The SD congestion comes from Location Update. As the indicator I get from NPO, during the hour that problem happen, SD_assign_request increase from 100-200 (normal value) to 16.000, and most of them is SD_assign_request_lu.
The problem happen almost everynight, in different BSCs,especially all the cells that have the problem are in the border of the LAC. I found in some cases, the neighbor cells( different LAC) are loss of all channel. But in some cases, no. Especially there’s a case that the cell have problem are all in the border of the country.
Noted that the problem happen only in 1 hour and disappear.