tommy,
external interference = interference from other systems (other operators, or other RF systems such as radards, TV, Radio, etc)
In my assumptions, i assume that NOTHING have changed on the radio interface (same freq. as before, same parameters, for all surrounding cells). Therefore there cannot be a sudden increase of SD Drop due to a sudden interference from a neighbour cell. Indeed, the frequencies were not changed!
JE – Ok. What about hw problems in the BTS?