The capability to allow media to be routed in an optimal manner between IMS operators when the subscriber is roaming was introduced in Rel-10. However, certain features may require that the media is controlled by the home network.
These features include conferencing, transcoding, tones insertion and announcement insertion.
In these cases, the media plane is typically re-directed back to the home network to perform these functions. For
transcoding, the media would have to stay routed through the home network for the duration of the call. However, for tone and announcement insertion, there is the possibility to temporarily re-direct the media plane though the home network only for the duration of the tone/announcement.
Unfortunately, neither of these solutions is ideal from a subscriber's perspective as it potentially introduces delay between the call parties. In addition, the subscriber will also experience a cut-out of media whilst the re-direction is performed (if redirected mid-call e.g. for tone/announcement insertion). This increases the risk of the call being cut-off (e.g. if the redirect fails for some reason).
This TR provides a study into the new (Rel-11) requirements identified by SA WG1 to allow a home
network to control a visited network to perform the following IMS functions for its subscribers:
conferencing (network hosted);
tone insertion; and