this indicator measures the SDCCH ASSIGNMENT FAILURE, not the call setup failure !! there are plenty of messages after the connection request that are needed in order to end the call setup.
what problem might lead to high value of S.A.F rate ?
3 typical causes :
– congestion (if there is sdcch congestion, the MS will try to retransmit the channel request several times : for one “real” attempt, you count three attempts, therefore three failures : instead of measuring 2% congestion, you measure 6% congestion…)
– system failures
but also :
– dupllicate BSIC BCCH
– duplicate BSIC TCH
– DUMMY RACH (generated by the BTS noisy EM emissions)
– ABIS or ATER over satelllite : by the time the answer ” IMMEDIATE ASSIGNMENT” arrives to the MS, the MS has already sent another CHANNEL REQUEST. This leads to 2 requests for 1 success ==> 50% failure.