5G Initial Registration Call Flow and Procedure

Introduction

The 5G Initial Registration procedure is the first signaling process that allows a device to connect to a 5G Standalone (SA) network. During this procedure, the User Equipment (UE) identifies itself, authenticates with the network, establishes security protection, and registers with the Access and Mobility Management Function (AMF).

Once the registration procedure is completed successfully, the UE becomes reachable by the network and can start using services such as:

• mobile data
• voice services (VoNR)
• messaging services
• network slicing

This procedure is defined in the specifications published by the 3rd Generation Partnership Project including TS 23.502 (5G procedures) and TS 24.501 (NAS signaling).

Network Elements Involved

Before examining the call flow, let's understand the key network components involved.

UE (User Equipment)

The UE initiates the registration procedure. It may be a smartphone, modem, or IoT device connecting to the 5G network.

gNB (5G Base Station)

The gNB provides radio access to the UE and forwards NAS signaling messages between the UE and the 5G Core.

AMF (Access and Mobility Management Function)

The AMF is the main control-plane node responsible for:

• registration management
• mobility management
• NAS signaling handling

AUSF (Authentication Server Function)

The AUSF performs authentication of the UE using subscriber credentials.

UDM (Unified Data Management)

The UDM stores subscriber information such as:

• authentication data
• subscription profiles
• allowed network slices

Interfaces Used

5G Initial Registration Call Flow Diagram

Below is the simplified message flow for the Initial Registration procedure.

UE               gNB               AMF             AUSF / UDM
 |                |                 |                 |
 |----RRC Setup-->|                 |                 |
 |                |                 |                 |
 |---Registration Request---------->|                 |
 |                |----N2---------->|                 |
 |                |                 |----Auth Req---->|
 |                |                 |<---Auth Vector--|
 |<--Authentication Request--------|                  |
 |---Authentication Response------>|                  |
 |                |                 |                  |
 |<--Security Mode Command---------|                  |
 |---Security Mode Complete------->|                  |
 |                |                 |                  |
 |<--Registration Accept-----------|                  |
 |---Registration Complete-------->|                  |

Step-by-Step Explanation

1. RRC Connection Setup

Before the NAS procedure begins, the UE establishes a radio connection with the gNB.

Typical RRC messages include:

• RRC Setup Request
• RRC Setup
• RRC Setup Complete

After this step, the UE can send NAS signaling messages to the core network.

Important parameters to check

Engineers should verify:

• Cell ID
• PLMN selection
• NR ARFCN
• RSRP / RSRQ measurements

If the RRC connection fails, the registration procedure cannot start.

2. Registration Request

Once the RRC connection is established, the UE sends a Registration Request message to the AMF.

The message contains key information including:

• Registration type (Initial Registration)
• UE identity (SUCI or 5G-GUTI)
• Security capabilities
• Requested network slices (NSSAI)

The message travels through:

UE → gNB → AMF

Important parameters to check

Engineers should check:

• Registration type value
• Mobile identity (SUCI / GUTI)
• Requested NSSAI
• UE security capability

3. Authentication Procedure

The network must verify that the UE is a legitimate subscriber.

The AMF requests authentication data from the AUSF and UDM.

Typical messages:

• Authentication Request
• Authentication Response

The UE calculates a response using credentials stored in the USIM.

Important parameters to check

During troubleshooting, verify:

• RAND and AUTN values
• RES* authentication response
• synchronization failures
• authentication reject causes

4. Security Mode Command

After successful authentication, the AMF activates NAS security.

Messages involved:

• Security Mode Command
• Security Mode Complete

These messages enable:

• NAS encryption
• NAS integrity protection

Important parameters to check

Look at:

• selected ciphering algorithm
• selected integrity algorithm
• security capability negotiation

5. Registration Accept

The AMF now accepts the registration and sends a Registration Accept message.

This message may contain:

• 5G-GUTI assignment
• allowed NSSAI (network slices)
• registration area information
• mobility parameters

Important parameters to check

Engineers should verify:

• registration result
• assigned 5G-GUTI
• allowed NSSAI
• TAI list

6. Registration Complete

The UE confirms successful registration by sending a Registration Complete message.

At this point:

• UE becomes reachable in the network
• location is updated in AMF
• services can be activated

What Happens After Registration?

After registration, the UE can perform additional procedures such as:

• PDU Session Establishment (data connection)
• IMS Registration (for VoNR calls)
• Network slice selection
• Policy control procedures

Key Messages in the Procedure

Differences Between LTE Attach and 5G Registration

References

The procedure is described in specifications from the 3rd Generation Partnership Project.

Key documents include:

• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 24.501 - NAS Protocol for 5G
• 3GPP TS 33.501 - 5G Security Architecture

Summary

The 5G Initial Registration procedure is the process through which a device securely connects to the 5G Core network. During this procedure, the UE performs authentication, activates security protection, and registers with the AMF.

Once registration is complete, the UE can establish data sessions, access voice services such as VoNR, and use other network services.