Preferential Closed User Group

Description

Preferential CUG (Closed User Group) is a supplementary service defined in 3GPP specifications 22.085, 24.454, and 24.654. A Closed User Group is a set of subscribers who are allowed to communicate only among themselves, with restrictions on communication to users outside the group. The Preferential CUG feature allows a user who subscribes to multiple CUGs to designate one of them as the default. When the user initiates a communication (e.g., a call or data session) without explicitly specifying a CUG index, the network uses the preferential CUG to identify the required CUG for that communication. This simplifies the user experience by automatically applying the preferred group without requiring manual selection each time.

The technical implementation of Preferential CUG involves the storage of subscription data in the Home Subscriber Server (HSS) or Unified Data Management (UDM). The subscription includes a list of CUGs to which the user belongs, and for each basic service group (e.g., telephony, data), a preferential CUG may be defined. When the user registers or initiates a communication, the network retrieves this information. If the outgoing request does not contain a CUG index, the network checks the preferential CUG for the relevant basic service group and uses that index to determine the CUG. The CUG index is then used to enforce the CUG restrictions, such as allowing or denying calls to other CUG members or non-members.

The Preferential CUG is particularly useful in enterprise environments where an employee might belong to multiple CUGs (e.g., one for the entire company, one for a specific department). By setting a preferential CUG, the employee can ensure that their default communications are treated as part of the most relevant group, without having to remember and input a CUG index each time. This reduces the risk of errors and enhances usability. The network handles the selection transparently, applying the appropriate CUG restrictions and privileges.

In terms of signaling, the Preferential CUG is managed through supplementary service procedures. The user can activate, deactivate, or modify the preferential CUG using Unstructured Supplementary Service Data (USSD) or other management interfaces. The network acknowledges these requests and updates the subscription data accordingly. When a communication is set up, the Mobile Switching Center (MSC) or Mobility Management Entity (MME) checks the preferential CUG if no explicit CUG index is provided. This check is part of the call/session setup process and ensures that the correct CUG is applied.

The concept has evolved over releases. In early releases (Rel-4 to Rel-7), the definition emphasized that a preferential CUG is the nominated default CUG to be used when no explicit CUG index is received. Starting from Rel-8, the wording changed to focus on the user nominating a CUG index that the network uses as a default. This shift reflects a more user-centric perspective, highlighting the subscriber's role in nominating the preferential CUG. The feature has been maintained through Rel-19, demonstrating its continued relevance in enterprise and group communication scenarios.

Purpose & Motivation

Preferential CUG was created to simplify the use of Closed User Groups for subscribers who belong to multiple CUGs. Before its introduction, users had to explicitly specify a CUG index for each communication if they wanted to use a particular CUG. This was cumbersome and error-prone, especially in enterprise settings where users might frequently switch between different groups. The Preferential CUG feature solves this by allowing users to nominate a default CUG that is automatically applied when no explicit index is provided. This enhances usability and reduces the likelihood of misdialing or incorrect group selection.

The feature also addresses the need for efficient network operation. By allowing a default CUG, the network can streamline call setup procedures, as it does not need to prompt the user for a CUG index in every case. This is particularly beneficial in high-traffic environments where quick call establishment is important. Additionally, it provides a way for operators to offer differentiated services to enterprise customers, who often require multiple CUGs for different departments or projects.

Historically, the concept was introduced in Release 4 as part of the GSM supplementary services. It was carried forward into UMTS and LTE, with adaptations to new core network architectures. The change in wording in Rel-8 reflects a refinement in the specification to better describe the user's action of nominating the preferential CUG. The feature remains relevant in 5G, where CUG-like concepts are used for network slicing and private networks, although the term Preferential CUG itself is primarily associated with legacy circuit-switched services.