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5G Network Slicing

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Author: Guo-Cheng Wu
Date: 2024/12/30

Network slicing is an innovative technology introduced in the 5G network, first proposed in the 3GPP Rel-15 specification. Its primary function is to support a wide range of diverse services within a single physical network. For instance, there are three major application scenarios in the 5G network: eMBB, mMTC, and URLLC. Given the varying resource requirements of these scenarios, network slicing plays a crucial role in efficiently allocating dedicated slices for each service.

How to Identify a Network Slice?

A Single Network Slice Selection Assistance Information (S-NSSAI) is used to identify a Network Slice. Each S-NSSAI consists of two parameters:

  • Slice/Service Type (SST): Defines the expected behavior of the Network Slice regarding features and services.

    • According to the 3GPP specification, the SST values are standardized as follows:
    Slice/Service type SST value Characteristics
    eMBB 1 Slice suitable for the handling of 5G enhanced Mobile Broadband.
    URLLC 2 Slice suitable for the handling of ultra-reliable low latency communications.
    MIoT 3 Slice suitable for the handling of massive IoT.
    V2X 4 Slice suitable for the handling of V2X services.
    HMTC 5 Slice suitable for the handling of High-Performance Machine-Type Communications.
    HDLLC 6 Slice suitable for the handling of High Data rate and Low Latency Communications.

  • Slice Differentiator (SD): An optional parameter that supplements the SST to distinguish between multiple Network Slices with the same Slice/Service Type.

Network Slice Selection Assistance Information (NSSAI)

The NSSAI is composed of multiple S-NSSAIs and can be categorized into the following types:

  • Requested NSSAI: A set of S-NSSAIs requested by the UE during connection setup, carried in the registration request sent by the UE. The Requested NSSAI can contain up to eight S-NSSAIs.
  • Allowed NSSAI: A set of S-NSSAIs approved by the core network from the Requested NSSAI, which the UE can utilize in the Serving PLMN within the current Registration Area. The Allowed NSSAI can include up to eight S-NSSAIs.
  • Rejected NSSAI: A set of S-NSSAIs rejected by the core network from the Requested NSSAI, often due to reasons such as the S-NSSAI being unsupported in the current registration area or unavailable due to reaching the maximum number of UEs.

  • Configured NSSAI: A set of S-NSSAIs provisioned in the UE by the network, applicable to one or more PLMNs, indicating which S-NSSAIs the UE is authorized to use.

    Note: If the Configured NSSAI does not contain any S-NSSAIs, a default S-NSSAI will be applied.

Network Slice Selection Function (NSSF)

Before delving into the procedure of network slice selection, it is important to understand the NSSF and its functionality. The primary role of the NSSF is to assist the AMF in selecting the appropriate network slice based on the current scenario. The NSSF provides two key services:

  • Nnssf_NSSelection: This service operation facilitates Network Slice selection in the Serving PLMN and the HPLMN. It enables the NSSF to provide the AMF with the Allowed NSSAI and the Configured NSSAI for the Serving PLMN. This service may be invoked during the Registration procedure, inter-PLMN mobility, PDU Session Establishment, or UE Configuration Update procedures.
  • Nnssf_NSSAIAvailability: This service allows the NSSF to update the S-NSSAIs supported by the NF service consumer (e.g., AMF) on a per tracking area basis. In addition to updating and deleting operations, it also supports operations like subscribing and notifying.

How does Core Network Select a Network Slice for UE?

The process of selecting a network slice for a UE primarily involves two key network functions:

  • Access and Mobility Management function (AMF): The AMF processes the registration request from the UE and forwards the Requested NSSAI to the NSSF.
  • Network Slice Selection Function (NSSF): The NSSF is responsible for selecting the appropriate set of Network Slice instances to serve the UE on behalf of the AMF. Specifically, it determines the Allowed NSSAI.

Procedure

This section focuses specifically on the steps involved in network slice selection. For a detailed explanation of the overall 5G registration procedure, please refer to this blog: Registration procedures

  1. UE to RAN: The UE initiates a Registration Request to the network through the RAN. Along with UE-specific information, the request includes the Requested NSSAI, indicating the network slices the UE intends to access.
  2. RAN (AMF Selection): If the UE doesn't provide 5G-S-TMSI or GUAMI in the Registration Request, the RAN will select a serving AMF based on the Requested NSSAI.
  3. AMF to NSSF: Upon receiving the registration request, the AMF sends a request to the NSSF to execute the network slice selection process. The AMF provides detailed information such as the Requested NSSAI, the PLMN ID of the SUPI, and the TAI.
  4. NSSF: The NSSF performs the Nnssf_NSSelection_Get service using the information provided by the AMF. It determines which S-NSSAIs the UE is eligible to use and aggregates them into the Allowed NSSAI. In addition, the NSSF may also identify the best AMF to serve the UE or provide a list of candidate AMFs.
  5. NSSF to AMF: After executing the Nnssf_NSSelection_Get service, the NSSF sends the Allowed NSSAI to the AMF.
  6. AMF to UE: If the registration request is successful, the AMF responds to the UE with a Registration Accept message, including the Allowed NSSAI. This enables the UE to identify the network slices available for use.

Network Slice-Specific Authentication and Authorization (NSSAA)

NSSAA is a procedure initiated by the AMF during the Registration process when certain network slices require authentication and authorization. This procedure is triggered when the AMF identifies that NSSAA is necessary for an S-NSSAI included in the current Allowed NSSAI.

Procedure

The AMF initiates a Nnssaaf_NSSAA_Authenticate request to the Network Slice-Specific and SNPN Authentication and Authorization Function (NSSAAF). The NSSAAF then forwards the request to the Authentication, Authorization, and Accounting Server (AAA Server) for authentication or authorization. Note that free5GC currently does not support this functionality. For more detailed information about the NSSAA procedure and NSSAAF service, you can refer to TS 23.502.

Conclusion

Network slicing is a fundamental capability for optimizing 5G networks, allowing them to support a diverse array of services with varying resource demands. The ability to define and manage these slices is critical to meeting the diverse needs of modern network systems. It ensures efficient resource utilization, dynamic adaptability, and end-to-end isolation, delivering guaranteed performance for a wide range of use cases.

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About me

Hi, I’m Guo-Cheng Wu, a newcomer to 5G and free5GC, and currently conducting research on Network Slicing. Let me know without hesitation if there is any mistake in the article.

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Github: https://github.com/leowu0407