Any experts? Regarding CSFB and TAC to LAC Mapping in MME

I have a general question regarding CSFB and TAC LAC Mapping.


In my eNodeB i have CSFB configured to both 2G and 3G for example. With different priorities. So, i have high priority 2G CSFB to Cell x LAC x, and low priority 3G CSFB to Cell y and LAC y. Lets call the TAC for the eNodeB cell TAC z


Then in my Cisco MME, i have Map TAC z to LAC x (where LAC x is the 2G LAC).

Now its impossible to map the same TAC to 2 LAC's, so the TAC is just mapped to LAC x.


Does this mean that its only possible to do the CSFB to LAC x??


Because as previously stated, all my eNodeB's have 2 CSFB options to different LAC's.

Thanks in advance

Hi, no it is not possible to map one TAC to multiple LAC, it has to be a one to one mapping.

Whether you can CSFB to LAC Y in your example will depend on a few things. If it is MO CSFB you can still do CSFB but the UE will perform a LAU first before the call setup and thus you will have a slightly longer call set up time.

If it is MT CSFB, it can also work if cell Y and cell X are under the same MSC (LAU will also be performed). It can also work if cell Y and cell X are under different MSCs but you have deployed MTRF (Mobile Terminated Roaming Forwarding) on you core.

If none of the above are true then MT CSFB will fail.

Thanks a lot for the information. Thats great.