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LTE CSFB Procedure Call Flow

call-flow LTE | CSFB | SGs | Legacy Interworking

LTE CSFB is the procedure that moves voice-related service from LTE toward a legacy circuit-switched domain when VoLTE is not the active service path.

This page follows the LTE-side trigger, SGs-side coordination, and the move out of LTE toward legacy voice handling.

Introduction

CSFB can start from a mobile-originated call attempt or a mobile-terminated paging path. The LTE side restores the UE if needed, coordinates with the legacy side through SGs-related handling, and then moves the UE out of LTE toward the target legacy system.

The main nodes are the UE, eNB, MME, and the legacy CS side represented by the MSC / VLR.

What Is CSFB Procedure in Simple Terms?

What starts the procedure: A voice service request needs legacy CS handling instead of staying fully on LTE.
What the UE and network want to achieve: Move the UE from LTE packet service toward a legacy voice-capable system without losing the voice attempt.
What success looks like: The UE leaves LTE and the voice attempt continues on the target legacy side.
What failure means: The voice attempt stops during LTE-side preparation, SGs handling, or the move to the target system.

Why this procedure matters

CSFB still matters in mixed deployments where VoLTE is unavailable or intentionally bypassed. A large number of voice failures start in the LTE preparation part rather than in the legacy side alone.

Quick Fact Sheet

Procedure name	LTE CSFB Procedure
Domain	Legacy voice fallback from LTE
Main trigger	Voice service needs CS-domain continuation instead of VoLTE
Start state	UE is attached to LTE and voice service must leave EPS-only handling
End state	UE moves toward legacy CS service handling
Main nodes	UE, eNB, MME, MSC / VLR
Main protocols	NAS, S1AP, SGs, RRC
Main success outcome	Fallback preparation completes and the UE reaches the target voice branch
Main failure outcome	Voice attempt fails before or during the move out of LTE
Most important messages	Extended Service Request, SGsAP messages, redirection or handover command
Main specs	TS 23.272, TS 24.301

LTE CSFB Procedure — Click the diagram to open the full-size in a new tab.

Preconditions

The UE is attached to LTE and has valid EPS mobility context.
CS fallback support and SGs coordination are provisioned in the network.
The legacy target system is available to receive the voice attempt.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Starts or receives the voice-related service trigger and executes the move out of LTE.
eNB	Carries the LTE access signaling and commands the UE toward the target legacy system.
MME	Coordinates the LTE and legacy voice path through SGs-related handling.
MSC / VLR	Represents the legacy CS side that will continue the voice attempt.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries the LTE access and redirection or handover command.
S1-MME	eNB <-> MME	Carries LTE control-plane support during fallback preparation.
SGs	MME <-> MSC / VLR	Carries the legacy coordination behind CS fallback.

End-to-End Call Flow

UE            eNB            MME            MSC / VLR
|--voice trigger---->|              |                 |
|                    |--S1 / NAS--->|--SGs handling-->|
|                    |<--redirect----|<--SGs result----|
|<--leave LTE--------|              |                 |

Major Phases

Phase	What happens
1. Voice trigger	The UE starts or receives a voice-related event that must use legacy service.
2. SGs coordination	The MME exchanges the fallback context with the legacy CS side.
3. LTE exit branch	The eNB or MME triggers the move out of LTE toward the target system.
4. Legacy continuation	The voice attempt proceeds on the target CS side if fallback succeeds.

Step-by-Step Breakdown

Step 1: Start the fallback trigger

Sender -> receiver: UE -> eNB / MME

Message(s): Extended Service Request or paging-driven continuation

Purpose: Start the LTE-side branch that indicates legacy CS handling is needed.

State or context change: The MME now knows the voice attempt should leave pure EPS service.

Note: This is the main LTE pivot for CSFB call correlation.

Step 2: Coordinate with the legacy side

Sender -> receiver: MME <-> MSC / VLR

Message(s): SGsAP coordination

Purpose: Prepare the legacy CS side for the fallback voice attempt.

State or context change: The target voice branch is now known before the UE leaves LTE.

Note: If this part is weak, the UE may leave LTE without a usable target continuation.

Step 3: Command the UE out of LTE

Sender -> receiver: eNB / MME -> UE

Message(s): Redirection or interworking handover command

Purpose: Tell the UE to leave LTE toward the target system.

State or context change: The UE exits LTE and starts target-side voice continuation.

Note: The exact branch can be redirection or a prepared interworking move depending on network design.

Step 4: Continue on the target side

Sender -> receiver: UE -> legacy target system

Message(s): Legacy access continuation

Purpose: Continue the voice attempt on the target CS side.

State or context change: The call no longer depends on LTE for voice continuity.

Note: The LTE page stops at the boundary where the legacy system takes over.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Extended Service Request	NAS	UE -> MME	Starts the CS fallback request from the LTE side.	Check whether the service type matches the expected CS fallback scenario.
Paging	S1AP	MME -> eNB	Useful for mobile-terminated fallback when the UE must first be made reachable.	Check whether LTE reachability was restored before the fallback branch continued.
SGsAP coordination	SGs	MME <-> MSC / VLR	Prepares the legacy side for the voice attempt.	Check whether the legacy side accepted and preserved the fallback context.
Redirection or target command	RRC / NAS	eNB / MME -> UE	Moves the UE out of LTE toward the legacy target system.	Check the target system details and timing of the move out of LTE.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Service type	The fallback-related reason carried in the LTE service request.	Extended Service Request	Shows that the request is really for CS fallback and not ordinary data service restoration.	The call is correlated as ordinary service request instead of fallback.
SGs association	The legacy coordination relationship between MME and MSC / VLR.	SGs handling	Explains whether the legacy side can actually continue the voice attempt.	The LTE side is ready, but the legacy side has no usable context.
Target RAT	The legacy system selected for fallback.	Redirection or target command	Confirms whether the UE was sent to the expected target technology.	The UE leaves LTE toward the wrong target side.
Fallback timing	The interval between LTE trigger and target-system move.	Full fallback branch	Important for late fallback and user-visible call setup delay.	The trigger is early enough, but the move out of LTE is still too slow.
Mobile-originated vs mobile-terminated context	Whether the voice attempt started from the UE or from network reachability restoration.	Early fallback branch	Helps explain the expected first trigger and message order.	The trace is interpreted using the wrong fallback direction.

Successful Completion

Success means the LTE-side fallback branch completes and the voice attempt continues on the target legacy system.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Fallback trigger appears, but the UE never leaves LTE	The SGs side or the target selection step did not complete correctly.	Extended Service Request, SGs handling, and the absence of the LTE exit command.	Extended Service Request, SGsAP coordination	NAS, SGs, LTE Uu	Check whether the legacy side was actually prepared before expecting the move.
UE leaves LTE but voice attempt still fails	The target legacy continuation failed after LTE-side preparation looked normal.	The target continuation branch after redirection or handover.	Target command	Legacy target side	Continue analysis on the target legacy system.
Mobile-terminated fallback looks like paging failure only	The LTE reachability branch is being read without the later CS fallback context.	Paging, service restoration, and SGs coordination together.	Paging, Extended Service Request	S1-MME, SGs	Correlate the paging branch with the later fallback branch rather than reading them separately.

What to Check in Logs and Traces

Decide first whether the trace is mobile-originated or mobile-terminated fallback.
Use Extended Service Request and SGs handling as the main LTE-side fallback checkpoints.
Do not stop analysis at the LTE exit command; verify that the target legacy side really continued the voice attempt.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

CSFB is the LTE-side bridge into legacy voice service. The exact target continuation changes by RAT, but the LTE preparation branch still determines whether the voice attempt gets that far.

FAQ

What is the main LTE message behind CSFB?

In many cases the main LTE-side trigger is Extended Service Request.

Is CSFB the same as ordinary handover?

No. It is a voice-service fallback procedure with SGs and legacy CS coordination behind it.