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LTE Emergency Attach Procedure Call Flow

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

LTE Emergency Attach is the attach path used when the UE needs emergency service access instead of normal subscriber service. It allows the network to create the minimum EPS context needed for emergency connectivity even when normal service conditions are restricted.

This procedure matters because emergency access rules, subscription handling, and PDN treatment differ from ordinary attach behavior.

Introduction

The LTE Emergency Attach Procedure starts when the UE requests emergency service through LTE and no normal service path is suitable or available. It still uses the attach framework, but the attach type, APN handling, and policy outcome are specific to emergency service treatment.

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

What Is LTE Emergency Attach in Simple Terms?

What starts the procedure: The UE requests emergency LTE access.
What the UE and network want to achieve: Create the emergency EPS context needed for emergency service continuation.
What success looks like: Emergency attach is accepted and the UE reaches the emergency service path.
What failure means: The UE does not reach emergency EPS service and another emergency handling path may be needed.

Why this procedure matters

Emergency attach uses attach-like signaling but should not be read as ordinary subscriber onboarding. The highest-value checks are the emergency attach type, emergency APN or PDN treatment, and the final service result.

Quick Fact Sheet

Procedure name	LTE Emergency Attach Procedure
Domain	Emergency EPS access
Main trigger	Emergency service request over LTE
Start state	UE needs emergency access and does not rely on normal service continuation
End state	Emergency EPS context is accepted or the emergency attempt is rejected
Main nodes	UE, eNB, MME, SGW, PGW
Main protocols	RRC, NAS, S1AP, GTPv2-C
Main success outcome	Emergency LTE access and emergency PDN handling are available
Main failure outcome	Emergency attach reject or incomplete emergency service setup
Most important messages	Attach Request, Authentication, Security Mode, Attach Accept
Main specs	TS 23.401, TS 24.301, TS 23.167, TS 36.300

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

Preconditions

The UE can access an LTE cell and start RRC signaling.
The network allows emergency-service processing for the scenario.
The MME can select the emergency service path and PDN handling.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Starts the emergency attach and asks for emergency EPS service.
eNB	Provides the LTE access leg and forwards the emergency NAS signaling.
MME	Controls the emergency attach path and chooses the allowed emergency service outcome.
SGW	Participates in the emergency session setup on the user-plane side.
PGW	Applies the emergency PDN and packet-service context.

Interfaces used

Interface	Endpoints	Role
LTE Uu	UE <-> eNB	Carries the access leg and first emergency NAS message.
S1-MME	eNB <-> MME	Carries emergency attach control signaling.
S11	MME <-> SGW	Carries emergency session creation signaling.
S5/S8	SGW <-> PGW	Carries the emergency PDN path on the EPC side.

End-to-End Call Flow

UE               eNB               MME               SGW               PGW
|                 |                 |                 |                 |
|--RRC access---->|                 |                 |                 |
|--Attach Request (emergency)----->|                 |                 |
|                 |--Initial UE Message------------->|                 |
|<--Authentication / Security Mode-|                 |                 |
|--Responses / Security Complete-->|                 |                 |
|                 |                 |--Create Session----------------->|
|                 |                 |                 |--Emergency PDN->|
|                 |                 |<--Create Session Response--------|
|<--Attach Accept / Initial Context Setup-----------|                 |
|--Attach Complete---------------->|                 |                 |

Major Phases

1. Emergency access entry

The UE opens an LTE signaling path and indicates emergency attach intent.

2. Identity, authentication, and security

The network validates the UE as required for the emergency scenario and protects the remaining NAS exchange.

3. Emergency session creation

The EPC creates the minimum session and PDN context needed for emergency service continuation.

4. Accept and completion

The UE receives the emergency attach result and confirms completion.

Step-by-Step Breakdown

Emergency access start

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

Message(s): Attach Request with emergency attach type

Purpose: Tell the network that the UE needs emergency EPS service.

State or context change: The attach path is handled with emergency rules.

Note: This is the first field to inspect before treating the trace as a normal attach.

Security continuation

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

Message(s): Authentication and Security Mode exchange

Purpose: Protect the emergency NAS path as required by the scenario.

State or context change: Protected NAS continuation becomes active.

Note: Some emergency scenarios still follow the same basic NAS security sequence as ordinary attach.

Emergency session creation

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

Message(s): Create Session signaling

Purpose: Build the emergency PDN context.

State or context change: The emergency bearer path becomes available.

Note: Policy or PDN failure here can block emergency continuity even when the early attach looked clean.

Important Messages

Message	Protocol	Sender -> Receiver	Purpose in this procedure	What to inspect briefly
Attach Request	NAS	UE -> MME	Starts the emergency attach path.	Emergency attach type and emergency PDN request context.
Authentication Request	NAS	MME -> UE	Authenticates the UE when required for the emergency path.	Whether the scenario uses full authentication or an emergency exception path.
Security Mode Command	NAS	MME -> UE	Protects the remaining NAS exchange.	Whether the protected continuation matches the emergency scenario.
Attach Accept	NAS	MME -> UE	Returns the emergency attach result.	Final service result and emergency bearer context.

Important Parameters to Inspect

Item	What it is	Where it appears	Why it matters	Common issues
Emergency attach type	NAS attach type indicating emergency access.	Attach Request	Shows that this is the emergency procedure, not normal attach.	Wrong attach type, wrong test scenario.
Emergency APN or PDN context	Packet-service context used for emergency continuity.	PDN request and session creation	Explains whether the emergency bearer path can be built.	Blocked APN, policy mismatch.
Returned attach result	Final NAS outcome for emergency attach.	Attach Accept or Attach Reject	Shows whether emergency EPS service was actually granted.	Ambiguous service result, reject cause confusion.

Successful Completion

A successful Emergency Attach gives the UE the EPS context needed to continue the emergency service path over LTE.

Common Failures and Troubleshooting

Emergency attach request is rejected

Likely cause: Emergency-service policy, radio failure, or emergency PDN setup failure.

Where to inspect: Attach Request type, reject cause, emergency APN or PDN handling.

Relevant message(s): Attach Request, Attach Reject

Relevant interface(s): NAS, S1-MME, S11

Likely next step: Decide whether the failure is radio-side, policy-side, or emergency PDN-side.

What to Check in Logs and Traces

Confirm that the UE really sent the emergency attach type.
Check whether authentication and security followed the expected emergency branch.
Inspect emergency PDN and session creation results before calling the attempt successful.
Use the final accept or reject outcome to separate radio access failure from emergency-policy failure.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

Emergency attach should be read as an emergency-service access path, not a normal attach with a different label. Start with the attach type and the emergency PDN result.

FAQ

Is Emergency Attach the same as normal Attach?

No. It reuses attach signaling but follows emergency-service rules and outcomes.

What is the first field to inspect?

The emergency attach type in Attach Request.

What usually causes failure?

Radio failure, policy restrictions, or emergency PDN setup failure are common break points.