hello Tounsi,
the immediate assignment is the message sent from BSC -> BTS and BTS -> MS during a normal assignment. This message is sent on AGCH, and describe the physical address of the SDCCH subchannel that is going to welcome the call. Once the MS gets this info, it will move to this SDCCH subchannel.
A normal assignment means a “call setup”, in opposite to handover assignment. It is also written “NA”.
Regards,