Yes immediate assignment is phase to allocate SDCCH channel but when MS request for channel to BTS then BTS will send message CHNRD(Channel Required) to BSC and then BSC send message CHNAV(channel activation SDCCH) to BTS. Here BSC will increment counter for attempted SDCCH and IMMEDIATE ASSIGNMENT as well.When BTS receive CHNAV message from BSC then it sends CHNAK message to BSC and T_MSRFPCI counter is started. Now BSC will assign immediate assignment message to BTS. If BTS does not receive immediate assignment message time between T_MSRFPCI then the call gets dropped. This means that SDCCH and IMMEDIATE ASSIGNMENT will be dropped. This means that yes there is SDCCH drop during immediate assignment. Please correct me if I am wrong.
We got the solution. Actually what happened was that, When we migrate to BR9 then there was bug in software. T200 timer handling capability was only 500msec but actually we were having 620 msec delay in abis.