Simulcast is a technique by which a WebRTC client encodes the same video stream twice in different resolutions and bitrates and sending these to a media router who then decides who receives which of the streams. Simulcast adds more flexibility to how media servers can operate at scale.
See also: Simulcast WebRTC in the WebRTC context and to get a more fuller picture about this technology.
Simulcast is closely related to SVC, where a single encoded video stream can be layered and each participant receives only the layers that he can process.
How Simulcast Works
In a simulcast scenario, the originating device, which is broadcasting data, sends multiple media streams towards what is known as the Selective Forwarding Unit (SFU). This means that each device will encode and transmit several media streams, each with varying levels of quality.
Typically, there are two or three streams sent to the server, each with a different bitrate and consequently, different resolutions, frame rates, and overall quality.
For the client device, this has an added cost in both CPU (the need to encode multiple separate streams) and network (the need to send to the SFU multiple separate streams).
The Role of the SFU
The SFU, or media server, plays a crucial role in the simulcast process:
The SFU receives from the various participants in a meeting their video streams. Each participant can send multiple streams – these streams have the same content in different bitrates (=quality). The SFU is is now responsible for deciding which stream to send to each participant in the call.
The SFU might choose to send the highest bitrate stream, which in our example could be 1.5 megabits per second, offering the best stream quality available. Alternatively, it could opt to send a lower quality stream if that’s more appropriate.
Decision-Making in Simulcast
The criteria the SFU uses to make these decisions can vary greatly depending on the implementation. Factors such as the available bitrate on both the sender and receiver ends, the capabilities of the devices involved, the current network quality, the layout on the screen, what is being shown, and the priorities set within the application all play a part in this decision-making process.
An important part of this work is handling BWA – Bandwidth Allocation.
Learn More About Simulcast
Simulcast is a complex but fascinating feature of WebRTC that caters to the dynamic needs of multi-party communication. For those interested in diving deeper into this topic or other terms related to WebRTC, resources like webrtcglossary.com offer a wealth of information.
Additionally, for a more structured learning path, courses are available that can provide further insight into simulcast and other advanced WebRTC functionalities.
