Multi-party audio/video conferencing

Another important feature of FreeSWITCH is delivered by the mod_conference conferencing module. The mod_conference provides dynamic conference rooms that can bridge together the audio and video from several users. It may mix video streams together, applying CG (computer graphics) transformations to them, such as composing a live feed of different conference participants together, over imposing a caption with the name and role to each users' video stream, sharing the screen of each participant computer (for example, a PowerPoint presentation), and so on. Also, a real-time chat can be added to the conference, so participants can exchange text messages out of band from the main audio/video stream. Obviously, this same module can also be used for plain regular audio conference calls.

Each new session that connects to the same conference room will join the others, and instantly be able to talk and see all of the other participants at the same time (as per the whim of the conference admin, who can choose who to display, who can talk, and so on). Using an example similar to the one we used for bridging to another phone, we can make an extension to join a conference room:

<extension name="example 4"> 
<condition field="destination_number"  expression="^3000$"> 
<action application="conference" data="3000@default"/> 
</condition> 
</extension> 

This is as simple as bridging a call, but with a conference application many callers can call the same extension (3000 in this case) and join the same conference room. If three people joined this conference and one of them decides to leave, the other two would still be able to continue their conversation.

The conference module also has other special features, such as the ability to play sound or video files or text-to-speech to the whole conference, or even to a single member of the conference. As you may have guessed, we are able to do this by using the TTS and video/sound file interfaces provided by their respective modules. The smaller pieces come together to extend the functionality without needing knowledge of each other.

The conference module also uses the event system in an additional way, employing what are called custom events. When it first loads, a module can reserve a special event namespace called a subclass. When something interesting happens, such as when a caller joins or leaves a conference, it fires those events on the CUSTOM event channel in the core queue. When we are interested in receiving such events, all we have to do is subscribe to the CUSTOM event by supplying an extra subclass string, which specifies the specific CUSTOM events we are interested in. In this case, it is conference::maintenance. This makes it possible to look out for important things such as when someone joins or leaves the conference, when they start and stop talking, when they are displayed on video, or what video layout (screen disposition) is currently in use. Conferencing is discussed in detail in Chapter 13, Conferencing and WebRTC Video-Conferencing.