24.1473GPPConferencing using the IP Multimedia (IM) Core Network (CN) subsystemRelease 16Stage 3TS
6.2 Functional entities
6.2.1 User Equipment (UE)
For the purpose of SIP based conferences, the UE shall implement the role of a conference participant as described in subclause 6.3.1.
6.2.2 Media Resource Function Controller (MRFC)
For the purpose of SIP based conferences, the MRFC shall support the procedures for media control of ad-hoc conferencing described in subclause 10.3 of 3GPP TS 24.229 .
6.2.3 Conferencing Application Server (Conferencing AS)
For the purpose of SIP-based conferences, the conferencing AS shall act as a conference focus, as described in subclause 6.3.2. The conferencing AS may implement the role of a conference participant as described in subclause 6.3.1.
The conferencing AS shall use the procedures for media control in subclause 10.2 of 3GPP TS 24.229  to implement SIP based conferences.
6.2.4 Media Gateway Control Function (MGCF)
The MGCF implements the role of Conference participant (see subclause 6.3.1).
6.3.1 Conference Participant
The conference participant shall support the procedures specified in 3GPP TS 24.229  appropriate to the functional entity in which the conference participant is implemented.
If the conference participant uses BFCP as specified in clause 8, then the conference participant shall support the procedures specified in RFC 4583 .
6.3.2 Conference Focus
In addition to the procedures specified in subclause 6.3.2, the conference focus shall support the procedures specified in 3GPP TS 24.229  appropriate to the functional entity in which the conference focus is implemented.
If the conference focus uses BFCP as specified in clause 8, then the conference focus shall support the procedures specified in RFC 4583 .
NOTE: RFC 4582  recommends the use of Transport Layer Security (TLS) for the secure exchange of BFCP. Whether this is followed, and the mechanism for the exchange of the certificates in association with SDP are outside the scope of this version of this document.
When the conference focus receives any SIP request or response containing SDP, the conference focus shall examine the media parameters in the received SDP.
Provided that the INVITE request received by the conference focus contains an SDP offer including one or more "m=" media descriptions, the SDP answer shall:
– reflect the media capabilities and policies as available for the conference; and
– contain a request confirmation for the result of the resource reservation at the originating end point for every "m=" media line if preconditions were required by the originator.
During session establishment procedure for a conference, SIP messages shall only contain SDP payload if that is intended to modify the session description.
For "video" and "audio" media types that utilize the RTP/RTCP, the conference focus shall specify the proposed bandwidth for each media stream utilizing the "b=" media descriptor in the SDP. For other media streams the "b=" media descriptor may be included. The value or absence of the "b=" parameter will affect the assigned QoS which is defined in 3GPP TS 29.208 .
If the conference focus supports the Volume Based Charging (VBC) functionality, then for each media stream used for the conference, the conference focus shall include the "a=content" media level attribute with value "g.3gpp.conf" as defined in annex B in the SDP.
The conference focus shall include the DTMF media format at the end of the "m=" media descriptor in the SDP for audio media flows that support both audio codec and DTMF payloads in RTP packets as described in RFC 2833 .
Upon receipt of a SDP answer or sending a SDP answer that changes the resource requirements for the conference, the conference focus shall provide the corresponding changes of conference resources.
Upon receipt of a SDP offer during conference creation, that confirms that the conference participant has reserved the required resources, the conference focus shall through-connect the conference resources.