CSSR = Number of TCH Assignment Successfull/Number of MOC + Number of MTC – Number of IMSI Detach – Number of SMS/2
IMSI detach is subrated because it has the same cause as of the orignating call and pegs in the MOC Counter.
SMS is subtracted because it also has the same cause as of the MOC call and it is divided by 2 becasue it takes 2 DTAP messages to send 1 sms.
Regards