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LTE SIP Session Establishment over IMS Call Flow

call-flow LTE | IMS | SIP | VoLTE

LTE SIP session establishment over IMS is the generic SIP session setup procedure used after IMS registration when the UE starts or accepts an IMS session such as a VoLTE call.

This page focuses on the SIP-side session ladder built around INVITE, provisional responses, reliability handling, and final session confirmation.

Introduction

After IMS registration is active, the session usually begins with INVITE. IMS can return 100 Trying, 180 Ringing, or 183 Session Progress before the final 200 OK arrives.

The main nodes are the UE and the IMS signaling path. LTE radio and EPC support still matter because the SIP exchange depends on the LTE packet and bearer context remaining stable.

What Is SIP Session Establishment over IMS in Simple Terms?

What starts the procedure: An IMS session such as voice or another SIP service needs to be created after registration is active.
What the UE and network want to achieve: Create a complete SIP dialog with successful offer and answer handling.
What success looks like: INVITE reaches final success and the dialog is confirmed with ACK.
What failure means: The SIP session never reaches stable establishment or fails before media or service continuity is ready.

Why this procedure matters

This is the core SIP setup ladder behind VoLTE call setup and other IMS services. If it breaks, later bearer or media analysis may never become relevant.

Quick Fact Sheet

Procedure name	LTE SIP Session Establishment over IMS
Domain	IMS dialog setup over LTE
Main trigger	A new SIP dialog must be created after registration
Start state	UE is IMS-registered and LTE packet service is stable
End state	SIP session is established and acknowledged
Main nodes	UE, IMS signaling path
Main protocols	SIP, SDP
Main success outcome	INVITE transaction reaches 200 OK and ACK
Main failure outcome	Session setup stalls, rejects, or times out
Most important messages	INVITE, 183 Session Progress, PRACK, 200 OK, ACK
Main specs	TS 24.229

LTE SIP Session Establishment over IMS — Click the diagram to open the full-size in a new tab.

Preconditions

IMS registration is already active.
The LTE packet path toward IMS is stable.
Policy and bearer context can support the service being started.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Starts or receives the LTE voice or messaging service and exchanges SIP signaling with IMS.
eNB	Carries the LTE radio side used for IMS-capable packet service.
MME / EPC	Preserve LTE access, bearer, and paging continuity behind the IMS transaction.
P-CSCF / S-CSCF	Handle the SIP signaling path used for registration, call control, and service continuity.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries the radio access needed before and during IMS service use.
S1-MME	eNB <-> MME	Carries LTE control-plane continuation behind paging, service request, or bearer handling.
Gm	UE <-> P-CSCF	Carries SIP requests and responses between the UE and IMS.

End-to-End Call Flow

UE                 IMS
|--INVITE---------->|
|<-100 Trying-------|
|<-183 Session Prog-|
|--PRACK----------->|
|<-200 OK-----------|
|--ACK------------->|

Major Phases

Phase	What happens
1. Session request	The initiating side sends INVITE toward IMS.
2. Provisional handling	IMS returns early responses that describe call progress.
3. Reliability handling	PRACK may confirm reliable provisional signaling.
4. Final confirmation	200 OK and ACK complete the established dialog.

Step-by-Step Breakdown

Step 1: Start the session

Sender -> receiver: UE -> IMS

Message(s): INVITE

Purpose: Create the SIP dialog and carry the offered session details.

State or context change: IMS begins dialog setup and route evaluation.

Note: This is the first message to inspect when a call or IMS session never really starts.

Step 2: Follow provisional progress

Sender -> receiver: IMS -> UE

Message(s): 100 Trying, 180 Ringing, 183 Session Progress

Purpose: Show early progress before final acceptance.

State or context change: The dialog exists, but final session establishment is still pending.

Note: The combination of 180 and 183 often explains whether ringing, early media, or both appeared.

Step 3: Acknowledge reliable provisional signaling

Sender -> receiver: UE -> IMS

Message(s): PRACK

Purpose: Acknowledge reliable provisional responses when used.

State or context change: IMS can continue final establishment with the provisional stage confirmed.

Note: Missing PRACK can break the session before the final success response.

Step 4: Complete the dialog

Sender -> receiver: IMS -> UE and UE -> IMS

Message(s): 200 OK and ACK

Purpose: Confirm the dialog and mark the session as established.

State or context change: The SIP dialog is now active and later media or service-specific handling can continue.

Note: If 200 OK appears without ACK, the dialog is still not cleanly completed.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
INVITE	SIP	UE -> IMS	Starts the session dialog.	Check the target identity, SDP offer, and route path.
183 Session Progress	SIP	IMS -> UE	Shows early session progress and often early media context.	Check whether early media or service progress was expected at this point.
PRACK	SIP	UE -> IMS	Acknowledges reliable provisional signaling.	Check whether reliable provisional handling completes cleanly.
200 OK	SIP	IMS -> UE	Confirms successful session establishment.	Check the final response timing and whether the dialog reaches stable confirmation.
ACK	SIP	UE -> IMS	Finalizes the successful INVITE transaction.	Check whether ACK follows 200 OK immediately and cleanly.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Request-URI / called party	The target identity of the session.	INVITE	Explains which endpoint or service the dialog is trying to reach.	Wrong target causes routing or service mismatch.
SDP offer / answer	The media description exchanged during setup.	INVITE / 200 OK / provisional responses	Shows whether media negotiation matches the expected service.	Codec or media mismatch breaks the session after SIP success.
Supported / Require	SIP capability and feature indicators.	INVITE and responses	They explain whether optional behavior such as reliable provisional signaling is in use.	The UE or network expects a feature that the other side does not handle cleanly.
Route / Record-Route	The SIP path through IMS.	INVITE and responses	Useful when the dialog is being routed through unexpected entities.	Unexpected route changes make later in-dialog requests fail.
Reliable provisional context	Whether PRACK is expected.	183 / PRACK	Explains whether the session must complete an extra acknowledgement step.	The session stalls because PRACK handling is missing or late.

Successful Completion

Success means the SIP dialog reaches 200 OK and the dialog is confirmed with ACK.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
INVITE gets no useful progress	Routing, IMS reachability, or policy blocks the session before it advances.	INVITE and the first returned response.	INVITE, 100 Trying	Gm	Check whether the dialog is routed into the expected IMS path.
Session stops at provisional stage	Reliable provisional handling, early media negotiation, or downstream service behavior is broken.	183, PRACK, and the gap before final success.	183 Session Progress, PRACK	Gm	Check whether reliable provisional signaling was completed.
200 OK appears but the session still does not stabilize	ACK is missing or later bearer or media handling is broken.	200 OK, ACK, and the next service-specific branch.	200 OK, ACK	Gm and LTE bearer context	Separate SIP dialog success from later media or bearer failure.

What to Check in Logs and Traces

Start with INVITE and follow the dialog through provisional responses to the final ACK.
Check whether PRACK is expected and whether it completes.
Do not assume SIP setup success means bearer or media success; those are later checks.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

This page is the generic SIP session reference. The dedicated mobile-originated and mobile-terminated pages add the LTE access and call-direction context around the same SIP ladder.

FAQ

What confirms SIP session establishment?

The session is considered established when the final 200 OK is followed by ACK.

Why is PRACK important?

It confirms reliable provisional signaling such as 183 Session Progress when that behavior is used.