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gNB (RAN)

Note

The gNB (gNodeB) is a 5G base station that handles message transfer between the core network and User Equipment (UE). It serves as the radio access network node in 5G networks.

Architecture Overview

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Basic Connections

Connections

The gNB (RAN) establishes four fundamental connections:

  • To Core Network:

    • Control Plane: NGAP (Next Generation Application Protocol) with AMF (Access and Mobility Management Function)
    • Data Plane: GTP-U (GPRS Tunneling Protocol - User Plane) with UPF (User Plane Function)

  • To UE (User Equipment):

    • Control Plane: NAS (Non-Access Stratum) signaling with UE
    • Data Plane: Raw packet transmission with UE over the radio interface

Procedures

  1. NG Setup with AMF (port: 38412)

    • The gNB establishes a connection with the AMF to set up basic operational parameters and register itself with the core network.
    • This procedure includes exchanging supported features, served PLMNs (Public Land Mobile Networks), and TAC (Tracking Area Code) information.

  2. GTP Tunnel Establishment with UPF (port: 2152)

    • The gNB establishes GTP-U tunnels with the UPF for user data forwarding.
    • These tunnels are identified by TEID (Tunnel Endpoint Identifier) values for proper packet routing.

  3. UE Connection Management

    Upon receiving a new UE control plane connection, the gNB initiates the following procedures:

    • UE Registration: Authenticates and registers the UE with the network
    • PDU Session Establishment: Creates data sessions for the UE's communication needs

Xn Interface

In the current implementation, the Xn interface is specifically designed for exchanging TEID information to support the NR-DC (New Radio Dual Connectivity) feature.

Now, the Xn interface on each gNB listens on the address and port specified in the YAML configuration file, for example:

xnIp: "10.0.1.2"
xnPort: 31415

This listener uses TCP, allowing any request to connect to it as needed. The processing function is defined in gnb/xn.go and can be extended in the swtch section.

GTP Forwarding

The gNB implements GTP-U forwarding to handle user plane data between the UE and the core network. This includes:

  • Encapsulation and decapsulation of user packets
  • TEID-based routing

For more detailed information about GTP-U implementation, please refer to: Userspace GTP-U