CSFB is a technique most 4G operators use to provide voice services over the traditional CS Core avoiding initial investment on IMS which should ideally provide the voice services in a pure 4G Network, The PS Services are carried by the EPC and whenever a 4G user attempts a voice call, it is transferred to the exisitng 2G/3G N/W & after the call, the UE falls up to 4G to resume PS Services.
HOW IS CSFB REALISED?
1. The MME connects to the MSC of the 2G/3G N/W over the SGs interface.
2. The UE's are required to attach to both EPC and GPRS/UMTS network, what we technically call a combined attach (EPS/IMSI attach).
HOW DOES A CSFB MOBILE ORIGINATED CALL OCCUR?
In Short:
1. The UE initiates a voice service request. The CSFB indication Information Element is encapsulated in the NAS signaling & transmitted to the MME. (Extended Service Request)
2. The MME instructs the eNodeB to handover the UE to 3G.
3. The UE receives the eNodeB fallback instruction (handover, reselection, or redirection to a specified 3G cell). After a signaling link is set up, a paging request is initiated, and the voice service starts.
NOTE: This process is seamless, if not, there needs to be detailed trace analysis to analyze the call flows.
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