Hello, CF!
I have the same problem (where are you from? – if you don’t mind) indeed more then 80% of the total congestion is due to MC541a (external_HO_prep_fail due to cong), MC551 (internal_HO_prep_fail due to cong), MC 561 (intracel_HO_prep_fail due to cong), but MC812 (Normal_TCH_assign_fail due to cong) is only 5-7%.
One of possibly cause, as we detected, is connected to GPRS and EDGE. Let’s imagine: cell has 3 carriers (21 TCHs) and at the certain moment we have 16 calls (MO, MT or HO) and 5 TS with packed data, so all of TS are busy. When the new normal TCH assignment request comes we have to reduce number of TS for packet data,but when HO reuest comes there is no GPRS downgrade – we have incoming HO failure due to congestion (MC541a, MC551 or MC561. We have no recommendations or solutions for this situation therefore I’ll be gratefull for your any comments.