Link degraded

Dear friend!
Link degradation refers to the deterioration of the quality of the communication link. The possible causes are low optical power, insufficient dispersion compensation, or non-linearity. The OSNR is the target.
The difference between link interruption and link interruption is as follows:
When the link deteriorates, users can still communicate with each other, but the communication effect is poor. The communication may be intermittent, the language and image may be intermittent, and the data packet may be lost.
If the link is disconnected, users cannot communicate with each other.
Thank you!