Closed User Group Interlock Code

Description

The CUG interlock code is a parameter used within the network to identify membership in a Closed User Group (CUG). Unlike the CUG index, which is user-facing and allows the originating user to select a CUG, the interlock code is used internally by the network to validate and enforce CUG restrictions. When a call is set up, the network uses the interlock code to determine whether the calling and called parties belong to the same CUG and whether the call should be allowed based on the CUG's rules.

In the 3GPP architecture, the CUG interlock code is part of the subscription data stored in the Home Location Register (HLR) or Home Subscriber Server (HSS). Each CUG is assigned a unique interlock code, which is included in the call setup signaling when a CUG call is made. The network compares the interlock codes of the calling and called parties to verify that they are members of the same CUG. If they are, the call proceeds; if not, the call may be rejected or handled according to the CUG's restrictions, such as allowing only outgoing calls or barring incoming calls.

The CUG interlock code is defined in 3GPP specifications such as TS 24.454 and TS 24.654. These specifications outline the procedures for CUG provisioning, activation, and invocation, including the use of the interlock code. The interlock code is typically a numeric value that is unique within the network. It is distinct from the CUG index, which is used by the user to select a CUG. The interlock code is not exposed to the user; it is used solely by the network for internal processing.

The concept of the CUG interlock code was introduced in Release 8, although CUG itself dates back to earlier releases. The interlock code provides a standardized way to identify CUG membership, ensuring interoperability between different network elements and vendors. It allows the network to enforce CUG restrictions consistently, regardless of the user's device or location.

Overall, the CUG interlock code is a critical component of the CUG service, enabling the network to manage group communications securely and efficiently. It ensures that only authorized users can communicate within a CUG and that the correct charging and routing rules are applied.

Purpose & Motivation

The CUG interlock code exists to provide a standardized identifier for CUG membership within the network. While the CUG index allows users to select a CUG, the network needs a reliable way to verify that the calling and called parties are members of the same CUG and to enforce the group's restrictions. The interlock code serves this purpose by uniquely identifying each CUG within the network, enabling consistent authorization and routing decisions.

Without the interlock code, the network would struggle to manage CUG calls effectively. For example, if a user belongs to multiple CUGs, the network must determine which CUG applies to a particular call. The interlock code, combined with the CUG index, allows the network to map the user's selection to the correct CUG and apply the appropriate rules. This is essential for services like closed user groups, where members may have different privileges, such as the ability to make calls to non-members or receive calls from non-members.

Historically, CUG services were implemented in a proprietary manner, leading to interoperability issues. The introduction of the CUG interlock code in 3GPP Release 8 standardized the identification of CUG membership, ensuring that different network elements and vendors could interoperate seamlessly. This standardization was crucial for the widespread adoption of CUG services and for enabling roaming between networks. The interlock code also simplifies the management of CUG subscriptions, as it provides a single, unambiguous identifier that can be used across the network.