Opus Packet Loss %

Understand Opus-level packet loss percentage — how CallMeter measures audio loss from the decoder's perspective, thresholds, and why codec-level loss differs from network-level loss.

Opus Packet Loss % reports the percentage of audio frames lost as seen by the Opus decoder. This is calculated as the ratio of gap frames (frames the decoder expected but never received) to the total frames pushed to the decoder. It is the most accurate measure of audio loss impact because it captures exactly what the codec experienced.

Think of it as the decoder's own loss report card. Network-level packet loss tells you what the network dropped. Opus packet loss tells you what the audio codec actually missed — which may be different, because the jitter buffer sits between the network and the decoder.

Opus only

This metric is specific to Opus-encoded audio streams. For codec-agnostic packet loss, see Packet Loss Rate.

How It Works

CallMeter tracks two counters at the Opus decoder:

Frames pushed — Audio frames successfully delivered to the decoder
Frames gapped — Frames the decoder expected (based on timing) but never received

The loss percentage is: (gapped / (pushed + gapped)) * 100

This is more precise than network-level loss because:

Packets that arrive but are too late for the jitter buffer count as lost here
Duplicate packets that the network counts as "received" are deduplicated before reaching the decoder
Burst loss patterns that affect multiple frames are captured accurately

Why It Matters

Opus Packet Loss % directly predicts the amount of PLC (Packet Loss Concealment) activity. Every lost frame triggers PLC, generating synthetic audio. While Opus's PLC algorithm is excellent for isolated losses, sustained loss above 3% produces audible degradation that users will notice.

For enterprise VoIP testing, this metric is the single best predictor of audio quality impact from packet loss. It removes the ambiguity of network-level metrics and tells you exactly how much audio the listener missed.

Thresholds

Level	Loss %	Interpretation
Good	0 - 1%	Loss is minimal and fully concealed by PLC
Warning	1% - 3%	Occasional audible artifacts, investigate network conditions
Critical	Above 3%	Noticeable quality degradation, PLC struggling to keep up

Codec-level loss compounds faster

At 3% Opus packet loss, approximately 60ms per second of audio is concealed. Combined with burst loss patterns where consecutive frames are lost, the PLC algorithm's accuracy drops rapidly, producing audible warbling and distortion.

Common Causes of High Opus Packet Loss

Cause	Explanation
Network packet loss	Packets dropped before reaching the receiver
Jitter buffer overflow	Buffer full, forcing late packets to be discarded
Jitter buffer underflow	Buffer too small, delivering gaps to the decoder
Late packet arrivals	Packets arriving after their playout deadline
Clock drift	Sender and receiver clocks diverging, causing systematic late arrivals

How to Fix It

Compare with network loss — If Opus loss is higher than network-level Packet Loss Rate, the jitter buffer is discarding packets. Review jitter buffer configuration.
Check jitter buffer sizing — An undersized buffer cannot absorb jitter spikes, converting late packets into codec-level loss.
Enable Forward Error Correction — Opus supports in-band FEC that lets the decoder recover from isolated losses without PLC.
Address network quality — If Opus loss matches network loss, the problem is on the network path. Focus on reducing packet loss at the network level.
Monitor clock drift — Significant Clock Drift can cause systematic jitter buffer issues that appear as Opus-level loss.

PLC Events — Each lost frame triggers a PLC event; directly driven by this metric
PLC Duration — Total concealed audio time, the duration-based view of Opus loss
Packet Loss Rate — Network-level loss for comparison with codec-level loss
Jitter — High jitter often precedes Opus-level loss due to jitter buffer overflow

RFC Reference

The Opus codec's loss handling is defined in RFC 6716 (Definition of the Opus Audio Codec), Section 4 (PLC) and Section 2.1.7 (loss robustness). The interaction between Opus FEC and packet loss is further described in RFC 7587 (RTP Payload Format for the Opus Speech and Audio Codec).