Description
The Network Slice Access Group (NSAG) is a RAN-level concept introduced in 3GPP Release 17 to optimize Radio Access Network (RAN) procedures for network slicing. It is an identifier that groups together one or more Network Slice Selection Assistance Information (S-NSSAI) values. The core idea is that from the RAN's perspective, UEs that are allowed to access the same set of slices (i.e., the same NSAG) can be treated similarly for certain RAN procedures, allowing for more efficient resource management and signaling. The NSAG is configured in the RAN by the management system (OAM) and is communicated to the UE and the core network.
Operationally, the NSAG is used in several key RAN procedures. During the initial access, a UE may indicate its supported NSAG to the gNB in the RRC message, helping the RAN understand the slice grouping context early. More importantly, NSAG is utilized in paging optimization. When the core network (AMF) needs to page a UE in RRC_IDLE or RRC_INACTIVE state, it includes the UE's NSAG in the paging message sent to the RAN. The RAN can then broadcast the paging message only in cells that support the slices associated with that NSAG, rather than in all cells. This significantly reduces unnecessary paging overhead and improves battery life for UEs not in the target slice group. Furthermore, the RAN can use NSAG for radio resource allocation policies, connection establishment prioritization, and mobility settings that are common to all slices within the group.
The NSAG identifier is part of the UE context in both the core network and the RAN. The AMF determines the NSAG for a UE based on the subscribed and allowed S-NSSAIs and includes it in the UE context established with the gNB (e.g., via the NGAP interface). This allows the RAN to maintain a mapping between the UE, its allowed slices, and the corresponding NSAG. The use of NSAG effectively abstracts the potentially large and dynamic set of S-NSSAIs into a smaller number of stable groups, simplifying RAN implementations and reducing the signaling burden of handling individual slice identifiers in every RAN procedure.
Purpose & Motivation
NSAG was introduced to address RAN scalability and efficiency challenges posed by a large number of fine-grained network slices. In early 5G releases, the RAN had to handle individual S-NSSAIs for each UE, which could lead to signaling overhead and complex processing, especially for procedures like paging and mobility that are agnostic to the specific service but related to the slice group a UE belongs to. The RAN needed a way to aggregate slice-specific information to apply common policies without processing each slice individually.
The primary problem NSAG solves is inefficient paging. Without NSAG, when paging a UE, the RAN might have to broadcast the page in all cells, wasting radio resources and causing interference, because it doesn't know which cells support the UE's specific slices. NSAG allows the RAN to perform 'slice-aware paging,' targeting only relevant cells. This is crucial for network slicing to be efficient in the RAN. Furthermore, NSAG simplifies RAN implementation for slice-based radio resource management, allowing the gNB to configure common parameters (like dedicated random access resources or scheduling policies) per NSAG rather than per S-NSSAI, which is more scalable and manageable.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (647 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the NSAG (Network Slice Access Group) function was newly introduced to manage the association of a UE to a specific set of network slices. It provides mechanisms for handling S-NSSAI mapping, updating subscribed and configured S-NSSAI, and performing S-NSSAI based congestion control. The release also introduced clarifications for modifying the set of network slices for a UE and for slice selection procedures.
- 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
- Correction to rejected S-NSSAI TS 23.501CR0007
- Correction to handling of S-NSSAI mapping information TS 23.501CR0020
- Change subscribed S-NSSAI in UE to configured NSSAI of HPLMN TS 23.501CR0040
- Slice selection cleanup TS 23.501CR0047
- Complete of IMS Emergency support in 5G including slice and local numbers TS 23.501CR0052
+ 55 more changes
In Release 16, the NSAG function was enhanced with the introduction of the Pending NSSAI for network slice-specific authentication and authorization, a new state allowing a UE to be registered while the network completes slice-specific procedures. Additionally, the release detailed mechanisms for registration rejection due to no available allowed S-NSSAI(s) and defined procedures for handling network slice-specific authentication and authorization failure and revocation.
- Enhancement on slice interworking--501 TS 23.501CR0850
- Further detailing of 5G LAN group management TS 23.501CR1052
- Introduction of Slice-Specific Authentication and Authorisation TS 23.501CR1174
- 23.501 part of PCF selection for PDU sessions with same DNN and S-NSSAI TS 23.501CR1375
- Adding UDR NF Group ID association functionality TS 23.501CR1384
- Assistance indication for WUS grouping TS 23.501CR2053
+ 103 more changes
In Release 17, the NSAG (Network Slice Access Group) function was newly introduced to enable the configuration of Network Slice AS Groups, which allows for the grouping of network slices for specific Radio Resource Management (RRM) purposes. This enhancement works in conjunction with other Release 17 features for slice admission control, such as enabling slice priority and slice groups for RRM. These additions provide the network with more granular tools for managing slice-based resource allocation and access control.
- 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
- Support of 5GC assisted cell selection to access network slice TS 23.501CR2719
+ 136 more changes
In Release 18, the Network Slice Access Group (NSAG) function was enhanced by introducing the concept of a Partially Allowed NSSAI, where each S-NSSAI is associated with a list of Tracking Areas where it is supported, and an Alternative S-NSSAI determined by the NSSF for use when a requested slice is unavailable. It also added support for graceful termination of PDU sessions during network slice decommissioning and optimized procedures for handling temporal validity conditions for groups of UEs. Furthermore, the release enabled improved network control of UE behavior for a network slice and introduced mechanisms for network slice replacement.
- N3IWF selection enhancement for support of S-NSSAI needed by UE TS 23.501CR3707
- Change of Network Slice instance for PDU sessions TS 23.501CR3867
- Service area provisioning and LADN aspects for enhanced group management TS 23.501CR3914
- Improved network control of the UE beahviour for a network slice TS 23.501CR3939
- TNGF selection enhancement for support of S-NSSAI needed by UE TS 23.501CR3953
- KI#3, NEF exposure for handling PDU Session Type change and managing temporal invalidity/validity condition for a group of UEs TS 23.501CR3964
+ 266 more changes
In Release 19, the enhancements for the Network Slice Access Group (NSAG) function focused on refining Application Function (AF) interaction and slice lifecycle management. This included new support for AF-triggered slice replacement and modifications to a UE's set of network slices, along with clarifications for handling scenarios like Partially Allowed S-NSSAI interactions and Configured NSSAI mismatches during these procedures. Furthermore, the release provided corrections and clarifications on operational aspects such as the slice deregistration inactivity timer and network slice handling in Indirect Network Sharing scenarios.
- Exposure enhancements for static UE IP address assignment and 5G VN group's User Plane Security Policy TS 23.501CR5492
- Support of Slice change based on AF request TS 23.501CR5764
- Resolve EN for Network Slice Selection in Participating Operator Network TS 29.531CR0223
- S-NSSAI selection while in EPS TS 23.501CR5866
- Support for S-NSSAI granularity energy consumption exposure TS 23.501CR5956
- Clarification on network slice handling for Indirect Network Sharing TS 23.501CR5560
+ 57 more changes
Explore further
Broader topics and technologies where NSAG plays a role.
Defining Specifications
3GPP specifications that define or reference NSAG, 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 29.531 vj50 | 3GPP TS 29531: Nnssf Service Based Interface | Rel-19 |
| TS 37.320 vj00 | Minimization of Drive Tests (MDT) Overview | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.304 vj00 | UE RRC_IDLE and RRC_INACTIVE Procedures | Rel-19 |
| TS 38.321 vj00 | NR MAC Protocol Specification | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |
| TS 38.413 vj10 | NG Application Protocol (NGAP) | Rel-19 |
| TS 38.423 vj10 | Xn Application Protocol (XnAP) specification | Rel-19 |
| TS 38.470 vj10 | F1 Interface Introduction | Rel-19 |
| TS 38.473 vj10 | 5G F1 Application Protocol (F1AP) | Rel-19 |