what is minimum DBM/RSSI for secure connection?

Question

I am wondering what is the lowest DBM/RSSI i should have to make sure my connection between devices is safe both with setting up a connection and datatransfer?

Bill Siever · Accepted Answer

There probably is some critical threshold where you could request that users move the devices closer together. This may be useful for certain use-cases, like updating firmware, but I'm not sure there's either a need or a good way to use RSSI to ensure a reliable transfer in general. 
 First, I don't think there's a need because individual packets already have a checksum. If you want to do a reliable transfer be sure to use the write with response (to confirm the write). 
 I don't think RSSI is a reliable indicator of robustness because it doesn't have much bearing on the continued reliability of the link. During a connection both devices involved could move causing RSSI to either decrease or increase. Furthermore, other factors, like outside interference on the channels or loss of power on either device, could cause the link to be lost.