16, STM-4, STM-1 frame, the overhead
bytes are classified into four modules:
Regenerator section overheads
Multiplex section overheads
Higher order path overheads
Lower order path overheads
If a fault occurs in the first two modules, it affects all the higher order paths. If a fault occurs in
the overhead bytes of a higher order path, however, it affects only this higher order path and its
lower order paths.
The following sections describe the signal flow and the processing of each overhead byte.

Downlink Signal Flow
In the higher order downstream signal flow, overhead bytes are extracted and terminated.
Frame Synchronizer and Regenerator Section Overhead Processor

Uplink Signal Flow
The overhead bytes are extracted and then terminated in the downlink signal flow of the higher
order path. Overhead bytes are generated and alarm signals are returned to the opposite NE in
the uplink signal flow of the higher order path.