the difference between CSSR & CSR is that;
CSSR call setup success rate is the rate of total call setup attempt you remove call fail divide by total call setup attemp *100
the basic formulat is
CSSR= ((Nb of call attempt- call fail)/ Nb of call)*100
for exemple
you have
nb of call attempt= 1000
call fail = 20
your cssr will be =((1000-20)/1000)*100
= 98%
CSR is call success rate
it is CSSR- call drop