IMS core

Description

The IMS core is the central session control part of the IP Multimedia Core Network Subsystem (IM CN Subsystem) in 3GPP. It consists of the Call Session Control Functions (CSCFs) and the Interconnection Border Control Function (IBCF). These elements are responsible for establishing, modifying and tearing down SIP-based multimedia sessions, authenticating users, invoking application services and interconnecting with external networks. The IMS core is defined in 3GPP TS 23.204 (Support of Short Message Service over generic IP access) and TS 23.824 (Study on IMS core), and it forms the foundation for services such as VoLTE, VoWiFi, RCS and IMS-based messaging.

The CSCFs are the heart of the IMS core and come in three roles. The Proxy-CSCF (P-CSCF) is the first point of contact for the UE; it acts as a SIP proxy, forwards SIP messages, performs policy and charging control interactions with the PCRF, and may provide security functions such as IPsec for SIP signalling. The Serving-CSCF (S-CSCF) is the central registrar and session control node; it authenticates the user via the HSS, downloads the user profile, decides which application servers are invoked, and routes SIP requests. The Interrogating-CSCF (I-CSCF) is the entry point from other networks and is responsible for querying the HSS to select the appropriate S-CSCF for a terminating session. Together, these three CSCF roles form the session control plane of the IMS core.

The IBCF is the interconnection element of the IMS core. It sits at the boundary between the operator's IMS network and external networks (other IMS operators, PSTN/PLMN gateways, or IP networks). The IBCF performs functions such as topology hiding, session screening, protocol interworking and media plane control. It works together with the Transition Gateway (TrGW) in the media plane to control the IP transport at the network border. The IBCF ensures that external signalling does not expose internal network topology and that only authorized sessions cross the boundary.

In operation, a UE registers with the IMS core by sending a SIP REGISTER to the P-CSCF, which forwards it to the I-CSCF, which selects an S-CSCF via the HSS. The S-CSCF authenticates the user and stores the registration. For a session, the originating UE sends a SIP INVITE to the P-CSCF, which forwards it to the S-CSCF; the S-CSCF invokes the appropriate application servers (e.g., MMTel, SCC AS) and routes the request toward the terminating network, possibly via the IBCF. The terminating side performs a similar process in reverse. The IMS core thus orchestrates the entire session lifecycle, including supplementary services, mid-call changes and session continuity.

The IMS core is access-agnostic: it can be reached over LTE, NR, Wi-Fi, fixed broadband or legacy CS access (via ICS). It relies on the HSS for subscriber data, the PCRF/PCF for policy and charging, and the DNS/ENUM for routing. Its separation of session control (CSCFs) from application logic (application servers) and from media transport (gateways) makes it a flexible, scalable architecture for delivering IP-based multimedia services. The IMS core has been part of 3GPP since Release 5 (initial IMS) and its definition as CSCFs plus IBCF was introduced in Release 7, remaining stable through Release 19.

Purpose & Motivation

The IMS core exists to provide a standardized, access-independent session control layer for IP-based multimedia services. Before IMS, each service (voice, messaging, video) tended to have its own control logic and its own network elements, leading to fragmentation, duplication and high cost. The IMS core solves this by defining a common SIP-based session control architecture that can be reused by many services. Application servers provide the service-specific logic, while the CSCFs handle registration, authentication, routing and session control in a uniform way.

The IBCF component addresses the need for secure, controlled interconnection between IMS networks and with external networks. Without a border element, internal topology and signalling would be exposed, and interworking with non-IMS networks would be difficult. The IBCF provides topology hiding, screening and interworking, making the IMS core a well-behaved citizen in a multi-operator environment.

The IMS core also enables service continuity and mobility. Because session control is centralized and access-agnostic, a session can be anchored in the IMS core and survive changes in access network, which is the basis for features such as SRVCC and IMS Service Continuity. The definition of the IMS core as CSCFs plus IBCF, introduced in Release 7, reflects the maturity of the architecture after the initial Release 5 IMS work, and it has remained stable because the fundamental roles of these elements have not changed.