Inter-IMS Network to Network Interface

Description

The Inter-IMS Network to Network Interface (II-NNI) is a critical standardized reference point within the 3GPP IMS architecture. It defines the interface between the IMS core networks of two different service providers or administrative domains. The primary purpose of the II-NNI is to facilitate the interconnection of IMS networks to support end-to-end IMS services between subscribers belonging to different operators. Technically, the II-NNI is not a single physical link but a logical interface encompassing a set of protocols and procedures. The core protocol used across the II-NNI is the Session Initiation Protocol (SIP), extended with 3GPP-specific headers and parameters (as defined in TS 24.229) for session control. Alongside SIP, the interface utilizes the Session Description Protocol (SDP) for media negotiation and the Real-time Transport Protocol (RTP) for media bearer transport. A key architectural component is the Interconnection Border Control Function (IBCF), which acts as the gateway node at the edge of each IMS network. The IBCF performs vital functions such as topology hiding, network address/port translation (NAPT), SIP message screening and adaptation, and interworking between IPv4 and IPv6. The Transition Gateway (TrGW), often co-located with the IBCF, handles the media plane functions like media relay and transcoding if necessary. The II-NNI enables the establishment of peer-to-peer sessions (voice, video, messaging), registration path optimization, and the exchange of service-related information between networks while maintaining security, policy control, and charging separation.

Purpose & Motivation

The II-NNI was created to solve the fundamental problem of inter-operator connectivity for advanced IP-based multimedia services delivered by the IMS. Prior to its standardization, operators could deploy IMS as an island within their own network, but offering services like VoLTE or video calling to subscribers on other networks required proprietary gateways or fallback to legacy circuit-switched networks. The motivation was to enable a seamless, all-IP service experience across operator boundaries, which is essential for widespread user adoption. It addresses the limitations of non-standardized interconnection, which leads to interoperability issues, limited service richness, and complex network management. By defining a standardized interface, 3GPP allowed operators to interconnect their IMS cores directly, preserving the full feature set of IMS services (e.g., HD voice codecs, video, supplementary services) end-to-end. This was a crucial step in the evolution from circuit-switched telephony to all-IP communication, supporting the industry's move towards Rich Communication Services (RCS) and ensuring IMS could fulfill its role as a global service platform.