Description
The IQ reference point is a critical interface within the 3GPP IMS (IP Multimedia Subsystem) architecture, specifically defined for scenarios where a User Equipment (UE) accesses IMS services through a non-3GPP IP access network. It connects the IMS Access Gateway (AGW), which acts as the entry point from the untrusted non-3GPP access, to the Proxy-Call Session Control Function (P-CSCF), which is the first contact point within the trusted IMS core network. The primary role of the IQ interface is to provide a secure and controlled pathway for SIP (Session Initiation Protocol) signaling and associated media streams.
From a protocol perspective, the IQ reference point carries SIP messages over UDP, TCP, or TLS. When security is required, TLS is mandated to protect the SIP signaling between the AGW and the P-CSCF. The interface also handles the negotiation and establishment of media flows. A key function enabled over IQ is the traversal of Network Address Translation (NAT) and firewall devices commonly found in fixed and public Wi-Fi networks. The IMS Access Gateway often incorporates Session Border Controller (SBC) functionalities, including SIP Application Level Gateways (ALGs) and Interactive Connectivity Establishment (ICE) support, to facilitate this NAT traversal and ensure media can flow directly or via relays between the UE and the IMS network.
Operationally, when a UE attaches via a non-3GPP access network (e.g., a home Wi-Fi router), it discovers the P-CSCF address, often provided via DHCP or other access-specific methods. The UE then sends its SIP REGISTER and subsequent INVITE messages towards this P-CSCF. These messages traverse the IMS Access Gateway via the IQ interface. The AGW may perform initial filtering, policy enforcement, and security checks before forwarding the SIP dialog to the P-CSCF. For media, the AGW may act as a media relay or simply allow the media to flow through it, depending on the network topology and NAT/firewall configurations. This architecture enables seamless service continuity for services like Voice over Wi-Fi (VoWiFi), where a call can be initiated or received on a Wi-Fi network with the same identity and service logic as on the LTE radio network.
Purpose & Motivation
The IQ interface was introduced to address a fundamental challenge: extending IMS-based services, particularly voice and messaging, to devices connected via non-cellular IP networks. Prior to its standardization, IMS access was largely confined to the 3GPP-defined packet-switched domain (e.g., over GPRS, HSPA, or LTE). However, operators sought to leverage ubiquitous Wi-Fi and fixed broadband networks to offload traffic, improve indoor coverage, and create converged service offerings. The public internet and private fixed networks presented new challenges not present in the controlled 3GPP access, namely pervasive NAT, diverse firewall policies, and a lack of inherent trust.
The creation of the IQ reference point, along with the IMS Access Gateway functional entity, provided a standardized solution. It defined a clear architectural boundary and interface specifications for securely integrating these untrusted access networks into the IMS core. This solved the problem of NAT and firewall traversal for SIP and RTP/RTCP streams, which is essential for establishing bidirectional media sessions. It also allowed operators to apply consistent authentication, authorization, and policy control (via the Rx interface to the PCRF) regardless of the access technology. The IQ interface was thus a key enabler for VoWiFi and other IMS services over non-3GPP access, forming a cornerstone of the 3GPP's Fixed-Mobile Convergence (FMC) strategy.
Key Features
- Standardized transport for SIP signaling between non-3GPP access networks and the IMS core
- Support for secure transport using TLS to protect SIP messages
- Enables NAT and firewall traversal for both signaling and media streams
- Facilitates policy enforcement and service control for IMS sessions originating from untrusted access
- Key architectural component for enabling Voice over Wi-Fi (VoWiFi) and other IMS services over fixed broadband
- Interworks with IMS Access Gateway functionalities, often integrating SBC capabilities
Evolution Across Releases
Initial introduction of the IQ reference point within the IMS architecture for non-3GPP access. It defined the basic interface between the newly specified IMS Access Gateway and the P-CSCF to support SIP-based IMS registration and session control over untrusted IP networks, enabling early VoWiFi deployments.
Defining Specifications
| Specification | Title |
|---|---|
| TS 29.079 | 3GPP TS 29.079 |
| TS 36.141 | 3GPP TR 36.141 |