NSSF — Network Slice Selection Function¶

Learning objectives¶

• Explain the role of the NSSF (a centralized support function for network slice selection).
• Understand the positioning of the flow by which the Allowed NSSAI is determined from the Requested NSSAI.
• Grasp the meaning of the collaboration in which the AMF requests the NSSF (N22 / Nnssf).
• Understand the division of responsibilities between this page (individual NF) and the Network Slicing chapter.

Prerequisites¶

• Basic concepts of S-NSSAI / NSSAI → Network Slicing
• The overall picture of SBA / SBI (Service Based Architecture) → Curriculum
• How to read interfaces → Interface dictionary

What you'll learn in this chapter¶

We explain Why (why a centralized function called the NSSF is needed) → What (the determination of the Allowed NSSAI and what it returns) → How (how it supports the AMF through N22/Nnssf), as an individual NF page focused on roles, APIs, and interfaces. The detailed procedures of slice selection (selection logic and sequences) are in the Network Slicing chapter, and this page directs you there.

Why — why it's needed¶

When there are many slices and there is cross-PLMN operation or complex slice configurations, the AMF may not be able to fully judge which slice (S-NSSAI) to permit based solely on the information it holds locally. In such cases, a function that centrally takes on the judgment of slice selection is needed. That is the NSSF.

Analogy: It is like a general reception desk (AMF) entrusting the specialized room assignment it cannot judge on its own to a specialist for floor guidance (NSSF). Without the NSSF, in complex slice configurations, the determination of the Allowed NSSAI and the re-selection of an appropriate AMF would become impossible.

Overview¶

Based on the Requested NSSAI passed from the AMF and subscription information, etc., the NSSF determines the Allowed NSSAI to permit for the UE. Together with that, it returns the selection of an appropriate AMF Set and information used for querying the NRF, supporting slice selection. Note that in a configuration where the AMF can resolve the slice on its own, a query to the NSSF is unnecessary.

Basic Concept — explanation for beginners¶

In a large building, the general reception desk (AMF) sometimes cannot handle even "which specialized floor to guide someone to." At that point, the information desk (NSSF) judges "this customer should go to this group of floors." The NSSF is like an information desk that helps with the slice assignment that reception alone cannot decide.

Network Function details¶

Architecture¶

flowchart LR
    AMF["AMF (requester)"] -->|"N22: Nnssf_NSSelection request"| NSSF["NSSF (slice selection support)"]
    NSSF -->|"return Allowed NSSAI / AMF Set info"| AMF
    NSSF -.->|"NRF Discovery, etc."| NRF["NRF (NF Repository)"]

How to read the diagram: When the AMF cannot resolve the slice on its own, it requests NSSelection from the NSSF over N22 (Nnssf). The NSSF determines information about the Allowed NSSAI and an appropriate AMF Set and returns it to the AMF, and provides information for querying the NRF as needed (the dotted line indicates the relationship with the NRF).

Interface¶

For details, see the Interface dictionary, and for the concept of slices, see Network Slicing. The SBI communicates over HTTP/2 + JSON over TLS.

Appearance in procedures¶

• Slice selection at Registration: When the AMF cannot resolve the Requested NSSAI on its own, it queries the NSSF over N22 and determines the Allowed NSSAI.
• At PDU session: The SMF within the slice is mainly selected using S-NSSAI + DNN. The degree of the NSSF's involvement in this context is configuration-dependent and to be confirmed.

For the detailed flows, see Network Slicing and Registration.

Comparison with EPC¶

4G/EPC has no standard concept of network slices, and there is no function corresponding to the NSSF. DECOR / eDECOR (Dedicated Core Networks) is regarded as a predecessor-like positioning, but its functions are limited and the direct correspondence is to be confirmed.

Release differences¶

• Rel-15: The basic functions of the NSSF (Allowed NSSAI determination, NSSelection / NSSAIAvailability) were specified.
• Rel-16: Slice-related extensions such as NSSAA (Network Slice-Specific Authentication and Authorization) and NSAC (Network Slice Admission Control) were added. The specific degree of the NSSF's involvement is to be confirmed.
• Rel-17: Further extensions are to be confirmed.

3GPP Specification¶

• TS 23.501 §6.2.14 — Functional definition of the NSSF
• TS 29.531 — Nnssf (NSSelection / NSSAIAvailability). Individual section numbers to be confirmed.
• TS 23.502 — Slice selection procedures (to be confirmed)
• TS 23.501 §5.15 — Network slicing (to be confirmed)

Summary¶

• The NSSF is an NF that centrally supports network slice selection.
• From the Requested NSSAI and subscription information, etc., it determines the Allowed NSSAI and supports AMF Set / NSI selection.
• In a configuration where the AMF can resolve the slice on its own, a query to the NSSF is unnecessary.
• It collaborates with the AMF over N22 (Nnssf). The provided APIs are Nnssf_NSSelection / Nnssf_NSSAIAvailability (TS 29.531).
• The detailed selection procedures are handled by the Network Slicing chapter, and this page focuses on roles, APIs, and interfaces.

Next Step¶

• Network Slicing — the detailed procedures of slice selection
• AMF — the main requester
• NRF — the collaboration partner for NF Discovery
• NF dictionary
• Interface dictionary