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LTE PS Handover / SRVCC Preparation Flows Call Flow

call-flow LTE | PS Handover | SRVCC | Preparation

LTE PS handover and SRVCC preparation flows are the preparation branches that happen before packet-switched interworking execution or voice continuity away from LTE.

This page focuses on the preparation state that exists before the UE is finally told to leave LTE.

Introduction

Before a PS interworking move or SRVCC continuity branch can succeed, the target side often needs preparation. This page collects that preparation layer so the source-side decision, target-side readiness, and later execution timing can be read together.

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

What Is PS Handover / SRVCC Preparation Flows in Simple Terms?

What starts the procedure: A later PS interworking move or SRVCC branch needs target-side preparation first.
What the UE and network want to achieve: Build the target-side readiness needed before execution away from LTE.
What success looks like: Target-side preparation completes and the later execution branch has usable context.
What failure means: The later move out of LTE begins with weak, missing, or stale target preparation.

Why this procedure matters

Preparation is where many continuity failures are born. By the time the UE is visibly leaving LTE, the target side may already be underprepared and the visible failure is only the last symptom.

Quick Fact Sheet

Procedure name	LTE PS Handover / SRVCC Preparation Flows
Domain	Interworking and voice-continuity preparation
Main trigger	A later PS or SRVCC execution branch needs target preparation first
Start state	The UE is still on LTE and execution has not started
End state	The target side is prepared for later PS or SRVCC execution
Main nodes	source LTE side, MME, target-side preparation context
Main protocols	Preparation signaling, S1 support
Main success outcome	The target branch is ready before the UE leaves LTE
Main failure outcome	The later continuity branch starts without reliable preparation
Most important messages	Preparation request, preparation transfer, preparation result
Main specs	Preparation and continuity context

LTE PS Handover and SRVCC Preparation Flows — Click the diagram to open the full-size in a new tab.

Preconditions

A later PS handover or SRVCC execution branch is already expected.
The target side can be prepared before execution starts.
The source LTE side still has time to build the target branch before the move.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
source LTE side	Starts the preparation needed for the later interworking or continuity move.
MME	Supports the preparation and preserves coordination between source and target branches.
Target-side preparation context	Represents the non-LTE or legacy side being made ready for execution.

Interfaces used

Interface	Path	Role
S1-MME	source LTE side <-> MME	Carries the LTE-side support for target preparation.
Preparation signaling path	source side / MME <-> target side	Carries request, transfer, and result information before execution.

End-to-End Call Flow

source LTE side          MME             target preparation
|--prep request------->|--prep request---->|
|--prep transfer------>|--prep transfer--->|
|<--prep result--------|<--prep result-----|

Major Phases

Phase	What happens
1. Decide the need for preparation	The source side knows a later PS or SRVCC branch is coming.
2. Request target preparation	The target side is asked to build the needed context.
3. Transfer supporting content	The target side receives the source-side preparation content it needs.
4. Confirm readiness	The target branch returns the result and becomes ready for execution.

Step-by-Step Breakdown

Step 1: Open the preparation branch

Sender -> receiver: source LTE side -> target side

Message(s): Preparation request

Purpose: Ask the target side to build readiness before the later move starts.

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

Note: This is the staging point behind later PS handover or SRVCC execution.

Step 2: Transfer source-side preparation context

Sender -> receiver: source LTE side -> target side

Message(s): Preparation transfer

Purpose: Send the supporting source-side content the target branch needs.

State or context change: The target side now has the source information needed for later use.

Note: This explains why the target side later behaves as it does.

Step 3: Receive the preparation result

Sender -> receiver: target side -> source LTE side

Message(s): Preparation result

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

State or context change: The later PS or SRVCC branch now has a known preparation quality.

Note: A weak preparation result often explains a later continuity failure that looks sudden.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Preparation request	Preparation signaling	source side -> target side	Starts target-side readiness before execution.	Check the target branch identity and the reason the target is being prepared.
Preparation transfer	Preparation signaling	source side -> target side	Delivers the source-side support content needed by the target branch.	Check whether the supporting context really reached the target side.
Preparation result	Preparation signaling	target side -> source side	Confirms readiness before PS handover or SRVCC execution begins.	Check whether the result is actually strong enough for later execution.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Preparation target	The exact branch being prepared for later execution.	Preparation request	Shows whether the source side prepared the correct target branch.	The wrong branch is prepared.
Transferred support context	The source-side information sent to the target branch.	Preparation transfer	Explains why the target side later behaves in a specific way.	The target side is missing the source context it needed.
Preparation result quality	How ready the target branch really is.	Preparation result	Separates visible preparation signaling from usable preparation.	A visible result is mistaken for a strong result.
Gap before execution	The time between preparation and the later move.	Preparation and later execution timing	Useful when prepared context may have become stale before use.	Preparation succeeds, but the later move happens too late to use it well.
Execution type	Whether the later branch is PS handover or SRVCC continuity.	Overall preparation context	Explains what kind of target readiness the preparation was supposed to create.	The preparation is judged using the wrong execution model.

Successful Completion

Success means the target-side preparation is ready and usable before the later PS handover or SRVCC execution begins.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Preparation signaling exists but later continuity still fails	The preparation was weak, incomplete, or stale by the time execution started.	Preparation request, transfer, result, and execution timing together.	Preparation request, transfer, result	S1-MME, preparation signaling path	Compare preparation quality with the later continuity timing rather than judging by signaling presence alone.
Later execution looks underprepared	The source side never built or transferred the needed context cleanly.	The full preparation stage before the UE was told to leave LTE.	Preparation request, transfer	Preparation signaling path	Confirm whether the target branch really received all the expected preparation steps.

What to Check in Logs and Traces

Read preparation before execution when SRVCC or PS interworking fails suddenly.
Use request, transfer, and result as one preparation set rather than separate isolated messages.
Compare the strength of the preparation result with the timing of the later move.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

This page is for the preparation layer only. The user-visible execution branch comes later, after the target side is supposed to be ready.

FAQ

What are PS handover and SRVCC preparation flows?

They are the source-side staging steps that make the target branch ready before the UE leaves LTE.

Why separate preparation from execution?

Because many visible failures happen during execution, but the real cause is hidden in the earlier preparation stage.