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LTE S1 Handover Procedure Call Flow

call-flow LTE | Mobility | S1AP | RRC | MME | SGW

LTE S1 handover is the intra-LTE mobility variant used when the source side cannot rely on direct X2 preparation and instead uses the MME to relay the target-preparation signaling. It is the core-assisted branch of intra-LTE handover.

This page focuses on the S1-specific control path and how it differs from X2 handover.

Introduction

The source eNB sends Handover Required to the MME, the MME asks the target eNB to prepare resources, and after the UE reaches the target cell the MME already knows about the target side so the flow differs slightly from X2-based mobility.

The main nodes are the UE, source eNB, MME, target eNB, and often the SGW for indirect forwarding when required.

What Is S1 Handover Procedure in Simple Terms?

What starts the procedure: A connected LTE move is needed but direct X2 preparation is not used.
What the UE and network want to achieve: Move the UE through an MME-relayed handover path.
What success looks like: The target is prepared through S1, the UE moves, and service continues on the target side.
What failure means: The S1-preparation leg fails, execution fails, or forwarding and continuity are incomplete.

Why this procedure matters

This page explains the core-assisted LTE mobility branch. It is the right reference when the MME is visible from the very beginning of the handover preparation sequence.

Quick Fact Sheet

Procedure name	LTE S1 Handover Procedure
Domain	Intra-LTE handover relayed through S1-MME
Main trigger	Connected LTE mobility without usable X2 preparation
Start state	UE is connected on the source eNB
End state	UE is connected on the target eNB through the S1-based mobility branch
Main nodes	UE, source eNB, MME, target eNB, SGW
Main protocols	RRC, S1AP, GTP-U
Main success outcome	MME-assisted target preparation and clean target takeover
Main failure outcome	Preparation failure, execution loss, or forwarding issue
Most important messages	Handover Required, Handover Request, Handover Command, Handover Notify
Main specs	TS 36.300, TS 23.401, TS 36.331

LTE S1 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.

Start via S1 to Confirm target side. The target side confirms that the UE reached the new serving cell.

Preconditions

The UE is connected on LTE.
The mobility decision requires target preparation through S1-MME.
The target eNB can accept the UE context relayed by the MME.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Executes the move after the source side receives the S1-based handover command context.
Source eNB	Starts the S1-assisted move with Handover Required.
MME	Relays preparation between source and target and manages continuity context.
Target eNB	Accepts the UE context through the MME and becomes serving after target access.
SGW	May support indirect forwarding when no direct forwarding path exists.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries the handover command and target access.
S1-MME	source eNB <-> MME <-> target eNB	Carries the preparation and confirmation path.
S1-U	eNB <-> SGW	Carries data continuity and any forwarding update.

End-to-End Call Flow

UE        Source eNB             MME              Target eNB          SGW
|            |--Handover Required------>|                   |               |
|            |                          |--Handover Request>|               |
|            |                          |<--HO Req Ack------|               |
|<--HO Cmd---|                          |                   |               |
|==== target access ======================================>|               |
|            |                          |<--Handover Notify-|               |
|            |<--UE Context Release-----|                   |               |

Major Phases

Phase	What happens
1. Source request	The source eNB asks the MME to start the handover.
2. Target preparation	The MME relays UE context to the target eNB.
3. UE execution	The source eNB commands the UE to access the target.
4. Confirmation and cleanup	The target confirms arrival and the source context is released.

Step-by-Step Breakdown

Step 1: Handover Required

Sender -> receiver: source eNB -> MME

Message(s): Handover Required

Purpose: Tell the MME that connected mobility should start.

State or context change: The MME becomes the control relay for target preparation.

Note: This is the signature message that separates S1 from X2 handover.

Step 2: Target preparation through the MME

Sender -> receiver: MME -> target eNB

Message(s): Handover Request and acknowledgement

Purpose: Prepare the target with UE context through S1-MME.

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

Note: Indirect forwarding decisions may also become visible here.

Step 3: UE move

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

Message(s): RRC Connection Reconfiguration and target access

Purpose: Move the UE to the prepared target side.

State or context change: The target eNB becomes the radio-serving side.

Note: Execution failure here becomes a recovery problem.

Step 4: Arrival confirmation

Sender -> receiver: target eNB -> MME

Message(s): Handover Notify

Purpose: Confirm that the UE reached the target side.

State or context change: The MME updates context and triggers source cleanup.

Note: The MME already knows the target because it relayed the preparation leg.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Handover Required	S1AP	source eNB -> MME	Starts the S1-based mobility branch.	Inspect target cause and forwarding context.
Handover Request	S1AP	MME -> target eNB	Prepares the target through the MME.	Check target admission and bearer context.
RRC Connection Reconfiguration	RRC	source eNB -> UE	Carries the handover command to the UE.	Inspect target details and execution timing.
Handover Notify	S1AP	target eNB -> MME	Confirms the UE reached the target side.	Check whether target arrival was confirmed before source release.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Forwarding mode	Direct or indirect forwarding decision during S1 mobility.	Handover Required and target prep	Shows how data continuity will be handled.	No direct forwarding path available.
Target eNB ID	The target cell and eNB selected by the MME-relayed path.	Handover Request and command	Needed to correlate the intended target.	Wrong target correlation.
Handover cause	Why the source eNB started the move.	Handover Required	Useful when the move looks unexpected.	Cause mismatch with measurement history.
Arrival confirmation	Whether the target side confirmed UE arrival.	Handover Notify	Confirms execution really completed.	Target access seen but notify missing.
Source cleanup timing	When the old context is released.	Release handling	Shows whether cleanup followed a valid target arrival.	Premature release or stale source state.

Successful Completion

Success means the MME-relayed preparation finishes, the UE reaches the target side, and the source context is released after target arrival is confirmed.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
MME relay branch stalls	The handover never gets past source-to-MME preparation.	Handover Required and target-preparation response.	Handover Required, Handover Request	S1-MME	Check whether the target accepted the context.
Forwarding path causes continuity loss	The target move happened but forwarding or bearer continuity was incomplete.	Forwarding mode and target arrival handling.	Handover Notify and related continuity state	S1-U	Inspect forwarding assumptions, not only radio execution.

What to Check in Logs and Traces

Check whether the move started with Handover Required.
Confirm that the target preparation was relayed through the MME.
Verify that Handover Notify appeared after target access.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

S1 handover brings the MME into the flow from the start. That is the main structural difference from X2 handover.

FAQ

What is S1 handover?

It is intra-LTE handover where the MME relays the target preparation instead of direct X2 coordination.

When is S1 handover used?

It is used when the mobility path cannot rely on direct X2 preparation between source and target.