Closed Access Group cell

Description

A CAG cell is a cell in a 5G network that restricts normal service to subscribers who are members of a specific Closed Access Group (CAG). The CAG is a list of subscribers (or devices) that are permitted to access the cell for regular communication. This concept is part of the 5G Non-Public Network (NPN) framework, specifically for public network integrated NPNs (PNI-NPNs), where a private network is hosted on a public network's infrastructure. The CAG cell is defined in 3GPP specifications 24.501 (Non-Access-Stratum (NAS) protocol for 5G System) and 38.413 (NG-RAN; NG Application Protocol (NGAP)).

In a CAG cell, the network broadcasts a CAG identifier (CAG ID) in the system information. Subscribers that are members of the CAG have this CAG ID in their subscription data (stored in the Unified Data Management, UDM). When a UE attempts to access the cell, it checks whether its allowed CAG list includes the broadcast CAG ID. If it does, the UE can access the cell for normal services. If not, the UE is not allowed to access the cell for normal services, but it may still be allowed to initiate emergency calls or emergency services fallback, depending on local regulations. This ensures that even non-members can reach emergency services when needed.

The CAG cell concept is designed to support private networks that are integrated into public networks. For example, a factory may deploy a private 5G network on a public operator's spectrum. The factory's CAG cell would only allow factory devices (with the appropriate CAG subscription) to use the cell for normal traffic, while public subscribers would be barred from normal service but could still make emergency calls. This provides isolation and security for the private network while leveraging the public network's coverage and infrastructure.

From a technical standpoint, the CAG cell is identified by a CAG ID, which is a 32-bit identifier. The cell broadcasts this ID in the SIB1 (System Information Block 1) along with an indication that it is a CAG cell. The UE's NAS layer manages the allowed CAG list, which is provided by the network during registration or configuration. When the UE is in a CAG cell, it performs access control checks: if the cell's CAG ID is in the allowed list, the UE can proceed with normal access; otherwise, it may only attempt emergency services. The NG-RAN (Next Generation Radio Access Network) enforces these restrictions at the radio level, and the core network (AMF) also performs checks during registration.

The CAG cell is a key enabler for 5G private networks, allowing enterprises to have dedicated coverage and services while still benefiting from the public network's capabilities. It also supports roaming scenarios where a subscriber from one CAG may access another CAG cell if they have the necessary permissions. The concept was introduced in Rel-19 as part of the ongoing evolution of 5G NPN features, building on earlier work in Rel-16 and Rel-17 on NPNs. It provides a standardized way to manage access control for private networks, ensuring that only authorized users can access the private resources, while maintaining regulatory requirements for emergency services.

In summary, a CAG cell is a cell that restricts normal service to members of a Closed Access Group, with exceptions for emergency services. It is a fundamental building block for public network integrated non-public networks, providing secure and controlled access for private network users.

Purpose & Motivation

The CAG cell concept exists to enable private 5G networks to be integrated into public networks while maintaining strict access control. Before CAG cells, private networks were often deployed as standalone networks with their own dedicated spectrum and infrastructure, which was costly and inefficient. Alternatively, they could be implemented using network slicing, but that did not provide cell-level access control. The CAG cell solves this by allowing a public network to host private cells that only authorized users can access for normal services, while still allowing emergency calls for all users as required by regulations.

The motivation for CAG cells comes from the growing demand for private networks in industries such as manufacturing, logistics, and healthcare. These industries need dedicated coverage and performance guarantees, but they also want to leverage the economies of scale and wide coverage of public networks. The CAG cell provides a way to create a private network within a public network without compromising security or isolation. It also simplifies the deployment and management of private networks, as the public operator can handle the infrastructure and spectrum, while the enterprise manages its own subscribers and services.

Historically, the concept of Closed Subscriber Groups (CSG) existed in 3GPP for 3G and 4G networks, allowing restricted access to specific cells (e.g., femtocells). However, CSG was primarily designed for residential or small enterprise use and did not scale well for large private networks. The CAG cell in 5G is an evolution of that idea, tailored for the 5G Service-Based Architecture and the needs of industrial private networks. It was introduced in Rel-19 to address the specific requirements of PNI-NPNs, providing a standardized and scalable solution for access control in private networks.