Outgoing Calls Barred within the CUG

Description

Outgoing Calls Barred (OCB) is a supplementary service defined within the 3GPP framework, operating as a specific restriction within the broader Closed User Group (CUG) service. A CUG is a subscription option that defines a group of subscribers who are permitted to communicate only amongst themselves, creating a logical private network over the public mobile infrastructure. The OCB feature is a specific barring condition applied to a CUG member, which prohibits that member from initiating (i.e., making) calls to destinations outside the boundaries of their assigned CUG. The enforcement of this barring is managed by the core network, specifically within the Home Location Register (HLR) or Home Subscriber Server (HSS) and the Mobile Switching Center (MSC) or Call Session Control Function (CSCF). When a subscriber with an active OCB restriction attempts to place a call, the originating MSC or CSCF queries the subscriber's profile from the HLR/HSS. Upon identifying the OCB flag and verifying that the dialed number is not a member of the subscriber's permitted CUG(s), the network rejects the call setup request, typically returning a tone or announcement to the caller. The service is highly configurable per subscriber, allowing network operators to define multiple CUGs with different barring characteristics, including combinations with Incoming Calls Barred (ICB) and other CUG interlock codes. This architecture ensures that service logic and subscriber data are centralized in the home network, allowing for consistent application of barring rules even when the subscriber is roaming. OCB is a fundamental tool for implementing controlled communication environments, directly impacting call control, billing (by preventing chargeable calls to external numbers), and service differentiation.

Purpose & Motivation

The primary purpose of OCB is to provide a mechanism for creating controlled, private communication groups within a public mobile network. It solves the problem of uncontrolled communication costs and unauthorized external calls for specific subscriber groups, such as employees within a company, members of a government agency, or devices in a machine-to-machine (M2M) fleet. Before such features were standardized, organizations had to rely on physical private networks or basic barring services that lacked the granularity of group-based logic. The creation of CUG services with OCB and ICB options was motivated by commercial demands from enterprise customers who required the cost control and privacy of a Private Branch Exchange (PBX) but with the mobility and coverage of a public cellular network. It allows operators to offer value-added services that cater to business segments, generating additional revenue streams beyond individual consumer plans. Historically, this concept originated in fixed-line telephony and was adapted into GSM and subsequent 3GPP systems, becoming a cornerstone of intelligent network (IN) based supplementary services. It addresses the limitation of simple barring (like barring all outgoing calls) by providing a more sophisticated, group-aware restriction that still permits essential internal communication.

Key Features

• Restricts outgoing calls to numbers outside the subscriber's designated Closed User Group(s).
• Network-enforced at call setup by the MSC or CSCF based on HLR/HSS profile data.
• Operates independently or in combination with Incoming Calls Barred (ICB) within a CUG.
• Supports multiple CUG subscriptions with different interlock codes for complex group structures.
• Applicable in both circuit-switched (CS) and IP Multimedia Subsystem (IMS) domains.
• Remains effective in roaming scenarios through interaction between visited and home networks.

Evolution Across Releases

Defining Specifications