Dynamic NR-DC

At gNB

If gNB get the signal to modify the UE's tunnel, it will iinteract with secondary gNB for retrieving the second tunnel's info, like TEID.

• For master gNB:

  1. Build the target PDUSessionResourceModifyIndicationTransfer message.

     pduSessionModifyIndicationTransfer, err := getPDUSessionResourceModifyIndicationTransfer(ranUe.GetDlTeid(), g.ranN3Ip, 1)
     

  2. Encapsulate into PDUSessionResourceModifyIndication NGAP message.

     pduSessionModifyIndication, err := getPDUSessionResourceModifyIndication(ranUe.GetAmfUeId(), ranUe.GetRanUeId(), constant.PDU_SESSION_ID, pduSessionModifyIndicationTransfer)
     

  3. Interact with secondary gNB.

     pduSessionModifyIndication, err = g.xnPduSessionResourceModifyIndication(ranUe.GetMobileIdentityIMSI(), pduSessionModifyIndication)
     

     This step will start communication with secondary gNB via Xn-interface.

  4. Send the modify indication message to AMF for core network DC setup.

     n, err := g.n2Conn.Write(pduSessionModifyIndication)
     

  5. Receive the confirm message and transmit it to secondary gNB

     _, err = g.xnPduSessionResourceModifyConfirm(ranUe.GetMobileIdentityIMSI(), ngapPduSessionResourceModifyConfirmRaw[:n])
     

  6. Send tunnel update message to UE

     modifyMessage := []byte(util.TUNNEL_UPDATE)
     n, err = ranUe.GetN1Conn().Write(modifyMessage)
     

     This message is just designed by free-ran-ue, not followed 3GPP. The purpose of the message is letting UE update the data plane configuration.

• For secondary gNB:

  1. Receive the modify indication NGAP message and insert its tunnel's information.
  2. Receive the confirm NGAP message and update the uplink TEID.

  For more details implemtation, please refer to: xn.go

At UE

After master gNB finishing the modify procedure, it will send an update message to UE. Once UE received this tunnel update message, it will update the data plane configuration.

• Modify from non-DC to DC:

  1. Dial a connection to secondary gNB's data plane connection.

     conn, err := util.TcpDialWithOptionalLocalAddress(u.nrdc.dcRanDataPlane.ip, u.nrdc.dcRanDataPlane.port, u.nrdc.dcLocalDataPlaneIp)
     

  2. Start the data plane read for secondaru gNB.

     buffer := make([]byte, 4096)
     for {
         n, err := u.dcRanDataPlaneConn.Read(buffer)
         u.readFromRan <- buffer[:n]
     }
     

• Modify from DC to non-DC

  1. Close the secondary gNB's data plane connection.

     u.dcRanDataPlaneConn.Close()
     

The data plane handler will based on the ue.nrdc.enable flag to do the traffic flow split.