Simulcast

Simulcast is a video conferencing architecture with the following data transmission procedure between the server and the endpoints:

  1. The server receives several (3-5) video streams from each client containing the camera image in various resolutions and quality.
  2. The server sends a video stream copy of the other participants that corresponds to the bandwidth and device capabilities of each endpoint.


Simulcast 1This architecture is a good choice when the layout puts one participant in the bigger video window, while others are displayed in smaller panes.

Simulcast 2

Simulcast architecture advantages

  • Several levels of quality allow for some flexibility as compared to earlier VC architectures.
  • Saves the server from processing  video.

Simulcast  architecture disadvantages

  • In real life Simulcast rarely manages to use channels and resources efficiently because the image in all streams is transmitted the same, simply with different levels of detail. Redundancy occurs because each participant in the conference sends a larger stream than necessary.
  • Simulcast-based systems require significant resources from user’s VC endpoints, but use them inefficiently. In such systems the main network load falls on the endpoints, which is caused by the need for simultaneous support of multiple levels of video quality.

Comparison: 4 participants, on the endpoint

MCU SFU Simulcast SVC
Outgoing streams 1 1 3 1
Incoming streams 1 3 3 3
Out channel, Mb/s 1,0 1,0 1,5 1,2
Incoming channel, Mb/s 1,0 3,0 1,0 1,0
CPU load 20% 60% 80% 30%

Comparison: 4 participants, on the server

MCU SFU Simulcast SVC
Outgoing streams 4 12 12 12
Incoming streams 1 4 12 4
Out channel, Mb/s 4,0 12,0 ~4,0 ~4,0
Incoming channel, Mb/s 4,0 4,0 6,0 4,8
CPU load 100% 0% 0% 0%

Simulcast has been replaced by SVC architecture, which allows changing video quality without creating explicit copies.

For a more in-depth understanding of how this and other video conferencing architectures work, we recommend watching this video:

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Source: trueconf.com