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LTE Dedicated EPS Bearer Establishment Call Flow

call-flowLTE | EPC | NAS | S1AP

LTE Dedicated EPS Bearer Establishment is the procedure that adds a dedicated bearer after the default bearer already exists. It is used when traffic needs a service profile different from the default bearer, such as separate QoS treatment.

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

Introduction

The dedicated EPS bearer is created on top of an existing PDN session and default bearer. It gives the network a separate bearer identity and service profile for traffic that needs different handling.

The main nodes are the UE, eNB, MME, SGW, and PGW.

What Is LTE Dedicated EPS Bearer Establishment in Simple Terms?

What starts the procedure: The network decides an existing session needs an additional bearer with its own service handling.
What the UE and network want to achieve: Add a dedicated bearer without recreating the whole PDN session.
What success looks like: The UE receives Activate Dedicated EPS Bearer Context Request and returns Activate Dedicated EPS Bearer Context Accept.
What failure means: The extra bearer cannot be activated or later traffic stays on the default bearer unexpectedly.

Why this procedure matters

This is the bearer path that explains why one session can carry multiple service profiles instead of using only the default bearer.

Quick Fact Sheet

Procedure name	LTE Dedicated EPS Bearer Establishment
Domain	Additional bearer setup on an existing PDN session
Main trigger	An existing session needs an extra bearer for separate service handling
Start state	UE already has a default bearer and active session context
End state	UE has an additional dedicated bearer
Main nodes	UE, eNB, MME, SGW, PGW
Main protocols	NAS, S1AP
Main success outcome	The dedicated bearer becomes active with its own EBI and QoS relation
Main failure outcome	The additional bearer is not activated or is rejected
Most important messages	Activate Dedicated EPS Bearer Context Request, Activate Dedicated EPS Bearer Context Accept
Main specs	TS 24.301, TS 23.401

LTE Dedicated EPS Bearer Establishment call flow — Click the diagram to open the full-size in a new tab.

Preconditions

The default bearer and PDN session already exist.
The network has a reason to add a separate bearer profile.
The eNB can apply the related radio-side bearer context.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Receives and accepts the dedicated bearer context.
MME	Coordinates bearer activation toward the UE.
eNB	Applies the radio-side bearer relation.
SGW / PGW	Provide the underlying session and traffic anchoring context.

Interfaces used

Interface	Path	Role
NAS	UE <-> MME	Carries the dedicated bearer activation pair.
S1-MME	eNB <-> MME	Transports bearer-related context to the access side.

End-to-End Call Flow

UE               eNB               MME
|<- Activate Dedicated Bearer Req -|
|-- Activate Dedicated Bearer Accept ->|
|   dedicated bearer becomes active  |

Major Phases

Phase	What happens
1. Bearer decision	The network decides to add a dedicated bearer.
2. Bearer activation	The dedicated bearer context is delivered to the UE.
3. UE acceptance	The UE confirms the added bearer.

Step-by-Step Breakdown

Step 1: Dedicated bearer request

Sender -> receiver: MME -> UE

Message(s): Activate Dedicated EPS Bearer Context Request

Purpose: Tell the UE to add a dedicated bearer.

State or context change: The UE now has a second bearer context to process.

Note: EBI, TFT, and QoS are the main checks.

Step 2: Dedicated bearer accept

Sender -> receiver: UE -> MME

Message(s): Activate Dedicated EPS Bearer Context Accept

Purpose: Confirm the dedicated bearer activation.

State or context change: The added bearer is now active if the access side also completed correctly.

Note: Accept timing is the fastest success checkpoint.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Activate Dedicated EPS Bearer Context Request	NAS	MME -> UE	Starts dedicated bearer activation.	EBI, TFT, and QoS relation.
Activate Dedicated EPS Bearer Context Accept	NAS	UE -> MME	Confirms dedicated bearer activation.	Timing and bearer identity match.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
EBI	The identity of the dedicated bearer.	Activation request and accept	Separates the added bearer from the default bearer.	Wrong bearer identity correlation.
TFT	Traffic-filter relation for the dedicated bearer.	Activation request	Explains which traffic should use the added bearer.	Wrong filter logic or mismatch.
QoS / QCI / ARP	The service profile of the dedicated bearer.	Activation request	Defines why the extra bearer exists at all.	Unexpected service profile.

Successful Completion

Success is confirmed when the UE returns Activate Dedicated EPS Bearer Context Accept and the added bearer is visible as active in the session context.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Request appears, but accept never returns	UE cannot apply the dedicated bearer parameters or uplink return failed.	TFT, QoS, and timing after the request.	Activate Dedicated EPS Bearer Context Request	NAS, LTE Uu	Check bearer profile consistency first.
Dedicated bearer exists in signaling, but traffic still stays on default bearer	Filter or access-side handling is incomplete.	TFT relation and later traffic behavior.	Request and accept pair	NAS and later data-path checks	Confirm that the bearer was not only signaled but also used.

What to Check in Logs and Traces

Confirm the default bearer already existed before this procedure started.
Check EBI, TFT, and QoS fields in the activation request.
Use the accept as the main UE-side success marker.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

The dedicated bearer is added on top of the default bearer. It does not replace the original bearer.