5G system domains
A 5G system includes UE, the NG-RAN access network, the 5G Core, and Data Networks. NG-RAN provides radio access and transport toward the core. 5GC controls registration, mobility, sessions, and policy. A DN can be the Internet, an operator service, or a private enterprise network.
The 5GC control plane uses a service-based architecture. Network functions expose services to other authorized functions. AMF terminates N1 and N2 and handles registration and mobility. SMF manages PDU sessions and controls the UPF through N4, while the UPF forwards packets between NG-RAN and the DN.
The domain model assigns functional responsibility without requiring every function to run on separate physical equipment.
NG-RAN and gNB
A gNB terminates the NR radio interface and implements PHY, MAC, RLC, PDCP, and RRC functions according to the deployment split. It schedules radio resources, processes measurements, supports mobility, and forwards user data over N3 toward UPF.
gNB functions can be divided between a Central Unit and Distributed Units connected by F1. A further split divides the CU into CU-CP and CU-UP. In this split, F1-C connects CU-CP to the DU, F1-U connects CU-UP to the DU, and E1 connects CU-CP to CU-UP. CU-CP hosts RRC and the control-plane part of PDCP, while CU-UP hosts the user-plane part of PDCP and SDAP.
Interfaces among domains
- N1 is the logical reference point for NAS signaling between UE and AMF. Its network-side termination is in the AMF.
- N2 carries control signaling between NG-RAN and AMF.
- N3 carries user packets between NG-RAN and UPF.
- N4 connects SMF and UPF and carries session and forwarding-rule control for the UPF.
- N5 connects PCF and AF, the Application Function, for application-related policy information.
- N6 connects UPF to a selected Data Network.
- Xn connects NG-RAN nodes for mobility and coordination.
N1 does not represent a direct physical connection between UE and AMF. NAS messages traverse the radio interface and NG-RAN.
Control and user planes
The control plane handles registration, authentication, slice selection, mobility, and PDU-session establishment. Its messages involve UE, gNB, AMF, and other 5GC functions.
The user plane carries application packets through gNB and UPF. A UPF can be placed near an application to reduce latency while control functions remain centralized.
Standalone and Non-Standalone
Standalone combines NR access with 5GC and enables the 5G service model, slicing, and native 5GS procedures.
Non-Standalone combines NR with existing LTE and EPC in defined 3GPP options. In EN-DC, the LTE eNB acts as the master node and the en-gNB as the secondary node. NR can add user-plane capacity. SA and NSA describe the deployment pairing of radio access and core network. They are not frequency-band names.
PDU sessions and quality of service
A connects UE to a DN through an appropriate UPF. Depending on supported session type, it can carry IPv4, IPv6, or Ethernet data.
Session traffic is classified into QoS flows identified by QFI. NG-RAN maps these flows to radio data bearers. One PDU session can contain several flows with different quality requirements.
Network slicing
A network slice is a logical network that provides specific network capabilities and network characteristics. The UE can provide a Requested NSSAI. The network selects functions that serve the requested slice, while the NSSF participates in determining the Allowed NSSAI and the serving AMF set.
A slice is not one tunnel or a dedicated radio frequency. It can share infrastructure and network functions with other slices. Isolation depends on resource, policy, core-function, and virtualization configuration.
System-architecture scope
The system view follows signaling and user traffic among UE, NG-RAN, 5GC, and DN. Detailed AMF, SMF, UPF, UDM, AUSF, PCF, NRF, and NSSF duties are covered in the 5G Core article.