Glossary
MOS
A 1–5 scale measuring voice call quality. 4.0+ is good; below 3.5 is degraded.
also known as: Mean Opinion Score, Call quality score
MOS (Mean Opinion Score) is a 1–5 scale for measuring voice call quality, originally developed by the International Telecommunications Union (ITU) as a way to rate audio codecs and network paths against human listener perception.
The scale
| MOS | Quality | Listener experience |
|---|---|---|
| 4.3–5.0 | Excellent | Very clear; no perceptible issues |
| 4.0–4.3 | Good | Clear; occasional minor issues |
| 3.5–4.0 | Fair | Listenable; some issues |
| 3.0–3.5 | Poor | Noticeable degradation; affects conversation |
| Below 3.0 | Bad | Hard to understand; calls regularly fail |
For business VoIP on Australian broadband, MOS 4.0–4.3 is what good deployments hit consistently. Below 3.5 means something’s degrading - internet path, codec, or network congestion.
How MOS is calculated
Two flavours:
Subjective MOS (original) - actual human listeners rate sample audio. Used for codec design and laboratory testing. Not practical in production.
Objective MOS (used in phone systems) - algorithms derive MOS from technical measurements: latency, jitter, packet loss, codec, and codec-specific characteristics. The most common is ITU’s E-Model. 3CX reports this kind of MOS in call detail records.
What drives low MOS
- Packet loss - biggest single factor. Even 1% loss is audible; 3%+ is disruptive.
- Jitter - variation in packet arrival times. Smooth out via jitter buffer; excessive jitter causes audio drops.
- Latency - one-way delay. Below 150ms is fine; 150–300ms is noticeable; above 300ms is bad.
- Codec choice - G.711 (uncompressed) has highest MOS ceiling; G.729 (compressed) is lower; Opus (modern) scales widely.
- Network congestion - peak-time bandwidth contention.
How to monitor MOS in your business
3CX call detail records include per-call MOS. Configure alerts for calls dropping below 3.5. Quarterly review trends - if MOS is drifting down, something in your network or trunk path is degrading.
CPS reviews customers’ MOS trends quarterly as part of the managed-service health-check.
