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5G Inter-gNB Handover Procedure Call Flow

call-flow 5G NR | Inter-gNB Mobility | Xn | N2 | RRC

5G Inter-gNB Handover is the connected-mode mobility procedure that moves an active UE from one gNB domain to another.

It is broader than a local cell change because radio, context, and traffic continuity all cross the gNB boundary.

Introduction

This page explains the end-to-end inter-node move: measurement trigger, target preparation, handover command, target access, and post-move continuity.

The strongest troubleshooting split is between preparation failure, radio execution failure, and post-completion continuity failure.

What Is Inter-gNB Handover in Simple Terms?

What starts the procedure: The network decides another gNB should become the new serving node.
What the UE and network want to achieve: Move the UE across the gNB boundary while preserving service.
What success looks like: The UE accesses the target gNB and service continues cleanly there.
What failure means: Preparation, target access, or path-switch continuity fails across the node boundary.

Why this procedure matters

Inter-gNB mobility is one of the places where radio issues, coordination issues, and transport continuity issues all look similar in alarms unless the engineer separates them carefully.

Quick Fact Sheet

Procedure name	5G Inter-gNB Handover
Domain	5G NR mobility between different gNBs
Main trigger	Connected UE must move from one gNB domain to another
Start state	UE is actively served by a source gNB
End state	UE continues service under a different target gNB
Main nodes	UE, source gNB, target gNB, AMF, UPF
Main protocols	RRC, Xn or N2 coordination, NGAP, user-plane path switch
Main success outcome	UE moves to the target gNB and service continues with preserved context
Main failure outcome	Preparation, target access, or path switch fails across the gNB boundary
Most important messages	Measurement Report, Handover Request, RRC Reconfiguration, RRC Reconfiguration Complete
Main specs	TS 23.502, TS 38.300, TS 38.331

Preconditions

The UE is already connected on the source gNB.
A different gNB is available as a valid target.
Neighbor and target preparation data are configured correctly.
The user-plane path can follow the UE once the move completes.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Reports measurements, executes the move, and completes service under the target gNB.
Source gNB	Owns the UE before handover and starts the inter-node mobility preparation.
Target gNB	Allocates resources and becomes the new serving node after execution.
AMF	Coordinates mobility when core assistance is required or the path is N2-based.
UPF	Supports user-plane continuity as traffic follows the new gNB.

Interfaces used

Interface	Path	Role
NR-Uu	UE <-> gNBs	Carries measurements, handover command execution, and target access.
Xn	Source gNB <-> target gNB	Preferred direct coordination path when available between gNBs.
N2	gNB <-> AMF	Used for core-assisted inter-gNB mobility when Xn is not used or sufficient.
N3	gNB <-> UPF	Carries or updates the user-plane anchor after the move.

End-to-End Call Flow

UE                Source gNB              Target gNB              AMF / UPF
|                     |                        |                    |
|-- Measurement Report ----------------------->|                    |
|                     |-- Handover Request --->|------------------->|
|                     |<-- Handover Ack -------|<-------------------|
|<-- RRC Reconfiguration ----------------------|                    |
|==== move to target gNB =========================================>|
|-- RRC Reconfiguration Complete -------------------------------->|
|==== traffic continues via target gNB ==========================>|

Major Phases

Phase	What happens
1. Measurement trigger	The source gNB learns that another gNB offers a better serving opportunity.
2. Inter-node preparation	The source and target coordinate resources across the gNB boundary.
3. UE execution	The source sends the mobility command and the UE accesses the target gNB.
4. Completion and path switch	The target confirms the UE and the user-plane path follows the move.
5. Stable target-side service	The UE continues on the new gNB without falling into recovery.

Step-by-Step Breakdown

The source gNB sees another gNB as the better target

Sender -> receiver: UE -> source gNB

Message(s): Measurement Report

Purpose: Trigger mobility across the gNB boundary based on real neighbor measurements.

State or context change: The move becomes an inter-node mobility case rather than a local cell change.

Note: Always classify whether the target is within the same gNB or a different gNB before reading the trace further.

The source and target prepare across the node boundary

Sender -> receiver: Source gNB -> target gNB or via AMF

Message(s): Handover Request and acknowledgment

Purpose: Prepare target radio and context resources before the UE is moved.

State or context change: The inter-gNB path becomes executable.

Note: Inter-node mobility has more failure surface than local cell change because context leaves the source gNB domain.

The UE receives the mobility command

Sender -> receiver: Source gNB -> UE

Message(s): RRC Reconfiguration

Purpose: Tell the UE which target gNB and target cell to move toward.

State or context change: The UE leaves the source-side radio context and begins target access.

Note: The source-side decision may be good while the target-side RF execution still fails.

The UE accesses the target gNB

Sender -> receiver: UE -> target gNB

Message(s): Target-cell synchronization and access sequence

Purpose: Establish the radio connection under the new gNB domain.

State or context change: The target gNB becomes the active radio serving node.

Note: Target access failure is one of the most common practical reasons a clean preparation still ends badly.

Completion and continuity are validated

Sender -> receiver: UE -> target gNB -> AMF / UPF

Message(s): RRC Reconfiguration Complete

Purpose: Confirm completion and shift the service continuity fully to the target side.

State or context change: The inter-gNB mobility branch closes with stable target-side service.

Note: Validate both completion signaling and post-move traffic before calling the handover healthy.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Measurement Report	RRC	UE -> source gNB	Starts the inter-gNB mobility reasoning.	Check target ranking and timing.
Handover Request	XnAP / NGAP	Source side -> target side	Prepares inter-node resources.	Confirms the move really crossed the gNB boundary.
RRC Reconfiguration	RRC	Source gNB -> UE	Carries the move toward the target gNB.	Use it to inspect target identity and mobility control info.
RRC Reconfiguration Complete	RRC	UE -> target side	Confirms the UE completed the move.	Main radio success checkpoint.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Target gNB identity	The node chosen to receive the UE.	Preparation and command stage	Proves the move is really inter-gNB.	Misclassification here causes bad debugging assumptions.
Preparation success	Whether the target gNB was ready.	Handover Request / Ack	Key for separating coordination failure from RF failure.	A missing or weak prep stage often hides upstream causes.
Target access timing	Time from command to successful access under the target gNB.	Execution correlation	Shows whether the UE moved efficiently.	Long gaps or no completion point to execution trouble.
Path-switch state	How the user-plane path followed the move.	Post-completion continuity	Separates radio completion from full service success.	Traffic continuity matters after RRC success.
Fallback or recovery signals	Whether the UE fell into reestablishment or another recovery branch.	Post-failure phase	Shows the handover did not complete cleanly.	Important for root-cause classification.

Success Criteria

The target gNB is correctly selected and prepared.
The UE executes the move and completes target access cleanly.
Completion signaling reaches the network successfully.
Traffic continuity remains stable under the target gNB.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Target gNB preparation fails	The source selects a target but the inter-node preparation does not complete correctly.	Handover Request, Ack, and target resource readiness.	Preparation stage	Xn or N2	Often hidden behind later generic handover failure alarms.
UE cannot access the target gNB	The command is delivered but target-side radio execution fails.	RRC command, target access steps, and absence of completion.	RRC Reconfiguration	NR-Uu	Classic execution failure branch.
Radio completion succeeds but traffic continuity fails	The user-plane path did not follow the inter-gNB move correctly.	Completion plus post-move traffic checks.	RRC Reconfiguration Complete	N3	This is a post-handover continuity failure, not a clean success.
UE falls into recovery after attempted move	The inter-gNB handover broke badly enough to trigger downstream recovery procedures.	Missing completion and later recovery signaling.	RRC Reconfiguration, recovery messages	NR-Uu	Treat recovery as a symptom, not the original cause.

What to Check in Logs and Traces

Verify the move is truly inter-gNB by checking the target identity first.
Inspect Handover Request / Ack to prove target preparation really happened.
Correlate RRC Reconfiguration and target access timing closely.
Use RRC Reconfiguration Complete as the radio success point, then keep going.
Validate post-move traffic continuity before declaring success.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

FAQ

What is 5G Inter-gNB Handover?

It is the connected-mode mobility procedure where the UE moves from one gNB to a different gNB.

How is it different from Intra-gNB Handover?

Inter-gNB crosses the gNB boundary, so preparation and continuity handling are broader and riskier.

What proves success?

The UE completes the move to the target gNB and traffic continuity remains healthy afterward.

What should I inspect first?

Start with the target gNB choice, preparation signaling, command timing, and post-move traffic continuity.

Is Xn always used?

Not always. The move may use Xn directly or rely more on N2/core assistance depending on network conditions.