Description
The CS-IBCF (Circuit Switched domain - Interconnection Border Control Function) is a critical component within the IMS (IP Multimedia Subsystem) architecture defined by 3GPP. It resides at the border between an IMS network and external circuit-switched (CS) networks, such as the Public Switched Telephone Network (PSTN) or legacy Public Land Mobile Networks (PLMN). Its primary role is to act as a gateway and a control point for all signaling traffic passing between these two distinct network domains. Architecturally, the CS-IBCF is part of the IMS Interconnection Border Control Function (IBCF) family, which also includes the TrGW (Transition Gateway) for media handling and the IBCF itself for IP-to-IP interconnection. The CS-IBCF specifically focuses on the CS domain interface, often interacting with a Media Gateway Control Function (MGCF) which controls the media gateways (MGW) that convert between IP-based media in IMS and TDM/CS-based media.
Operationally, the CS-IBCF works by intercepting and processing SIP (Session Initiation Protocol) signaling messages that are destined for or originate from CS networks. When an IMS user initiates a call to a PSTN number, the SIP INVITE is routed to the CS-IBCF. The CS-IBCF performs several key functions: it may act as a SIP Back-to-Back User Agent (B2BUA), terminating the SIP dialog from the IMS side and originating a new SIP dialog towards the MGCF. This B2BUA model allows for topology hiding, where the internal IMS network topology and node addresses are concealed from the external CS network. It also enables protocol adaptation, as the CS-IBCF may translate between different SIP profiles or incorporate information for interworking with ISUP (ISDN User Part) signaling handled by the MGCF.
Key components of the CS-IBCF's functionality include security enforcement, where it can apply filtering policies, validate incoming requests, and protect against malicious traffic from the CS domain. It also provides charging correlation by associating charging identifiers between the IMS and CS domains, which is vital for billing. Furthermore, the CS-IBCF manages network address and port translation, and can control the usage of resources. Its role is defined in 3GPP TS 29.235, which specifies the SIP profile and procedures for this interface. By centralizing these border control functions, the CS-IBCF simplifies network architecture, enhances security, and ensures reliable service interworking, forming a cornerstone for the migration from legacy CS networks to all-IP IMS networks.
Purpose & Motivation
The CS-IBCF was created to address the fundamental challenge of interconnecting next-generation, all-IP IMS networks with the vast installed base of legacy circuit-switched telephony networks. Prior to IMS, mobile and fixed networks were predominantly CS-based, using protocols like ISUP for call control. The introduction of IMS, with its SIP-based signaling and IP-based media, created a protocol and architectural mismatch. Simple gateways could handle media conversion, but a intelligent control function was needed at the signaling border to manage security, hide network topology, and ensure proper service interworking.
The primary problems the CS-IBCF solves are security vulnerabilities from direct exposure of IMS core elements, complexity in managing inter-operator connections, and a lack of control over the interworking process. Without a CS-IBCF, an IMS S-CSCF (Serving Call Session Control Function) would need to communicate directly with an MGCF in another operator's domain, exposing its IP address and capabilities. The CS-IBCF acts as a secured demarcation point, implementing topology hiding to protect the internal network. It also centralizes interworking logic, simplifying the addition of new interconnection partners and ensuring consistent policy enforcement for all CS-bound traffic. Its creation was motivated by the industry's gradual transition to all-IP networks, requiring a stable, standardized border function that could facilitate this coexistence period, enabling operators to deploy IMS while maintaining connectivity to the global PSTN.
Key Features
- Acts as a SIP Back-to-Back User Agent (B2BUA) for signaling between IMS and CS domains
- Provides topology hiding to conceal internal IMS network structure from external CS networks
- Enforces security policies including filtering and validation of incoming CS-domain signaling
- Facilitates protocol adaptation and interworking between SIP and legacy CS signaling (e.g., towards ISUP via MGCF)
- Supports charging correlation by managing charging identifiers across the IMS-CS boundary
- Centralizes control for CS interconnection, simplifying network management and policy enforcement
Evolution Across Releases
Introduced as part of the initial IMS architecture for fixed and mobile convergence. Defined the core function of the CS-IBCF as the border control entity for interconnection between the IMS and external CS networks. Specified its role as a SIP B2BUA, topology hiding agent, and security enforcement point, with detailed procedures in TS 29.235.
Introduced support for SRVCC (Single Radio Voice Call Continuity) from E-UTRAN to GERAN/UTRAN, requiring the CS-IBCF to handle specific session transfer procedures and coordinate with the MSC Server enhanced for SRVCC to ensure seamless voice handovers from LTE to 2G/3G CS networks.
Ongoing support and potential deprecation studies. As the industry moves towards all-IP, the role of the CS-IBCF is maintained for remaining CS interconnections, but its specifications are stable with a focus on ensuring backward compatibility and operational stability for operators still reliant on CS interworking.
Defining Specifications
| Specification | Title |
|---|---|
| TS 29.235 | 3GPP TS 29.235 |