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LTE Detach Procedure Call Flow

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

LTE Detach is the procedure that releases EPS registration and ends the active mobility or service context for the UE. It can be UE-initiated or network-initiated depending on why the context is being removed.

Introduction

The LTE Detach Procedure cleanly removes the UE from EPS service when the UE or network decides the registration should end. It also drives the release of the related bearer and control-plane context.

What Is LTE Detach in Simple Terms?

What starts the procedure: The UE or network decides EPS registration must end.
What the UE and network want to achieve: Remove registration and release related EPC context cleanly.
What success looks like: The detach request is accepted and the related context is released.
What failure means: Context may remain partially active or the detach does not complete cleanly.

Why this procedure matters

Detach is the clean exit path for EPS registration. It explains why later paging, service request, or mobility procedures should no longer continue from the old context.

Quick Fact Sheet

Main trigger	UE power-off, SIM removal, network release decision, or explicit EPS detach need
Start state	UE is registered in EPS
End state	EPS registration is removed and related bearer context is released
Most important messages	Detach Request, Detach Accept

LTE Detach procedure flow across UE, eNB, MME, and SGW — Click the diagram to open the full-size in a new tab.

Preconditions

The UE is currently registered in EPS.
The detach direction and type are valid for the scenario.
The network can release the related bearer and control-plane context.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Starts UE-initiated detach or responds to network-initiated detach.
eNB	Relays the NAS detach exchange.
MME	Controls EPS registration release.
SGW	Releases related bearer context.

Interfaces used

Interface	Endpoints	Role
LTE Uu	UE <-> eNB	Carries the NAS detach signaling.
S1-MME	eNB <-> MME	Carries control-plane detach handling.
S11	MME <-> SGW	Carries bearer and session release coordination.

End-to-End Call Flow

UE               eNB               MME               SGW
|                 |                 |                 |
|--Detach Request----------------->|                 |
|                 |--uplink NAS--------------------->|
|                 |                 |--Release bearer/context------->|
|                 |                 |<--Release result---------------|
|                 |<-Detach Accept-----------------------------------|
|<--NAS delivery-------------------|                 |

Major Phases

1. Detach trigger

The UE or network decides that EPS registration should end.

2. Detach signaling

Detach Request is delivered in the correct direction for the scenario.

3. Context release

The MME and SGW remove the related service context.

4. Completion

Detach Accept confirms the closing step when required.

Step-by-Step Breakdown

Detach start

Sender -> receiver: UE -> MME or MME -> UE

Message(s): Detach Request

Purpose: Start EPS registration release.

State or context change: The old registered context is marked for removal.

Note: Start with detach type and direction before reading anything else.

Context release

Sender -> receiver: MME -> SGW

Message(s): Bearer and context release signaling

Purpose: Remove the related service context.

State or context change: EPS mobility and bearer state are torn down.

Note: Detach is not complete until the context release side also closes.

Important Messages

Message	Protocol	Sender -> Receiver	Purpose in this procedure	What to inspect briefly
Detach Request	NAS	UE -> MME or MME -> UE	Starts EPS registration release.	Detach type, cause, and whether the flow is UE- or network-initiated.
Detach Accept	NAS	UE -> MME or MME -> UE	Confirms detach completion when applicable.	Whether the final acknowledgement appears in the expected direction.

Important Parameters to Inspect

Item	What it is	Where it appears	Why it matters	Common issues
Detach type	Indicates the detach scope and reason.	Detach Request	Shows what kind of context should be removed.	Wrong detach interpretation.
Direction	Whether the flow is UE-initiated or network-initiated.	Detach Request / Accept	Changes the expected message ordering.	Reading the flow in the wrong direction.

Successful Completion

A successful detach removes the old EPS registration and tears down the related bearer and mobility context.

Common Failures and Troubleshooting

Detach signaling appears but context is not really released

Likely cause: Bearer or session release did not complete.

Where to inspect: SGW-side release handling and later residual service behavior.

Relevant message(s): Detach Request, Detach Accept

Relevant interface(s): NAS, S11

Likely next step: Confirm whether the old context still survives on the EPC side.

What to Check in Logs and Traces

Start with the detach direction and detach type.
Confirm whether Detach Accept is expected in the scenario.
Check that bearer and context release followed the NAS detach signaling.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

Detach is the controlled exit from EPS registration. The main checks are the detach type, direction, and whether the related bearer and context release actually followed.

FAQ

Can Detach be network-initiated?

Yes. The network can start detach, and the direction changes the later message order.

What should I inspect first?

Detach type and direction.