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LTE Handover From E-UTRA Preparation Request Call Flow

call-flow LTE | Interworking | Preparation Signaling

Handover From E-UTRA Preparation Request is the preparation signaling used when LTE needs to build the target-side context before the UE leaves E-UTRA for another system.

This page focuses on the preparation stage rather than the full target-system execution branch.

Introduction

In legacy interworking and some inter-RAT scenarios, the LTE side does not send the UE away immediately. It first prepares the target system so the move has a better chance of succeeding when the actual handover or fallback branch starts.

The main nodes are the source eNB, MME, and the target-side preparation context.

What Is Handover From E-UTRA Preparation Request in Simple Terms?

What starts the procedure: The LTE side decides a target system must be prepared before the UE leaves E-UTRA.
What the UE and network want to achieve: Build target-side preparation before actual interworking execution begins.
What success looks like: The target-side preparation is accepted and a usable target context exists.
What failure means: The UE may still be told to leave LTE later, but the target branch is not properly prepared.

Why this procedure matters

Preparation failures are often hidden because the visible user problem appears only later, when the UE finally leaves LTE and the target side is not ready.

Quick Fact Sheet

Procedure name	LTE Handover From E-UTRA Preparation Request
Domain	Interworking preparation
Main trigger	The target system needs preparation before the UE leaves LTE
Start state	The UE is still on LTE and the move has not yet been executed
End state	Target-side preparation exists for later interworking execution
Main nodes	source eNB, MME, target-side preparation context
Main protocols	S1AP / interworking support
Main success outcome	The target branch is prepared before execution
Main failure outcome	The later move out of LTE lacks a usable prepared target
Most important messages	Preparation request and preparation result
Main specs	Interworking procedure context

LTE Handover From E-UTRA Preparation Request — Click the diagram to open the full-size in a new tab.

Preconditions

The LTE side has already identified the need to leave E-UTRA.
A target system exists that can be prepared before execution.
The UE is still in LTE when the preparation branch starts.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
source eNB	Starts the target-side preparation before the UE leaves LTE.
MME	Supports the preparation path and preserves coordination toward the target side.
Target-side preparation context	Represents the non-LTE system being prepared for later execution.

Interfaces used

Interface	Path	Role
S1-MME	source eNB <-> MME	Carries the LTE-side preparation support.
Interworking preparation path	MME / source side <-> target side	Carries the target preparation request and result.

End-to-End Call Flow

source eNB          MME             target side
|--prep request----->|--prep request---->|
|<--prep result------|<--prep result-----|

Major Phases

Phase	What happens
1. Need for target preparation	The source side decides the target must be prepared first.
2. Preparation request	The request is sent toward the target side.
3. Preparation result	The target side returns acceptance or failure.
4. Ready-for-execution state	The later move out of LTE can use the prepared context.

Step-by-Step Breakdown

Step 1: Start the preparation branch

Sender -> receiver: source eNB -> MME / target side

Message(s): Handover From E-UTRA Preparation Request

Purpose: Ask the target side to create the context needed before the UE leaves LTE.

State or context change: Target-side preparation begins while the UE is still on LTE.

Note: This is the highest-value message for understanding whether the later interworking move was properly staged.

Step 2: Receive the preparation result

Sender -> receiver: target side -> source side

Message(s): Preparation result

Purpose: Confirm whether the target side is ready for later execution.

State or context change: The target branch is now either usable or already known to be weak.

Note: A bad preparation result often explains a later visible failure that otherwise looks sudden.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Handover From E-UTRA Preparation Request	Interworking support	source side -> target side	Starts target-side preparation before the UE leaves LTE.	Check the target identity and whether the request actually matches the intended target system.
Preparation result	Interworking support	target side -> source side	Confirms whether the target branch is ready.	Check whether the target side accepted the request and returned usable context.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Target identity	The exact system being prepared.	Preparation request	Shows which target side the source branch expects to use later.	The wrong target is prepared.
Prepared context	The target-side information created for later execution.	Preparation result	Explains whether the source side really has a usable target branch.	The source assumes preparation succeeded when it did not.
Timing before execution	The gap between preparation and the later move.	Preparation branch and later execution	Useful when prepared context ages or becomes stale.	Preparation succeeds too early and becomes unusable later.
Source-side decision reason	Why the target branch was being prepared.	Preparation request context	Helps explain the broader interworking motive.	Preparation is read without understanding the mobility or fallback reason behind it.
Preparation result quality	Whether the target response is positive, partial, or weak.	Preparation result	Separates usable preparation from merely visible signaling.	The target response exists but is not actually good enough.

Successful Completion

Success means a usable target-side preparation context exists before the actual interworking execution begins.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Preparation request exists but later execution still fails	The target branch was prepared weakly or the preparation became stale before use.	Preparation request, result, and the later execution timing.	Preparation request, preparation result	S1-MME, target preparation path	Check whether the target context was still valid when execution began.
Execution starts without clear preparation evidence	The expected preparation step was missing or not captured in the trace.	The period before the LTE exit command.	Preparation request	S1-MME	Confirm whether the network used a less prepared or alternate execution path.

What to Check in Logs and Traces

Check preparation before checking execution when the target side looks unexpectedly weak.
Use the request and the result as a pair rather than reading only the source-side request.
Compare the preparation timing to the actual move out of LTE later.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

This page is a preparation-stage reference. It explains how the target is made ready before execution, not the final target-system move itself.