I am familiar with this issue in Nokia system for more than 5 years. SDCCH Drop Rate near to 1-1.5% is usual in NSN. I worked in 2 operators of my country that is using NSN BSS and the average value of SDCCH Drop rate is 1.5% both operators.
After several analysis we are using below formula now; SDCCH_ABIS_FAIL_CALL removed from nominator and SDCCH_ASSIGN_57 is being used in denominator; SDCCH Drop value decreaed about 0.5% after this change:
100 * ((SDCCH_RADIO_FAIL + SDCCH_RF_OLD_HO + SDCCH_USER_ACT + SDCCH_BCSU_RESET + SDCCH_NETW_ACT +
SDCCH_BTS_FAIL + SDCCH_LAPD_FAIL + SDCCH_A_IF_FAIL_CALL + SDCCH_A_IF_FAIL_OLD + SDCCH_ABIS_FAIL_OLD)/
(SDCCH_ASSIGN_57 + SDCCH_HO_SEIZ))
BR – Mehrdad