Entry point

Description

In the IP Multimedia Core Network (IM CN) subsystem, the term "entry point" refers to the network entity that serves as the initial ingress point for signaling and media traffic entering a particular network domain. The entry point is responsible for receiving incoming requests from external networks or from user equipment and routing them appropriately within the IM CN subsystem. The specific node designated as the entry point depends on the deployment architecture and the application of border control concepts.

When border control concepts are applied, as specified in 3GPP TS 23.228, the Interconnection Border Control Function (IBCF) acts as the entry point for the network. The IBCF provides capabilities such as topology hiding, media and signaling control, and security functions at the network border. In this scenario, the IBCF replaces the Interrogating Call Session Control Function (I-CSCF) as the entry point. The IBCF and I-CSCF can be co-located in a single physical node, allowing for efficient resource utilization and simplified network architecture.

If border control concepts are not applied, the I-CSCF is considered the entry point of the network. The I-CSCF is a core node in the IP Multimedia Subsystem (IMS) that handles incoming session initiation requests, performs queries to the Home Subscriber Server (HSS) to determine the appropriate Serving CSCF (S-CSCF), and routes requests accordingly. It acts as a stateless proxy for initial registration and session establishment.

Additionally, when the Proxy CSCF (P-CSCF) is located in the home network, the I-CSCF is considered the entry point for the purposes of the specification. This ensures consistent handling of signaling regardless of the P-CSCF's location. The entry point concept is crucial for interoperability between different IMS networks and for enforcing security and policy controls at network boundaries.

The entry point function is defined in 3GPP specifications such as TS 23.228 (IP Multimedia Subsystem architecture), TS 24.229 (IMS call control protocols), and TS 31.131 and TS 34.131 (related to IMS security and testing). These specifications detail the procedures and interfaces that the entry point must support to ensure proper operation within the IM CN subsystem.

Purpose & Motivation

The entry point concept exists to provide a clear and standardized demarcation point for signaling and media entering an IM CN subsystem. It solves the problem of routing incoming traffic to the correct network functions while enabling border control, security, and topology hiding. Before the introduction of this concept, IMS networks lacked a unified approach to handling ingress traffic, leading to potential security vulnerabilities and inefficient routing.

The entry point also facilitates interconnection between different IMS networks and between IMS and external networks. By designating either the IBCF or I-CSCF as the entry point, network operators can apply consistent policies for authentication, authorization, and traffic management. This is particularly important for roaming scenarios and for ensuring that only authorized traffic enters the network.

Furthermore, the entry point concept supports the co-location of IBCF and I-CSCF, which reduces deployment complexity and cost. It allows operators to choose whether to apply border control concepts based on their specific requirements, providing flexibility in network design. The entry point is thus a foundational element for secure and efficient IMS interconnection.