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LTE Handover Failure and Recovery Call Flow

call-flowLTE | Mobility | RRC | Recovery

LTE Handover Failure and Recovery describes what happens when a connected mobility attempt does not complete cleanly and the UE must recover service through re-establishment, source-side fallback, idle return, or a later NAS-side continuation. It is the recovery view of interrupted connected mobility, not the normal handover success path.

This page helps separate handover execution failure from later service-restoration behavior.

Introduction

A failed handover can break the connected path even when the handover preparation itself looked correct. The UE may then try re-establishment, return to the source side, or later continue through a recovery branch once radio access is stable again.

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

What Is LTE Handover Failure and Recovery in Simple Terms?

What starts the procedure: A connected-mode handover is interrupted or does not complete successfully.
What the UE and network want to achieve: Restore usable service and a stable signaling path after the failed mobility attempt.
What success looks like: The UE regains a stable path through re-establishment or another valid recovery branch and later service continues.
What failure means: Recovery also fails, service is interrupted longer, or the UE must restart from a broader entry path.

Why this procedure matters

This page explains why a mobility failure can later appear as a radio recovery problem or a NAS service-recovery problem instead of as a clean handover reject.

Quick Fact Sheet

Procedure name	LTE Handover Failure and Recovery
Domain	Failed mobility continuation and recovery
Main trigger	Connected handover does not complete cleanly
Start state	UE was in a connected handover branch
End state	Service is restored on a stable path or the UE falls back to broader recovery
Main nodes	UE, source eNB, target eNB, MME
Main protocols	RRC, S1AP, NAS
Main success outcome	UE regains a stable serving path after failed mobility
Main failure outcome	Repeated interruption or full service loss
Most important messages	RRC Connection Reestablishment Request, Mobility From E-UTRA Command
Main specs	TS 36.331, TS 36.300, TS 23.401

LTE Handover Failure and Recovery call flow — Click the diagram to open the full-size in a new tab.

Preconditions

The UE was already in a connected mobility branch.
A handover command or execution branch had already started.
The failed mobility still leaves at least one possible recovery path.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Attempts mobility execution and later starts the recovery branch if handover fails.
Source eNB	May remain the fallback point if target-side continuation fails.
Target eNB	May be the intended destination but not the final stable serving cell if mobility fails.
MME	Becomes relevant again if later service restoration continues through NAS.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> cells	Carries handover execution and any later radio recovery branch.
S1-MME / X2	Between eNBs and MME	Provide the mobility context behind the failed branch.
NAS	UE <-> MME	May appear later if service restoration continues after radio recovery.

End-to-End Call Flow

UE         Source eNB       Target eNB        MME
|-- handover execution ---------------------->|
|   handover does not complete               |
|-- Reestablishment / fallback branch ------>|
|<-- recovery result ------------------------|
|---------------------- later NAS recovery ------------------------>|

Major Phases

Phase	What happens
1. Mobility execution	The UE starts the connected handover branch.
2. Handover failure	The mobility path breaks before stable target-side continuation.
3. Radio recovery	The UE tries re-establishment or another fallback branch.
4. Service recovery	Later NAS or access continuation restores service on a stable path.

Step-by-Step Breakdown

Step 1: Handover execution starts

Sender -> receiver: eNB -> UE

Message(s): Often Mobility From E-UTRA Command or another handover execution branch

Purpose: Move the UE from the source serving path to the intended target path.

State or context change: The UE enters a connected mobility branch.

Note: A prepared handover can still fail during execution.

Step 2: Handover fails

Sender -> receiver: UE / radio behavior

Message(s): No stable target-side completion

Purpose: Recognize that the intended mobility path did not complete.

State or context change: The UE loses stable connected continuity.

Note: This is where mobility trouble turns into recovery trouble.

Step 3: Recovery branch starts

Sender -> receiver: UE -> eNB

Message(s): Often RRC Connection Reestablishment Request

Purpose: Recover a stable radio path after the failed mobility attempt.

State or context change: The UE either regains a radio control path or falls back again.

Note: The final serving cell after recovery may not be the original handover target.

Step 4: Later service continuation

Sender -> receiver: UE -> MME

Message(s): Later NAS recovery or fresh access branch

Purpose: Restore end-to-end service after the radio path stabilizes.

State or context change: Mobility failure is absorbed into a working service path again.

Note: This is where the trace often stops looking like a handover issue and starts looking like a recovery issue.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Mobility From E-UTRA Command	RRC	eNB -> UE	Starts a mobility execution branch that may later fail.	Which mobility scenario was being attempted.
RRC Connection Reestablishment Request	RRC	UE -> eNB	Common recovery start after failed handover.	Cause and context relation after the interrupted mobility branch.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Mobility execution context	The target-side handover branch that was being attempted.	Handover command and later failure point	Shows which mobility path actually broke.	Wrong assumption about source or target cell.
Recovery cause	The reason used for the later recovery branch.	Reestablishment Request	Links the recovery to the failed mobility branch.	Recovery path interpreted as generic radio failure only.
Final serving side	The stable cell after recovery finishes.	Later recovered signaling	Shows where service actually resumed.	Target assumed successful even though recovery returned elsewhere.

Successful Completion

Success means the UE regains a stable serving path after the failed handover and later service resumes on that recovered path.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Handover looks prepared, but service disappears during execution	The mobility branch failed during execution rather than during preparation.	Handover command and the first failed recovery action.	Mobility From E-UTRA Command, Reestablishment Request	LTE Uu, X2 or S1 context	Check whether recovery returned to source-side service or another fallback path.
Recovery succeeds, but target-side continuity never appears	The final stable path is different from the intended handover target.	Recovered serving cell and later service continuation.	Recovery messages and later NAS service branch	LTE Uu, NAS	Trace the final stable serving path instead of assuming target success.

What to Check in Logs and Traces

Separate handover preparation from handover execution failure.
Check which recovery branch followed the interrupted mobility attempt.
Confirm the final stable serving side before deciding whether mobility really succeeded.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

Handover failure recovery is not always target-side success with delay. The final stable path after recovery may be different from the originally intended target path.

FAQ

What is LTE Handover Failure and Recovery?

It is the recovery workflow that follows when a connected mobility attempt does not complete cleanly.

Does failed handover always lead to re-establishment?

No. Re-establishment is common, but the UE may also fall back to idle return or another re-entry path.