LTE: Why does "CS Fallback Indicator" IE present in UE Context Modification Request ?

Question

What happens for an UE at MME, which gets triggered "UE Context Modifcation Procedure" for "CS fall back" ?

Answer 1

During the CSFB procedure after the MSC sends over SGs the Paging Request wit Indicator of incoming call. This triggers the CSFB procedure.

Then the MME sends an S1-AP UE Context Modification Request message to eNodeB. This message indicates to the eNodeB that the UE should be moved to UTRAN/GERAN. The registered PLMN for CS domain is identified by the PLMN ID included in the LAI, which is allocated by the MME.

If UE can switch (because of coverage) to UTRAN/GERAN it does so the S1 Context is deleted and bearer are suspended over LTE/4G.

You will find more here
http://www.lteandbeyond.com/2013/09/CSFB-Mobile-Terminating-Call-in-Active-mode.html