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LTE X2 Handover Procedure Call Flow

call-flow LTE | Mobility | X2AP | RRC | S1AP | GTP-U

LTE X2 handover is the intra-LTE mobility variant where the source eNB and target eNB coordinate the move directly over the X2 interface. It is the preferred path when direct X2 connectivity is available and the move can stay local between the two eNBs until the later EPC path switch.

This page focuses on the interface-specific behavior that makes X2 handover different from S1 handover.

Introduction

The source eNB uses X2 signaling to prepare the target eNB, then sends the handover command to the UE, and the target side later performs the path switch toward the MME after successful target access.

The main nodes are the UE, source eNB, target eNB, MME, and SGW.

What Is X2 Handover Procedure in Simple Terms?

What starts the procedure: A connected LTE move is needed and the source and target eNBs have usable X2 connectivity.
What the UE and network want to achieve: Move the UE to the target eNB with direct source-to-target preparation.
What success looks like: The target eNB accepts the UE, then performs a path switch toward the EPC.
What failure means: X2 preparation fails, target access fails, or the path switch does not complete.

Why this procedure matters

This page explains the direct eNB-to-eNB branch of LTE mobility. It is the main reference when the source eNB, not the MME, leads the early handover coordination.

Quick Fact Sheet

Procedure name	LTE X2 Handover Procedure
Domain	Intra-LTE handover using direct X2 coordination
Main trigger	Connected LTE mobility with X2 connectivity available
Start state	UE is connected on the source eNB
End state	UE is connected on the target eNB and the EPC path has switched
Main nodes	UE, source eNB, target eNB, MME, SGW
Main protocols	RRC, X2AP, S1AP, GTP-U
Main success outcome	Direct source-to-target preparation and clean target takeover
Main failure outcome	Preparation reject, failed execution, or bad path switch
Most important messages	Handover Request, SN Status Transfer, RRC Connection Reconfiguration, Path Switch Request
Main specs	TS 36.300, TS 36.423, TS 36.331, TS 23.401

LTE X2 Handover Procedure call flow — Click the diagram to open the full-size in a new tab.

Handover Concept

This illustration shows the basic handover concept used in this procedure: the UE leaves the serving side after the mobility decision and continues on the target side once the target path is ready.

Prepare over X2 to Switch EPC path. Only after successful target access does the EPC update the downlink path to the target eNB.

Preconditions

The UE is already connected on LTE.
Source and target eNBs have usable X2 connectivity.
The target cell can admit the UE and carry the active bearers.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Executes the target move after the X2-prepared handover command.
Source eNB	Starts handover and prepares the target directly over X2.
Target eNB	Admits the UE and later becomes the active serving side.
MME	Handles the later path switch instead of the early preparation leg.
SGW	Maintains user-plane continuity after the target takes over.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries the handover command and target access.
X2	source eNB <-> target eNB	Carries direct preparation, bearer information, and status transfer.
S1-MME	target eNB <-> MME	Carries the later path switch after target access.
S1-U	eNB <-> SGW	Carries user-plane forwarding before and after the move.

End-to-End Call Flow

UE         Source eNB            Target eNB              MME
|             |--X2 Handover Request---->|                   |
|             |<--X2 Handover Ack--------|                   |
|<--HO Cmd----|                          |                   |
|==== target access ====================>|                   |
|             |--SN Status Transfer----->|                   |
|             |                          |--Path Switch Req->|
|             |<--UE Context Release---- |                   |

Major Phases

Phase	What happens
1. X2 preparation	The source eNB prepares the target directly over X2.
2. UE execution	The source eNB sends the command and the UE moves to the target.
3. Status and path switch	The target side receives remaining state and updates the EPC path.
4. Source release	The source eNB releases its old UE context.

Step-by-Step Breakdown

Step 1: X2 target preparation

Sender -> receiver: source eNB -> target eNB

Message(s): X2 Handover Request and Handover Request Acknowledge

Purpose: Prepare the target directly without involving the MME in the early leg.

State or context change: The target eNB is ready before the UE moves.

Note: This is the clearest difference from S1 handover.

Step 2: UE receives the move command

Sender -> receiver: source eNB -> UE

Message(s): RRC Connection Reconfiguration

Purpose: Tell the UE to leave the source and access the target cell.

State or context change: The UE starts the execution phase.

Note: If execution fails here, the trace shifts into recovery.

Step 3: Target access and status transfer

Sender -> receiver: UE -> target eNB and source eNB -> target eNB

Message(s): Target access and SN Status Transfer

Purpose: Bring the UE onto the target side and preserve bearer continuity details.

State or context change: The target becomes the active radio side.

Note: Status transfer is useful when packet continuity matters.

Step 4: EPC path switch

Sender -> receiver: target eNB -> MME

Message(s): Path Switch Request

Purpose: Move EPC delivery to the target eNB.

State or context change: The target-side EPC path becomes active.

Note: This step happens after the UE is already on the target side.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Handover Request	X2AP	source eNB -> target eNB	Starts direct target preparation over X2.	Inspect UE context, bearer list, and admission details.
SN Status Transfer	X2AP	source eNB -> target eNB	Preserves bearer sequence context during the move.	Check whether the transfer was present for continuity-sensitive flows.
RRC Connection Reconfiguration	RRC	source eNB -> UE	Carries the X2-based handover command.	Inspect target info and execution timing.
Path Switch Request	S1AP	target eNB -> MME	Finalizes EPC continuity after target access.	Check target IDs and bearer mapping.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
X2 availability	Whether direct source-to-target connectivity exists.	Mobility context and eNB relation data	Explains why X2 rather than S1 is used.	No X2 route or unstable X2 relation.
Target admission	Resource acceptance at the target eNB.	X2 Handover Ack	Confirms the target is ready.	Admission reject or partial bearer issue.
SN status	Bearer sequence continuity information.	SN Status Transfer	Important for minimizing packet disorder during the move.	Missing or stale status transfer.
Path switch state	EPC update after target access.	Path Switch Request	Shows whether the EPC followed the move.	Target access succeeded but EPC remained stale.
Source release timing	When the old source context is removed.	Context release handling	Helps detect premature source cleanup.	Source released too early or not at all.

Successful Completion

Success means the target eNB is prepared over X2, the UE reaches the target, and the later path switch updates the EPC to the new serving side.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
No X2 preparation possible	Source and target cannot use direct X2 signaling.	Neighbor relation and early mobility branch.	Handover Request missing or rejected	X2	Use S1 handover if that is the actual path.
Target move succeeds but packets stall	The EPC did not follow the move or bearer continuity broke.	Path switch and SN status handling.	SN Status Transfer, Path Switch Request	X2, S1-MME, S1-U	Check both radio execution and bearer continuity together.

What to Check in Logs and Traces

Check that the early preparation really used X2 rather than S1.
Confirm whether SN Status Transfer appeared for active bearers.
Verify that the target-side path switch happened after successful access.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

X2 handover keeps the early preparation local between eNBs. The MME becomes active later during the path-switch step rather than at the start of the move.

FAQ

What is X2 handover?

It is intra-LTE handover where source and target eNBs coordinate directly over X2.

Why is it different from S1 handover?

In X2 handover the MME does not relay the early target-preparation signaling.