Home / Call Flows / LTE / Default EPS Bearer Establishment

LTE Default EPS Bearer Establishment Call Flow

call-flowLTE | EPC | NAS | S1AP | GTPv2-C

LTE Default EPS Bearer Establishment is the procedure that creates the first usable EPS bearer for packet service. It is the bearer path that appears with initial PDN connectivity and gives the UE its first active bearer identity, IP context, and user-plane route.

This bearer is usually established during attach or a related initial PDN setup path.

Introduction

The default EPS bearer is the baseline bearer that anchors packet connectivity for the PDN session. It is activated through NAS signaling and depends on the corresponding EPC session context already being created through MME, SGW, and PGW interaction.

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

What Is LTE Default EPS Bearer Establishment in Simple Terms?

What starts the procedure: The network has accepted PDN connectivity and is ready to activate the first bearer.
What the UE and network want to achieve: Create the first usable bearer for packet service.
What success looks like: The UE receives Activate Default EPS Bearer Context Request and returns Activate Default EPS Bearer Context Accept.
What failure means: The first PDN session exists only partially or the UE cannot activate the default bearer cleanly.

Why this procedure matters

This is the first bearer the UE needs before normal LTE packet service becomes usable.

Quick Fact Sheet

Procedure name	LTE Default EPS Bearer Establishment
Domain	Initial EPS bearer and PDN service setup
Main trigger	Successful initial PDN session creation
Start state	UE is progressing through initial PDN connectivity without an active default bearer
End state	UE has an active default EPS bearer for the PDN session
Main nodes	UE, eNB, MME, SGW, PGW
Main protocols	NAS, S1AP, GTPv2-C
Main success outcome	Default bearer is active and packet service can continue
Main failure outcome	PDN setup is incomplete or bearer activation fails
Most important messages	Activate Default EPS Bearer Context Request, Activate Default EPS Bearer Context Accept
Main specs	TS 24.301, TS 23.401, TS 29.274

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

Preconditions

The UE already has the access-side signaling path needed for NAS continuation.
The initial PDN session has been accepted at the EPC side.
The MME has bearer parameters ready to deliver to the UE.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Receives and accepts the first bearer context.
eNB	Applies the radio-side bearer relation needed for service delivery.
MME	Coordinates bearer activation toward the UE.
SGW	Provides user-plane anchor relation.
PGW	Provides PDN-side session context and addressing.

Interfaces used

Interface	Path	Role
S11	MME <-> SGW	Carries session creation for the default bearer path.
S5/S8	SGW <-> PGW	Links the bearer to the PDN side.
S1-MME	eNB <-> MME	Transports the NAS bearer activation toward the UE.

End-to-End Call Flow

UE               eNB               MME               SGW/PGW
|                 |                 |-- Create Session -->|
|                 |                 |<-- Session result ---|
|<- Activate Default Bearer Req ---|                     |
|-- Activate Default Bearer Accept ->|                    |
|   default bearer becomes active    |                    |

Major Phases

Phase	What happens
1. EPC session creation	The core creates the initial session context.
2. Bearer activation delivery	The MME sends the default bearer activation to the UE.
3. UE acceptance	The UE confirms the bearer context.

Step-by-Step Breakdown

Step 1: Create Session

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

Message(s): GTPv2-C Create Session Request and Response

Purpose: Build the initial PDN and bearer context in the EPC.

State or context change: The core has the default bearer template ready for activation.

Note: APN and subscription problems often surface here first.

Step 2: Default bearer activation

Sender -> receiver: MME -> eNB -> UE

Message(s): Activate Default EPS Bearer Context Request

Purpose: Tell the UE which bearer context to activate.

State or context change: The UE receives its first active bearer parameters.

Note: EBI, APN, QoS, and PDN address are the main checks.

Step 3: UE accept

Sender -> receiver: UE -> eNB -> MME

Message(s): Activate Default EPS Bearer Context Accept

Purpose: Confirm the bearer is accepted.

State or context change: The default bearer becomes operationally active.

Note: This message is the clearest user-side success proof.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Activate Default EPS Bearer Context Request	NAS	MME -> UE	Activates the first bearer context.	EBI, APN, PDN address, and QoS fields.
Activate Default EPS Bearer Context Accept	NAS	UE -> MME	Confirms activation of the first bearer.	Timing and EBI continuity.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
APN	The PDN name used for initial service.	Create Session and bearer request	Defines which PDN service is being built.	Unknown APN, wrong APN policy.
EPS Bearer ID	The identity of the default bearer.	Bearer activation request and accept	Links the activation to later bearer handling.	EBI mismatch.
PDN address	The IP context assigned to the UE.	Default bearer activation	Shows whether packet service has usable addressing.	Wrong address family or missing address.
QoS / QCI	The baseline bearer service parameters.	Default bearer activation	Defines the service profile of the default bearer.	Unexpected QCI or policy mismatch.

Successful Completion

Success is confirmed when the UE returns Activate Default EPS Bearer Context Accept and the default bearer is active for the PDN session.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
PDN session exists, but bearer activation fails	UE cannot accept the default bearer parameters or radio-side context is incomplete.	Bearer request content and later access-side setup.	Activate Default EPS Bearer Context Request	NAS, S1-MME	Check EBI, APN, and UE-side acceptance.
Accept never returns	UE did not finish activation or uplink NAS delivery failed.	Timing and air/core correlation after the request.	Activate Default EPS Bearer Context Accept	LTE Uu, S1-MME	Separate UE-side rejection from transport loss.

What to Check in Logs and Traces

Check Create Session success before blaming the UE-side bearer activation.
Inspect APN, EBI, QoS, and PDN address in the bearer request.
Confirm that the accept returns to the MME.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

The default bearer is the first usable EPS bearer. Later dedicated bearers depend on this baseline bearer context already existing.

FAQ

What is LTE Default EPS Bearer Establishment?

It is the procedure that activates the first usable EPS bearer for packet service.

What confirms success?

Activate Default EPS Bearer Context Accept is the clearest UE-side success message.