How MME notify to LTE UE for CS call to perform CSFB when UE is in connected mode or idle mode?

Question

Suppose MME is getting CS call notification for MSC then How MME will notify to UE when
1. UE is idle mode. ?
2. UE is in connected mode?

Answer 1

1) UE is in Idle Mode: First send the Paging with CN Domain : CS. After that UE will send the RRC Request then try to send the CS Service Notification for MT CSFB Call.

2)UE is in connected mode? : If UE is already in Connected Mode there is no need of paging. Just directly send CS Service Notification Message to Initiate a MT CSFB Call.

Answer 2

There are two different scenario when UE is in ECM-CONNECTED and another state i.e. ECM-IDLE.
- when UE is in ECM-CONNECTED (UE is registered and holds radio connection with eNodeB), MME sends UE Context Modification Request message with "CS Fall back Indicator" to eNodeB for particular UE . On receiving "UE Context Modification Request" message, eNodeB issues RRC connection release with "redirected carrier info".

• When UE is ECM-IDLE state (state when UE doesn't have rrc connection with eNodeB), MME issues paging message for UE with CS indicator. On receiving paging message, UE initiates "Extended Service Request" towards MME and MME sends "CSFB indicator" in Initial Context Setup Request message to eNodeB and ask re-direct to UE.