IMS Application Server Interface (MSC Server enhanced for ICS to IMS)

Description

The I2 reference point is a critical interface within the 3GPP IMS Centralized Services (ICS) architecture. It is defined between the Mobile Switching Centre (MSC) Server that has been enhanced with ICS capabilities (often called an ICS-enhanced MSC Server or MSC Server AS) and the home IMS network, specifically the IMS core comprising elements like the Serving-Call Session Control Function (S-CSCF). Its primary function is to enable a user connected via the legacy Circuit-Switched (CS) radio access (e.g., GERAN, UTRAN) to access and utilize services hosted in the IP-based IMS domain. The I2 interface allows the MSC Server to behave as an IMS Application Server (AS) towards the IMS core.

Architecturally, the ICS-enhanced MSC Server contains an internal IMS Service Centralization and Continuity (SCC) function and an IMS Application Server function. When a CS-attached UE initiates or receives a session, the MSC Server uses the I2 reference point to interact with the home IMS network. For a mobile-originated call, the MSC Server leverages I2 to perform an IMS registration on behalf of the user (if not already done) and to route the session signaling (e.g., SIP INVITE) into the IMS core. The S-CSCF in the IMS then processes the request, applies service logic from other ASs (like Telephony Application Server), and routes the call. For a mobile-terminated call to an ICS user, the IMS routes the SIP session to the MSC Server AS via the I2 interface, which then converts the SIP signaling into CS signaling (e.g., ISUP) to deliver the call to the UE over the CS domain.

The I2 interface is fundamentally a SIP-based interface, typically using the Diameter-based Cx interface (between the MSC Server AS and the Home Subscriber Server (HSS)) for subscriber authentication and profile retrieval. It enables service consistency, allowing a user to experience the same IMS-based services (like voice call continuity, supplementary services) whether attached via CS or PS access. The MSC Server uses I2 to report the user's CS attachment status to the IMS, which is crucial for terminating call routing decisions. The interface also supports capabilities like network-initiated de-registration and registration updates. By providing this bridge, I2 is a key enabler for the migration strategy from CS networks to all-IP IMS networks, allowing operators to introduce and centralize services in IMS while still supporting a large base of legacy CS-only handsets and radio coverage areas.

Purpose & Motivation

The I2 interface was created to solve a major transition challenge in the evolution from circuit-switched telephony to the IP Multimedia Subsystem (IMS). Prior to ICS, users attached to the CS domain could only access traditional CS services, while IMS services were exclusive to users attached via Packet-Switched (PS) access like LTE. This created a service dichotomy and hindered the deployment of new IMS-based services, as operators had to maintain duplicate service platforms for CS and PS users. IMS Centralized Services (ICS) was conceived to centralize all service logic and execution in the IMS, regardless of the user's access type.

The I2 reference point is the technical realization that enables this vision for CS-attached users. It addresses the limitation of the legacy CS network's inability to interact directly with the SIP-based IMS core. By defining a standardized interface where the MSC Server can act as an IMS Application Server, the CS network becomes a mere access leg to the IMS service platform. This allows operators to decommission legacy CS service platforms (like traditional IN systems) and rapidly deploy new services once in the IMS, making them instantly available to both CS and PS users. It was a strategic solution to accelerate network transformation, reduce operational costs, and deliver a uniform service experience, thereby motivating the creation and standardization of the I2 interface in 3GPP Release 12.