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NR-DC(New Radio-Dual Connectivity)

Note

DC

Types

In real 5G networks, the NR-DC feature can be triggered in two ways:

  1. Static Approach: Two tunnels are established when the gNB and UE's PDU session starts.
  2. Dynamic Approach: After the PDU session is established, the gNB can dynamically offload QoS traffic to the secondary gNB or retrieve the traffic at any time.

Static Approach

Expected Behavior

  • After PDU session established, there will be TWO TUNNELs in this PDU session:
    1. The first tunnel (handled by Master-gNB): forward the default(non-specified QoS traffic) traffic.
    2. The second tunnel (handled by Secondary-gNB): forward the specified QoS traffic configured via webconsole.

How to trigger NR-DC?

In the PDU session establishment procedure, we need to prepare TWO TUNNELs' information (one is for Master-gNB, and te other is for Secondary-gNB) after step 12(N2 PDU Session Request) and encapsulate them in step 14(N2 PDU Session Response):

procedure

Warning

You may need to do Xn-interface communication between gNBs or hardcode the informations in development environment.
Communications include:

  • Master-gNB sends UPLINK Tunnel information to Secondary-gNB.
  • Master-gNB retrieve DOWNLINK TEID information from Secondary-gNB.

Here we take PacketRusher's buildPDUSessionResourceSetupResponseTransfer function as example, this function will be called before step 14 for building the data of the response:

  • For basic, no NR-DC, it only contains FIRST_DOWNLINK_TEID and MASTER_GNB_IP:

    func buildPDUSessionResourceSetupResponseTransfer() (data customNgapType.PDUSessionResourceSetupResponseTransfer) {
    // QoS Flow per TNL Information
    qosFlowPerTNLInformation := &data.QosFlowPerTNLInformation
    qosFlowPerTNLInformation.UPTransportLayerInformation.Present = ngapType.UPTransportLayerInformationPresentGTPTunnel
    // UP Transport Layer Information in QoS Flow per TNL Information
    upTransportLayerInformation := &qosFlowPerTNLInformation.UPTransportLayerInformation
    upTransportLayerInformation.Present = ngapType.UPTransportLayerInformationPresentGTPTunnel
    upTransportLayerInformation.GTPTunnel = new(ngapType.GTPTunnel)
=>  dowlinkTeid := binary.BigEndian.AppendUint32(nil, <FIRST_DOWNLINK_TEID>)
    upTransportLayerInformation.GTPTunnel.GTPTEID.Value = dowlinkTeid
=>  upTransportLayerInformation.GTPTunnel.TransportLayerAddress = ngapConvert.IPAddressToNgap(<MASTER_GNB_IP>, "")
    // Associated QoS Flow List in QoS Flow per TNL Information
    associatedQosFlowList := &qosFlowPerTNLInformation.AssociatedQosFlowList
    associatedQosFlowItem := ngapType.AssociatedQosFlowItem{}
    associatedQosFlowItem.QosFlowIdentifier.Value = qosId
    associatedQosFlowList.List = append(associatedQosFlowList.List, associatedQosFlowItem)
    return
}

  • For NR-DC, we need to add the SECOND_DOWNLINK_TEID and SECONDARY_GNB_IP(ignore the basic part which is same with above):

    func buildPDUSessionResourceSetupResponseTransfer() (data customNgapType.PDUSessionResourceSetupResponseTransfer) {
    // QoS Flow per TNL Information
    ...
    // UP Transport Layer Information in QoS Flow per TNL Information
    ...
    // Associated QoS Flow List in QoS Flow per TNL Information
    ...
    // DC QoS Flow per TNL Information
    DCQosFlowPerTNLInformationItem := ngapType.QosFlowPerTNLInformationItem{}
    DCQosFlowPerTNLInformationItem.QosFlowPerTNLInformation.UPTransportLayerInformation.Present = ngapType.UPTransportLayerInformationPresentGTPTunnel
    // DC Transport Layer Information in QoS Flow per TNL Information
    DCUpTransportLayerInformation := &DCQosFlowPerTNLInformationItem.QosFlowPerTNLInformation.UPTransportLayerInformation
    DCUpTransportLayerInformation.Present = ngapType.UPTransportLayerInformationPresentGTPTunnel
    DCUpTransportLayerInformation.GTPTunnel = new(ngapType.GTPTunnel)
=>  DCUpTransportLayerInformation.GTPTunnel.GTPTEID.Value = aper.OctetString(<SECOND_DOWNLINK_TEID>)
=>  DCUpTransportLayerInformation.GTPTunnel.TransportLayerAddress = ngapConvert.IPAddressToNgap(<SECONDARY_GNB_IP>, "")
    // DC Associated QoS Flow List in QoS Flow per TNL Information
    DCAssociatedQosFlowList := &DCQosFlowPerTNLInformationItem.QosFlowPerTNLInformation.AssociatedQosFlowList
    DCAssociatedQosFlowItem := ngapType.AssociatedQosFlowItem{}
    DCAssociatedQosFlowItem.QosFlowIdentifier.Value = qosId
    DCAssociatedQosFlowList.List = append(DCAssociatedQosFlowList.List, DCAssociatedQosFlowItem)
    // Additional DL QoS Flow per TNL Information
    data.AdditionalDLQosFlowPerTNLInformation = new(ngapType.QosFlowPerTNLInformationList)
    data.AdditionalDLQosFlowPerTNLInformation.List = append(data.AdditionalDLQosFlowPerTNLInformation.List, DCQosFlowPerTNLInformationItem)
    return
}

After the response received by AMF, core network will create the DC Tunnel for specifed QoS traffics and update the corresponding forwarding rules in UPF via N4 session.

Dynamic Approach

Under development, expected to support dynamic traffic offloading in future SMF releases.

Reference