In "FOSDEM 2015 - part I" I made an overview of this Conference and a few comments on the talk "Python, WebRTC and you", where the a peer-to-peer WebRTC service was also described.
In this case we’re considering video-conference scenarios, possibly with many participants, and the ability to add some presentation features, like highlighting the current speaker.
The architecture behind Jitsi VideoBridge aims to avoid a centralized mixer (MCU), but at the same time prevent the complexities of a Full Mesh approach.
Enter the SFU (Selective Forwarding Unit) concept: the server component is “simply” a router of media streams among conference participants. A IETF Draft describes the behaviour expected for an SFU. Each participant receives one stream per each other user: it’s then up to the receiving client to take care of stream presentation.
The SFU doesn't need to decode, mix, and then re-encode all the streams, and for this reason it can use just a small amount of resources, and scales well.
The critical point, as Emil is aware too, is dealing with mobile devices: low available bandwidth and relatively limited computational resources make it prohibitive for applications running on such devices to handle many different streams, some of each potentially carrying high-quality video.
What can help in this case is the concept of “simulcast”: compatible applications (like Chrome) are able to generate streams at different quality levels (resolution and framerate) at the same time. There are still some difficulties in using simulcast, as witnessed in this post. Different topologies involved in WebRTC networks are also well described in this other article.
Anyway the receiver of such streams can then instruct the server about which quality it is capable of receiving. Apps on mobile devices will get the low-quality streams; browsers on desktops will require the high-quality ones.