5G architecture: RAN, core and the service-based architecture
In short: A 5G system has three main parts: the radio access network (RAN), the transport network and the 5G Core. The core is built from software network functions that talk over HTTP/2 APIs, known as the service-based architecture. Control and user planes are separated, so traffic can be handled close to users.
The big picture
A 5G system connects user equipment (UE), such as phones, routers and sensors, to data networks such as the internet or a company network. Between them sit the radio access network (RAN), which handles the radio link, and the 5G Core (5GC), which handles authentication, sessions, mobility and policy. A transport network of fibre, microwave or other links joins them.
The radio access network
The 5G base station is called a gNB. It is often split into three parts so that work can be placed where it fits best:
- Radio Unit (RU): antennas and radio hardware at the mast.
- Distributed Unit (DU): time-critical processing, usually near the RU.
- Centralised Unit (CU): less time-critical functions, which can be pooled in a regional site. It is further split into control plane (CU-CP) and user plane (CU-UP).
The O-RAN approach, promoted by the O-RAN Alliance, defines open interfaces between these parts so operators can mix vendors, and adds RAN Intelligent Controllers (RICs) that host optimisation apps. Openness brings flexibility but also more interfaces to secure.
Non-standalone and standalone
| Mode | How it works | What you get |
|---|---|---|
| Non-standalone (NSA) | 5G radio added to a 4G network; the 4G core (EPC) and 4G signalling anchor the connection | Faster speeds, quick to deploy, but few new 5G core features |
| Standalone (SA) | 5G radio connected to a 5G Core | Full 5G features: network slicing, better identity protection, service-based core |
Many early 5G launches used NSA. Features such as network slicing need SA.
The 5G Core and service-based architecture
In earlier generations, core elements were boxes joined by fixed point-to-point interfaces. The 5G Core instead uses a service-based architecture (SBA): each network function (NF) offers services over HTTP/2 APIs, and functions find each other through the Network Repository Function (NRF). NFs are designed to run as cloud-native software, often in containers.
| Function | Role |
|---|---|
| AMF (Access and Mobility Management) | Registration, connection and mobility of devices; entry point for signalling |
| SMF (Session Management) | Creates and manages data sessions, assigns IP addresses, controls the UPF |
| UPF (User Plane Function) | Forwards user traffic between RAN and data network; can be placed at the edge |
| AUSF and UDM | Authentication service and subscriber data and credentials |
| PCF (Policy Control) | Quality of service and charging policy |
| NSSF and NEF | Slice selection, and controlled exposure of network capabilities to external apps |
Control and user plane separation (CUPS) means the SMF (control) can be central while UPFs (user traffic) sit close to users, which is the basis for multi-access edge computing. Key reference points to recognise: N2 (RAN to AMF), N3 (RAN to UPF, user data in GTP-U tunnels), N4 (SMF to UPF) and N6 (UPF to data network).
Tip: for security, think of the 5G Core as a cloud application. API security, container hardening, identity between functions and segmentation all apply, as covered in 5G security.
Practice
Which 5G Core function forwards user data packets?
How do network functions find each other in the 5G SBA?
Why does network slicing need standalone 5G?
Key takeaways
- A 5G system is UE, RAN, transport and the 5G Core.
- The gNB can be split into RU, DU and CU; O-RAN opens these interfaces.
- Standalone 5G uses the 5G Core and unlocks features such as slicing.
- The 5G Core is service-based: network functions talk over HTTP/2 APIs.
- Separated control and user planes let UPFs run at the edge.
Original content written by Mohamed Fayas for FM TecHub. © 2026 FM TecHub. You're welcome to share a link to this page; please don't copy the text elsewhere without permission.