Closed User Group Index

Description

The CUG index is a parameter used in the context of Closed User Groups (CUGs), which are a feature of GSM and UMTS networks that allows groups of subscribers to communicate among themselves and restrict access from outside the group. The CUG index is provided by the originating user to select a specific CUG when originating a communication. This parameter is essential for users who belong to multiple CUGs, as it enables them to choose which group's privileges and restrictions apply to a particular call.

In the network architecture, the CUG index is part of the call setup signaling. When a user initiates a call, the mobile station includes the CUG index in the setup message if the user wishes to use a specific CUG. The network then validates the CUG index against the user's subscription data, which is stored in the Home Location Register (HLR) or Home Subscriber Server (HSS). If the user is authorized for the specified CUG, the call proceeds with the CUG's characteristics, such as allowed destinations and charging rules. If the CUG index is invalid or the user is not authorized, the call may be rejected or handled according to the CUG's restrictions.

The CUG index is defined in 3GPP specifications such as TS 22.085, TS 24.454, and TS 24.654. These specifications outline the procedures for CUG provisioning, activation, and invocation. The CUG index is typically a numeric value that identifies a particular CUG within the user's subscription profile. It is distinct from the CUG interlock code, which is used within the network to identify CUG membership. The CUG index is user-facing, allowing the user to select a CUG, while the interlock code is used internally for routing and authorization.

The concept of CUG was introduced in early GSM releases to provide a private networking capability for businesses and groups. The CUG index was initially defined as a value assigned by the network to identify a CUG. However, in Release 8, the definition was refined to emphasize its role as a parameter used by the originating user to select a particular CUG. This change reflected a more user-centric approach, highlighting the user's ability to choose among multiple CUGs.

Overall, the CUG index is a key element in the CUG service, enabling flexible and secure group communication. It allows users to manage their CUG memberships and select the appropriate group for each call, ensuring that the correct privileges and restrictions are applied.

Purpose & Motivation

The CUG index exists to provide a user-friendly way for subscribers to select a specific Closed User Group (CUG) when making a call. CUGs are used to create private communication groups within a public network, allowing members to communicate freely while restricting access from non-members. Without a CUG index, a user belonging to multiple CUGs would have no way to indicate which group's rules should apply to a particular call, leading to ambiguity and potential service disruptions.

The CUG index solves the problem of multiple CUG memberships by giving each CUG a unique identifier that the user can specify during call setup. This is particularly important in business environments where an employee might belong to several CUGs, such as a department-specific group and a company-wide group. By selecting the appropriate CUG index, the user ensures that the call is subject to the correct set of permissions and restrictions, such as allowed destinations and charging rates.

Historically, the CUG index was defined as a value assigned by the network to identify a CUG. This definition focused on the network's perspective, where the index was used internally to manage subscriptions. However, as CUG services evolved, the need for user control became apparent. The definition was updated in Release 8 to emphasize the user's role in selecting a CUG, reflecting a shift towards more personalized and flexible services. This change aligned with the broader trend in telecommunications towards user-centric service delivery.