m=audio 38202 RTP/AVP 96 a=rtpmap:96 t38/8000

a=fmtp:96 T38FaxVersion=2;T38FaxRateManagement=transferredTCF

9.7 Conferencing

SIP conferencing is an important application of SIP. The ability to provide a better experience than today’s conventional PSTN teleconferences is an important driver and differentiator for SIP services and endpoints. While SIP conferencing can use the same model as the PSTN, where a centralized mixer/conference bridge accepts media from each participant and mixes the resulting media, there are improvements possible with SIP. Some of the alternative topologies, uses, and applications are described in the SIP conferencing framework document [23]. The resulting SIP extensions and best current practices are described in RFC 4579 known as call control for conferencing [24].

9.7.1Focus

The centralized point of control for a SIP conference is known as a focus. The focus performs SIP authentication and authorization on behalf of all conference participants, and controls the media mixing. Note that this is not exactly the same thing as saying the focus performs media mixing. In some cases, the focus can cause participants in the conference to perform their own mixing if there is a full mesh of media streams among participants in the conference. However, mixing control is done by the focus. The focus also provides information about participants in the conference: their identities and capabilities. This information can be shared with participants in the conference. One SIP method of doing this is using the SIP conference event package [25]. A participant in a conference sends a SUBSCRIBE to the focus, which creates a subscription, resulting in NOTIFYs sent containing information about the conference.

A conference focus uses the isfocus feature tag to indicate that a particular dialog is associated with a conference. This allows a UA to automatically learn that a given point-to-point call is actually part of a conference. This allows behavior such as:

•Use of a conference-specific user interface;

•Automatic subscription for conference events;

•Use of conferencing call control functions.

Some of the conferencing call control functions are described in [24]. The call flows described in this document are listed in Table 9.4.

An example call flow is shown in Figure 9.4 where a conference is created, joined, and two other participants added, using automated SIP approaches.

9.7.2Mixer

The mixer creates the media combination that allows each party to participate in the group session. For audio, this means mixing in the loudest speakers at any instant. For video, it can mean showing the video of the loudest speaker, or combining all video pictures into a single screen. For text, it can mean sharing all text messages typed by each participant with attribution. The mixer is under the control of the focus, and often it will be a part of the focus.

9.7.3Non-SIP Conference Control

In addition to the SIP call control means described in [24], the IETF has standardized other conference control protocols. For example, the ability to send media to the group (i.e., act as the presenter) can been controlled as a floor, with only a single floor holder at a time. A floor control protocol provides a method for requesting and granting the floor. The Binary Floor Control Protocol (BFCP) [26] has been developed to enable shared access to a resource within a conference. The Centralized Conference Manipulation Protocol (CCMP) [27] has been defined to create, schedule, and define media types and resources for a SIP conference. CCMP also supports conference control operations during a conference such as muting a participant, and removing or adding a participant. CCMP supports multimedia conferences including voice, video, and text sessions, and can support a full set of multiuser chats. CCMP, along with SIP conference aware UAs and a focus, allows a full standards-based conferencing system to be built.

Table 9.4

Call Control for Conferencing Functions

Joining a conference, dial out and dial in

Creating a conference, manually and automatically

Adding or deleting a participant

Requesting a participant join a conference

Switching UAs during a conference

Transferring a point-to-point session into a conference

Deleting a conference

Discovery of a conference