5G Network Slice Selection Procedure Explained

Introduction

The Network Slice Selection procedure in 5G networks determines which network slice or slices a UE is allowed to use after registration and during session setup.

In practical terms, this procedure decides whether the UE can access the expected S-NSSAI, whether the serving network accepts the requested slice, and whether later procedures such as PDU Session Establishment can continue on the correct slice.

This page is most useful when read together with 5G Initial Registration, Mobility Registration Update, and PDU Session Establishment.

The procedure is mainly defined in:

• 3GPP TS 23.501 - System Architecture for the 5G System
• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 24.501 - NAS Protocol

What Network Slice Selection Does

Slice selection is the process that maps the UE and its service request to one or more allowed S-NSSAIs.

Network Functions Involved

Interfaces Used

Network Slice Selection Call Flow Position

UE            gNB             AMF             UDM / NSSF
 |              |               |                  |
 |-- Registration Request ----->|                  |
 |   (Requested NSSAI)          |                  |
 |              |-------------->|                  |
 |              |               |-- subscription ->|
 |              |               |-- slice query -->|
 |              |               |<-- slice result -|
 |<-- Registration Accept ------|                  |
 |   (Allowed NSSAI)            |                  |

In many traces, slice selection becomes visible during registration, but its operational impact often appears later when a PDU Session Establishment Request is accepted, remapped, or rejected.

Step-by-Step Network Slice Selection Procedure

Step 1: UE Sends Registration Request with Requested NSSAI

The UE starts or updates registration and may include Requested NSSAI in the NAS signaling.

This is the network’s first strong indication of which slice or slices the UE wants to use in the current PLMN.

What to inspect

• Requested NSSAI content
• Serving PLMN
• Whether the UE is using configured or mapped slice data

Step 2: AMF Checks Subscription and Slice Policy

The AMF evaluates the request using subscriber data and, where relevant, interacts with other functions such as the NSSF.

This is where the network decides whether the requested slices are allowed, partially allowed, remapped, or rejected.

What to inspect

• AMF selection context
• Subscription profile
• NSSF decision or slice mapping logic

Step 3: Allowed NSSAI Is Returned to the UE

The network returns the result in the registration response, typically as Allowed NSSAI and, in some cases, Rejected NSSAI or related slice information.

This defines which slices the UE can actually use from that point onward.

What to inspect

• Allowed NSSAI list
• Rejected or unavailable S-NSSAI entries
• Whether the expected slice was replaced by another one

Step 4: Later Procedures Use the Selected Slice Context

The outcome becomes operationally important during procedures such as PDU Session Establishment.

If the chosen slice does not match subscriber policy, DNN configuration, or SMF expectations, failures often appear there instead of during registration.

What to inspect

• PDU session requested S-NSSAI
• DNN and slice combination
• SMF selection
• Session reject causes

Where Engineers Usually See Problems

What to Check in Logs and Traces

• Requested NSSAI in the Registration Request.
• Allowed NSSAI and any rejected slice information in the Registration Accept.
• Whether the AMF changed after slice-related selection logic.
• Subscription data and roaming policy for the affected subscriber.
• The S-NSSAI used later in PDU Session Establishment Request.
• Whether the issue is really slice selection or a later SMF / DNN / policy failure.

Common Troubleshooting Scenarios

Requested Slice Missing from Allowed NSSAI

This usually points to a subscription restriction, roaming limitation, or serving-area policy mismatch.

PDU Session Reject After Successful Registration

Often the registration path succeeded, but the selected slice is not valid for the requested DNN, SMF, or service policy.

Slice Changes After Mobility

After a Mobility Registration Update, the available slices may change because the UE moved into a different tracking area or policy region.

Roaming Slice Mapping Confusion

In roaming deployments, the requested home-network slice and the visited-network slice mapping may not be one-to-one, which makes trace review harder.

Related Procedures

• 5G Initial Registration
• Mobility Registration Update
• PDU Session Establishment
• Network Requested PDU Session Establishment

Recommended Reference Specifications

• 3GPP TS 23.501 - System Architecture for the 5G System
• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 24.501 - NAS Protocol
• 3GPP TS 29.531 - NSSF Services and APIs