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LTE SRVCC Procedure Call Flow

call-flow LTE | VoLTE | SRVCC | Mobility

LTE SRVCC is the voice continuity procedure that moves an active VoLTE call from LTE and IMS handling toward legacy voice access when LTE radio conditions or policy require that transition.

This page focuses on the continuity path itself: the LTE measurement trigger, the move out of LTE, and the point where voice service continues on the target legacy side.

Introduction

SRVCC begins while a VoLTE call is already active. LTE mobility logic decides that the call should leave LTE, the UE receives the command to move out of LTE, and the voice session continues through legacy access.

The main nodes are the UE, source eNB, MME, and the legacy voice side reached through the continuity path.

What Is SRVCC Procedure in Simple Terms?

What starts the procedure: An active VoLTE call needs continuity away from LTE because of coverage or mobility conditions.
What the UE and network want to achieve: Preserve the ongoing voice session while leaving LTE for legacy voice access.
What success looks like: The UE leaves LTE and the voice call continues on the target legacy side.
What failure means: The LTE call leaves the radio layer but the voice session does not continue cleanly on the target side.

Why this procedure matters

SRVCC is one of the highest-value continuity procedures because the user already has an active voice session when the move happens. Late handover, wrong target preparation, or broken continuity handling become immediately visible.

Quick Fact Sheet

Procedure name	LTE SRVCC Procedure
Domain	VoLTE voice continuity to legacy access
Main trigger	Active VoLTE call must leave LTE while preserving continuity
Start state	VoLTE call is active on LTE
End state	Voice call continues on the target legacy access
Main nodes	UE, source eNB, MME, legacy target side
Main protocols	RRC, mobility support, IMS continuity context
Main success outcome	Voice continuity is preserved during the move out of LTE
Main failure outcome	The call drops or continuity breaks during the transition
Most important messages	Measurement Report, Mobility From E-UTRA Command
Main specs	TS 23.216, TS 36.331

LTE SRVCC Procedure — Click the diagram to open the full-size in a new tab.

Preconditions

A VoLTE call is already active on LTE.
SRVCC support and target legacy continuity are provisioned.
The LTE side has a valid mobility reason or policy trigger for leaving LTE.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Maintains the active voice session while executing the move out of LTE.
Source eNB	Detects the mobility need and starts the move toward the target legacy side.
MME	Supports the continuity path and related control-plane handling.
Legacy target side	Takes over the voice session after LTE is left.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> source eNB	Carries the measurement result and the LTE exit command.
S1-MME	source eNB <-> MME	Carries continuity support on the EPC side.
Legacy access path	UE <-> target side	Carries the continued voice session after LTE is left.

End-to-End Call Flow

UE              source eNB / MME           legacy side
|--Measurement Report------------->|
|<-Mobility From E-UTRA Command----|
|   leave LTE and continue call -->|

Major Phases

Phase	What happens
1. Mobility trigger	LTE detects that the active VoLTE call should leave LTE.
2. Continuity preparation	The network prepares the target legacy side for the voice move.
3. LTE exit command	The UE receives the command to leave LTE.
4. Voice continuation	The call continues on the target side if continuity succeeds.

Step-by-Step Breakdown

Step 1: Detect the continuity trigger

Sender -> receiver: UE -> source eNB

Message(s): Measurement Report

Purpose: Show that the active call is reaching the conditions where LTE continuity is no longer preferred.

State or context change: The source side now has the measurement input used for the continuity decision.

Note: The reporting point is often where late SRVCC decisions first become visible.

Step 2: Prepare the move out of LTE

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

Message(s): SRVCC continuity preparation

Purpose: Prepare the target side so the ongoing voice session can continue after LTE is left.

State or context change: The continuity branch exists on the target side before the UE is told to move.

Note: If target preparation is weak, the call may leave LTE but still drop.

Step 3: Command the UE to leave LTE

Sender -> receiver: source eNB -> UE

Message(s): Mobility From E-UTRA Command

Purpose: Tell the UE to exit LTE toward the target continuity system.

State or context change: The UE leaves LTE and starts continuity on the target side.

Note: This is the most visible LTE-side confirmation that SRVCC really started.

Step 4: Continue the voice call

Sender -> receiver: UE -> legacy target side

Message(s): Target-side continuation

Purpose: Preserve the call on the target system.

State or context change: The active voice session is no longer on LTE but should remain active to the user.

Note: The LTE page stops at the transition boundary; target-specific legacy signaling continues afterward.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Measurement Report	RRC	UE -> source eNB	Shows why the source side triggered continuity away from LTE.	Check the reported radio condition and whether the trigger was early, late, or expected.
Mobility From E-UTRA Command	RRC	source eNB -> UE	Commands the UE to leave LTE for the target continuity system.	Check target-system details and continuity timing.
BYE	SIP	IMS side	Useful when the IMS dialog release must be correlated around the continuity move.	Check whether IMS release happened at the expected point relative to continuity.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Trigger timing	The point where LTE decided to leave the current serving state.	Measurement Report and command timing	Explains whether continuity was started too late or too early.	Late trigger causes dropped continuity.
Target legacy system	The system chosen for continuity after LTE is left.	Mobility From E-UTRA Command	Needed to confirm the UE was sent to the right target side.	Wrong target preparation breaks voice continuity.
Active call state	The fact that the voice session is already active during the move.	Before and during SRVCC	Distinguishes SRVCC from a normal idle or non-call mobility branch.	The continuity move is mistaken for a normal mobility event.
IMS release timing	When the IMS side is released relative to continuity.	Around target continuation	Helps explain whether the IMS call leg ended at the right point.	IMS release is too early or too late for smooth continuity.
Target continuation result	Whether the voice session stayed active after leaving LTE.	Target-side continuation	This is the real user-visible outcome of SRVCC.	The UE leaves LTE but the call still drops.

Successful Completion

Success means the active VoLTE call leaves LTE and continues on the target legacy side without user-visible call loss.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
SRVCC starts too late	The measurement trigger or continuity preparation happened too late for clean preservation.	Measurement Report, command timing, and the point of radio degradation.	Measurement Report, Mobility From E-UTRA Command	LTE Uu, S1-MME	Check whether the trigger was delayed relative to deteriorating radio conditions.
UE leaves LTE but call still drops	Target-side preparation or voice continuity failed after the LTE exit command.	The target continuation branch after the LTE command.	Mobility From E-UTRA Command	LTE exit plus target legacy side	Move into target-side continuity analysis.
Trace looks like normal mobility instead of SRVCC	The active voice state was not considered during correlation.	The active call state before the move and the continuity context.	Measurement Report, Mobility From E-UTRA Command	LTE Uu	Re-classify the move as voice continuity, not generic mobility.

What to Check in Logs and Traces

Start with the active call state, then find the measurement trigger and LTE exit command.
Check whether the command timing was late relative to deteriorating radio conditions.
Confirm continuity on the target side rather than stopping analysis at the LTE exit command.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

SRVCC should be read as a voice continuity procedure first and a mobility procedure second. The presence of an already-active call changes how the timing and failure impact should be interpreted.

FAQ

What is the most important LTE-side SRVCC message?

Mobility From E-UTRA Command is the clearest LTE-side command showing that the UE is leaving LTE for continuity.

Is SRVCC the same as a normal inter-RAT handover?

No. It is a voice continuity procedure with an already-active call, so the continuity outcome matters more than generic mobility success alone.