Hi guys.
Do you have any useful document to
One: explain TCH _ASSIGNMENT / IMM ASSIGNMEMT in clear way
two: learn how we can optimize them
The IMMEDIATE ASSIGN message facilitates the assignment of the MS onto an SDCCH
channel. Correspondingly, the IMMEDIATE ASSIGN REJECT message facilitates the
refusal of the access. If the SDCCH reservation or activation fails, the BSC sends the
IMMEDIATE ASSIGN REJECT message to the MS. The GSM timer T3101 supervises the
immediate assignment procedure.
Successful TCH assignment indicates the procedure from the BSC receiving the Channel Request message sent by the MSC to receiving the Assignment Complete message.
So, in the case of Immediate Assignment, my recommendation is to monitor how many rejects occurs, and this is done by checking the counter related to Immediate Assignment Rejects which most of the times should be zero. If not, this indicates congestion in a very early stage of a call (for instance) since not even the SDCCH can be assigned. The cause of rejects needs to be analized but in a first and basic approach, sometimes it can be solved by downtiltting the antenna to eliminate far access, however. If the site has a satellite Abis there is a totally different approach. For more info please check on 3GPP TS 04.08.
Regarding TCH assignments, basically a failure means that the UE could not size a TCH, let´s say, for a call or HO, so, you need to find the reason causing this event which can be TCH congestion or blocking due to many reasons like, lack of resources (HW) or poor RF quality, so this analysis will lead you to other counters or KPIs that you will need to check.
For more info check 3GPP TS 04.08 and look for traffic channel assignment.
Hope this will give you a good starting point to plan optimization actions 
including the extraction of GSM sites to use spectrum for another RAT.
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