Hi! Akefa,
Immediate assignment means SDCCH if assignment type is “Early Assignment(EA)” type and TCH if assignment type is “Very Early Assignment(VEA)”. Most of the opeartor use “Early Assignment” so that TCH can effectively use. I hope that your type is EA.
If timer T3101 is fixed in your network, I think it is optimized value. SDCCH congestion may be due to another reason.
regards,
NK