Hosting Service Centralization and Continuity Application Server

Description

The Hosting Service Centralization and Continuity Application Server (SCC AS) is a key functional entity in the IP Multimedia Subsystem (IMS) that manages the dialog with the remote party for a Collaborative Session. This term is defined in 3GPP specifications 23.237 and 23.831. The SCC AS is responsible for providing service centralization and continuity, enabling users to transfer sessions between different access networks and devices seamlessly. In the context of a Collaborative Session, which allows a user to use multiple devices simultaneously within a single session (e.g., video on one device and audio on another), the Hosting SCC AS acts as the anchor point. It maintains the session state, controls the media flows, and ensures that the remote party perceives a single session. The Hosting SCC AS is typically located in the home network and is involved in session setup, modification, and termination.

The architecture of the SCC AS includes two roles: the Hosting SCC AS and the Controlling SCC AS. The Hosting SCC AS is the one that manages the dialog with the remote party, meaning it handles the SIP signaling towards the remote end and controls the media resources. The Controlling SCC AS, on the other hand, manages the access leg towards the user's devices and coordinates the collaborative session. In a collaborative session, the Controlling SCC AS may be in the same or different SCC AS instance as the Hosting SCC AS. The Hosting SCC AS uses the IMS Service Control (ISC) interface to communicate with the Serving Call Session Control Function (S-CSCF) and the Ut interface for service configuration. It also interacts with the Media Resource Function (MRF) for media mixing or processing if needed. The Hosting SCC AS relies on the IMS core for registration, authentication, and routing.

How it works: When a user initiates a collaborative session, the Controlling SCC AS sets up the session and may decide to involve the Hosting SCC AS. The Hosting SCC AS then establishes a dialog with the remote party using SIP. It receives session description information from the Controlling SCC AS and may modify it before sending to the remote party. The Hosting SCC AS also handles mid-session changes, such as adding or removing media flows from different devices. It ensures that the remote party sees a consistent session, even though the user may be using multiple devices. The Hosting SCC AS can also perform access transfer procedures, such as when the user moves from one access network to another, by updating the remote leg accordingly. This requires coordination with the Controlling SCC AS and the access network elements.

The Hosting SCC AS is essential for enabling advanced multi-device services and session continuity. It allows operators to offer services like multi-device collaboration, where a user can seamlessly switch between devices or use them simultaneously. It also supports regulatory requirements for emergency calls and lawful interception. The concept was introduced in Rel-10 as part of the IMS Service Centralization and Continuity (SC) work. Over releases, the definition has remained stable, indicating its foundational role. In 5G, similar concepts are being adapted for the 5G Core, but the SCC AS remains a key IMS entity for voice and multimedia services.

Purpose & Motivation

The Hosting SCC AS was created to address the need for service continuity and centralization in IMS networks, particularly with the rise of multi-device usage and heterogeneous access networks. Before its introduction, IMS sessions were typically tied to a single device and access network, making it difficult to transfer sessions or use multiple devices collaboratively. The Hosting SCC AS solves this by acting as a central anchor for the session, managing the dialog with the remote party and coordinating media flows across devices. This enables features like Collaborative Sessions, where a user can combine devices for a richer experience, and session continuity, where a session can be transferred between access networks without interruption. It also simplifies the network architecture by centralizing session control in the home network, ensuring consistent service delivery and policy enforcement. The Hosting SCC AS is a key enabler for operators to offer value-added services and compete with over-the-top providers.