Outgoing Access (CUG SS)

Description

Outgoing Access (OA) is a specific capability within the Closed User Group Supplementary Service (CUG SS) defined by 3GPP. A Closed User Group is a service where a defined group of subscribers can communicate with each other, typically with restrictions on calls to and from the public network. The OA feature specifically governs the permissions for a CUG member to initiate calls to destinations outside their own CUG. The network determines whether an outgoing call attempt is permitted based on the subscriber's CUG subscription data, which includes an 'Outgoing Access' indicator. This data is stored in the Home Location Register (HLR) or Home Subscriber Server (HSS) and provided to the serving Mobile Switching Centre (MSC) or call session control function during registration and call setup.

When a CUG subscriber with OA enabled attempts to make a call, the serving network node (e.g., MSC) performs a CUG interrogation. It checks the called party number against the subscriber's CUG membership list. If the called number is not a member of any of the subscriber's CUGs, the OA indicator is evaluated. If OA is allowed ('OA allowed'), the call proceeds as a normal call to the public network or another CUG (if inter-CUG communication is permitted). If OA is not allowed ('OA not allowed'), the call attempt is rejected by the network. This enforcement is a core part of the call control logic in the circuit-switched or IMS core network.

The OA feature works in conjunction with other CUG features like Incoming Access (IA), which controls the ability to receive calls from outside the CUG. The combination of OA and IA settings defines the communication profile for a CUG member, such as 'fully closed' (no incoming or outgoing external calls) or 'hybrid' (outgoing allowed but incoming blocked, or vice-versa). This service is crucial for enterprise and vertical market solutions, allowing organizations to create secure, cost-controlled telephony groups within the public mobile network infrastructure. Its implementation relies on standardized signaling between network elements to transport CUG information and authorization results.

Purpose & Motivation

The Outgoing Access feature was created to provide granular control over communication within Closed User Group services, a fundamental requirement for business and private network offerings. Prior to standardized CUG services, operators offered basic group calling features with limited flexibility, often requiring complex and proprietary switch configurations. The OA specification solved the problem of uncontrolled communication costs and security by allowing service providers to explicitly define whether group members could call outside the private group.

This capability addresses several key needs: cost management for enterprises (preventing unauthorized expensive calls), security for sensitive organizations (restricting communication channels), and service differentiation for operators. By standardizing OA as part of the CUG SS in 3GPP Release 4, it ensured interoperability across different network vendors and operators, enabling seamless national and international CUG services. The feature allows for the creation of various service tiers, from completely isolated groups to groups with controlled external access, meeting diverse customer requirements without requiring non-standard network implementations.

Key Features

• Controls permission for CUG members to initiate calls to numbers outside their subscribed CUG(s).
• Defined by an 'OA allowed' or 'OA not allowed' indicator in the subscriber's CUG subscription data.
• Enforced by the MSC or call control function during call setup based on data from HLR/HSS.
• Works in combination with Incoming Access (IA) to define comprehensive CUG communication profiles.
• Supports multiple CUG index values for subscribers belonging to more than one group.
• Utilizes standardized MAP (Mobile Application Part) or Diameter signaling to transport CUG information.

Evolution Across Releases

Defining Specifications