VoLTE Conference Call

Maybe you remember what I said about Group Messaging. That all the RCS deployments would be done faster without this feature. A similar thing we can say about VoLTE Conferencing. Ad-Hoc Multi Party Conference Call (CONF) is one of the basic requirements we have on VoLTE calling. Simply put each VoLTE network has to support conference calling. But to troubleshoot this great functionality can be a nightmare.

Ad-Hoc Multi Party Conference is one of the Supplementary Services supported by Telephony Application Server (TAS) (a dedicated Conference AS is an option too) and it is described in GSMA IR.92, which then refers to 3GPP TS 24.605 and 24.147. Today we’ll take a look at the conference call flow, along with the Mr’ interface between TAS and Media Resource Function (MRF).

Add participant button

Although we talk about conferencing, in fact it’s just a multi-party call. We don’t schedule any conference call for a given list of participants. We can only add additional numbers to an existing call. That’s why we describe the service as an ad-hoc conference. From the mobile operator point of view the conferencing service provides the means for a user to create, manage, terminate, join and leave conferences as well as the ability to update the involved parties. But most of the stuff is truly hidden to the end subscribers.

In general both voice and video conference can be supported, but only the support of audio media is required by VoLTE standard. The maximum number of participants differs network to network, usually it is between 6 and 10. Note, that the functionality is not limited to VoLTE users only, we can add to the call the CS users too.

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Group … Messaging

RCS could be probably deployed much faster if it wasn’t that general right from the beginning. To implement a simple 1-1 chat is not a big deal. The same applies for voice and video. And honestly one doesn’t expect much from a new product as long as the basic functionality is working fine.

Well, in case of RCS/RCSe group messaging was always referred as a basic one. But sometimes it was not without struggles. There are many ways how a Conference can be established. Just to find out all the relevant RFCs is not an easy task. RCS Group Chat technical realization is based on the “Ad-Hoc Session Mode messaging”. RCS is referencing to OMA-SIMPLE or OMA-CPM. OMA is defining their ways how to handle the chats and pointing to particular RFCs. Many of them. The basic one is the RFC 4353. In this document it is described the logical architecture. Simply put group messaging is a multi 1:1 chat. All the participants will establish a 1:1 chat with a center point called “focus”. This logical role is implemented as a part of RCS (CPM/SIMPLE) server functionality.

Group Chat – focus

As mentioned, in case of RCS we mostly talk about ‘Ad-hoc’ conferencing. That means that the conference does not need to be scheduled or reserved, but is created “on the fly”. The benefit of this approach is that the conference URI doesn’t need to be known to the user; instead it is created by the focus (RCS server) and used by the participants’ UAs. The SIP URI of the Conference Factory is usually provisioned in the client (as in a “create new conference” button).