5G Emergency Registration Procedure Explained

Introduction

The Emergency Registration procedure in 5G networks allows a User Equipment (UE) to access emergency services even when normal network registration is not available.

This procedure ensures that users can make emergency calls such as:

• 911
• 112
• other emergency service numbers

even if:

• the UE is not fully registered
• the subscriber does not have valid credentials
• the network does not allow normal services

The procedure is defined by the 3rd Generation Partnership Project in:

• TS 23.502 - 5G System Procedures
• TS 24.501 - NAS Protocol

When Emergency Registration Is Triggered

Emergency Registration occurs when:

• the UE attempts an emergency call
• the UE is not registered with the network
• the network rejects normal registration

Examples include:

• SIM not provisioned
• roaming restrictions
• subscription expired
• UE not authenticated

Despite these conditions, the network must still allow emergency access.

Network Elements Involved

UE (User Equipment)

The UE triggers emergency registration when the user initiates an emergency call.

gNB (5G Base Station)

Provides radio access and forwards emergency signaling to the core network.

AMF (Access and Mobility Management Function)

Handles emergency registration and allows limited service access.

IMS Emergency Services

Once registration succeeds, emergency calls are routed through the IMS emergency services architecture.

Interfaces Used

Emergency Registration Call Flow

Below is the simplified message sequence.

UE                gNB                AMF
 |                 |                  |
 |----RRC Setup--->|                  |
 |                 |                  |
 |---Registration Request (Emergency)-->|
 |                 |----N2----------->|
 |                 |                  |
 |<--Registration Accept (Emergency)--|
 |---Registration Complete----------->|

This procedure is simpler than normal registration because:

• authentication may be skipped
• limited service access is allowed

Step-by-Step Explanation

Step 1: Emergency Call Trigger

The UE detects that the user dialed an emergency number.

Examples include:

• 911
• 112

The UE then attempts Emergency Registration if it is not already registered.

Important parameters to check

Engineers should verify:

• emergency call indicator
• emergency service category
• emergency number detection

Step 2: RRC Connection Setup

The UE establishes a radio connection with the gNB.

Typical messages include:

• RRC Setup Request
• RRC Setup
• RRC Setup Complete

The establishment cause is usually set to:

emergency

Important parameters to check

Look for:

• RRC establishment cause = emergency
• serving cell information
• radio signal quality

Step 3: Emergency Registration Request

The UE sends a Registration Request with the registration type set to emergency.

The message includes:

• UE identity (if available)
• emergency indicator
• UE capabilities

This message is forwarded to the AMF.

Important parameters to check

Check:

• registration type = emergency
• emergency indicator flag
• UE identity used

Step 4: Limited Network Authentication

In many cases, the network skips full authentication to allow emergency access quickly.

Instead, the network:

• validates the request
• allows limited registration

This ensures emergency services remain accessible.

Important parameters to check

Engineers should verify:

• emergency service authorization
• restricted service mode
• emergency service category

Step 5: Registration Accept

The AMF sends Registration Accept indicating emergency registration.

The UE now enters limited service mode.

This allows:

• emergency voice calls
• location reporting

but blocks normal services.

Important parameters to check

Check:

• registration result = emergency services
• allowed services list
• emergency indication flag

Step 6: Registration Complete

The UE sends Registration Complete to confirm the procedure.

After this step:

• emergency services are available
• the UE can initiate an emergency call

What Happens Next?

After emergency registration, the UE performs:

• Emergency PDU Session Establishment
• IMS Emergency Call Setup

This connects the call to public safety answering points (PSAP).

Differences Between Normal and Emergency Registration

Common Troubleshooting Scenarios

Emergency Registration Rejected

Possible causes:

• emergency access disabled
• network configuration error
• gNB configuration issue

UE Cannot Initiate Emergency Call

Possible causes:

• IMS emergency services not configured
• emergency PDU session failure
• location reporting failure

Registration Succeeds but Call Fails

Possible reasons:

• IMS emergency routing problem
• PSAP connectivity issue

Key Messages in Emergency Registration

Relevant 3GPP Specifications

This procedure is defined in:

• TS 23.502 - 5G System Procedures
• TS 24.501 - NAS Protocol
• TS 33.501 - 5G Security

Published by the 3rd Generation Partnership Project.

Summary

The Emergency Registration procedure ensures that users can always access emergency services in 5G networks, even when normal registration fails.

The network allows limited connectivity so that emergency calls can still be routed to the appropriate public safety services.

This is a critical feature of modern mobile networks that ensures public safety and regulatory compliance.