Combinational Service ICS capable User Equipment

Description

The CSICS-capable UE represents a pivotal evolution in mobile device technology, designed to support the coexistence and combination of Circuit-Switched (CS) and IP Multimedia Subsystem (IMS) services within a single terminal. Before the advent of such capabilities, mobile devices were typically capable of supporting either traditional voice calls over CS or basic IP services, but not in a truly integrated, combinational manner. A CSICS-capable UE is defined as a User Equipment that supports Combinational Service, which is the ability to have a CS service (like voice) and an IMS service (like file sharing or video) active simultaneously over the same radio connection. This is achieved through the concept of the 'Combinational Service' (CSICS), where the UE maintains a CS connection for voice while also using packet-switched bearers for IMS-based services. The technical foundation relies on the UE's capability to handle multiple parallel service contexts. In a typical scenario, a user could establish a voice call over the CS domain, and during that call, initiate a video sharing session or a file transfer over the PS domain via IMS. The UE must manage the radio resources, signalling, and user interface to support such parallel operations seamlessly. This capability is particularly important in the context of the Core Network, as it allows operators to leverage their existing CS infrastructure while gradually introducing IMS-based services, thus protecting investments and ensuring a smooth transition. The CSICS-capable UE interacts with various network elements, including the Mobile Switching Centre (MSC) for CS services and the Call Session Control Function (CSCF) in the IMS for IP-based services. The UE's capabilities are advertised to the network during registration or session establishment, allowing the network to allocate appropriate resources and manage the combined services. From a standards perspective, the CSICS-capable UE is specified in 3GPP Technical Specifications 22.279 and 22.979, which outline the service requirements and implementation aspects. These specifications define the mechanisms for combining CS and IMS services, including the necessary signalling flows, security considerations, and quality of service (QoS) requirements. The UE must support multiple radio access bearers (RABs) or evolved RABs (ERABs) to handle the concurrent CS and PS traffic, and it must implement the necessary protocols for session management, such as SIP for IMS services and the CS domain protocols for voice. In terms of architecture, the CSICS-capable UE operates within a network that includes a CS domain (MSC, GMSC, etc.) and an IMS domain (CSCF, HSS, etc.), connected via gateways and interfaces. The UE may use a single radio interface to maintain both connections, which requires careful resource management to avoid interference and ensure QoS. The combinational service concept was introduced to address the growing demand for multimedia services while leveraging existing CS infrastructure, and it represents an important step towards the all-IP network of later releases.

Purpose & Motivation

The CSICS-capable UE was created to address the need for simultaneous support of circuit-switched and IP-based services on a single device, which was not possible with earlier UE designs that could only handle one service at a time. As mobile networks evolved from pure CS systems to mixed CS/PS systems, operators wanted to offer innovative multimedia services (like video sharing during a voice call) without completely overhauling their CS infrastructure. The CSICS-capable UE enables this by allowing a voice call over CS while using IMS over PS for supplementary services, thus providing a richer user experience. This approach also facilitated a smooth migration path towards all-IP networks (like VoLTE) by allowing users to experience IMS services while still relying on the reliable CS domain for voice. The limitations of previous approaches included the inability to combine services, leading to interrupted voice calls when initiating data sessions or vice versa. The CSICS-capable UE solves this by enabling parallel service contexts.