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Voice over LTE (VoLTE) is a technology for providing existing voice and SMS services over LTE which does not have a circuit-switched domain. It is implemented using IP Multimedia Subsystem (IMS), which is a standard 3GPP technology for IP-based multimedia services. One of the key requirements of the Roaming Architecture for Voice over LTE with Local Breakout (RAVEL) as described in 3GPP TS 23.228, 3GPP TS 24.229, and as proposed in 3GPP TR 23.850, is that the  Interconnection Border Control Function (IBCF) supports Optimal Media Routing (OMR) procedures and allows bypassing the Transition Gateways (TrGWs).

The 3GPP TS 29.079 describes the procedures for optimizing the media path for call within IP Multimedia Subsystem. The purpose of OMR is to identify and remove unnecessary media functions from the media path for each media stream associated with a session. The OMR procedures identify and name the IP address realm used for each media path segment among UAs and TrGWs. An IP realm name is associated with each set of IP endpoints that are mutually reachable through IP routing and without address translation. Endpoints in different IP realms usually require allocation of a TrGW between those IP realms for connectivity and possibly for NAT.

Prior to OMR procedures, the