Hi Ayat,
CSSR – Call Set-Up Success Rate ,
To have big value like 99%, it is impossible to have TCH congestion or assignment fail 50%.
The main things that affect the CSSR is the SDCCH drop and SDCCH congestion and TCH congestion.
Some operator ignor the SDCCH congestion from there calculations when they calculate teh CSSR, so the TCH congestion will affect the TCH_ASS_SUCC rate, another thing could affect the assignment of the TCH is the bad quality.
Hoppe this can help you…