IMS Centralized Services User Equipment

Description

An ICS UE is a User Equipment that has been enhanced to support IMS Centralized Services (ICS), a 3GPP feature that allows IMS services to be delivered over both circuit-switched (CS) and packet-switched (PS) access networks. The primary goal of ICS is to provide a consistent set of services to the user regardless of the access technology, and to enable service continuity when the UE moves between different access networks. The ICS UE is a key component in this architecture, as it must support the necessary protocols and procedures to interact with the ICS network elements, such as the Service Centralization and Continuity Application Server (SCC AS) and the MSC Server enhanced for ICS.

The ICS UE is defined in 3GPP specifications TS 23.292 and TS 23.719. It is essentially an IMS-capable UE with additional ICS-specific functionality. This includes the ability to use the CS access network for media transport while maintaining IMS-based service control. The ICS UE can initiate and receive sessions using either CS or PS access, and it can perform handovers between them without interrupting the service. The UE supports the ICS-specific signalling procedures, such as the use of the CS Access Adaptation Function (CAAF) and the I1 interface between the MSC Server and the IMS network. The ICS UE also supports the necessary security mechanisms, including the use of IPsec for IMS signalling and the CS security for CS access.

In terms of architecture, the ICS UE interacts with several network entities. When using CS access, the UE connects to the MSC Server enhanced for ICS, which in turn communicates with the IMS core network via the I1 interface. The MSC Server acts as a gateway between the CS and IMS domains, translating CS signalling (e.g., Call Control) into SIP messages towards the IMS. The ICS UE must support the CS call control procedures and the IMS SIP procedures, and it must be able to handle the interworking between them. When using PS access, the ICS UE communicates directly with the IMS core via the Gm interface, using SIP for session control. The ICS UE also supports the ICS-specific procedures for service centralization, such as the use of the SCC AS for terminating calls and for service continuity.

Key components of the ICS UE include the IMS client, the CS protocol stack, and the ICS adaptation layer that coordinates between them. The UE must support the necessary protocols for both domains, including SIP, SDP, and the CS protocols (e.g., Call Control, Mobility Management). It also needs to support the ICS-specific enhancements, such as the ability to include the ICS indicator in SIP messages and to handle the ICS-specific headers. The ICS UE may also support other features like Simultaneous CS and PS services, which allow the UE to have both CS and PS connections active at the same time.

The role of the ICS UE in the network is to enable operators to offer IMS services to users who are on CS-only access networks or who roam into such networks. It allows for a smooth migration from CS to IMS, as users can continue to use their existing CS access while benefiting from IMS services. The ICS UE also facilitates service continuity, ensuring that ongoing sessions are not dropped when the UE moves between access networks. This is particularly important for voice calls, which are typically handled by CS in legacy networks but are increasingly being delivered via IMS (e.g., VoLTE). The ICS UE bridges this gap by supporting both.

In summary, the ICS UE is a specialized UE that supports IMS Centralized Services, enabling access to IMS services over both CS and PS networks. It is defined in 3GPP Release 8 and has been maintained in subsequent releases. Its key features include support for ICS-specific signalling, interworking between CS and IMS, and service continuity. It plays a crucial role in the evolution of mobile networks towards all-IP, allowing operators to leverage their existing CS infrastructure while offering advanced IMS services.

Purpose & Motivation

The ICS UE exists to address the challenge of providing consistent IMS services across different access networks, particularly during the transition period when both CS and PS networks coexist. In the early days of IMS, services were only available over PS access, which limited their reach to users with PS connectivity. However, many networks still relied on CS for voice services, and users expected to have access to their services regardless of the access technology. The ICS UE, along with the ICS architecture, was created to bridge this gap by enabling IMS services to be delivered over CS access as well. This allows operators to offer a unified service experience, where users can access IMS services such as voice, video, and messaging even when they are on a CS-only network or roaming into one.

Historically, before ICS, if a user wanted to use IMS services, they needed to be on a PS access network. If they moved to a CS-only network, the IMS services would be unavailable, and they would have to rely on CS services, which might not have the same features. This led to a fragmented user experience and made it difficult for operators to migrate to IMS. The ICS UE solves this by allowing the UE to use CS access for media while keeping the service control in the IMS domain. This means that the user can have a consistent set of services, and the operator can centralize service control in IMS, simplifying network architecture and reducing costs.

The ICS UE also enables service continuity, which is essential for real-time services like voice. When a user moves between CS and PS access, the ICS UE and the network work together to ensure that the session is not dropped. This is achieved through the SCC AS, which anchors the session and performs the necessary handovers. The ICS UE supports the procedures for these handovers, making the transition seamless to the user. This is particularly important as networks evolve towards VoLTE and beyond, where service continuity between different access technologies is a key requirement.

In essence, the ICS UE was motivated by the need to provide a smooth migration path from CS to IMS, ensuring that users can always access their services and that operators can leverage their existing investments while moving towards an all-IP network. It is a critical enabler for the vision of IMS Centralized Services, which aims to centralize service control in IMS while using the best available access network for media transport.