CS fallback is a generic name. 1xCS fallback is when the fallback CS network is CDMA 1xRTT.
And simply CSFB when the CS network is GSM/UMTS. Basically, for CSFB to be supported the UE should be performing Combined attach procedure (EPS attach type should be Combined EPS/IMSI attach)
1xCSFB needs a 1x-CS IWS(Interworking gateway between MME and MSC server).
MME<---S102---->1xCS IWS <----A1----->3GPP2 MSC
For 1xRTT CSFB feature, CSFB can be done with RRC connection release with redirection information or 1xSRVCC based signaling (known as enhanced 1xCSFB). 1xRTT CSFB UE may also have dual-Rx/dual-Tx or Dual-Rx/Single-Tx capability. Dual-Rx/dual-Tx 1xRTT CSFB UE can simultaneously transmit and receive on both EPS and 1x at the same time. This allows the UE to obtain 1x voice service from 1xRTT system while maintaining the data stream over EPS at the same time. This is also based on network configuration and deployment option, and UE capability. Dual-Rx/Single-Tx 1xRTT CSFB UE allows simplification in EPS network deployment because there is no coordination is required between the E UTRAN and 1xRTT network (i.e. S102 is not required).
In 1xCSFB, SIB8 informs UE about 1xCSFB support. The information in the SIB-8, together with additional information delivered to the device using Radio Resource Control (RRC) signaling,allows the UE to form a 1x Registration Message (RGM), encapsulated into a Generic Circuit Switched Notification Application (GCSNA) message. This GCSNA message is transparently carried by the eNodeB and the MME to the 1xCS IWS through the S102 tunnel. The 1xCS IWS extracts the RGM and updates the 1x MSC about the location of the UE using an A1-Location Update message. The MSC then knows how to reach the device when it receives a mobile terminated voice or SMS.
UE should support multiple radio interfaces LTE, UTRAN and GERAN
MME has SGs interface to the 3G MSC/VLR
For the GSM-UMTS CSFB feature, EPS can perform the CSFB procedure with PS handover procedure, RRC connection release with redirection information, or cell change order with NACC (for GERAN only). This is based on network configuration and deployment option
The below description is for the Redirection procedure
When a MO call is attempted, UE triggers CSFB by sending EMM_Extended_service_request (with CSFB indicator). The nw will trigger RRC connection release with target RAT info(UTRAN/GERAN). Based on this info and SIB info, UE will start finding the target cells. Establishes the RRC with the target RAT, initiates the RAU procedure (Location update procedure in case of GSM) and simultaneously sends CM_service_req(with MO orig call req).This will trigger the Auth/Security procedures and further the call setup will proceed.
MT call procedure is same as above except that, UE will receive CS_Service_Notification with Paging Identity for incoming CS call(MSC ->MME->UE) before sending EMM_Extended_Service_Request (with CSFB Indicator).
MSC maintains a mapping of IMSI-MMEID, thus enabling MSC to select the right MME to send the Paging msg.