The high-level split
5G separates the network into two domains:
The RAN (Radio Access Network) handles everything wireless — cell search, radio scheduling, beamforming, and the radio protocol stack (PHY, MAC, RLC, PDCP, SDAP, RRC). The RAN is what this site covers in depth.
The 5GC (5G Core Network) handles identity, authentication, session management, and routing of user data to the internet. The 5GC is a service-based architecture — network functions expose HTTP/2 APIs to each other, replacing the point-to-point interfaces of the EPC.
RAN nodes — gNB and ng-eNB
The gNB (next-generation Node B) is the 5G base station. It terminates all radio protocols (PHY through RRC) and connects to the 5GC via the NG interface. A single gNB cell serves our example UE on PCI 442.
A gNB can optionally be split into a gNB-CU (Central Unit, handles RRC/PDCP) and a gNB-DU (Distributed Unit, handles RLC/MAC/PHY), connected via the F1 interface. This split enables centralised processing and fronthaul flexibility.
Core network functions
| NF | Full name | Role | Interface to gNB |
|---|---|---|---|
| AMF | Access and Mobility Mgmt Function | UE registration, authentication orchestration, NAS signalling, paging, mobility | N2 (NG-C) |
| UPF | User Plane Function | Data plane anchor, packet routing, QoS enforcement, traffic measurement | N3 (NG-U) |
| SMF | Session Mgmt Function | PDU session creation, IP address allocation, UPF selection and control | Via AMF (N11) |
| UDM | Unified Data Mgmt | Subscriber data, authentication credentials, subscription management | N8, N10 |
| AUSF | Authentication Server Function | 5G-AKA and EAP-AKA' authentication, key derivation | N12 |
| PCF | Policy Control Function | QoS policy, charging rules, slice policy | N7, N15 |
| NRF | NF Repository Function | Service discovery — NFs register here and find each other | N27 |
The two NG interfaces
The gNB connects to the 5GC via two logically separate interfaces, both running over the same physical transport:
N2 (NG-C) — the control plane. Carries NGAP (NG Application Protocol) messages between gNB and AMF. Used for UE registration, handover, paging, PDU session setup requests. Protocol stack: SCTP/IP.
N3 (NG-U) — the user plane. Carries GTP-U (GPRS Tunnelling Protocol User Plane) encapsulated user data between gNB and UPF. One GTP-U tunnel per PDU session per UE. Protocol stack: UDP/IP.
Protocol stack — control and user plane
The full end-to-end protocol stack from UE to internet involves both radio protocols (covered in detail throughout this site) and core network protocols:
Network slicing
5G introduces network slicing — the ability to run multiple logical networks on shared physical infrastructure. Each slice has its own AMF, SMF, and UPF instances (or shared instances) with different QoS profiles.
Slices are identified by an S-NSSAI (Single Network Slice Selection Assistance Information): a Slice/Service Type (SST) and optional Slice Differentiator (SD). The UE requests specific slices in the Registration Request. The AMF selects the appropriate SMF/UPF for each slice.
// Standardised Slice/Service Type (SST) values: SST=1 → eMBB (enhanced Mobile Broadband) SST=2 → URLLC (Ultra-Reliable Low-Latency) SST=3 → MIoT (Massive IoT) SST=4 → V2X (Vehicle-to-X) // Example S-NSSAI in Registration Request: requestedNSSAI: S-NSSAI 1: SST=1, SD=0x000001 ← eMBB, operator-specific S-NSSAI 2: SST=2, SD=absent ← URLLC, no differentiator