Circuit Switched IMS Combinational Service

Description

Circuit Switched IMS Combinational Service (CSICS) is a 3GPP standardized architecture that enables simultaneous operation of circuit-switched (CS) voice calls and IP Multimedia Subsystem (IMS) multimedia sessions over the same radio connection. The technology works by establishing a CS voice call through traditional mobile switching centers while simultaneously setting up an IMS-based multimedia session through the packet-switched domain. This dual-domain operation allows users to maintain a high-quality voice call while accessing supplementary IMS services like video sharing, file transfer, or presence information.

Architecturally, CSICS involves coordination between multiple network elements including the Mobile Switching Center (MSC), Serving GPRS Support Node (SGSN), Gateway GPRS Support Node (GGSN), and IMS core elements like the Call Session Control Function (CSCF) and Application Servers. The MSC handles the circuit-switched voice path while the IMS core manages the multimedia session establishment and control. A key component is the Service Centralization and Continuity Application Server (SCC AS) which coordinates between the CS and IMS domains to ensure service continuity and proper synchronization between the two sessions.

The technology operates through specific signaling procedures defined in 3GPP specifications. When a user initiates a CS voice call, the network can simultaneously trigger IMS session establishment using Session Initiation Protocol (SIP) signaling. The UE maintains two separate bearers: one for CS voice traffic and another for IMS multimedia traffic. The radio access network must support concurrent CS and PS bearers, which is achieved through mechanisms like Dual Transfer Mode (DTM) in GERAN or simultaneous CS and PS operation in UTRAN. The synchronization between voice and multimedia components is managed through timing references and session coordination protocols.

CSICS plays a crucial role in network evolution by allowing operators to introduce IMS-based services without requiring immediate migration to full Voice over LTE (VoLTE) or Voice over NR (VoNR) deployments. It provides a transitional solution that leverages existing CS infrastructure while enabling new multimedia services. The technology supports various service combinations including CS voice with IMS-based video sharing, CS voice with IMS-based instant messaging, and CS voice with IMS-based file transfer, creating enriched communication experiences while maintaining reliable voice service quality.

Purpose & Motivation

CSICS was developed to address the challenge of introducing IMS-based multimedia services while maintaining compatibility with existing circuit-switched voice networks. As mobile operators began deploying IMS infrastructure in Release 7, they faced the dilemma of either waiting for full VoLTE deployment or finding an intermediate solution that could deliver enhanced services immediately. CSICS provided this bridge by allowing operators to offer IMS multimedia services alongside traditional CS voice calls, enabling faster time-to-market for enriched communication services.

The technology solved several specific problems: First, it addressed the coverage limitations of early packet-switched networks by leveraging the ubiquitous coverage of CS voice networks. Second, it enabled service providers to monetize their IMS investments sooner by offering value-added services without requiring complete network overhaul. Third, it provided users with enhanced communication experiences while maintaining the reliability and quality of traditional voice calls. This was particularly important during the transition period when packet-switched voice quality and coverage were still developing.

Historically, CSICS emerged during a period when mobile networks were transitioning from pure circuit-switched architectures to all-IP networks. Previous approaches required separate sessions for voice and multimedia services, which was inefficient and provided poor user experience. CSICS introduced the concept of combinational services where voice and multimedia components could be synchronized and presented as a unified service to the user. This addressed limitations of earlier technologies like circuit-switched video telephony, which required dedicated channels and offered limited flexibility compared to IMS-based multimedia services.

Key Features

• Simultaneous CS voice and IMS multimedia session establishment
• Backward compatibility with legacy circuit-switched networks
• Synchronization between voice and multimedia components
• Support for various service combinations (video sharing, file transfer, messaging)
• Utilization of existing CS coverage for reliable voice service
• Transitional architecture enabling IMS service introduction

Evolution Across Releases

Defining Specifications