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 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 toward UPF.
gNB functions can be divided between a Central Unit and Distributed Units connected by F1. A further CU split separates control-plane RRC processing from selected user-plane PDCP functions.
Interfaces among domains
- N1 is the logical NAS signaling relationship between UE and AMF.
- N2 carries control signaling between NG-RAN and AMF.
- N3 carries user packets between NG-RAN and UPF.
- N6 connects UPF to a selected Data Network.
- Xn connects NG-RAN nodes for mobility and coordination.
N1 is not a direct physical wire 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. LTE can anchor control signaling while NR adds user-plane capacity. SA and NSA describe access-to-core relationships, not frequency bands.
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 capability set for a service class or customer group. UE provides requested-slice information and the network selects functions capable of serving it.
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.