Closed Subscriber Group Whitelist

Description

The CSG whitelist is a subscriber-specific list maintained in the UE that contains the CSG identities and corresponding PLMN identities of all Closed Subscriber Groups to which the subscriber has authorised access. It is provided by the Non-Access Stratum (NAS) layer, specifically as part of the subscriber's subscription information stored in the USIM and managed by the network. The whitelist is used by the Access Stratum (AS) during cell selection, reselection, and handover procedures to determine whether a cell broadcasting a particular CSG ID is a CSG member cell for that UE. The list is defined in 3GPP TS 25.304 and TS 45.008, and its content is derived from the CSG subscription data in the HSS/HLR.

Operationally, the CSG whitelist is stored in the UE's non-volatile memory and can be updated by the network via NAS signalling procedures such as the ATTACH ACCEPT, TAU ACCEPT, or a dedicated CSG list management procedure. The network may add, remove, or modify entries in the whitelist based on changes to the subscriber's CSG subscription. The UE uses the whitelist in conjunction with the CSG ID broadcast by a cell: if the cell's CSG ID and PLMN identity match an entry in the whitelist, the cell is considered a CSG member cell. This comparison is performed during cell reselection in RRC_IDLE and during measurement reporting and handover evaluation in RRC_CONNECTED.

The CSG whitelist is distinct from the Allowed CSG List, which is a similar concept but applies to UEs that are not members but have been temporarily allowed access, for example, for emergency services. The whitelist is also different from the operator-controlled CSG list used by the network for access control decisions. The UE's whitelist is the authoritative source from the UE's perspective, ensuring that access decisions are made based on the subscriber's actual entitlements.

In Release 17, the term 'CSG whitelist' was renamed to 'Permitted CSG list' in the specifications, reflecting updated terminology in NAS specifications. The functionality remains the same, but the new name better aligns with the concept of a list of permitted CSGs. This change was part of a broader effort to harmonise terminology across 3GPP specifications and to avoid the term 'whitelist' due to its potential for misinterpretation. Despite the renaming, the underlying mechanisms and procedures for managing and using the list remain unchanged.

The CSG whitelist is a critical enabler for CSG access control and mobility. Without it, the UE would have no way to know which CSG cells it is allowed to access, leading to either failed access attempts or security breaches. The list also supports manual CSG selection, where the user can choose from available CSGs, and the UE will only allow selection of those in the whitelist. Overall, the CSG whitelist is a fundamental component of the CSG architecture, bridging NAS subscription data and AS access control.

Purpose & Motivation

The CSG whitelist was introduced to provide a standardised, subscriber-specific mechanism for the UE to determine its authorised CSG cells. Before its introduction, there was no uniform way for a UE to know which restricted cells it could access, leading to either unnecessary access attempts to non-permitted cells or inability to access permitted ones. The whitelist solves this by storing the necessary information in the UE and making it available to the access stratum for decision-making.

It also enables efficient mobility management. By comparing a cell's broadcast CSG ID and PLMN against the whitelist, the UE can quickly classify a cell as a member or non-member, allowing it to prioritise member cells for reselection and handover. This improves user experience and network efficiency. The whitelist is also used by the network to manage CSG subscription data and to ensure that only authorised UEs access CSG cells.

Historically, the CSG whitelist was defined in Release 9 as part of the Home NodeB/Home eNodeB specifications. It was later renamed to Permitted CSG list in Release 17 to align with evolving terminology and to emphasise that it represents a list of CSGs from which the subscriber is permitted to access. The concept has been extended to 5G with the introduction of the CAG (Closed Access Group) list, which serves a similar purpose for 5G non-public networks.