5GC to EPS Handover Using N26 with Indirect Data Forwarding Explained
5GC to EPS Handover Using N26 with Indirect Data Forwarding
Transitioning from 4G LTE (Evolved Packet System – EPS) to 5G Core (5GC) can be tricky when it comes to keeping users mobile. People want their services to keep running smoothly, even as they switch networks. To make that happen, there needs to be tight coordination between the Access and Mobility Management Function (AMF) in 5GC and the Mobility Management Entity (MME) in EPS.
The diagram attached illustrates the handover process using the N26 interface and indirect data forwarding, detailing how control and user plane messages are exchanged between network components to ensure service doesn't drop.
In this blog post, we break down the architecture, the messages that get exchanged, and the data paths involved in this handover.
Why N26 Matters in 5G to EPS Handover
The N26 interface serves as a connection point between the AMF (5GC) and the MME (EPS). Its key functions include:
Supporting mobility procedures for a UE moving from 5GS (5G System) to EPS.
Allowing interworking without needing the UE to fully re-register.
Handling GTPv2-C message exchanges for transferring session and bearer context.
Without N26, the UE would have to reattach when switching to EPS, which could lead to service interruptions and a drop in QoE (Quality of Experience).
Indirect Data Forwarding
There’s a chance of data loss during the handover since the user plane paths are changing. Indirect data forwarding ensures that packets in transit keep moving from the source to the destination network.
First, data is buffered at the source gNB (5G) and sent via the UPF to the target eNB (4G).
This method helps stop packet drops and keeps the session going.
Architecture Overview
The handover involves several network functions from both 5GC and EPS:
UE (User Equipment): Moves from 5GS to EPS coverage.
gNB (Next-Gen NodeB): The source 5G base station.
eNB (Evolved NodeB): The target 4G base station.
AMF (Access and Mobility Management Function): The control plane entity for 5GC.
MME (Mobility Management Entity): The control plane entity for EPS.
SMF + PGW-C: Session management in 5GC together with PGW control.
UPF + PGW-U: User plane in 5GC integrated with PGW user plane.
S-GW: Serving Gateway in EPS.
DN (Data Network): Provides external data connection.
Key interfaces include:
N26 (AMF ↔ MME)
N2, N3, N6, N11 (5GC reference points)
S1, S5, S11 (EPS reference points)
Signaling Flow in N26-Based Handover
The handover process includes control plane signaling and user plane switching.
Step-by-Step Breakdown
Handover Command (RRC message): The gNB sends a handover command to the UE when it needs to switch from 5G to 4G coverage.
Downlink User Plane Data (DL UP): Data packets are still sent to the UE via the UPF (5GC).
UPF → gNB DL Path: UPF forwards DL data to the source gNB using the N3 interface.
Forwarding DL Data (gNB → UPF): Packets buffered in gNB are sent upstream to the UPF.
Forwarding via S-GW (FDL data): UPF sends the forwarded data to S-GW using S5-U.
Forwarding to Target eNB (S1-U): S-GW passes the buffered DL packets to the eNB so the UE gets them during transition.
Handover Complete (UE → eNB): The UE connects to the eNB and sends a handover complete message.
Data Continuity: Downlink traffic starts flowing normally from DN → UPF/PGW-U → S-GW → eNB → UE.
GTPv2-C Signaling Between AMF and MME
The N26 interface employs GTPv2-C messages for session control:
Forwarding Relocation Request (AMF → MME)
Forwarding Relocation Response (MME → AMF)
Forwarding Relocation Complete Notification (AMF → MME)
Forwarding Relocation Complete Acknowledge (MME → AMF)
These messages ensure that the UE context, bearer, and QoS flows are handed off correctly.
Control and User Plane Separation
The diagram shows three types of signaling paths:
RRC (Radio Resource Control): Between UE and gNB/eNB.
CP (Control Plane): Dashed lines represent signaling exchanges (AMF ↔ MME, AMF ↔ SMF, etc.).
UP (User Plane): Solid lines show actual data forwarding (UPF, S-GW, DN).
This separation allows for more efficient resource management and clear mapping during interworking.
Benefits of N26 with Indirect Forwarding
✅ Seamless Handover: Stops session drops and re-registration delays.
✅ QoS Continuity: Keeps service quality up for ongoing calls, video streaming, and more.
✅ Efficient Context Transfer: AMF and MME can work together via N26 without needing full UE re-authentication.
✅ Minimized Packet Loss: Buffered forwarding helps prevent data loss during the transition.
✅ Backward Compatibility: Facilitates interaction between older EPS and modern 5GC.
Comparison: Direct vs Indirect Data Forwarding
Aspect Direct Forwarding Indirect Forwarding Path Source gNB → Target eNB Source gNB → UPF → S-GW → eNB Complexity Lower Higher Data Loss Risk Medium Lower Use Case Co-located networks Heterogeneous setups with 5GC/EPS
Indirect forwarding is a better choice when the source and target nodes aren’t directly connected, making it more reliable for mixed networks.
Deployment Considerations
Transport Latency: Extra hops in indirect forwarding add a little delay, but it’s usually fine for most services.
Interworking Support: Operators need to set up N26 correctly for smooth signaling between AMF and MME.
Policy and Charging Control (PCF + PCRF): Coordination between 5GC and EPS is necessary for consistent policy application.
HSS and UDM Integration: Identity and subscription info must stay in sync during handover.
Real-World Use Cases
Urban 5G Rollouts:
Dense urban areas with both 4G and 5G coverage.
Users often switch between networks; smooth handoffs help keep streaming and VoNR calls steady.
Rural Deployments:
5G coverage might be spotty; UEs frequently drop back to EPS.
Indirect data forwarding ensures services stay uninterrupted during these switches.
Enterprise Networks:
Private 5G systems integrated with public LTE/EPS.
N26 helps devices maintain connectivity as they shift between domains.
Conclusion
The 5GC to EPS handover using N26 with indirect data forwarding is crucial for smooth mobility between 5G and 4G LTE networks.
By using GTPv2-C signaling between the AMF and MME, along with buffered forwarding through the UPF and S-GW, networks can achieve:
Minimal packet loss
Continuous service
Consistent QoS
For telecom operators, getting N26 right is essential for delivering a truly seamless experience for users in mixed 4G/5G environments. As networks progress toward standalone 5G, these interworking strategies will continue to be vital during the transition phase.