Non-Closed Access Group Cell

Description

In 5G networks, the concept of Closed Access Groups (CAG) was introduced to support restricted access for specific user groups, similar to Closed Subscriber Group (CSG) in LTE. A CAG cell is an NR cell that broadcasts one or more CAG identities, and only UEs that are authorized for at least one of those CAGs are allowed to access the cell. Conversely, a Non-CAG Cell is defined as an NR cell that does not broadcast any CAG identity, or an E-UTRA cell connected to the 5G Core Network (5GCN). This means that Non-CAG cells are open to all UEs that are otherwise permitted by the network, without CAG-based access restrictions.

The distinction between CAG and Non-CAG cells is important for network deployment and UE behavior. When a UE performs cell selection or reselection, it must consider whether it is authorized to access a CAG cell. For Non-CAG cells, no such authorization check is needed. The network can configure the UE with a list of allowed CAGs (e.g., via NAS signalling), and the UE will only attempt to access CAG cells that broadcast an allowed CAG ID. Non-CAG cells are always considered accessible from a CAG perspective, provided other criteria (e.g., PLMN, tracking area) are met.

In terms of architecture, the CAG concept is part of the 5G System (5GS) and is specified in 3GPP TS 24.501 (NAS protocol for 5GS) and TS 38.413 (NGAP). The NG-RAN (gNodeB) broadcasts CAG identities in system information for CAG cells. For Non-CAG cells, no CAG identity is broadcast. The Access and Mobility Management Function (AMF) in the core network is responsible for managing CAG-related subscription data and for enforcing access control during registration procedures. When a UE registers with the network, it may provide a CAG identifier if it is accessing a CAG cell; the AMF then checks whether the UE is authorized for that CAG.

The introduction of CAG in Release 16 (and further refined in later releases) was motivated by the need to support private networks, such as those in factories, campuses, or venues, where access should be limited to specific devices or users. Non-CAG cells represent the traditional public mobile network cells that are open to all subscribers. The coexistence of CAG and Non-CAG cells allows operators to deploy mixed networks where some cells provide restricted access while others offer public access.

From a UE perspective, the behavior in Non-CAG cells is similar to that in any regular cell: the UE performs normal cell selection and registration procedures. The UE does not need to check CAG authorization for Non-CAG cells. However, the UE may still be configured with CAG-related parameters (e.g., allowed CAG list) for use when it encounters CAG cells. The network can also indicate whether a cell is a CAG cell or not via broadcast information, allowing the UE to make appropriate decisions.

In summary, Non-CAG Cells are the standard cells in 5G networks that do not restrict access based on CAG. They are essential for providing ubiquitous coverage and public services, while CAG cells cater to private or restricted access scenarios. The definition in TS 24.501 and TS 38.413 ensures consistent understanding across core and access network specifications.

Purpose & Motivation

The concept of Non-CAG Cells exists to clearly delineate cells that do not enforce Closed Access Group restrictions, thereby providing a baseline for open access in 5G networks. As 5G expanded to support private networks and restricted access groups, it became necessary to define both CAG and Non-CAG cells to ensure that UEs and network nodes can distinguish between them. This distinction is crucial for access control, cell selection, and mobility management.

Before the introduction of CAG in Release 16, access control in cellular networks was primarily based on PLMN and CSG (for LTE). However, CSG was limited to LTE and did not fully address the needs of 5G private networks. The CAG concept was introduced to provide a more flexible and scalable mechanism for restricted access in 5G, supporting multiple CAGs per cell and per UE. Non-CAG Cells were defined to represent the majority of cells that remain open to all subscribers, ensuring that the default behavior of UEs is not disrupted.

The definition of Non-CAG Cell also helps in interworking scenarios, such as when an E-UTRA cell is connected to the 5G Core Network. In such cases, the E-UTRA cell is considered a Non-CAG Cell because CAG is an NR-specific concept. This ensures that UEs can access E-UTRA cells via 5GCN without CAG-related checks, simplifying the integration of LTE and NR in 5G deployments. Overall, the purpose is to provide clarity and consistency in access control across different radio access technologies and deployment scenarios.