Add N2 handover documentation
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Guido Casati
+91
-2
@@ -210,7 +210,7 @@ sequenceDiagram
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end
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```
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## Handover
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## Inter-DU Handover (F1)
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The basic handover (HO) structure is as follows. In order to support various
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handover "message passing implementation" (F1AP, NGAP, XnAP), RRC employs
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@@ -264,6 +264,94 @@ sequenceDiagram
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sdu->>cucp: F1AP UE Context Release Complete
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Note over ue,tdu: UE active on target DU
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```
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## Inter-gNB Handover (N2)
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This is an inter-NG-RAN procedure. The N2 handover specification is defined in the following documents:
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* 3GPP TS 23.502, 4.9.1.3 Inter NG-RAN node N2 based handover:
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- Outlines detailed handover signaling flows for N2-based handovers.
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- Covers both intra-system (between 5G gNBs) and inter-system (between 5G and LTE eNBs) handovers.
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* 3GPP TS 38.300, section 9 Mobility and State Transitions:
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- describes mobility procedures at the NG-RAN level, depending on the RRC state.
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* 3GPP TS 38.413 (NGAP), section 8.4 UE Mobility Management Procedures:
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- Specifies the signaling procedures over the N2 interface.
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- Includes messages like Handover Request, Handover Command, and Handover Preparation.
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* 3GPP TS 38.331 (RRC): details the UE-level RRC procedures involved during handovers
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### End-to-end flow
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```mermaid
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sequenceDiagram
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participant ue as UE
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participant sdu as source DU
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participant scucp as source CU-CP
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participant scuup as source CU-UP
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participant tdu as target DU
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participant tcucp as target CU-CP
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participant tcuup as target CU-UP
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participant amf as AMF
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Note over ue,sdu: UE active on source DU
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alt HO triggered through A3 event
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ue->>sdu: RRC Measurement Report
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sdu->>scucp: F1AP UL RRC Msg Transfer (RRC Measurement Report)
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Note over scucp: Handover decision (A3 event trigger)
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else Manual Trigger
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Note over scucp: Handover decision (e.g., telnet)
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end
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Note over scucp: nr_rrc_trigger_n2_ho() ("on source CU")
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scucp->>amf: HANDOVER REQUIRED
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amf->>tcucp: HANDOVER REQUEST
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Note over tcucp: rrc_gNB_process_Handover_Request
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Note over tcucp: trigger_bearer_setup
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tcucp->>tcuup: Bearer Context Setup Request
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tcuup->>tcucp: Bearer Context Setup Response
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Note over tcucp: rrc_gNB_process_e1_bearer_context_setup_resp
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Note over tcucp: nr_rrc_trigger_n2_ho_target() ("on target CU")
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Note over tcucp: nr_initiate_handover()
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tcucp->>tdu: F1AP UE Context Setup Req
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Note over tdu: Create UE context
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tdu->>tcucp: F1AP UE Context Setup Resp (incl. CellGroupConfig)
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Note over tcucp: rrc_CU_process_ue_context_setup_response() ("on target CU")
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Note over tcucp: e1_send_bearer_updates()
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tcucp->>tcuup: E1AP Bearer Context Modification Req
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tcuup->>tcucp: E1AP Bearer Context Modification Resp
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tcucp-->>tcucp: callback: ho_req_ack()
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Note over tcucp: nr_rrc_n2_ho_acknowledge() ("on target CU")
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tcucp->>amf: HANDOVER REQUEST ACKNOWLEDGE (data forwarding info)
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amf->>scucp: HANDOVER COMMAND
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scucp->>sdu: F1AP UE Context Modification Req (RRC Reconfiguration)
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sdu->>ue: RRC Reconfiguration
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sdu->>scucp: F1AP UE Context Modification Resp
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Note over sdu: Stop scheduling UE
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Note over scucp: rrc_CU_process_ue_context_modification_response()
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Note over scucp: e1_send_bearer_updates()
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scucp->>scuup: E1 Bearer Context Modification Req (PDCP Status Request)
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scuup->>scucp: E1 Bearer Context Modification Resp (PDCP Status Info)
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scucp->>amf: NG Uplink RAN Status Transfer
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amf->>tcucp: NG Downlink RAN Status Transfer
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tcucp->>tcuup: E1 Bearer Context Modification Req (PDCP Status Info)
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tcuup->>tcucp: E1 Bearer Context Modification Resp
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Note over ue: UE attachment to target DU
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Note over ue,tdu: RA (Msg1 + Msg2)
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ue->>tdu: RRC Reconfiguration Complete
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tdu->>tcucp: F1AP UL RRC Msg Transfer (RRC Reconfiguration Complete)
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tcucp-->>tcucp: callback: ho_success()
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Note over tcucp: nr_rrc_n2_ho_complete() ("on target CU")
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Note over tcucp: handle_rrcReconfigurationComplete() ("on target CU")
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tcucp->>amf: HANDOVER NOTIFY
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amf->>scucp: UE Context Release Command
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note over scucp: ngap_gNB_handle_ue_context_release_command
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note over scucp: rrc_gNB_process_NGAP_UE_CONTEXT_RELEASE_COMMAND
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scucp->>scuup: E1 Bearer Context Release Command
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scuup->>scucp: E1 Bearer Context Release Complete
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Note over scucp: rrc_gNB_generate_RRCRelease
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scucp->>sdu: F1 UE Context Release Command
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sdu->>scucp: F1 UE Context Release Complete
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note over scucp: rrc_CU_process_ue_context_release_complete
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note over scucp: rrc_remove_ue
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Note over ue,tdu: UE active on target DU
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```
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# Structures
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@@ -308,7 +396,8 @@ corresponding CU.
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`nr_ho_source_cu_t` contains notably a function pointer `ho_cancel` for
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handover cancel. `nr_ho_target_cu_t` contains function pointers `ho_req_ack`
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for handover request acknowledge and `ho_success` for handover success. As can
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for handover request acknowledge, `ho_success` for handover success,
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`ho_failure` for handover failure (N2 only).
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be seen in the sequence diagram above, either the "target CU" or "source CU"
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needs to do an operation, and a "switch" from target to source CU is done using
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these function pointers. For instance, in F1, the handover request acknowledge
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+21
-1
@@ -99,6 +99,26 @@ the [MAC configuration](../MAC/mac-usage.md) as well for SIB configuration.
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### Neighbor-gNB configuration
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TBD
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#### What is a gNB neighbor?
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Network continuity is a key aspect of 5G. In the 5G architecture, gNB neighbors play a central role in maintaining service continuity through mechanisms such as handover and load balancing. By definition, a gNB neighbor is another gNB that can be measured and linked by the UE. If the current serving gNB is no longer optimal, the UE may connect to a neighbor gNB.
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To support this behavior, the network configuration specifies additional frequencies and cells that the UE should measure. The UE reports these measurements to the network, which then decides whether or not to initiate a handover.
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Neighbor types include:
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- **Intra-gNB neighbors** - cells belonging to the same gNB
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- **Inter-gNB neighbors** - cells belonging to different gNBs
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- **Inter-RAT neighbors** - cells belonging to another RAT (e.g., LTE)
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#### Required configuration parameters
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To define a neighbor cell in the configuration file, the following parameters are typically needed:
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- **gNB ID** - unique identifier of the gNB
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- **Cell ID** - identifier of the cell within the gNB
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- **Physical Cell ID** - identifier of the cell’s synchronization signal (PCI)
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- **Absolute Frequency** - frequency used by the SSB (absoluteFrequencySSB)
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- **Subcarrier Spacing** - numerology (e.g., 15 kHz, 30 kHz)
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- **PLMN configuration** - MCC, MNC, and MNC length
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- **Tracking Area Code (TAC)** - identifier of the tracking area
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Refer to the [handover tutorial](../handover-tutorial.md) for more information.
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