sorry my previous post didn’t make much sense, i’m glad you understood it ! (i was writing in a hurry)
so no – i don’t think the problem has to do with ciphering. If that was the case, most subscribers would experience it.
It would be great if you could find out what is the “trigger” or the “peg event” of the counter “ciphering sd drop”.
Anyway, are you certain you can rule out coverage or interference problems in those cells ? The best way to be sure is to compare the sd drop % with the tch drop %, trx per trx.
If radio problems are ruled out, then you might want to check “location update rejects”, which might not be visible in the BSS counters, but most probably available in the MSC/VLR counters. LU rejections (during IMSI attach) seem to be happening a lot for the past 2 years in many networks.
Finally, can you try to activate the SDCCH Handover ? That would definitely rule out radio problems.
If it’s possible, you should do a drive test, see if the problem occurs during your tests. If that doesn’t work, you can do A interface traces, where you will exactly see what’s going on for those 2%.
This value is really low, it’s way too low to suspect a major misconfiguration in your network. So I can tell you that this “minor” (but mind bogging) problem is going to be a major headache… 🙂
Will do my best to help, but you’ll have to do most of the work now…