Network Slice Access Group

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.