5G NSSF (Network Slice Selection Function)

In the 5G system, the NSSF (Network Slice Selection Function) is a key component in the 5GC architecture, responsible for enabling and managing network slices. It provides two services: Nnssf_NSSelection (slice selection) and Nnssf_NSSAIAvailability (slice availability), which are defined as follows:

I. Network slicing allows operators to create multiple virtual networks on top of a shared physical infrastructure. Each slice can be customized according to specific service requirements, such as enhanced mobile broadband (eMBB), ultra-reliable low-latency communication (URLLC), or massive machine-type communication (mMTC). The NSSF plays a core role in selecting the appropriate network slice for a given User Equipment (UE) and ensuring the correct resources are allocated.

II. The responsibilities of the NSSF, as defined in 3GPP TS 29.531, are:

• Selecting a set of network slice instances: Based on the UE's subscription, requested Network Slice Selection Assistance Information (NSSAI), and operator policies, the NSSF determines which slice instances should serve the UE.
• Determining allowed NSSAI and configured NSSAI mapping: Based on the UE's subscription (subscribed S-NSSAI from UDM), requested NSSAI, current service area (TA/PLMN), operator policies, and network constraints, the NSSF determines which S-NSSAI are available to the UE.

The specific tasks of the NSSF include:

• Calculating allowed NSSAI – selecting the set of S-NSSAI authorized for the UE in the current serving PLMN and registration area from the requested or subscribed list.
• Providing configured NSSAI mapping information – the NSSF returns the configured NSSAI mapping for the serving PLMN, which the AMF then passes to the UE via a registration accept message or UE configuration update message.

III.Roaming Scenarios: In this scenario, the NSSF provides S-NSSAI mapping between the VPLMN and HPLMN to ensure network slice compatibility and determine the AMF set – in some cases, the NSSF can also help determine the appropriate set of AMFs (Access and Mobility Management Functions) to serve the UE, especially when AMF reallocation is required.

IV. NSSF Services In 5GC, the NSSF provides services to AMF, SMF, NWDAF, and other NSSF instances in different PLMNs through a service-based interface (SBI) based on the Nnssf service. The main function of the NSSF is to provide network slice information to the AMF; the NSSF exposes two main services via SBI:

• Nnssf_NSSelection: Used by the AMF to retrieve network slice selection information.
• Nnssf_NSSAIAvailability: Used by the AMF to update the NSSF with information on the S-NSSAI supported within each tracking area (TA) and to subscribe to availability change notifications.