Video Bitrate
Understand video bitrate measurement in VoIP calls — how CallMeter tracks encoding bitrate, what bitrate changes reveal about network adaptation, and how to interpret bitrate trends.
Video Bitrate measures the encoding rate of the video stream in bits per second (bps). It represents how much data the video codec is producing per second. Higher bitrate generally means better visual quality, but it also demands more network bandwidth.
Think of it as the "quality dial" on the video encoder. When network conditions are good, the encoder turns it up for sharper video. When congestion hits, it turns it down to avoid overwhelming the network — trading visual quality for a stable connection.
How It Works
The video encoder continuously adjusts its output bitrate based on several factors:
- Target bitrate configuration — The maximum bitrate the encoder is allowed to produce
- Video content complexity — Fast motion and detailed scenes require more bits than a static talking head
- Congestion feedback — REMB (Receiver Estimated Maximum Bitrate) or Transport-CC signals from the remote side telling the encoder to reduce its output
- Keyframe events — Keyframes temporarily spike the bitrate because they encode a complete picture
CallMeter reports the instantaneous encoding bitrate each second, allowing you to track how the encoder adapts over the call's duration.
Both directions
Video bitrate is measured separately for send and receive directions. The send bitrate reflects your encoder's output. The receive bitrate reflects what the remote encoder is producing and what arrives at your decoder.
Why It Matters
Video bitrate is the central indicator of video quality adaptation. In enterprise VoIP testing, bitrate trends reveal:
- Bandwidth adequacy — A stable bitrate at or near the target indicates sufficient bandwidth. A dropping bitrate indicates the encoder is backing off due to congestion.
- Quality consistency — Large bitrate swings produce visible quality fluctuations for the viewer.
- Congestion onset — Bitrate reduction is often the first sign of network congestion, appearing before packet loss or freezes.
- Infrastructure capacity — Load tests that drive bitrate down across many endpoints reveal the bandwidth ceiling of your infrastructure.
Common Causes of Low or Dropping Video Bitrate
| Cause | Explanation |
|---|---|
| Network congestion | Encoder reducing output in response to congestion signals |
| Bandwidth limitation | Link capacity insufficient for the target video bitrate |
| REMB throttling | Remote receiver estimating low available bandwidth |
| Resolution reduction | Encoder dropping resolution, which naturally reduces bitrate |
| CPU overload on sender | Encoder unable to maintain target bitrate due to processing constraints |
How to Fix It
- Verify available bandwidth — Ensure the network path can sustain the target video bitrate with 20-30% headroom for overhead and burst.
- Check for congestion signals — Monitor whether REMB values from the remote side are driving the bitrate reduction.
- Review QoS configuration — Apply DSCP marking (e.g., AF41 for video) to ensure video traffic is prioritized over best-effort flows.
- Adjust target bitrate — If the encoder consistently cannot reach its target, set a more realistic target that the network can sustain.
- Monitor CPU usage — If the sending device is CPU-constrained, the encoder may reduce bitrate or framerate to keep up.
Related Metrics
- Resolution — Resolution changes often accompany bitrate changes during adaptation
- Video FPS — Frame rate may drop alongside bitrate during congestion
- Video Freeze Events — Extreme bitrate drops can lead to decoder starvation and freezes
- Send Bitrate — Total network-level bitrate including overhead
- Packet Loss Rate — Loss often triggers the congestion feedback that reduces bitrate
Keyframes Received
Understand the keyframes received metric in video streams — how CallMeter tracks inbound I-frames, and how to use the count to assess video recovery and remote encoder behavior.
Video FPS
Understand video frame rate measurement in VoIP calls — how CallMeter tracks FPS, what frame rate drops mean for visual quality, and common causes of choppy video.