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5g nsa handover call flow

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The 5G Non-Standalone (NSA) architecture allows for the integration of 5G with existing 4G LTE networks. In the NSA deployment, the control plane signaling goes through the LTE radio network, while the user plane traffic can be routed through either LTE or 5G, depending on the availability and capability of the 5G network. Here, I'll explain the handover call flow in a 5G NSA scenario.

1. Initial Connection Setup:

  • The UE (User Equipment) initially connects to the LTE eNodeB (eNB) and registers with the LTE core network (EPC - Evolved Packet Core).
  • The 5G NR (New Radio) is not yet involved in the connection at this stage.

2. 5G NR Connection Setup:

  • The UE establishes a connection with the 5G NR gNodeB (gNB) using the LTE connection as an anchor.
  • The 5G NR connection is established over the NR radio interface, but the control plane signaling is still handled by the LTE eNB.

3. Handover Trigger:

  • A handover trigger occurs, either initiated by the network (e.g., due to 5G coverage improvement) or by the UE (e.g., moving out of LTE coverage into 5G coverage).

4. Path Switch to 5G NR:

  • The decision for handover is made, and the control plane signaling for the handover is initiated in the LTE core network.
  • The data path (user plane) is switched from LTE to 5G NR, enabling the UE to communicate directly with the 5G NR gNB.

5. Xn Interface Handover:

  • The Xn interface, which connects the source (old) and target (new) gNBs, is used for the handover.
  • The Xn interface facilitates the transfer of context information and signaling messages between the source and target gNBs.

6. Data Forwarding and Control Plane Handover:

  • The data forwarding path is established from the LTE eNB to the 5G NR gNB.
  • The control plane handover messages are exchanged between the LTE eNB and the 5G NR gNB to transfer the UE's context and state information.

7. Data Path Switch Completion:

  • Once the data path is successfully switched to 5G NR, the LTE eNB releases the resources allocated for the UE in the LTE network.

8. LTE Handover Preparation and Completion:

  • The LTE eNB prepares for the handover release, signaling the completion of the handover to the LTE core network.
  • The UE is now fully connected to the 5G NR network, and the LTE connection is released.

This handover call flow ensures a seamless transition for the UE from LTE to 5G NR while maintaining an active connection throughout the process. The involvement of both LTE and 5G NR components in the handover process is a key characteristic of the 5G NSA architecture.