Optimal Media Routeing

Description

Optimal Media Routeing (OMR) is a network-based optimization mechanism defined across several 3GPP specifications (e.g., TS 23.894, TS 24.229) that determines the most efficient path for media traffic (e.g., RTP streams for voice and video) in an IP-based communication session. Its primary goal is to avoid unnecessary traversal of core network elements, particularly the media gateways and session border controllers, by enabling a more direct media path between the two communicating endpoints when possible and permissible by policy. This is often referred to as achieving "local switching" or a "direct media path."

Architecturally, OMR involves several key network functions within the IMS and packet core. The main decision point typically resides in the Proxy-Call Session Control Function (P-CSCF) or the Service Centralization and Continuity Application Server (SCC AS), which analyze the session establishment signaling (SIP/SDP). These functions examine parameters like the IP addresses and port numbers offered by the endpoints in the SDP offer/answer exchange. Based on network topology knowledge, operator policies, and potentially input from the Policy and Charging Rules Function (PCRF), they determine if a direct media path is optimal and allowed. If so, the signaling path may be modified to instruct the endpoints to send media packets directly to each other's IP addresses, bypassing intermediary media processing nodes.

How it works involves a coordinated process during session setup. When a call is initiated, the originating UE sends a SIP INVITE with an SDP offer containing its media capabilities and proposed IP address/port for receiving media. This signaling traverses the IMS core (P-CSCF, S-CSCF, etc.). The OMR-enabled function analyzes this information. Upon receiving the SDP answer from the terminating side, it compares the two IP addresses. If they are determined to be on the same or closely connected IP subnet (e.g., within the same operator's network, or both behind the same NAT), and policy allows, the network may decide to enable a direct media path. The network functions may then potentially modify the SDP bodies in the signaling messages to reflect the optimal route or simply allow the endpoints to use the addresses they have already exchanged. The media (RTP/RTCP) then flows directly between the UEs, while the control signaling (SIP) continues to traverse the IMS core for session control and service logic.

Purpose & Motivation

OMR was developed to address significant inefficiencies in early VoIP and IMS deployments where media traffic was often "tromboned" through central media gateways or session border controllers, even when the two communicating parties were geographically close or within the same network. This tromboning increased latency, jitter, and packet loss, degrading the quality of experience for real-time services like voice and video calls. It also consumed unnecessary bandwidth on transport links and added load to expensive media processing resources, increasing both capital and operational expenditures for operators.

The motivation for OMR stemmed from the commercial deployment of services like Voice over LTE (VoLTE), where high-quality voice is a key requirement. The traditional model of anchoring all media at a central point (like a Media Gateway Control Function - MGCF or a Media Resource Function - MRF) was suboptimal for many call scenarios, especially mobile-to-mobile calls within the same network. OMR solves this by enabling intelligence in the session control layer to analyze the endpoints' locations and dynamically select a media path that minimizes hops and network resource usage. This directly improves call setup times, reduces end-to-end delay, and enhances voice/video quality. It also allows operators to scale their networks more efficiently by offloading media traffic from core nodes onto the more distributed IP transport network.