Third Party Call Control

Description

Third Party Call Control (3PCC) is a standardized service architecture defined in 3GPP that enables an application server (AS) to act as a controlling entity for establishing and managing multimedia sessions between user endpoints. The AS functions as a back-to-back user agent (B2BUA) or third-party call controller that initiates separate SIP dialogs with each participating endpoint, coordinating the session parameters and media flows between them. This architecture allows the AS to maintain complete control over the session lifecycle, including setup, modification, and teardown, without requiring the endpoints to have direct signaling connectivity with each other.

The 3PCC architecture operates using SIP (Session Initiation Protocol) as the primary signaling protocol, with the application server implementing the 3PCC controller function. When establishing a session, the AS first initiates a SIP INVITE to one endpoint (the calling party) with an SDP offer describing the desired media characteristics. After receiving the response from this endpoint, the AS then initiates a separate SIP INVITE to the other endpoint (the called party), using the SDP answer from the first endpoint as the offer to the second endpoint. This creates two independent SIP dialogs that the AS bridges together at the signaling level, allowing it to control media negotiation and session parameters for both parties.

Key components of the 3PCC architecture include the 3PCC Application Server, which implements the call control logic; the Serving Call Session Control Function (S-CSCF) in the IMS core network, which routes SIP messages between endpoints and the AS; and the User Equipment (UE) endpoints that participate in the controlled sessions. The AS maintains separate SIP dialogs with each endpoint and can modify session parameters independently, enabling features like call transfer, call waiting, and conference bridging. The AS also handles media resource functions when needed, such as for announcements or media mixing in conference scenarios.

3PCC plays a critical role in enabling network-based services where the service logic resides in the network rather than in the endpoints. This allows service providers to deploy advanced telephony services without requiring client-side intelligence or software updates on user devices. The architecture supports various service scenarios including click-to-dial services (where a web application initiates a call between two parties), call center applications, prepaid calling services, and multimedia conferencing solutions. 3PCC also enables lawful interception capabilities and billing integration by providing the AS with complete visibility and control over the session establishment and teardown processes.

Purpose & Motivation

3PCC was developed to address the limitations of traditional peer-to-peer SIP session establishment, where both endpoints must have direct signaling capability and intelligence to negotiate session parameters. In early VoIP and multimedia services, advanced features like call transfer, conferencing, and network-initiated sessions required complex endpoint implementations or proprietary solutions. 3PCC provides a standardized mechanism for network-based service providers to offer value-added services without depending on endpoint capabilities.

The technology solves several key problems in telecommunications service delivery. First, it enables service providers to implement complex call control logic in the network rather than requiring intelligent endpoints, allowing for consistent service delivery across diverse device types and capabilities. Second, it provides a mechanism for third-party applications (such as web services or enterprise applications) to initiate and control communications sessions, enabling integration between communication services and other applications. Third, it allows for centralized billing, policy enforcement, and service logic that can be updated independently of endpoint software.

Historically, 3PCC emerged as part of the IMS (IP Multimedia Subsystem) architecture in 3GPP Release 8, building upon earlier work in SIP-based services and call control. It addressed the growing need for network operators to offer advanced services in a standardized, interoperable manner while maintaining control over service quality, billing, and network resources. The architecture also supports regulatory requirements such as lawful interception and emergency services by providing a centralized point of control for communications sessions.

Key Features

• Network-based session control without endpoint intelligence
• Support for third-party application-initiated sessions
• Back-to-back user agent (B2BUA) architecture
• Independent SIP dialogs with each endpoint
• Centralized billing and policy enforcement
• Integration with IMS core network elements

Evolution Across Releases

Defining Specifications