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LTE Handover to GERAN Call Flow

call-flow LTE | GERAN | Inter-RAT | RRC

LTE handover to GERAN is the outbound inter-RAT mobility path used when the UE leaves LTE and continues on GSM. It is a target-specific continuation of the general inter-RAT handover family.

This page focuses on the LTE-to-GERAN branch rather than the umbrella inter-RAT description.

Introduction

The source LTE side prepares the GSM branch, sends Mobility From E-UTRA Command, and the UE continues with GSM-side signaling after leaving LTE.

Use this page when the target technology is specifically GERAN.

What Is Handover to GERAN in Simple Terms?

  • What starts the procedure: The network selects GSM as the target system.
  • What the UE and network want to achieve: Leave LTE and continue service on GERAN.
  • What success looks like: The UE exits LTE and begins the expected GSM continuation.
  • What failure means: The GSM branch never starts or never stabilizes.

Why this procedure matters

This page gives the GSM-specific view of outbound inter-RAT mobility from LTE.

Quick Fact Sheet

Procedure name LTE Handover to GERAN
Domain Outbound inter-RAT mobility from LTE to GSM
Main trigger GERAN selected as the target RAT
Start state UE is connected on LTE
End state UE continues on GERAN
Main nodes UE, source eNB, GERAN
Main protocols LTE RRC and GERAN control signaling
Main success outcome Service leaves LTE and continues on GSM
Main failure outcome Inter-RAT transition to GERAN fails
Most important messages Mobility From E-UTRA Command and GSM target continuation
Main specs TS 36.331 and GERAN mobility specs
LTE Handover to GERAN call flow
Click the diagram to open the full-size in a new tab.

Handover Concept

This illustration shows the basic handover concept used in this procedure: the UE leaves the serving side after the mobility decision and continues on the target side once the target path is ready.

Handover Concept Concept illustration of a UE moving from the serving side to the target side. Source eNB GERAN UE moving to target Serving side Target side
Prepare GSM branch to Continue on GSM. The GSM control path becomes active if the move succeeds.
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Preconditions

  • The UE is connected on LTE.
  • GERAN is chosen as the target system.
  • The GSM target branch is prepared.

Nodes and Interfaces

Nodes involved

Node Role in this procedure
UE Leaves LTE and continues on GSM.
Source eNB Delivers the outbound mobility command.
GERAN Takes over the service branch after LTE exit.

Interfaces used

Interface Path Role
LTE Uu UE <-> source eNB Carries the LTE-side exit command.
GERAN target interface UE <-> GERAN Carries the post-LTE GSM continuation branch.

End-to-End Call Flow

UE             Source eNB             GERAN
|<--Mobility From EUTRA Command--|      |
|==== leave LTE =======================>|
|------------ GERAN continuation ------->|

Major Phases

Phase What happens
1. Prepare GERAN branch The LTE side chooses GSM and prepares the target move.
2. Deliver command The UE receives the LTE-side exit command.
3. Exit LTE The current LTE serving path stops.
4. Continue on GERAN The GSM target branch starts.

Step-by-Step Breakdown

Step 1: GERAN target preparation

Sender -> receiver: LTE side

Message(s): Inter-RAT preparation

Purpose: Prepare the GSM continuation branch.

State or context change: The UE is still on LTE.

Note: The target-specific path is already decided here.

Step 2: Mobility command

Sender -> receiver: source eNB -> UE

Message(s): Mobility From E-UTRA Command

Purpose: Tell the UE to leave LTE and continue on GERAN.

State or context change: The UE has the GSM target information.

Note: This is the LTE trace anchor for GSM continuation.

Step 3: LTE exit

Sender -> receiver: UE

Message(s): LTE exit execution

Purpose: Stop the LTE serving path.

State or context change: LTE connected service ends at this point.

Note: After this, the target trace becomes GSM-specific.

Step 4: GERAN continuation

Sender -> receiver: UE -> GERAN

Message(s): GERAN target access and completion

Purpose: Continue service on GSM.

State or context change: GERAN becomes active if the move succeeds.

Note: The next detailed analysis belongs on the GSM side.

Important Messages

Message Protocol Direction Purpose in this procedure What to inspect briefly
Mobility From E-UTRA Command RRC source eNB -> UE Carries the GERAN target move. Inspect the target-system payload and the LTE exit timing.

Important Parameters to Inspect

Parameter What it is Where it appears Why it matters Common issues
Target RAT GERAN as the destination system. Mobility command Confirms this is the GSM-specific branch. Wrong target assumption.
Exit timing The last LTE control point before GSM signaling. Air trace Useful for cross-system correlation. Timing mismatch.
First GERAN message The first target-side continuation message. GERAN trace Confirms the move reached GSM. No target continuation.
Service expectation What service is expected to continue on GSM. Mobility context Helps interpret the continuation branch. Wrong continuity expectation.
Recovery result What happened if GSM entry failed. Later trace Shows whether the UE recovered or lost service. Failure hidden as generic radio loss.

Successful Completion

Success means the UE leaves LTE and the GSM continuation branch begins as expected.

Common Failures and Troubleshooting

Symptom Likely cause Where to inspect Relevant message(s) Relevant interface(s) Likely next step
Command appears but GSM continuation is missing The LTE exit branch was visible but the GERAN target branch never stabilized. The LTE command and the first expected GSM message. Mobility From E-UTRA Command LTE Uu and GERAN target interface Check whether the target GERAN preparation matched the expected system.
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What to Check in Logs and Traces

  • Confirm that GERAN is the target named by the LTE command.
  • Use the command timing to align LTE and GSM traces.
  • Check whether the first GERAN message appears immediately after LTE exit.

Related Pages

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

Handover to GERAN is the GSM-specific branch. Treat the LTE command as the pivot point into GSM analysis.

FAQ

What is LTE handover to GERAN?

It is the inter-RAT path that moves connected service from LTE to GSM.