Network Slice Admission Control

Description

Network Slice Admission Control (NSAC) is a fundamental control plane function introduced in 3GPP Release 17 to manage the scale and capacity of network slices. It operates by enforcing a maximum allowed number of User Equipments (UEs) that can be simultaneously registered to a given network slice instance (NSI). This is not a resource admission control for data traffic, but rather a registration-level admission control. The primary goal is to prevent a slice from becoming overloaded with registered UEs, which could degrade the quality of service for all users of that slice and potentially impact other slices sharing the same underlying infrastructure. NSAC is a policy-based mechanism, where the maximum number of UEs per slice is a key policy parameter defined by the network operator, often aligned with commercial or technical service level agreements (SLAs).

The NSAC function is typically implemented within the core network's control plane, interacting with key network functions like the Access and Mobility Management Function (AMF) and the Network Slice Admission Control Function (NSACF). When a UE initiates a registration procedure and requests a network slice, the AMF consults the NSAC mechanism to check if the UE can be admitted to the requested slice. This check involves verifying whether the current registration count for that slice is below the configured maximum limit. If the limit has not been reached, the registration is allowed, and the counter is incremented. If the limit is reached, the registration request for that specific slice may be rejected, or the UE may be directed to an alternative slice, depending on network policies and UE capabilities.

NSAC works in conjunction with the Network Slice Admission Control Function (NSACF), which is the entity responsible for maintaining the registration counts and enforcing the admission policies. The AMF communicates with the NSACF via service-based interfaces (e.g., Namf_Communication) to report registration and deregistration events. The NSACF maintains per-slice counters, updates them based on these events, and provides admission decisions to the AMF. This architecture centralizes the admission control logic, allowing for consistent policy enforcement across the network. NSAC also supports monitoring and reporting capabilities, enabling operators to track slice usage and make informed decisions about capacity planning and slice lifecycle management.

Purpose & Motivation

NSAC was created to address a critical gap in the initial 3GPP network slicing architecture defined in Release 15. While early releases provided the mechanisms to create and manage slices, they lacked robust controls to limit the scale of a slice in terms of connected users. Without such control, a popular slice (e.g., for a massive IoT service or a popular enterprise application) could experience an uncontrolled influx of UEs, leading to signaling storms, resource exhaustion, and performance degradation for all users on that slice and potentially on shared network functions. This posed a significant risk to network stability and the ability to guarantee isolated performance as promised by slicing.

The introduction of NSAC in Release 17 was motivated by the need for operators to offer reliable and predictable slice-based services with enforceable SLAs. It allows operators to define a 'capacity' for a slice not just in bandwidth, but in terms of supported devices, which is essential for commercial models (e.g., selling a slice to an enterprise for up to 10,000 devices). It solves the problem of slice overload by providing a hard limit, protecting the slice from being overwhelmed. Furthermore, it enables more efficient resource planning and prevents one slice from monopolizing common control plane resources, thereby upholding the principle of resource isolation that is foundational to network slicing.