To French..
SDCCH congestion could be as simply as low SDCCH dimensioning based on the number of radios you have on that sector…how is your SDCCH radio dimension done …also are those sectors having high SDCCH in BSC boarders or high traffic area?
Please review dimensining part first and increase # of SDCCH assignment as necessary..try it out increasing one or two additional..and monitor.There are location upate parameters also which may need to be looked at. Good luck !!
Thanks