LTE CSFB to 2G Call Flow

call-flow LTE | CSFB | GSM Interworking

LTE CSFB to 2G is the fallback branch where LTE sends the UE toward GSM for legacy voice handling.

This page focuses on the LTE-to-GSM target branch and the moment where the UE leaves LTE for 2G continuation.

Introduction

Once fallback selects GSM as the target voice system, the LTE side must still choose a usable 2G target and command the UE toward it. The analysis here is mainly about the target details, the redirection timing, and whether the voice attempt really reached the 2G side.

The main nodes are the UE, eNB, MME, and the GSM target side.

What Is CSFB to 2G in Simple Terms?

What starts the procedure: The generic CSFB branch selects GSM as the target voice system.
What the UE and network want to achieve: Move the UE from LTE to a 2G target for legacy voice continuation.
What success looks like: The UE leaves LTE, reaches GSM, and the voice attempt continues.
What failure means: The GSM target is wrong, weak, too late, or the voice context is lost before continuation.

Why this procedure matters

GSM fallback often looks simple on paper, but target quality and timing still decide whether the user experiences a clean voice setup or a failed call attempt.

Quick Fact Sheet

Procedure name	LTE CSFB to 2G
Domain	LTE to GSM fallback
Main trigger	CSFB selects GSM as the target voice system
Start state	Generic LTE fallback is already active
End state	UE moves from LTE to GSM for voice continuation
Main nodes	UE, eNB, MME, GSM target side
Main protocols	RRC, NAS, SGs
Main success outcome	The UE reaches GSM and the voice attempt continues
Main failure outcome	The UE leaves LTE but does not reach stable GSM continuation
Most important messages	Extended Service Request, redirect to GSM, target continuation
Main specs	TS 23.272

LTE CSFB to 2G — Click the diagram to open the full-size in a new tab.

Preconditions

The generic CSFB branch has already started.
A usable GSM target exists for fallback.
The UE can move from LTE to the selected GSM target layer.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Leaves LTE and moves to the selected GSM target.
eNB	Commands the LTE-side move out of LTE toward GSM.
MME	Keeps the legacy coordination alive behind the fallback branch.
GSM target side	Takes over the voice attempt after LTE is left.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries the LTE-side move command toward GSM.
SGs	MME <-> MSC / VLR	Preserves the legacy coordination behind fallback.
2G target access	UE <-> GSM target side	Carries the target-system continuation after LTE exit.

End-to-End Call Flow

UE            eNB / MME             GSM target
|--CSFB trigger---->|                   |
|<--redirect to 2G--|                   |
|------leave LTE and move to GSM------->|

Major Phases

Phase	What happens
1. Target selection	The generic fallback branch selects GSM as the target side.
2. LTE exit command	The UE receives the command to move toward 2G.
3. GSM acquisition	The UE leaves LTE and reaches the GSM target.
4. Voice continuation	The voice attempt proceeds on the 2G side.

Step-by-Step Breakdown

Step 1: Select the 2G fallback branch

Sender -> receiver: MME / eNB

Message(s): Fallback target selection

Purpose: Choose GSM as the legacy target technology.

State or context change: The branch now has a concrete target system.

Note: This is the key pivot between generic CSFB and GSM-specific continuation.

Step 2: Send the GSM target command

Sender -> receiver: eNB -> UE

Message(s): Redirect to GSM or related interworking command

Purpose: Tell the UE where to go after leaving LTE.

State or context change: The UE exits LTE and starts the move toward the GSM target.

Note: The target details here decide whether the UE reaches a usable 2G side.

Step 3: Continue on GSM

Sender -> receiver: UE -> GSM target side

Message(s): 2G target continuation

Purpose: Start the voice attempt on the GSM side after LTE is left.

State or context change: The voice branch is now outside LTE.

Note: From this point, LTE mainly provides historical context for why the move happened.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Extended Service Request	NAS	UE -> MME	Useful early trigger behind the fallback branch before GSM is selected.	Check whether the service request is really a fallback trigger.
Redirect to 2G	RRC	eNB -> UE	Commands the UE to leave LTE toward GSM.	Check the target frequency or system details for the 2G move.
GSM continuation	Target RAT	UE -> GSM target side	Shows that the UE really reached the 2G branch.	Check whether the selected GSM target was reachable and expected.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
GSM target details	The 2G target information used by the LTE branch.	Redirect command	Explain where the UE was told to go after leaving LTE.	The UE moves to the wrong 2G side.
Fallback timing	The delay between LTE trigger and the GSM move.	Fallback branch	Useful when the call setup feels slow or late.	The redirect starts too late for a clean call attempt.
Legacy coordination state	The fallback context behind the GSM move.	Before target continuation	Shows whether the call attempt is still valid when the UE reaches 2G.	The UE reaches 2G but the voice attempt has already decayed.
Target acquisition result	Whether the UE reached the intended 2G side.	GSM continuation	Separates target selection problems from later GSM service problems.	The LTE side looks correct, but the UE never stabilizes on the GSM target.
Service direction	Whether the call is MO or MT behind the fallback path.	Early branch	Helps explain whether paging or direct service request was expected earlier.	The trace is interpreted with the wrong fallback direction.

Successful Completion

Success means the LTE fallback branch sends the UE to the correct GSM target and the voice attempt continues there.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
GSM redirect exists but the target side never stabilizes	The target GSM side was wrong, weak, or unavailable.	Redirect details and the result of the 2G acquisition.	Redirect to 2G	LTE Uu, 2G target access	Check the exact target-system details before treating it as a generic CSFB failure.
Fallback branch is slow before GSM acquisition	The trigger, legacy coordination, or target command is delayed.	Extended Service Request, SGs handling, and redirect timing.	Extended Service Request, redirect command	NAS, SGs, LTE Uu	Compare the delay against user-visible call setup timing.

What to Check in Logs and Traces

Use the generic fallback branch first, then pivot to the exact GSM target details.
Check whether the UE received a usable 2G target command.
Separate LTE preparation from GSM target-side failures after the UE leaves LTE.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

This page explains the GSM-specific target branch of fallback. Start with the generic CSFB page when the target RAT is not yet confirmed.

FAQ

What is LTE CSFB to 2G?

It is the fallback branch where the voice attempt leaves LTE and continues on GSM.

Why separate CSFB to 2G from generic CSFB?

Because the real problem is often in the GSM target selection and acquisition details, not in the early fallback trigger alone.