MOS Score
Mean Opinion Score — the headline VoIP quality metric that rates call quality on a 1 to 5 scale using the ITU-T E-model.
MOS Score
| Property | Value |
|---|---|
| Key | mos |
| Unit | 1-5 scale |
| Type | Gauge |
| Direction | Send and Receive |
| RFC | ITU-T P.800, ITU-T G.107 |
What It Measures
MOS (Mean Opinion Score) rates the quality of a voice call on a scale from 1 (bad) to 5 (excellent). Think of it as a single number that answers the question: "How good does this call sound?"
Technically, CallMeter computes MOS using the E-model defined in ITU-T Recommendation G.107. The E-model takes network impairments — jitter, packet loss, and round-trip time — and feeds them through a mathematical formula that produces a transmission rating factor (R-Factor), which is then mapped to the MOS scale. This is a computational estimate, not a subjective human rating, which makes it repeatable, consistent, and suitable for automated monitoring.
Why It Matters
MOS is the single most widely used metric in the VoIP industry for communicating call quality. When a carrier reports SLA compliance, when an enterprise evaluates a new SIP trunk, or when a DevOps team gates a deployment on quality — MOS is the number they reference.
| MOS Range | Quality | User Experience |
|---|---|---|
| 4.0 - 5.0 | Excellent | Toll-quality or better. No perceptible issues. |
| 3.5 - 4.0 | Good | Minor impairments. Acceptable for business calls. |
| 3.0 - 3.5 | Fair | Noticeable quality issues. Some users will complain. |
| 2.5 - 3.0 | Poor | Frequent issues. Not suitable for professional use. |
| 1.0 - 2.5 | Bad | Unusable. Call quality is severely degraded. |
How CallMeter Measures It
CallMeter computes MOS every second for each endpoint in your test. The calculation uses real-time jitter, packet loss, and round-trip time values measured from the actual RTP stream — not estimates or sampling. This gives you a continuous time-series of quality throughout the call rather than a single summary value.
The E-model computation accounts for codec-specific impairment factors. A G.711 call with 1% packet loss will produce a different MOS than an Opus call with the same loss, because each codec handles degradation differently.
Thresholds
| Level | Value | Meaning |
|---|---|---|
| Good | 4.0 or above | No action needed |
| Warning | 3.5 - 4.0 | Investigate contributing factors |
| Critical | Below 2.5 | Immediate attention required |
What Causes Low MOS
MOS is a composite metric. When it drops, look at the contributing factors:
- High jitter — Packet arrival time variation forces the receiver to compensate, degrading quality. See Jitter.
- Packet loss — Even 1% loss is audible. At 3% or above, quality degrades significantly. See Packet Loss Rate.
- High round-trip time — Latency above 200ms causes conversational overlap and awkward pauses. See Round Trip Time.
- Codec limitations — Narrowband codecs (PCMA/PCMU) have a lower MOS ceiling than wideband codecs (G.722, Opus) even under perfect conditions.
How to Fix It
- Identify the dominant impairment. Check jitter, packet loss, and RTT individually to find which one is driving MOS down.
- Apply QoS markings. Ensure RTP packets carry proper DSCP markings so network equipment can prioritize voice traffic.
- Reduce network hops. Each intermediate device adds latency and jitter opportunity. Use direct peering where possible.
- Upgrade codecs. If you are using G.711, consider G.722 or Opus for better resilience to network impairments.
- Check the path. Use CallMeter's per-second timeline to correlate MOS drops with specific network events.
MOS Is Not Everything
A MOS of 4.0 does not guarantee a perfect call. MOS does not capture issues like echo, one-way audio, DTMF failures, or codec negotiation problems. Always cross-reference with other metrics and call timing data for a complete picture.
Related Metrics
- R-Factor — The linear-scale precursor to MOS from the same E-model calculation
- Jitter — Primary contributor to MOS degradation
- Packet Loss Rate — Primary contributor to MOS degradation
- Round Trip Time — Affects MOS at higher latencies
Metrics Reference
Complete reference for CallMeter's 150+ real-time VoIP quality metrics. Understand every measurement across quality, network, feedback, jitter buffer, audio, video, and call timing categories.
R-Factor Score
Transmission rating factor from the ITU-T E-model — a linear 0-100 scale that quantifies voice transmission quality and maps directly to MOS.