Do you know what is the exact “cause” indicator for SDCCH Drop ? I assume you have a split for the SDCCH Drop, such as “Radio drop”, “Timer Expiry”, “Hardware Drop” etc…?
(indics are vendor dependent, which vendor are you working with ?)
What is the value of SD Drop Rate ? (split ?)
What is the amount of SDCCH erlang at busy hour ?
Thanks to your actions, we can safely say that:
– interference are not an issue
– hardware in the BTS is not faulty (not so sure yet)
– no other QoS indicators are degraded
Path balance ?
There is still a possibility that all calls too far away are getting a SDCCH drop due to poor path balance. While all calls close enough are kept until TCH calls is established. However, which such assumption, you should notice high call drop rate as well.
What you should try to do is a drive test over there, perform several call setups and try to obtain a sdcch drop 🙂 Move around in the cell coverage.
Try to send very long SMSs to extensively test the SDCCH.