5G Core Network (5GC) Explained
The 5G Core, or 5GC, is the control and data network behind 5G access. It separates mobility, session control, forwarding, policy, discovery, subscriber data, and authentication more explicitly than LTE EPC, and many of those interactions are expressed through a service-based architecture.
This hub is the right starting point when you want to understand how AMF, SMF, UPF, UDM, AUSF, PCF, NRF, NSSF, AF, and SBA fit together in real 5G procedures.
Quick facts
| Domain | 5GC |
|---|---|
| Access and mobility anchor | AMF |
| Session-control anchor | SMF |
| User-plane forwarding | UPF |
| Core design style | Service-Based Architecture for control functions |
| Key interfaces | N1, N2, N3, N4, N6, N9 and service-based internal interfaces |
Contents
Why this matters
The 5GC is where many “5G network” problems actually become specific: registration logic in the AMF, session setup in the SMF, policy decisions from the PCF, subscriber data from the UDM, authentication with AUSF, or forwarding behavior in the UPF.
Because the 5GC is more modular than EPC, architectural clarity matters more. Without it, traces and logs can look busy without being easy to interpret.
Where it fits in the network
| 5GC area | Role in the network |
|---|---|
| Access-facing side | Receives UE and RAN control via N1 and N2 through the AMF, and user-plane traffic via N3 through the UPF. |
| Mobility and registration control | AMF-centered functions maintain access-side control context. |
| Session control | SMF manages PDU session control and programs UPF forwarding behavior. |
| User plane | UPF anchors and forwards traffic toward data networks over N6 or other UPF paths over N9. |
| SBA control ecosystem | UDM, AUSF, PCF, NRF, NSSF, and AF support subscriber, security, policy, discovery, slicing, and application-driven behavior. |
Main nodes / functions / entities
AMF
Access and mobility management function for registration-side control.
SMF
Session management function for PDU session and UPF control.
UPF
User-plane function for traffic forwarding and anchoring.
AUSF
Authentication function in the 5GC security path.
UDM
Subscriber data and identity-related core function.
PCF
Policy control function for session and service behavior.
NSSF
Slice-selection support function.
NRF
Service discovery and repository function.
AF
Application function that can influence policy and service handling.
SBA
Service-based architecture view across the whole 5GC.
Interfaces used
| Interface | 5GC role |
|---|---|
| N1 | UE to AMF NAS path for registration and session-related signaling. |
| N2 | gNB to AMF control path, typically NGAP. |
| N3 | gNB to UPF user-plane path. |
| N4 | SMF to UPF control path, typically PFCP. |
| N6 | UPF to external data networks. |
| N9 | UPF to UPF user-plane continuity for distributed paths. |
| Service-based interfaces | Internal 5GC control interactions between SBA functions. |
Protocols used
| Protocol family | Common 5GC use |
|---|---|
| 5G NAS | Registration, mobility, and session signaling between UE and 5GC control functions. |
| NGAP | RAN-to-core control signaling on N2. |
| GTP-U | User-plane transport on N3 and N9. |
| PFCP | Forwarding and session control between SMF and UPF. |
| HTTP/2 + JSON SBA signaling | Service-based control interactions between many 5GC functions. |
| SIP / IMS | Voice-service signaling layered above 5GC transport and policy support. |
Used in procedures
- Initial registration places AMF and subscriber/authentication functions into context.
- Authentication procedure highlights AUSF and subscriber-related functions.
- PDU session setup highlights SMF and UPF roles.
- PDU session release shows session teardown and forwarding cleanup.
- Inter-AMF mobility shows mobility across distributed control functions.
- VoNR and IMS shows how core transport and policy support voice services.
Common troubleshooting notes
- Registration failure usually points first to AMF, subscriber data, or authentication branches rather than user plane.
- Good registration with missing data service often points toward SMF, UPF, N3, N4, N6, or policy problems.
- Slice-related issues often need AMF, NSSF, policy, and session handling to be read together.
- Because 5GC is modular, correlation across logs often matters more than a single trace point.
Related pages / next steps
AMF in 5G Explained
Start with the main 5G access and mobility control function.
SMF in 5G Explained
Continue into session management and PDU control.
UPF in 5G Explained
Continue into the data-forwarding side.
Service-Based Architecture Explained
Open the architectural principle that connects many 5GC functions.
Key takeaways
- The 5GC is more modular than LTE EPC, so function ownership matters more during troubleshooting.
- AMF, SMF, and UPF form the backbone of registration, session, and user-plane behavior.
- SBA is not just a design term; it changes how core functions discover each other and exchange control information.
FAQ
What is the 5GC?
The 5G Core is the core network behind 5G access, built from modular functions such as AMF, SMF, UPF, UDM, AUSF, PCF, NRF, and others.
Which 5G core function controls the UPF?
The SMF controls UPF forwarding behavior, commonly over the N4 interface using PFCP.
Why is the 5GC called service-based?
Many 5GC control functions interact through service-based interfaces rather than only point-to-point legacy style interfaces.