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LTE Radio Link Failure and Recovery Call Flow

call-flowLTE | RRC | Recovery | NAS

LTE Radio Link Failure and Recovery describes what happens when the UE loses the current connected radio path and then tries to return to service. It includes the detection of failure, the attempt to recover by RRC Re-establishment, and the fallback paths when the direct recovery branch does not succeed.

This page focuses on the full recovery workflow rather than one single message pair.

Introduction

Radio link failure does not always end in the same way. The UE may recover quickly through re-establishment, fall back to idle behavior, restart access, or later continue through a NAS-side recovery branch such as service request or fresh attach.

The main nodes are the UE and eNB, with later recovery often involving the MME.

What Is LTE Radio Link Failure and Recovery in Simple Terms?

What starts the procedure: The connected radio path breaks and normal signaling can no longer continue.
What the UE and network want to achieve: Restore usable service as quickly as possible through the right recovery branch.
What success looks like: The UE either completes re-establishment or reaches a stable fallback branch such as idle return and later service recovery.
What failure means: Recovery loops continue, context is lost, or later signaling still cannot resume.

Why this procedure matters

This is the page that explains why a connected LTE session suddenly breaks and how to separate radio failure from later NAS or core-side consequences.

Quick Fact Sheet

Procedure name	LTE Radio Link Failure and Recovery
Domain	Connected-path failure and recovery workflow
Main trigger	Loss of the active connected radio path
Start state	UE was in RRC_CONNECTED and loses stable radio continuity
End state	Service is restored through re-establishment or fallback to another valid branch
Main nodes	UE, eNB, MME
Main protocols	RRC, NAS
Main success outcome	UE regains stable signaling and reaches a valid next procedure
Main failure outcome	Repeated recovery attempts, lost context, or full service interruption
Most important messages	RRC Connection Reestablishment Request, RRC Connection Reestablishment, RRC Connection Reestablishment Reject, Service Request
Main specs	TS 36.331, TS 36.304, TS 23.401, TS 24.301

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

Preconditions

The UE had an active connected radio context before the failure.
The failure leaves enough information for the UE to try a recovery branch.
The later NAS or EPC context may still be valid even if the radio path is broken.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Detects the failure and decides whether to recover, fall back, or restart access.
eNB	Accepts or rejects re-establishment and provides the next radio-side result.
MME	Becomes visible again if recovery continues through NAS service restoration.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries the failure and radio recovery branch.
S1-MME	eNB <-> MME	Becomes relevant again if the UE later resumes NAS signaling.
NAS	UE <-> MME	Carries the later service recovery or re-entry branch.

End-to-End Call Flow

UE               eNB               MME
|   connected path active           |
|--- radio link failure ----------->|
|-- Reestablishment Request ------->|
|<- Reestablishment / Reject -------|
|   if success: continue signaling  |
|   if reject: idle return / access |
|---------------------- later Service Request ---------------------->|

Major Phases

Phase	What happens
1. Failure detection	The active connected path becomes unstable or unusable.
2. Radio recovery attempt	The UE tries re-establishment if the failure case allows it.
3. Fallback branch	If direct recovery fails, the UE returns to idle or fresh access behavior.
4. Service continuation	Later NAS service recovery or re-entry may restore end-to-end service.

Step-by-Step Breakdown

Step 1: Radio link failure is detected

Sender -> receiver: UE internal processing

Message(s): No successful connected continuation

Purpose: Recognize that normal connected signaling cannot continue.

State or context change: The active radio path is considered broken.

Note: This is the first branch point between quick recovery and full fallback.

Step 2: Re-establishment attempt

Sender -> receiver: UE -> eNB

Message(s): RRC Connection Reestablishment Request

Purpose: Ask the network to restore the earlier radio context.

State or context change: The eNB decides whether recovery is possible.

Note: If this step fails, the next useful branch is usually idle return or fresh access.

Step 3: Recovery result or fallback

Sender -> receiver: eNB -> UE

Message(s): Reestablishment or Reestablishment Reject

Purpose: Decide whether the connected branch is restored or abandoned.

State or context change: The UE either resumes connected signaling or falls back to another path.

Note: Reject is not the end of the full recovery story. The next branch still matters.

Step 4: Later service recovery

Sender -> receiver: UE -> eNB -> MME

Message(s): Often Service Request after a new access attempt

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

State or context change: The UE regains service continuity if stored EPS context is still usable.

Note: This is where radio failure starts to look like a NAS issue in layered traces.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
RRC Connection Reestablishment Request	RRC	UE -> eNB	Starts the direct radio recovery attempt.	Failure cause and earlier context relation.
RRC Connection Reestablishment	RRC	eNB -> UE	Restores the radio branch if recovery succeeds.	Whether the restore branch was really accepted.
RRC Connection Reestablishment Reject	RRC	eNB -> UE	Rejects direct radio recovery.	When the UE must move into fallback or fresh access.
Service Request	NAS	UE -> MME	Common later NAS continuation once the UE regains a stable access path.	Whether stored context remained valid after the failure.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Failure cause	The reason the UE uses for re-establishment.	Reestablishment Request	Shows why the connected path broke.	Wrong scenario assumption.
Context continuity	The relation to the lost connected path and stored EPS state.	RRC recovery and later NAS recovery	Explains whether the UE can recover directly or must restart elsewhere.	Stale context after failure.
Later NAS identity and KSI	The stored EPS context used if service request follows.	Later Service Request	Shows whether end-to-end recovery can continue without fresh attach.	Invalid stored context.

Successful Completion

Success means the UE reaches a stable next state after failure, either through completed re-establishment or through a clean fallback branch that restores service again.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Re-establishment keeps failing	The earlier radio context is no longer recoverable.	Reestablishment Request and reject timing.	Reestablishment Request, Reestablishment Reject	LTE Uu	Move into idle fallback and later access analysis.
Radio path recovers, but service still does not return	The later NAS continuation fails or stored EPS context is no longer valid.	Follow the next NAS branch after the radio path stabilizes.	Service Request or fresh attach start	LTE Uu, S1-MME, NAS	Separate radio recovery success from NAS recovery failure.

What to Check in Logs and Traces

Start by deciding whether the radio failure ended in re-establishment, idle fallback, or fresh setup.
Check whether later NAS signaling reused old EPS context or restarted registration.
Do not stop at the first reject. Follow the next valid recovery branch.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

Radio recovery and service recovery are not always the same branch. The radio path may recover first, while NAS service restoration still fails later.

FAQ

What is LTE Radio Link Failure and Recovery?

It is the broader recovery workflow that starts when the connected radio path breaks and ends when the UE reaches a stable next branch.

Does failure always mean fresh attach?

No. The UE may first try re-establishment or later restore service with stored EPS context.