I am working in a GSM operating at 1800 using Nokia BTS/BSC, we are having high call drop on the border cell i mean on circle boundry ,as these are open cells so their call drop and handover failure is high.So there is a parameter known as DISB which is used to relase the call after Timing advance has exceded the threshold.So can any body tell me any thing about this or any other way by which KPI regarding the border sites can be improved