IP Multimedia Routeing Number

Description

The IP Multimedia Routeing Number (IMRN) is a critical identifier within the 3GPP IMS architecture, specifically defined for scenarios involving IMS roaming. It is a temporary, globally unique E.164 number assigned by the visited network's IMS entity, typically the Visited Application Server (V-AS) or a dedicated IMRN allocation function. Its primary function is to serve as a routable contact point for incoming SIP sessions (like voice or video calls) destined for a roaming subscriber when the home and visited networks do not have a direct IMS-level roaming agreement or the necessary Gm/SIP interface between the User Equipment (UE) and the home network's P-CSCF is unavailable.

When a user roams into a network without IMS roaming, the home network cannot route SIP signaling directly to the UE's IP address in the visited network. To solve this, the visited network allocates an IMRN and associates it with the roaming user's temporary registration. This IMRN is then communicated back to the user's home network, often via the user's registration procedure. Subsequently, when a call is placed to the user's permanent Public User Identity (e.g., their SIP URI), the home network's IMS entities (like the S-CSCF) perform ENUM/DNS query translation. Upon determining that an IMRN is associated for routing, the session request is redirected to this IMRN.

The IMRN routes the call into the visited network's circuit-switched (CS) domain or a media gateway control function. The visited network, recognizing the IMRN, maps it back to the associated user's IP connectivity and delivers the SIP session to the UE over the packet-switched (PS) domain. This mechanism effectively creates a tromboning or anchoring point, allowing session control to be handled by the home network while enabling media delivery to the roamer. It decouples the routing identifier from the user's permanent identity for the duration of the roaming session, ensuring service continuity. The IMRN is released after the user deregisters or its lifetime expires, returning to a pool for reuse.

Purpose & Motivation

The IMRN was introduced to solve a fundamental problem in early IMS deployments and roaming scenarios: how to deliver rich IP-based multimedia services (VoIP, video, messaging) to users roaming in networks that lacked direct IMS-level peering. Initial 3GPP releases focused on home network service delivery. For roaming, the initial model relied on the visited network providing IP connectivity (PS domain), but assumed the SIP signaling path (Gm reference point) would always be established back to the home network's Proxy-CSCF. This required complex security and policy agreements between operators.

In practice, many operators, especially in early deployments, did not implement IMS roaming interfaces due to cost, security concerns, or phased rollout strategies. Without IMRN, a roaming user would be unreachable for incoming IMS sessions, severely limiting the utility of IMS services. The IMRN mechanism provided an interim, pragmatic solution. It leveraged existing, widely deployed CS roaming infrastructure (which uses E.164 numbers for routing) as a transport 'carrier' for IMS session signaling. This allowed operators to offer IMS roaming services without requiring immediate, full-scale IMS interconnection, thereby accelerating service availability and adoption.

It addressed the technical limitation of routing a SIP request to a dynamically assigned IP address in a foreign network that the home network's IMS core could not directly route to. By translating the problem into a well-understood telephony routing problem (routing via a temporary E.164 number), it bridged the gap between legacy circuit-switched roaming and next-generation IP multimedia services, ensuring backward compatibility and facilitating a smoother transition to all-IP networks.

Key Features

• Temporary E.164 number format for global telephony routing compatibility
• Dynamically allocated and bound to a roaming user's registration context
• Enables routing of SIP sessions via CS domain or media gateways when direct IMS roaming is absent
• Supports session continuity for roaming users without home-visited network IMS interconnection
• Released after user deregistration for pool management and number conservation
• Facilitates ENUM/DNS-based routing resolution in the home IMS network

Evolution Across Releases

Defining Specifications