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.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (788 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the NSAC function was introduced with capabilities for S-NSSAI-based congestion control, including the sending of congested S-NSSAI lists during access network signaling. It also defined mechanisms for slice availability updates, such as providing a Partially Allowed NSSAI with associated tracking area lists, and introduced the concept of an Alternative S-NSSAI for use when a requested slice is unavailable or congested.
- Including S-NSSAI received in EPS in Requested NSSAI and in PDU session establishment request upon inter-system change from S1 mode to N1 mode TS 24.501CR0082
- Network control for always-on PDU sessions TS 24.501CR0107
- Clarification on congestion control upon intersystem change TS 24.501CR0604
- Correction to rejected S-NSSAI TS 23.501CR0007
- Correction to handling of S-NSSAI mapping information TS 23.501CR0020
- Control of the Messages triggering Paging at AMF TS 23.501CR0033
+ 84 more changes
In Release 16, the NSAC function was enhanced to support an Alternative S-NSSAI, which the AMF can use to replace a requested S-NSSAI when it is unavailable or congested. This release also introduced the concept of a Partially Allowed NSSAI, where each listed S-NSSAI is associated with specific Tracking Areas where it is supported. Furthermore, Release 16 defined mechanisms for slice interworking and network slice-specific authentication and authorization, as indicated by the pending NSSAI provided during registration.
- Introducing Rate Control for 5G CIoT TS 23.501CR0752
- Introduction of data transfer in Control Plane CIoT 5GS Optimisation TS 23.501CR0889
- CIoT Introduction of Overload Control TS 23.501CR0894
- Introduction of Service Gap Control TS 23.501CR1014
- UL CL/BP controlled by I-SMF TS 23.501CR0848
- Enhancement on slice interworking--501 TS 23.501CR0850
+ 162 more changes
In Release 17, the NSAC function was enhanced with new capabilities including support for roaming scenarios, control based on access type, and the ability to support multiple NSACF instances for a single network slice during UE mobility. It also introduced support for emergency and priority services within admission control and the ability to use network data analytics for slice restriction decisions. Furthermore, the release added support for key GSMA NG.116 attributes like maximum throughput per slice per UE and the simultaneous use of a network slice.
- Support of different slices over different Non 3GPP access TS 23.501CR2525
- Network Slice restriction based on NWDAF analytics TS 23.501CR2567
- TS23.501 KI#1 Network Slice Admission Control Function (NSACF) definition TS 23.501CR2679
- TS23.501 KI#2 Network Slice Admission Control Function (NSACF) definition TS 23.501CR2680
- Support for UE-Slice-MBR TS 23.501CR2706
- Adding the usage of Session Management Congestion Control Experience analytics TS 23.501CR2708
+ 160 more changes
In Release 18, the NSAC function was enhanced with a hierarchical architecture to support centralized admission control across multiple service areas and to provide counting for EPS. It also introduced the concept of a Partially Allowed NSSAI, where an S-NSSAI is associated with specific Tracking Areas where it is supported, and Alternative S-NSSAI for replacement when a requested slice is unavailable or congested.
- N3IWF selection enhancement for support of S-NSSAI needed by UE TS 23.501CR3707
- Multiple NSACF architecture enhancement TS 23.501CR3785
- KI#4: Support for Centralized NSACF in a PLMN with multi-service areas TS 23.501CR3822
- KI#4: Support for HPLMN admission mode while Roaming TS 23.501CR3823
- Policy control enhancements to support multi-modal flows TS 23.501CR3864
- Change of Network Slice instance for PDU sessions TS 23.501CR3867
+ 255 more changes
In Release 19, the NSAC function was enhanced to support slice change based on an AF request and to utilize an Alternative S-NSSAI for replacement when a requested slice is unavailable or congested. Furthermore, the release introduced support for S-NSSAI level exposure related to energy efficiency and clarified network slice handling for Indirect Network Sharing. These additions provide more dynamic and policy-driven control over network slice admission and accessibility.
- Control Plane and User Plane Protocol stacks involving the MWAB node TS 23.501CR5561
- KI#2: UE subscription and policy control for energy efficiency and energy saving TS 23.501CR5739
- Functional Description of Energy Efficiency Control Functionality TS 23.501CR5740
- Support of Slice change based on AF request TS 23.501CR5764
- Control of UE access to MWAB TS 23.501CR5468
- Support Alternative QoS profile with PDU Set QoS and Notification Control TS 23.501CR5524
+ 95 more changes
In Release 20, the NSAC function was enhanced to incorporate energy consumption information exposure and policy control, as indicated by the Change Request titles. This update included architectural refinements and the specification of an EIF function to support policy control specifically for network energy saving. These additions integrated energy efficiency considerations directly into the network slice admission control framework.
Explore further
Broader topics and technologies where NSAC plays a role.
Defining Specifications
3GPP specifications that define or reference NSAC, 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 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 28.203 vi10 | Charging management | Rel-18 |
| TS 29.122 vj40 | T8 Reference Point for Northbound APIs | Rel-19 |
| TS 29.522 vj40 | 5G NEF Northbound APIs Stage 3 | Rel-19 |
| TS 29.536 vj30 | NSACF Service Based Interface Protocol | Rel-19 |