Network Slice Selection Assistance Information (NSSAI)

The atomic unit of slice identification, consisting of two mandatory components:

• Slice/Service Type (SST): An 8-bit standardized or non-standardized value defining the expected network slice behavior (e.g., SST=1 for eMBB, SST=2 for URLLC).
• Slice Differentiator (SD): An optional 24-bit field that complements the SST to distinguish among multiple slices with the same SST but different tenants or service requirements.

An S-NSSAI can be expressed as either SST-only or SST+SD, providing granularity for up to 256 standardized service types.

A list of S-NSSAIs provisioned in the UE by the serving network, typically containing one or more entries. This list defines which network slices the UE is authorized to access within the current registration area.

Key behaviors:

• The UE stores the Configured NSSAI per Public Land Mobile Network (PLMN).
• The Access and Mobility Management Function (AMF) may update this list during registration or configuration update procedures.
• A Default Configured NSSAI is applied when no specific configuration exists for a PLMN.

The set of S-NSSAIs that the UE includes in its Registration Request message to the AMF. This list is derived from the Configured NSSAI and represents the specific slices the UE intends to connect to.

Selection logic:

• The UE's Non-Access Stratum (NAS) layer constructs this list based on application requirements and the available Configured NSSAI.
• The AMF validates the Requested NSSAI against the UE's subscription information retrieved from the Unified Data Management (UDM).
• If validation fails, the AMF may reject the request or redirect the UE to a default slice.

The definitive list of S-NSSAIs that the AMF authorizes the UE to use within the current registration area. This is the output of the network's slice admission control process.

Critical properties:

• The Allowed NSSAI is a subset of the Requested NSSAI, filtered by subscription and network capability.
• It may include S-NSSAIs with different SST or SD values than those requested, if the network applies slice remapping.
• The UE uses this list to determine which Protocol Data Unit (PDU) sessions can be established on which slices.

The permanent list of S-NSSAIs stored in the UE's subscription profile within the UDM. This represents the contractual agreement between the network operator and the subscriber.

Enforcement mechanism:

• During registration, the AMF retrieves the Subscribed S-NSSAIs from the UDM.
• Any S-NSSAI in the Requested NSSAI that is not present in the subscription is rejected.
• For each subscribed S-NSSAI, the UDM may specify whether it is the default S-NSSAI for the subscriber.

The 3GPP standard defines several modes governing how NSSAI is transmitted during signaling procedures:

• Mode A: UE includes S-NSSAI in Radio Resource Control (RRC) messages, enabling the gNB to perform slice-aware admission control before establishing RRC connection.
• Mode B: UE includes S-NSSAI only in NAS messages, keeping slice information transparent to the RAN.
• Mode C: UE includes S-NSSAI in both RRC and NAS messages, providing slice awareness to both the RAN and core network.
• Mode D: Network triggers slice re-selection by providing a new Allowed NSSAI.