"There is no off the shelf software that would allow us to support calls of that size while ensuring that all communication is end-to-end encrypted, so we built our own open-source Signal Calling Service to do the job," the company said in a blogpost.

Signal mentioned that when building support for group calls, it evaluated many open sourceASelective Forwarding Unit (SFUs), but only two had adequate congestion control.

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"We launched group calls using a modified version of one of them, but shortly found that even with heavy modifications, we couldn't reliably scale past eight participants due to high server CPU usage," the company said.

To scale to more participants, the company wrote a new SFU from scratch in Rust.

It has now been serving all Signal group calls for nine months, scales to 40 participants with ease and is readable enough to serve as a reference implementation for an SFU based on the WebRTC protocols, the company said.

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