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LTE E-RAB Setup Procedure Call Flow

call-flowLTE | S1AP | E-RAB | eNB

LTE E-RAB Setup Procedure establishes one or more E-RABs at the access side so packet service can use the radio path that matches the bearer context. It commonly appears during initial context setup, default bearer activation, or later dedicated bearer addition.

The main S1AP pair is E-RAB Setup Request and E-RAB Setup Response.

Introduction

The E-RAB setup path is where bearer intent becomes active radio-access state at the eNB. It gives the UE and eNB the access-side resources needed for the bearer that the EPC has already decided to establish.

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

What Is LTE E-RAB Setup Procedure in Simple Terms?

What starts the procedure: The network wants the eNB to create a new E-RAB for a bearer.
What the UE and network want to achieve: Build the access-side bearer resources needed for service delivery.
What success looks like: The eNB receives E-RAB Setup Request and returns E-RAB Setup Response.
What failure means: The bearer may exist in core signaling but not become usable on the radio side.

Why this procedure matters

This procedure is the bridge between bearer creation in the core and actual packet-service usability at the access side.

Quick Fact Sheet

Procedure name	LTE E-RAB Setup Procedure
Domain	Access-side bearer creation
Main trigger	Need to create one or more E-RABs for service
Start state	Bearer intent exists, but the radio-access bearer is not active yet
End state	The target E-RAB is active at the eNB side
Main nodes	MME, eNB, UE
Main protocols	S1AP
Main success outcome	eNB establishes the requested E-RAB resources
Main failure outcome	Bearer setup remains incomplete at the access side
Most important messages	E-RAB Setup Request, E-RAB Setup Response
Main specs	TS 36.413, TS 23.401

LTE E-RAB Setup Procedure call flow across MME, eNB, and UE — Click the diagram to open the full-size in a new tab.

Preconditions

The EPC already has a reason to create a bearer at the access side.
The UE context is available at the eNB or is being created together with the procedure.
The MME can identify the target E-RAB and UE context cleanly.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
MME	Starts E-RAB setup toward the eNB.
eNB	Creates the requested radio-access bearer resources.
UE	Uses the new access-side bearer after setup succeeds.

Interfaces used

Interface	Path	Role
S1-MME	MME <-> eNB	Carries E-RAB Setup Request and Response.
LTE Uu	UE <-> eNB	Provides the later live radio path for the new bearer.

End-to-End Call Flow

UE               eNB               MME
|                 |<-- E-RAB Setup Req ---|
|                 |-- create E-RAB ctx ---|
|                 |--- E-RAB Setup Resp -->|
|   new radio bearer path becomes active   |

Major Phases

Phase	What happens
1. Setup decision	The MME decides the eNB must create a new E-RAB.
2. Access-side bearer creation	The eNB allocates the requested E-RAB resources.
3. Result reporting	The eNB reports success or failure back to the MME.

Step-by-Step Breakdown

Step 1: E-RAB Setup Request

Sender -> receiver: MME -> eNB

Message(s): E-RAB Setup Request

Purpose: Ask the eNB to create one or more E-RABs.

State or context change: The eNB begins creating the access-side bearer relation.

Note: E-RAB IDs, transport relation, and QoS information are the main checks.

Step 2: E-RAB Setup Response

Sender -> receiver: eNB -> MME

Message(s): E-RAB Setup Response

Purpose: Confirm which E-RABs were successfully created.

State or context change: The requested bearer becomes usable at the access side if setup succeeded.

Note: Partial success cases matter when several bearers are requested together.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
E-RAB Setup Request	S1AP	MME -> eNB	Requests creation of one or more E-RABs.	E-RAB IDs, transport-related values, and QoS parameters.
E-RAB Setup Response	S1AP	eNB -> MME	Reports whether the requested E-RAB setup succeeded.	Successful list, failed list, and any partial completion details.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
E-RAB ID	The radio access bearer being created.	Setup Request and Response	Shows exactly which bearer is being built.	Wrong item mapping or duplicate use.
QoS information	The service profile tied to the E-RAB.	Setup Request	Explains how the eNB should treat the bearer.	Unexpected QoS or mismatch with bearer intent.
S1AP UE IDs	The UE identifiers on S1-MME.	Request and Response	Ensure the setup applies to the correct UE.	Context mismatch or stale UE mapping.

Successful Completion

Success is confirmed when the eNB returns E-RAB Setup Response and the requested bearer becomes active and usable on the access side.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Core signaling looks complete, but user plane never becomes usable	The E-RAB was not established correctly at the access side.	Setup response result details and later traffic behavior.	E-RAB Setup Request, E-RAB Setup Response	S1-MME, LTE Uu	Check whether the requested bearer actually became active at the eNB.
Only some requested E-RABs were created	The eNB completed only part of the requested setup list.	Successful and failed item lists.	Setup Response	S1-MME	Separate partial setup from full setup failure.

What to Check in Logs and Traces

Check which E-RAB IDs were requested and which were successfully created.
Align the setup branch with the earlier bearer-control reason such as initial context setup or dedicated bearer activation.
If service still fails, decide whether the problem is access-side setup or later bearer use.

Related sub-procedures

Related message reference pages

LTE Dedicated EPS Bearer Establishment — later dedicated bearer relation

Related troubleshooting pages

Notes

E-RAB setup makes bearer intent usable on the radio side. If this step fails, the bearer may look correct in core signaling but still remain unusable for traffic.

FAQ

What is LTE E-RAB Setup Procedure?

It is the S1AP procedure used to create radio access bearer resources for a bearer that needs to become active.

Where does it often appear?

It commonly appears during initial context setup, default bearer activation, and later dedicated bearer addition.