Roaming Architecture for VoicE over IMS with Local breakout

Description

RAVEL (Roaming Architecture for VoicE over IMS with Local breakout) is a standardized architecture defined by 3GPP for enabling IP Multimedia Subsystem (IMS) services, primarily voice and video telephony, for roaming users. It specifically employs the Local Breakout (LBO) model for the user plane. In traditional home-routed roaming, all user plane traffic (voice packets) is tunneled back to the home network's Packet Data Network Gateway (PGW) before reaching the IMS core or the internet. RAVEL changes this by anchoring the user plane in the visited network's PGW (vPLMN PGW). This means media packets for a call between two roaming users in the same visited country can flow directly between them without traversing their respective home networks.

The architecture involves several key network functions. In the visited network, the Serving Gateway (SGW) and PDN Gateway (PGW) handle the user plane. The visited network also hosts a Proxy-Call Session Control Function (P-CSCF), which is the UE's first point of contact for IMS signaling. The P-CSCF can be located in the visited network (Visited P-CSCF) or remain in the home network. For RAVEL with full local breakout, the Visited P-CSCF is used. IMS signaling, however, is typically still routed to the home network's IMS core (e.g., the home S-CSCF) for service control and subscriber profile interrogation. This creates a split: control plane signaling goes home, while the user plane media path is established locally.

This separation is managed via the Policy and Charging Control (PCC) architecture. The visited network's Policy and Charging Rules Function (vPCRF) interacts with the home PCRF (hPCRF) via the S9 roaming interface to obtain policy rules for the subscriber. These rules are then enforced at the visited PGW by the Policy and Charging Enforcement Function (PCEF). This ensures that the home operator retains control over service quality and charging even when the user plane is broken out locally. RAVEL is crucial for efficient VoLTE/VoNR roaming, as it minimizes latency (critical for voice quality), reduces backhaul costs on inter-operator links, and allows optimal media routing, especially for localized calls and access to visited network services.

Purpose & Motivation

RAVEL was developed to solve the technical and economic inefficiencies of home-routed roaming for IMS-based real-time communication services like VoLTE. Early IMS roaming architectures often relied on home routing, where all traffic was funneled through the home operator's network. This model introduced significant latency (due to the 'tromboning' effect of packets traveling long distances), increased transport costs for both operators, and created suboptimal media paths (e.g., a call between two users roaming in the same city would have its media routed via their respective home countries). These issues were detrimental to the user experience of voice and video services, which are highly sensitive to delay and packet loss.

The motivation for RAVEL was to enable a 'local experience' for roaming subscribers, matching the quality and efficiency of domestic service. By adopting local breakout, RAVEL addresses the limitations of the home-routed model. It allows visited operators to leverage their own local network resources and peering agreements for media delivery. This is particularly important for the commercial success of VoLTE/VoNR roaming, as it makes it cost-effective for operators to offer and ensures high quality for end-users. The development of RAVEL in Release 11 was part of a broader effort to make IMS-based voice a viable and superior replacement for circuit-switched fallback (CSFB) during roaming, paving the way for ubiquitous HD voice and future communication services.