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LTE UL Handover Preparation Transfer Call Flow

call-flow LTE | Interworking | Preparation Transfer

UL Handover Preparation Transfer is the uplink preparation information branch used when LTE needs to send target-relevant preparation content before the UE leaves E-UTRA.

This page focuses on the transfer of preparation content, not the final interworking execution itself.

Introduction

Some interworking paths depend on an uplink preparation transfer that carries target-relevant preparation information before the actual move begins. This stage helps the target side understand the source-side decision and be ready for continuation.

The main nodes are the source LTE side and the target-side preparation context.

What Is UL Handover Preparation Transfer in Simple Terms?

What starts the procedure: The target preparation branch needs source-side uplink transfer content before execution.
What the UE and network want to achieve: Deliver the source-side preparation information needed by the target branch.
What success looks like: The preparation transfer reaches the target side and is usable later.
What failure means: The target side starts later execution without the source-side preparation it expected.

Why this procedure matters

This step is easy to overlook because it is not the final move, but it can explain why a target branch later looks underprepared or inconsistent.

Quick Fact Sheet

Procedure name	LTE UL Handover Preparation Transfer
Domain	Interworking preparation content transfer
Main trigger	The target side needs source-side preparation content before execution
Start state	Preparation is already underway, but execution has not started
End state	The target side has the uplink preparation transfer content
Main nodes	source side, target-side preparation context
Main protocols	Interworking support signaling
Main success outcome	Target preparation has the source-side content it needs
Main failure outcome	The target side lacks the needed transfer context later
Most important messages	Preparation transfer and target-side receipt
Main specs	Interworking preparation context

LTE UL Handover Preparation Transfer — Click the diagram to open the full-size in a new tab.

Preconditions

A target preparation branch already exists.
The source side has relevant information to transfer before execution.
The target side can accept that preparation content.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
source LTE side	Creates and sends the preparation-transfer content.
Target-side preparation context	Receives and stores the transfer content for later execution.

Interfaces used

Interface	Path	Role
Preparation transfer path	source side <-> target side	Carries the source-side preparation content before execution.

End-to-End Call Flow

source side                  target side
|--UL preparation transfer------->|
|<--transfer receipt / result-----|

Major Phases

Phase	What happens
1. Create transfer content	The source side gathers the information needed by the target branch.
2. Send preparation transfer	The content is sent before the later move out of LTE.
3. Target-side handling	The target side receives and stores the transfer content.
4. Ready-for-use state	The preparation transfer is available for later execution.

Step-by-Step Breakdown

Step 1: Build the transfer content

Sender -> receiver: source LTE side

Message(s): UL Handover Preparation Transfer content

Purpose: Create the information that the target branch needs before execution.

State or context change: The preparation-transfer payload is now ready to be sent.

Note: This content is valuable because it explains what the target side was supposed to know before the UE arrived.

Step 2: Send the transfer

Sender -> receiver: source side -> target side

Message(s): UL Handover Preparation Transfer

Purpose: Deliver the preparation content to the target side.

State or context change: The target branch can now use that content in later execution.

Note: If this step is weak or missing, the later target behavior may look surprisingly incomplete.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
UL Handover Preparation Transfer	Interworking support	source side -> target side	Delivers source-side preparation content before execution.	Check whether the transfer reached the expected target branch and carried the right context.
Transfer result or receipt	Interworking support	target side -> source side	Confirms target-side handling of the transfer content.	Check whether the target side actually accepted the transferred content.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Transfer payload intent	The purpose of the source-side content sent to the target.	Preparation transfer	Explains why the target side later behaves the way it does.	The target branch is analyzed without knowing what was transferred.
Target branch identity	The exact target-side branch receiving the content.	Preparation transfer	Shows whether the transfer was routed to the correct target side.	The content is sent to the wrong target branch.
Transfer timing	The gap between the transfer and the later execution step.	Preparation transfer and later move	Useful when transferred context may be stale later.	The transfer happens too early and ages out.
Receipt quality	Whether the target side actually accepted the content.	Transfer result	Separates visible signaling from truly usable preparation.	The transfer exists in the trace but is not usable.
Execution dependency	How much the later move depends on this transfer.	Preparation context	Explains whether a transfer issue can really break the later move.	The transfer is assumed irrelevant when it was actually critical.

Successful Completion

Success means the target side receives and stores the preparation-transfer content needed for later execution.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Preparation transfer is present but later execution still looks weak	The content was incomplete, stale, or not actually used by the target side.	Transfer content, receipt, and later execution timing.	UL Handover Preparation Transfer	Preparation transfer path	Check whether the target side treated the transfer as usable context.
Later execution lacks expected target readiness	The preparation transfer was missing or never accepted.	The period before execution and the target-side receipt.	UL Handover Preparation Transfer	Preparation transfer path	Confirm whether the source side actually sent the content expected by the target branch.

What to Check in Logs and Traces

Check whether the target side ever received the expected transfer content.
Compare the transfer timing with the later execution timing.
Do not assume the transfer was irrelevant just because it is not the user-visible step.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

This page is a preparation-transfer reference. It explains how target-side preparation content moves before execution, not the visible end-user move itself.

FAQ

What is UL Handover Preparation Transfer?

It is the transfer of source-side preparation content to the target branch before execution.

Why does this matter if the UE has not left LTE yet?

Because the target side may rely on that content later when the UE finally leaves LTE.