Description
A Multimedia Call (MMC) is a session established within a 3GPP network, specifically via the IP Multimedia Subsystem (IMS), that supports the simultaneous exchange of multiple media components. Unlike a traditional circuit-switched voice call, which is limited to a single audio stream, an MMC can dynamically combine audio, video, text, and potentially other data streams (like file sharing or screen sharing) within a single, coordinated session. The call is orchestrated using the Session Initiation Protocol (SIP) for signaling and the Session Description Protocol (SDP) for negotiating the media types and codecs to be used. The core network entities involved include the Proxy-Call Session Control Function (P-CSCF), Serving-CSCF (S-CSCF), and Media Resource Function (MRF) for potential media processing or mixing.
The establishment of an MMC begins with a SIP INVITE message that contains an SDP offer listing the media types the caller wishes to use. The recipient's device responds with an SDP answer, agreeing on a common set of media streams. This negotiation allows devices with different capabilities to find a compatible set of media for the call. During the call, additional media streams can be added or removed using SIP re-INVITE transactions, enabling features like upgrading a voice call to a video call mid-session. The user plane for these media streams typically uses Real-time Transport Protocol (RTP) over IP, routed directly between the endpoints or through media gateways if interworking with legacy networks is required.
Key to the MMC concept is the role of the IMS core in providing service logic, authentication, and policy control. The S-CSCF applies initial Filter Criteria (iFC) to trigger appropriate Application Servers (AS) that may provide value-added services for the multimedia call, such as call forwarding based on presence, or interactive voice/video response systems. Furthermore, the Policy and Charging Rules Function (PCRF) interacts with the P-CSCF to ensure the bearer network provides the appropriate Quality of Service (QoS) for the latency-sensitive real-time media streams. This end-to-end coordination between the signaling plane (IMS) and the transport plane (access and core network) is what enables a reliable and high-quality multimedia call experience.
MMC is not a single service but a foundational capability that enables a family of services. It is the underlying mechanism for Video Telephony, Multimedia Telephony (MMTel) services, and Rich Communication Services (RCS). The management and charging for these complex sessions are handled by specific network functions, which is why MMC is detailed in management specifications like TS 32.408. This specification defines the information model and procedures for charging data generation related to multimedia calls, tracking the usage of different media components for billing purposes.
Purpose & Motivation
The Multimedia Call was created to move beyond the limitations of traditional telephony, which was designed exclusively for bidirectional voice communication. As networks evolved to packet-switched IP architectures with higher bandwidth capabilities, user demand grew for more engaging and productive communication methods that integrated video and data sharing. The purpose of MMC is to leverage the IMS framework to provide a standardized, interoperable way to establish and manage sessions containing any combination of real-time media, thereby enabling a new generation of communication services.
The historical context stems from the separation of call control and media in VoIP systems and the desire to bring this flexibility to cellular networks. Prior approaches, like circuit-switched video telephony in 3G networks (3G-324M), were rigid, inefficient, and isolated from internet services. MMC, built on IMS, solves this by using IP for both signaling and media, allowing for seamless integration with internet applications, dynamic service composition, and convergence between fixed and mobile networks. It addresses the problem of service silos by providing a single control framework for multiple media types.
Furthermore, MMC enables network operators and third-party providers to create innovative, billable services. By having a standardized model for what constitutes a multimedia call, operators can implement sophisticated charging models—for instance, charging differently for voice-only segments versus video segments of the same call, or offering bundled media packages. This commercial flexibility, combined with the technical capability for rich interaction, motivated the standardization of MMC within the 3GPP framework, making it a cornerstone of modern IP-based communication services.
Key Features
- Supports simultaneous, negotiated media streams (audio, video, text) within a single SIP session
- Utilizes SIP and SDP for dynamic session establishment, modification, and termination
- Enables mid-call modality changes (e.g., adding video) through SIP re-INVITE procedures
- Integrated with IMS for service control, authentication, and application triggering
- Interworks with Policy and Charging Control (PCC) for appropriate QoS enforcement on media bearers
- Generates detailed charging data records (CDRs) for each media component, as defined in TS 32.408
Evolution Across Releases
Introduced as a formalized concept within the IMS management framework, particularly for charging purposes in TS 32.408. It defined the information model for Multimedia Call charging, enabling operators to bill for complex sessions involving multiple media types based on the IMS-based Multimedia Telephony (MMTel) service suite.
Defining Specifications
| Specification | Title |
|---|---|
| TS 32.408 | 3GPP TR 32.408 |