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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.

The supported NGAP types are:

  1. ngapType.NGAPPDUPresentInitiatingMessage

    1. ngapType.ProcedureCodePDUSessionResourceSetup: used for static NR-DC set up.
    2. ngapType.ProcedureCodePDUSessionResourceModifyIndication: used for dynamic NR-DC initial set up.

  2. ngapType.NGAPPDUPresentSuccessfulOutcome

    1. ngapType.ProcedureCodePDUSessionResourceModifyIndication: used for dynamic NR-DC final setup.

UE types

  • RanUe

    • Structure

      type RanUe struct {
    amfUeNgapId int64
    ranUeNgapId int64

    mobileIdentity5GS nasType.MobileIdentity5GS

    ulTeid aper.OctetString
    dlTeid aper.OctetString

    n1Conn        net.Conn
    dataPlaneConn net.Conn

    nrdcIndicator    bool
    nrdcIndicatorMtx sync.Mutex
}

    • Function

      RanUE is used to record all infomation about the UE controlled be gNB.

  • XnUE

    • Struct

      type XnUe struct {
    imsi string

    ulTeid aper.OctetString
    dlTeid aper.OctetString

    dataPlaneConn net.Conn
}

    • Function

      XnUE is used to record the infomation about the UE offload from master gNB. XnUE will only handle data plane message for forwarding the packets between core network and UE.

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