Description
The Network Slice Selection Function (NSSF) is a critical component within the 5G Core (5GC) network architecture, specifically part of the control plane. Its primary role is to assist in the selection of a suitable Network Slice Instance (NSI) for a User Equipment (UE) during registration and session establishment procedures. The NSSF operates by evaluating slice selection information, which includes the Subscribed Network Slice Selection Assistance Information (S-NSSAI) requested by the UE, the Allowed NSSAI provided by the network, and the UE's subscription profile stored in the Unified Data Management (UDM) function. Based on this information, local operator policies, and the availability of network slice instances, the NSSF determines the specific NSI to serve the UE's session.
Architecturally, the NSSF interacts with other core network functions through service-based interfaces (SBIs), primarily using the Nnssf service. Key interactions include communication with the Access and Mobility Management Function (AMF) during the UE registration procedure. When an AMF receives a registration request containing a Requested NSSAI, it may query the NSSF to determine the Allowed NSSAI for the UE and to identify the set of Network Slice and AMF Set(s) or the specific AMF that can serve the requested slices. The NSSF's decision-making process considers the mapping between S-NSSAIs and the actual deployed NSIs, which are composed of managed Network Slice Subnet Instances (NSSIs).
The NSSF's functionality is vital for the practical realization of network slicing, a foundational 5G concept. It ensures that a UE is connected to a network slice that aligns with its service requirements, whether for enhanced Mobile Broadband (eMBB), Ultra-Reliable Low-Latency Communications (URLLC), or massive Machine-Type Communications (mMTC). By performing this selection, the NSSF enables efficient resource utilization, isolation between slices, and the delivery of tailored Quality of Service (QoS). Its operations are defined in 3GPP specifications such as TS 23.501 for the overall system architecture and TS 29.520 for its service-based interface definition.
Purpose & Motivation
The NSSF was introduced with 5G in 3GPP Release 15 to address the fundamental challenge of efficiently mapping diverse user service requests to logically isolated network slices. Pre-5G networks offered a largely monolithic architecture where all services were delivered over a single, uniform network infrastructure. This approach was insufficient for supporting the wide range of 5G use cases, which have drastically different technical requirements—from gigabit speeds for video streaming to millisecond latency for industrial automation and high connection density for IoT sensors.
The creation of the NSSF was motivated by the need for a dedicated, intelligent function to manage slice selection as a first-class procedure in the core network. Without it, the AMF or other functions would need embedded, complex logic to understand slice deployment topologies and subscriber entitlements, leading to scalability and management issues. The NSSF centralizes this intelligence, allowing for dynamic, policy-driven slice selection that can adapt to network load, slice availability, and commercial agreements. It solves the problem of how to automatically and accurately connect a device to the correct virtualized network segment out of many possible options, which is a prerequisite for delivering network slicing as a service.
Architecture
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (349 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the NSSF was newly introduced as a core network function responsible for network slice selection. The foundational specifications defined its key service-based interface (Nnssf) and reference points, such as N22 to the AMF and N31 for inter-PLMN communication, establishing its role in the architecture. Initial work focused on its core procedures, including AMF selection based on network slices and clarifying the mechanisms for NSSF discovery.
- Selection of NAS procedures for E-UTRA connected to both EPC and 5GC TS 23.501CR0147
- Clarification on PCF selection TS 23.501CR0015
- Adding the new clause about SMSF selection TS 23.501CR0016
- Clarify NSSF discovery TS 23.501CR0039
- Corrections to UPF selection and resolution of related Editor's Note TS 23.501CR0044
- Slice selection cleanup TS 23.501CR0047
+ 48 more changes
In Release 16, the NSSF's scope was expanded to support new selection and discovery mechanisms, including enhancements for slice interworking and the introduction of slice-specific authentication and authorization. It also gained new capabilities to influence selection procedures based on analytics, such as for SMF and UPF selection, and to support delegated discovery via the Service Communication Proxy (SCP) as part of the enhanced Service-Based Architecture (eSBA). Furthermore, the release defined new interactions, such as the N34 reference point to the NWDAF, to utilize network data analytics for slice-related functions.
- CIoT Introduction of CN Selection and Steering TS 23.501CR0896
- ATSSS-SMF and UPF selection TS 23.501CR0761
- Trusted non-3GPP Access Network Selection TS 23.501CR0783
- eSBA communication schemas related to general discovery and selection TS 23.501CR0799
- eSBA communication schemas related to UDM and UDR discovery and selection TS 23.501CR0800
- Use of analytics for SMF selection TS 23.501CR0940
+ 77 more changes
In Release 17, the NSSF was enhanced to support Network Slice Admission Control Function (NSACF) discovery and selection, enabling slice-level resource management. It also gained the ability to utilize analytics from the NWDAF for network slice restriction and to support different slices over different Non-3GPP accesses. Furthermore, new capabilities were introduced, such as support for the GSMA NG.116 attribute "Simultaneous Use of a Network Slice" and UE-Slice-MBR.
- Support of different slices over different Non 3GPP access TS 23.501CR2525
- Network Slice restriction based on NWDAF analytics TS 23.501CR2567
- NWDAF discovery and selection TS 23.501CR2575
- NWDAF discovery and selection based on provided ML models TS 23.501CR2585
- UP path selection enhancement based on analytics info provided by NWDAF TS 23.501CR2586
- NWDAF discovery and selection for model sharing TS 23.501CR2614
+ 66 more changes
In Release 18, the NSSF's new capabilities include the introduction of an Alternative S-NSSAI replacement determined by the NSSF and support for network slice replacement. Furthermore, enhancements were made for improved network control of UE behaviour for a network slice and to support graceful network slice PDU sessions release during slice decommissioning. These additions work within the existing framework of the N22 reference point and Network Slice instance management.
- N3IWF selection enhancement for support of S-NSSAI needed by UE TS 23.501CR3707
- Discovery and Selection of the NWDAF Supporting Federated Learning in 5GC TS 23.501CR3772
- KI#4 23.501 AF traffic influence for common EAS, DNAI selection TS 23.501CR3788
- Common EAS/DNAI selection by AF TS 23.501CR3789
- Change of Network Slice instance for PDU sessions TS 23.501CR3867
- Assistance to Member Selection Functionality for Application Operation TS 23.501CR3910
+ 86 more changes
In Release 19, the NSSF saw enhancements for network slice handling in scenarios of Indirect Network Sharing and received clarifications for S-NSSAI selection while a UE is connected to EPS. Furthermore, the release introduced support for slice change based on an AF request and refined procedures for NSSF selection itself.
- RVAS with target NF selection enhancement. TS 23.501CR5364
- NF discovery and selection by target PLMN TS 23.501CR5399
- Support of UPF selection according to the conclusion in FS_UPEAS_Ph2 TS 23.501CR5441
- I-SMF selection/insertion based on local offloading allowed indication TS 23.501CR5604
- Support of Slice change based on AF request TS 23.501CR5764
- Support of L-PSA UPF Selection Considering N6 Delay TS 23.501CR5437
+ 41 more changes
In Release 20, the NSSF's enhancements were focused on enabling its interaction with the Service Communication Proxy for delegated discovery and selection, as indicated by the new "Routing Binding Indication" for SCP use. Furthermore, the release introduced new reference points, specifically N34 between the NSSF and the NWDAF, to support data analytics. These updates integrated the NSSF more deeply into the service-based architecture for improved network slice management.
- SMF and PCF Selection for UIA_ARC TS 23.501CR6504
Explore further
Broader topics and technologies where NSSF plays a role.
Defining Specifications
3GPP specifications that define or reference NSSF, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 26.891 vg00 | Media Distribution Services in 5G System | Rel-16 |
| TS 29.520 vj40 | 5G Network Data Analytics Services Stage 3 | Rel-19 |
| TS 29.524 vj00 | 5G Cause Code Mapping Specification | Rel-19 |
| TS 29.552 vj40 | 5G Network Data Analytics Signalling Flows | Rel-19 |
| TS 29.574 vj40 | 5G Data Collection Coordination Services Stage 3 | Rel-19 |
| TS 29.576 vj40 | 5G Messaging Framework Adaptor Services Stage 3 | Rel-19 |
| TS 32.255 vk10 | Telecom Management; Charging for 5G Data Connectivity | Rel-20 |
| TS 33.127 vj50 | Lawful Interception Architecture and Functions | Rel-19 |