CONference calling

Description

CON (CONference calling) is a standardized supplementary service defined in 3GPP specifications that enables multi-party voice communication within cellular networks. It operates as a service layer feature managed by the core network, specifically within the IP Multimedia Subsystem (IMS) for packet-switched domains and the circuit-switched core for legacy networks. The service allows a user, known as the conference controller or initiator, to establish a conference call by sequentially or simultaneously adding other participants, who can be mobile subscribers or fixed-line users. The conference can be ad-hoc, created dynamically during a call, or pre-arranged with scheduled start times and participant lists. Key network elements involved include the Serving Call Session Control Function (S-CSCF) in IMS, which handles session control, and the Media Resource Function (MRF), which provides media mixing capabilities to combine audio streams from all participants into a single coherent output for each attendee.

The architecture of CON relies on signaling protocols such as SIP (Session Initiation Protocol) in IMS-based implementations and ISUP (ISDN User Part) in circuit-switched scenarios. When a user initiates a conference, the network allocates conference resources, including a conference bridge in the MRF, which mixes real-time audio packets. Participants are connected via bearer paths that transport voice data, with the MRF ensuring synchronization and managing talker identification, echo cancellation, and volume leveling. The service supports various conference states, such as active, held, or terminated, and allows the controller to manage the call by adding, removing, or isolating participants. In IMS, CON is integrated with other supplementary services like call hold and call transfer, enabling seamless user interactions through Ut interface-based management or in-call signaling.

CON's implementation includes features like conference locking to prevent unauthorized additions, sub-conferencing for private side conversations, and billing differentiation for initiators versus participants. The service is defined across multiple 3GPP specifications, including TS 23.279 for architecture, TS 24.508 for signaling procedures, TS 29.235 for media control, and TS 32.808 for charging aspects. It supports interoperability with non-3GPP networks through gateways, ensuring conference calls can include participants from PSTN or other VoIP systems. The service's robustness is enhanced by error handling mechanisms, such as fallback to bilateral calls if conference resources are unavailable, and quality of service (QoS) prioritization to maintain low latency and high audio clarity during multi-party sessions.

Purpose & Motivation

CON was introduced to address the growing need for efficient group communication in mobile networks, particularly for business collaboration, family discussions, and emergency coordination. Prior to its standardization, multi-party calling was often limited to proprietary solutions or required external conference bridges, leading to interoperability issues and inconsistent user experiences. By defining CON as a supplementary service in 3GPP Release 7, it provided a uniform, network-native capability that could be seamlessly integrated into existing cellular infrastructures, reducing reliance on third-party services and enhancing service reliability.

The creation of CON was motivated by the expansion of mobile telephony into professional environments, where conference calls became essential for remote meetings and decision-making. It solved limitations of earlier ad-hoc methods, such as call waiting and manual call merging, which were cumbersome and lacked features like participant management or centralized media mixing. CON's standardization ensured consistent behavior across operators and devices, supporting both circuit-switched and packet-switched domains, and later evolved to align with IMS architectures for all-IP networks. This allowed operators to offer value-added services, generate additional revenue through conference call charges, and improve user satisfaction by enabling intuitive, multi-party interactions directly from mobile handsets.

Key Features

• Multi-party voice session establishment with up to several participants
• Support for ad-hoc and pre-arranged conference types
• Conference controller capabilities for adding, removing, and isolating participants
• Integration with IMS media resources via MRF for audio mixing
• Interworking with circuit-switched and packet-switched networks
• Charging differentiation between initiator and participants

Evolution Across Releases

Defining Specifications