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LTE Bearer Modification Procedure Call Flow

call-flowLTE | EPC | NAS | GTPv2-C

LTE Bearer Modification Procedure updates an existing EPS bearer without fully releasing and recreating it. It is used when the network needs to change the bearer profile, QoS relation, TFT content, or other session-side handling while keeping the bearer active.

The main NAS pair is Modify EPS Bearer Context Request and Modify EPS Bearer Context Accept.

Introduction

The bearer modification path appears after a bearer already exists and the network wants to adjust how that bearer is treated. The change may come from policy control, service adaptation, application behavior, or later EPC-side updates.

The main nodes are the UE, MME, SGW, and PGW. The eNB may also need matching radio-side updates if the change affects the access side.

What Is LTE Bearer Modification Procedure in Simple Terms?

What starts the procedure: The network decides an existing bearer needs updated handling.
What the UE and network want to achieve: Refresh the live bearer context without rebuilding the whole session.
What success looks like: The UE receives Modify EPS Bearer Context Request and returns Modify EPS Bearer Context Accept.
What failure means: The bearer stays on the old profile, enters an inconsistent state, or is released later.

Why this procedure matters

This procedure explains how a live bearer changes after initial setup, especially when QoS or TFT handling does not match the earlier bearer profile.

Quick Fact Sheet

Procedure name	LTE Bearer Modification Procedure
Domain	Live EPS bearer update
Main trigger	Policy, QoS, or filter change on an existing bearer
Start state	UE already has an active EPS bearer
End state	Existing bearer remains active with updated parameters
Main nodes	UE, MME, SGW, PGW
Main protocols	NAS, GTPv2-C
Main success outcome	Bearer stays active with the new context
Main failure outcome	Bearer update is not accepted or later service becomes inconsistent
Most important messages	Modify EPS Bearer Context Request, Modify EPS Bearer Context Accept
Main specs	TS 24.301, TS 23.401, TS 29.274

LTE Bearer Modification Procedure call flow across UE, MME, SGW, and PGW — Click the diagram to open the full-size in a new tab.

Preconditions

The UE already has an active default or dedicated bearer.
The EPC already has a live session context for the bearer being modified.
The network has a reason to refresh QoS, TFT, or related session attributes.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Applies the updated bearer parameters.
MME	Delivers the NAS bearer modification toward the UE.
SGW	Keeps the serving session context aligned.
PGW	Provides the PDN-side change source for the bearer profile.

Interfaces used

Interface	Path	Role
S11	MME <-> SGW	Updates bearer handling between control-plane nodes.
S5/S8	SGW <-> PGW	Propagates the PDN-side bearer change.
NAS	UE <-> MME	Carries the bearer modification toward the UE.

End-to-End Call Flow

UE               MME               SGW               PGW
|                 |                 |-- Bearer update -->|
|                 |                 |<-- Update result ---|
|<- Modify EPS Bearer Ctx Req ------|                    |
|-- Modify EPS Bearer Ctx Accept --->|                   |
|   bearer stays active with changes  |                  |

Major Phases

Phase	What happens
1. EPC update decision	The core decides the live bearer needs updated handling.
2. NAS bearer modification	The MME sends the new bearer context toward the UE.
3. UE confirmation	The UE confirms the updated context if the change is accepted.

Step-by-Step Breakdown

Step 1: Bearer update in the EPC

Sender -> receiver: PGW -> SGW -> MME

Message(s): GTPv2-C bearer update handling

Purpose: Refresh the live bearer profile before the UE-side update.

State or context change: The core now holds the new bearer parameters.

Note: This is where policy-triggered QoS changes usually become visible first.

Step 2: Modify bearer context

Sender -> receiver: MME -> UE

Message(s): Modify EPS Bearer Context Request

Purpose: Tell the UE which live bearer parameters have changed.

State or context change: The UE receives the updated EBI, TFT, QoS, or related context.

Note: EBI continuity matters because the bearer is being modified, not recreated.

Step 3: UE accept

Sender -> receiver: UE -> MME

Message(s): Modify EPS Bearer Context Accept

Purpose: Confirm that the UE accepted the live update.

State or context change: The bearer remains active on the new profile.

Note: Missing uplink confirmation often leaves traces that look only partially updated.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Modify EPS Bearer Context Request	NAS	MME -> UE	Updates the live bearer context.	EBI, TFT changes, QoS values, and transaction timing.
Modify EPS Bearer Context Accept	NAS	UE -> MME	Confirms that the update was accepted.	Return timing and correlation to the same bearer identity.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
EPS Bearer ID	The identity of the bearer being modified.	Bearer modification request and accept	Keeps the update tied to the correct live bearer.	Wrong bearer correlation.
TFT	Traffic filter rules applied to the bearer.	Modify EPS Bearer Context Request	Explains which traffic should use the updated bearer path.	Malformed or unexpected filter changes.
QoS / QCI / ARP	The refreshed service profile for the bearer.	Modify request and EPC update path	Shows why the modification happened.	Policy mismatch or unsupported values.
PTI	The ESM transaction identity for the procedure.	NAS bearer modification	Helps correlate the live transaction cleanly.	PTI mismatch or stale transaction context.

Successful Completion

Success is confirmed when the UE accepts the modify request and the bearer remains active with the updated parameters across NAS and EPC context views.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Modify request appears, but service behavior does not change	TFT or QoS update was not applied correctly.	Compare old and new bearer fields.	Modify EPS Bearer Context Request	NAS, S11, S5/S8	Check whether the new profile actually differs from the previous one.
Accept does not return	UE rejected or did not complete the update.	Look for missing uplink confirmation and later bearer cleanup.	Modify request, later deactivation or retry handling	NAS, LTE Uu	Separate UE-side rejection from transport loss.

What to Check in Logs and Traces

Confirm the bearer already existed before the modify branch started.
Check EBI, PTI, TFT, and QoS continuity between the old and new context.
Align the NAS modify request with the EPC-side bearer update trigger.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

Bearer modification keeps the bearer identity alive. It changes a live context instead of creating a new bearer from the beginning.