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5G SA N2-NGAP Handover Call flow

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The 5G Standalone (5G SA) N2-NGAP (Next Generation NodeB Application Part) Handover Call Flow involves the transfer of a data session from one cell to another in a 5G network. The N2 reference point connects the Access and Mobility Management Function (AMF) to the Next Generation NodeB (gNB) in the 5G architecture. The N2-NGAP Handover process is a crucial aspect of mobility management in 5G networks.

Here's a technical explanation of the 5G SA N2-NGAP Handover Call Flow:

  1. UE Context Preparation (Before Handover):
    • The User Equipment (UE) is initially connected to a source cell (gNB1).
    • The AMF (Access and Mobility Management Function) is responsible for managing the mobility of the UE and establishing the initial connection.
    • The AMF stores the UE context information, including the radio and core network states.
  2. Handover Triggering:
    • The handover process is triggered due to certain conditions, such as poor signal quality or load balancing requirements.
    • The AMF decides that a handover is necessary and selects a target cell (gNB2) for the handover.
  3. Path Switch Request (AMF to Source gNB):
    • The AMF sends a Path Switch Request (NGAP message) to the source gNB (gNB1).
    • This message includes information about the target gNB (gNB2) and the necessary parameters for the handover.
  4. Path Switch Request Acknowledge (Source gNB to AMF):
    • The source gNB acknowledges the Path Switch Request by sending a Path Switch Request Acknowledge message to the AMF.
    • This message confirms that the source gNB is ready for the handover.
  5. Xn-based Handover Preparation (Between gNBs):
    • The source gNB and target gNB establish an Xn interface connection to prepare for the handover.
    • The Xn interface is the interface between gNBs in the 5G architecture.
  6. Data Forwarding (From Source gNB to Target gNB):
    • The source gNB starts forwarding the user data to the target gNB over the Xn interface.
    • The target gNB begins to buffer the incoming data.
  7. Handover Command (Source gNB to UE):
    • The source gNB sends a Handover Command to the UE, instructing it to switch to the target gNB.
    • This message contains information about the target gNB and the radio resources to be used.
  8. Handover Complete (Target gNB to AMF):
    • The target gNB informs the AMF about the successful handover by sending a Handover Complete message.
  9. UE Context Transfer (AMF to Target gNB):
    • The AMF transfers the UE context to the target gNB to ensure a seamless transition.
    • This includes information about the UE's radio context and established data sessions.
  10. Data Forwarding Continues (Target gNB to UE):
    • The target gNB continues to forward the buffered data to the UE.
  11. Path Switch Request Acknowledge (AMF to Target gNB):
    • The AMF acknowledges the successful handover by sending a Path Switch Request Acknowledge to the target gNB.

The handover is now complete, and the UE is effectively connected to the target gNB. This process ensures continuity of service and improved mobility management in 5G Standalone networks. The N2-NGAP protocol plays a crucial role in facilitating communication between the AMF and gNBs during the handover process.