congestion means that when ms try for sd channel and all the sd channels are busy it means there is sd congestion
SD block means ms want sd channel but not able to get it due to congestion,hardware issue,interference, or any other issue that case is sd block
means sd congestion is one of the reason for sd block
Sd drop is that case when ms request for the sd fulfill by bts but not able to convert that sd into tch
Same in case of tch drop,block,congestion