I am surprised that so many networks are using this formula to calculate the CSSR where CSSR is degrading with increase in SDCCH Assignment Failures or decrease in Immediate Assignment Success rate .
In our network we are using CSSR formula as
(TCH seized/SD seized for MO/MT calls)*100
So in our case with any degradation in SD assignment does’t cause any degradation in CSSR which seems to me more logical as not all SD Accesses can be for making an MO or MT call.