to summary your problem : (No on SD attemps)/(No of SD request – (SD fail due to cong)) = 20%
it means that for instance, there are 1000 CHANNEL REQUIRED on the ABIS interface, and only 200 IMMEDIATE ASSIGNMENT.
Did you see this in the Abis trace ?
If that’s the case : could you isolate the CHANNEL REQUIRED that are not followed by the IMMEDIATE ASSIGNMENT (check the request reference in the channel required -> the same reference should be used in the I.A. that answers this C.R.)
In those un-answered Channel required :
— >are they a retransmission of a previous channel required ? (you know that the MS will transmit the same channel request max three times)
–> what is the cause value (emergency call ? normal call ? GPRS ?)
–> what is the time advance ?
… frankly the solution to your problem lies in the Abis interface ! just look deeper…