Hi All.
What is VoLTE EPS Fallback in 5G SA?
Is it like SRVCC or CSFB type?
EPSFB from NR SA means network do not support VoNR or UE do not support VoNR.
Then Voice call fall back to lower RAT, i.e. LTE.
So, same like CSFB.
Yes, kind of. But CSFB means no CS support in LTE time.
Here EPSFB means CS support in LTE.
Thanks, got it.
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About Voice service continuity within mobile network 5G SA, VoNR and VoLTE, there are two types of fallback features in 5G :
-
In EPS fallback, if a UE tries to use voice services in a 5G network that does not support VoNR, gNB redirects or performs a handover to the LTE network, and consequently the UE is able to use voice services via VoLTE. After the voice session is terminated, the UE can move back to the 5G network.
-
RAT fallback is similar to EPS fallback except that the UE falls back to the eLTE network controlled by the 5GC during the voice session.
EPS fallback call flow Step by step in 5G
Imagine you’re trying to make a voice call using your 5G-capable smartphone (
), but the 5G network does not support voice calls in your area. The EPS fallback mechanism will be triggered to switch your device to the 4G LTE network for the call.
- Initial UE Message:
gNodeB
You dial a number on your 5G smartphone, which sends an Initial UE Message to the 5G base station (gNodeB). This message includes the UE’s context information and the voice call request.
- NGAP Initial Context Setup Request: gNodeB➡️5GC
The gNodeB forwards the Initial UE Message to the 5G Core Network (5GC) in an NGAP Initial Context Setup Request message, indicating the need for EPS fallback.
- Fallback Decision: 5GC🔄MME
The 5GC decides to initiate EPS fallback and sends a Context Transfer message to the 4G Mobility Management Entity (MME).
- Create Session Request: MME➡️SGW
The MME sends a Create Session Request to the 4G Serving Gateway (SGW) for setting up the connection.
- Create Session Request: SGW➡️PGW
The SGW forwards the Create Session Request to the 4G Packet Data Network Gateway (PGW) for data session setup.
- Create Session Response: PGW➡️SGW
The PGW sends a Create Session Response back to the SGW, confirming the session setup.
-
Create Session Response: SGW➡️MME
The SGW forwards the Create Session Response to the MME. -
Initial Context Setup Response: 5GC➡️gNodeB
The MME sends an Initial Context Setup Response back to the 5GC, which includes the 4G bearer context. The 5GC then forwards the message to the gNodeB.
- RRC Connection Reconfiguration: gNodeB:arrow_right:
The gNodeB sends an RRC Connection Reconfiguration message to your smartphone, instructing it to switch to the 4G LTE network.
10 Voice Call on 4G LTE: 
Your device connects to the 4G LTE network and establishes a voice call connection.
- Call End & Switch Back to 5G:
Once the call ends, your device disconnects from the 4G LTE network and switches back to the 5G network for data services.
Throughout this process, the EPS fallback mechanism ensures a seamless experience by allowing you to make voice calls even when the 5G network doesn’t support them in your area. 
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