IMS Centralized Services

Description

IMS Centralized Services (ICS) is a 3GPP architectural concept introduced in Release 8 that enables an operator to deliver consistent, IMS-based communication services to users regardless of the access network they are attached to. The core idea is that the IMS (IP Multimedia Subsystem) becomes the single service control layer, while the actual access can be circuit-switched (CS), packet-switched (PS), wireless or wireline. This allows an operator to offer a uniform service experience — the same supplementary services, the same service configuration, the same identity — across 2G/3G CS access, HSPA/LTE packet access, and fixed broadband access.

Architecturally, ICS introduces a functional entity called the ICS UE (or IMS Centralized Services UE) and a network element known as the Service Centralization and Continuity Application Server (SCC AS), which resides in the IMS. The SCC AS is the anchor point for service control and provides service continuity between access networks. When a user is attached via CS access, the ICS UE uses the CS bearer for media transport but performs service control over the IMS by establishing a signalling path to the IMS. This signalling path can be realized in several ways: via the CS domain using Unstructured Supplementary Service Data (USSD) or a dedicated CS signalling channel, via a PS access in parallel with the CS access, or via a Gm-like interface over the CS domain. The chosen mechanism depends on UE and network capabilities.

For a CS-attached ICS user, the ICS UE registers with the IMS and uses IMS procedures for session setup, supplementary service invocation and mid-call service control. The media itself is carried over the CS bearer, but the session control is handled by the SCC AS in the IMS. This means that services such as call forwarding, call waiting, conference calling and mid-call modifications are executed in the IMS, providing a single point of service logic. When the user moves between access networks — for example, from CS access to PS access — the SCC AS can anchor the session and provide service continuity, ensuring the call is not dropped and services remain available.

ICS also supports the concept of a single phone number and a single service profile across all accesses. The user's IMS subscription and service settings are stored in the HSS and applied by the IMS application servers, so the same services are available whether the user is on a CS-only 2G network or an LTE network with VoLTE. This is particularly valuable for operators with a mix of legacy CS infrastructure and newer IMS deployments, as it allows them to migrate services to IMS gradually while maintaining service parity for users on legacy access.

The specifications for ICS are found in 3GPP TS 22.101 (Service aspects; Service principles) and TS 22.892 (Study on IMS Centralized Services), with detailed stage-2 and stage-3 procedures in other specifications. The concept is closely related to IMS Service Continuity (SRVCC and vSRVCC) and to the broader IMS architecture. ICS was a key enabler for operators to centralize service control in IMS while continuing to serve CS-only devices and networks, and it laid groundwork for later work on VoLTE and access-agnostic service delivery.

Purpose & Motivation

IMS Centralized Services was created to address a fundamental problem faced by mobile operators during the transition from circuit-switched to all-IP networks: how to provide a consistent set of services to users regardless of whether they are attached via CS access or IP-based access. Before ICS, service logic was duplicated: CS networks implemented supplementary services in the MSC/VLR, while IMS implemented them in application servers. This duplication led to inconsistent service behaviour, complex interworking and high operational cost.

ICS solves this by centralizing service control in the IMS. The CS access is used only as a bearer for media, while all service logic — call setup, supplementary services, mid-call control — is executed in the IMS. This means an operator can define a service once in the IMS and have it apply to users on CS access, PS access and fixed access alike. It also enables service continuity between access networks, so a user can move between accesses without losing services or dropping calls.

The motivation was both operational and strategic. Operationally, centralizing services reduces the need to maintain and evolve service logic in legacy CS nodes. Strategically, it allows operators to migrate users and services to IMS at their own pace, preserving investment in CS infrastructure while building toward an all-IP future. ICS also supports regulatory and emergency service requirements by ensuring consistent service availability across accesses.