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LTE 1xCSFB / CDMA2000 Fallback Procedure Call Flow

call-flow LTE | 1xCSFB | CDMA2000

LTE 1xCSFB / CDMA2000 fallback is the legacy voice branch where LTE sends the UE toward CDMA2000-based voice handling instead of staying on EPS voice service.

This page focuses on the LTE-side fallback path, target-system move, and the point where voice service leaves LTE for CDMA2000 continuation.

Introduction

The generic fallback logic is still similar to other CSFB branches, but the target system here is CDMA2000 and the interworking context is different from UMTS or GSM fallback. The LTE side prepares the move, sends the UE toward the CDMA2000 target, and stops being the main voice service layer.

The main nodes are the UE, eNB, MME, and the CDMA2000 target side.

What Is 1xCSFB / CDMA2000 Fallback Procedure in Simple Terms?

What starts the procedure: Legacy voice fallback selects CDMA2000 as the target system.
What the UE and network want to achieve: Move the UE from LTE toward CDMA2000 for voice continuation.
What success looks like: The UE leaves LTE and the voice attempt continues on the CDMA2000 side.
What failure means: The LTE preparation looks normal, but the target move or target continuation fails.

Why this procedure matters

This branch matters in networks that still depend on CDMA2000 voice continuity. The target-system details are different enough that generic CSFB analysis is often not sufficient.

Quick Fact Sheet

Procedure name	LTE 1xCSFB / CDMA2000 Fallback Procedure
Domain	LTE to CDMA2000 voice fallback
Main trigger	Fallback selects CDMA2000 as the target voice system
Start state	Generic LTE fallback branch is already active
End state	UE moves from LTE toward CDMA2000 voice service
Main nodes	UE, eNB, MME, CDMA2000 target side
Main protocols	RRC, NAS, interworking support
Main success outcome	The UE reaches the CDMA2000 side and voice continuation proceeds
Main failure outcome	The UE leaves LTE but the CDMA2000 continuity branch fails
Most important messages	Fallback trigger, target command, CDMA2000 continuation
Main specs	TS 23.272 and interworking context

LTE 1xCSFB and CDMA2000 Fallback Procedure — Click the diagram to open the full-size in a new tab.

Preconditions

The generic fallback branch has already started.
A usable CDMA2000 target exists for continuation.
The UE can move from LTE to the CDMA2000 target side.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Leaves LTE and moves toward the selected CDMA2000 target system.
eNB	Commands the LTE-side move away from LTE.
MME	Maintains the LTE-side fallback preparation before target continuation.
CDMA2000 target side	Takes over the voice attempt after LTE is left.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries the LTE-side command that starts the target move.
S1-MME	eNB <-> MME	Carries fallback coordination on the LTE side.
CDMA2000 target access	UE <-> target side	Carries the target-system continuation after LTE exit.

End-to-End Call Flow

UE            eNB / MME             CDMA2000 target
|--fallback trigger->|                     |
|<--move to target---|                     |
|------leave LTE and continue on target--->|

Major Phases

Phase	What happens
1. Target selection	The fallback branch selects CDMA2000 as the target system.
2. LTE exit command	The UE receives the target move command.
3. Target acquisition	The UE leaves LTE and reaches the CDMA2000 side.
4. Voice continuation	The voice attempt continues on the target side.

Step-by-Step Breakdown

Step 1: Identify the CDMA2000 target branch

Sender -> receiver: MME / eNB

Message(s): Fallback target selection

Purpose: Choose CDMA2000 as the continuation system.

State or context change: The fallback branch now has a specific target technology.

Note: This is the point where analysis becomes CDMA2000-specific rather than generic CSFB.

Step 2: Send the target move command

Sender -> receiver: eNB -> UE

Message(s): Interworking move command toward CDMA2000

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

State or context change: The UE exits LTE and starts target acquisition.

Note: The exact target information here decides whether the move can work at all.

Step 3: Continue on the target side

Sender -> receiver: UE -> CDMA2000 target side

Message(s): Target continuation

Purpose: Move the voice attempt onto the CDMA2000 system.

State or context change: The call is now outside LTE.

Note: The LTE trace explains the preparation, but the target side proves the continuity result.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Extended Service Request	NAS	UE -> MME	Useful early fallback trigger behind the LTE branch.	Check whether the fallback reason matches the expected voice continuation path.
Target move command	RRC / interworking support	eNB -> UE	Commands the UE away from LTE toward CDMA2000.	Check the target-system details and timing of the move.
Target continuation	Target RAT	UE -> CDMA2000 target side	Shows that the UE really reached the target side after leaving LTE.	Check whether the intended CDMA2000 target was reachable and valid.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
CDMA2000 target details	The target-system information used after LTE exit.	Target move command	Explains where the UE was sent after LTE.	The UE is sent to an unusable or wrong target.
Fallback timing	The gap between LTE trigger and target move.	Full branch	Useful for late fallback or user-visible delay.	The target move is too slow for smooth voice continuation.
Legacy continuity state	Whether the target voice context stayed valid until the move.	Before target continuation	Shows whether the voice attempt was still alive when the UE reached the target side.	The UE reaches the target side too late for continuation.
Target acquisition result	Whether the UE actually reached the intended target system.	Target continuation	Separates wrong target command from later target-service problems.	The LTE side looks normal, but the target side never stabilizes.
Service direction	Whether the branch started from MO or MT voice context.	Early branch	Helps explain the earlier trigger and expected message order.	The wrong service direction is assumed during analysis.

Successful Completion

Success means the LTE fallback branch sends the UE to the intended CDMA2000 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
Target move exists but the UE never reaches stable continuation	The CDMA2000 target side is wrong, unreachable, or not ready.	Target move details and the result after LTE exit.	Target move command	LTE Uu, target access	Check the exact target information before treating it as a generic LTE-side failure.
Fallback branch is slow before target acquisition	The LTE trigger or target preparation is delayed.	Fallback trigger, early coordination, and move timing.	Extended Service Request, target move command	NAS, LTE Uu	Compare the branch timing against the user-visible failure moment.

What to Check in Logs and Traces

Use the generic fallback branch first, then pivot to the exact CDMA2000 target details.
Check whether the UE received a usable target-system command.
Separate LTE-side preparation from target-side continuity failure after LTE exit.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

This page focuses on the CDMA2000-specific target branch. Start with generic CSFB analysis when the exact target system is still unclear.

FAQ

What is LTE 1xCSFB?

It is the fallback branch where LTE sends the UE toward CDMA2000 voice handling.

Why separate this from generic CSFB?

Because the target-system preparation and continuation details differ from UMTS and GSM fallback.