1. By checking the service configuration of the bound channels, we find the timeslots bound at the two ends are consistent without any problem, and the ports are configured normally.
2. Check the optical power of the lines and find no anomaly.
3. A further check finds that the SDH service cross connection is of the VC4 level on the OSN 3500 side, and of the VC3 level on the 2500+ side.
4. Modify the SDH service cross connection on the OSN 3500 side to the VC3 level, and the service becomes normal.