User & Control Plane Essentials
User Plane (Data Plane)
Purpose: Transports user-generated data (e.g., internet traffic, voice packets via VoNR).
Protocols: Uses protocols like GTP-U (GPRS Tunneling Protocol for User Plane) for tunneling data between nodes (e.g., between UPF and gNodeB).
No APIs: These connections are not RESTful APIs - they are optimized for high-speed, low-latency data transfer.
Control Plane
Purpose: Manages signaling between network functions (e.g., AMF, SMF, NRF) for tasks like authentication, session setup, or service discovery.
Interfaces: These are called Service-Based Interfaces (SBIs) and are RESTful APIs (HTTP/2 + JSON).
Examples:
- Namf: API exposed by AMF for communication with other functions.
- Nsmf: API exposed by SMF for session management.
- Nnrf: API exposed by NRF for service discovery.
How it works?: Functions like AMF or SMF act as HTTP clients/servers, invoking or exposing RESTful endpoints to exchange control messages (e.g. AMF queries NRF via Nnrf_NFDiscovery to find suitable SMF).
When a device connects to 5G:
- Control Plane: AMF (Access and Mobility Management Function) uses RESTful SBIs to communicate with NRF (Network Repository Function) to discover SMF (Session Management Function).
- User Plane: SMF selects a UPF (User Plane Function), and GTP-U tunnels are established to route user data.
Bibliography: