Group Video Calls: SFU, Simulcast & Thermal Adaptation

Picture this: your app has 8 participants, after 10 minutes the iPhone heats to 42°C, the call drops. The issue is architecture: without an SFU (Selective Forwarding Unit), each mobile client sends video to all others — in a group of 8, each handles 7 incoming streams and 1 outgoing. The CPU and network load scales quadratically. The proper solution is an SFU with simulcast. We design the architecture for your scenario: 2 to 20 participants, with adaptation to device thermal state.

Why SFU over MCU?

MCU (Multipoint Control Unit) mixes all streams into one on the server and sends each participant a single video. Client load is minimal, but mixing requires powerful CPU and introduces 100–300 ms latency. Suitable for webinars where most watch one speaker. SFU (Selective Forwarding Unit) only routes streams without decoding. Each client receives N streams (one per participant) and chooses which to decode and display. Higher client load but lower latency and greater flexibility.

For mobile apps with conferences up to 20 participants, SFU is optimal. Ready solutions: Livekit (open-source, self-hosted), Mediasoup, Jitsi Videobridge, or managed services — Daily.co, 100ms, Twilio Video Rooms. Livekit is a solid choice for self-hosted: Go server, WebRTC SFU, support for simulcast and dynacast, native SDKs for iOS (LiveKit-iOS), Android (LiveKit-Android), Flutter, and React Native. MIT license.

How Simulcast Reduces CPU Load

Without simulcast, on a 10+ person conference a mobile device sends one 720p stream to all participants, even those with small tiles. With simulcast, the client sends three qualities simultaneously (e.g., 180p/360p/720p), and the SFU sends each recipient the quality matching their tile size. Resource savings are significant: CPU load can drop up to 40% with many participants.

On a recent telemedicine project, we reduced CPU load by 35% on iOS devices by implementing simulcast and thermal state adaptation, enabling stable 12-person conferences without overheating.

On iOS, simulcast is configured via RTCRtpEncodingParameters with three layers:

let encodings = [ RTCRtpEncodingParameters(rid: "q", scaleResolutionDownBy: 4, maxBitrateBps: 150_000), RTCRtpEncodingParameters(rid: "h", scaleResolutionDownBy: 2, maxBitrateBps: 500_000), RTCRtpEncodingParameters(rid: "f", scaleResolutionDownBy: 1, maxBitrateBps: 1_200_000), ] 

On Android — similarly via RtpParameters.Encoding. This reduces network and CPU load on receivers with many participants.

How to Display Participants: Grid and Dominant Speaker

Grid for 2–4 participants is a static layout. For 5–16 participants, use a dynamic grid that changes on join/leave. Rule: do not recreate RTCVideoRenderer on every grid update — only reassign the track to the existing renderer. Recreating causes flickering and re-render.

Dominant speaker detection — identify who is speaking and show them larger. Livekit and 100ms provide this out of the box via onActiveSpeakersChanged events. In raw WebRTC, analyze audioLevel from RTCPeerConnection.getStats().

On iOS, render video through RTCMTLVideoView (Metal) — mandatory for conferences. The old RTCEAGLVideoView (OpenGL ES) does not support multiple instances with good performance on A15+. With 6 participants, RTCEAGLVideoView gives 40 FPS, RTCMTLVideoView a stable 60 FPS.

Battery and Thermal State

Group conferencing is the most battery-intensive mode. Encoding 720p at 30 FPS consumes ~15–20% battery per hour on an iPhone 14. With 4+ participants, decoding multiple streams adds more.

We react to ProcessInfo.thermalState (iOS) — at .serious or .critical, we lower resolution to 360p and reduce FPS to 15. On Android, PowerManager.getThermalHeadroom() (Android 11+). This is not degradation, but adaptation: a stable 360p conference is better than overheating and forced CPU throttling.

When the app is backgrounded on iOS, we stop camera capture (AVCaptureSession.stopRunning()) while audio continues via AVAudioSession. On Android, a ForegroundService with android:foregroundServiceType="camera|microphone" holds permissions. This saves battery and reduces heat.

Process and Timelines

Requirements audit → SFU selection (self-hosted or managed) → SDK integration → UI (grid, dominant speaker, controls) → simulcast → thermal state adaptation → load testing. After delivery, we provide SDK documentation, management console access, and brief team training. Our team has 7+ years of mobile development experience and over 15 commercial WebRTC projects.

Conference up to 8 participants via managed SFU (100ms, Daily) with ready SDK — 2–4 weeks. Self-hosted Livekit with custom UI and simulcast — 4–8 weeks. Cost is determined after requirements analysis. Contact us — we'll assess your project and propose a turnkey architecture.

What's Included

• Technical audit and architecture recommendations
• Selection and integration of SFU (self-hosted or managed)
• UI development: grid, dominant speaker, controls
• Simulcast and adaptive quality configuration
• Battery and thermal state optimization
• Load testing for up to 20 participants
• SDK integration documentation
• Management console access
• Brief team training

Step-by-Step Simulcast Setup on iOS

1. Create RTCRtpEncodingParameters objects for each layer with rid, scaleResolutionDownBy, maxBitrateBps.
2. Apply these encodings to the video track via RTCRtpSender.setParameters().
3. Enable simulcast on the SFU side (in Livekit, the SimulcastConfig flag).
4. Verify that receivers correctly switch layers when tile size changes.
5. Profile CPU and network load with 10+ participants.

According to WebRTC documentation, proper simulcast implementation can reduce bandwidth usage by up to 40%.

Get a consultation on integration — we'll help implement simulcast in your project.