Network Slice Selection Assistance Information

Description

Network Slice Selection Assistance Information (NSSAI) is a fundamental concept and data structure in 5G System (5GS) architecture, introduced in 3GPP Release 15. It is the mechanism by which a User Equipment (UE) communicates its desired network slice(s) to the network during registration and session establishment procedures. An NSSAI is not a single identifier but a collection, typically containing one or more Single Network Slice Selection Assistance Information (S-NSSAI) elements. Each S-NSSAI is the unique identifier for a specific network slice instance or slice type that a UE is permitted to access.

An S-NSSAI itself is composed of two parts: a Slice/Service Type (SST) and an optional Slice Differentiator (SD). The SST is a mandatory 8-bit field that indicates the expected network slice behavior in terms of features and services (e.g., enhanced Mobile Broadband - eMBB, Ultra-Reliable Low Latency Communications - URLLC, Massive IoT - MIoT). The SD is an optional 24-bit field used to differentiate among multiple network slice instances of the same SST, allowing an operator to deploy several slices for the same service type but for different tenants or use cases. The UE obtains the S-NSSAIs it is allowed to use from its subscription profile, often configured via the home operator.

During the initial registration procedure, the UE includes a Requested NSSAI in the Registration Request message sent to the Access and Mobility Management Function (AMF) via the (R)AN. The Requested NSSAI contains the S-NSSAIs the UE wishes to register for in the current registration area. The AMF, in conjunction with the Network Slice Selection Function (NSSF) and subscription data from the Unified Data Management (UDM), validates these requests. The network then responds with an Allowed NSSAI, which is the set of S-NSSAIs the UE is permitted to use in the current registration area. The UE stores this Allowed NSSAI and includes a subset of it, called the Configured NSSAI for the serving Public Land Mobile Network (PLMN), in subsequent registration and PDU Session Establishment requests. This process ensures the UE is connected to the correct AMF and Session Management Function (SMF) instances that support the requested slices, enabling the tailored connectivity and resource allocation that defines network slicing.

Purpose & Motivation

NSSAI was created to solve the critical problem of slice selection and routing in a 5G network where multiple logical networks (slices) coexist on shared physical infrastructure. In pre-5G systems, a UE simply attached to "the network." With network slicing, a UE may need access to several distinct logical networks simultaneously—for example, one for regular internet browsing (eMBB), one for a corporate VPN, and one for a low-latency gaming service. The network needed a standardized way for the UE to signal which of these logical networks it required for a given communication context.

The NSSAI mechanism provides this signaling. It addresses the limitation of earlier networks which lacked a native, standardized identifier for service-specific network partitions. Without NSSAI, implementing slicing would require non-standard, proprietary signaling or inefficient over-the-top solutions. NSSAI enables efficient network resource utilization by allowing the (R)AN and core network to direct traffic to the specific set of network functions (AMF, SMF, UPF, etc.) instantiated for a slice. This is essential for meeting the diverse Key Performance Indicators (KPIs) of different services, as it ensures the UE's data plane and control plane are handled by functions configured with the appropriate policies (e.g., latency, bandwidth, security). Its introduction was a cornerstone in making the 5G vision of customized logical networks a practical reality.