The Immediate Assignment messages are sent over the AGCH. You have a certain AGCH capacity, defined by the parameter BS_AG_BLKS_RES. It is the number of blocks (out of nine) reserved for AGCH. Default value is 4.
The 5 other blocks are used for PCH.
In your case, the number of immediate assignment is too high : in other words, the number of SDCCH establishments in your cell is too high.
The IA msg are stored in a queue for too long while waiting for their turn, and then they are discarded.
The AGCH blocks are just not enough to “flush” all the IA messages through the radio interfaces.
What you could do is increase the BS_AG_BLKS_RES to 4. Then 5.
Alternatives, but i don’t know if it works with Nokia-Siemens:
1/ activate AGCH Preemption over PCH blocks
2/ Increase the timer that controls how long a message can remain in the AGCH queue. It is not 3GPP standard, and it is called T_IA n ALU systems (def : 2.5s)
Wooow… i had to dig deep to find those answers, I hope you’ll make good use of it :))
It was a good refreshing process, thanks for your question.