in the ADSL network the backbone core network is NE40 routers, the interconnection between this two router NE40 is ok the ping is ok also but when the traffic increase we found some disconnection and packet lost 9% .
check with ping to the host form two side , loopback .
when the traffic increase we check again by the same way,
check the fiber ,check the connector (change them and see if there is difference).
clean the connector by recommanded tools and method.
the problem is solved after that.
we did check the that interface is up ,protocol is up ,so link is ok .
the transsmission is (STM-1 link throught transsmission equipement).
when we did check the fiber connector ,we found little dust .
when the traffic increase the temperature increase in optical modul on the interface so the dust will have more effect and create noise on the signal.
always you need to do routine check for the equipement hardware and software ,this is proactive measure action and can show you the problem before happening.