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Glossary · Voice technology

What is MOS (mean opinion score)?

  • Also called: mean opinion score, voice quality score

Reviewed by Sujan ThapaliyaLast updated

Definition

MOS is a one-to-five rating of call audio quality, originally from listening panels and now usually calculated from network measurements. Roughly 4.3 and above is considered toll quality; below about 3.6 most listeners describe the call as poor.
The three STIR/SHAKEN attestation levels, which underpin how mos (mean opinion score) behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, mos (mean opinion score) included.

In practice

How mos (mean opinion score) actually works

Calculated MOS is an estimate derived from latency, jitter, packet loss and the codec in use. It is useful as a single number to trend, and it hides which of those inputs is actually causing a problem.

The codec sets the ceiling. An uncompressed codec can approach the top of the scale; a heavily compressed one cannot reach it even on a perfect network, which is why comparing MOS across different codecs is misleading.

As a monitoring metric it works best as a distribution rather than an average — the calls at the bottom of the range are the ones customers remember.

MOS is capped by the codec: a compressed codec cannot reach the top of the scale on any network.
MOSQualityTypically means
4.3–4.5Toll qualityG.711 on a clean network
4.0–4.3GoodCompressed codec, healthy network
3.6–4.0Usable, noticeably imperfectSome loss or jitter
Below 3.6PoorPeople complain, and are right to

Worked example

G.711 on a clean network scores around 4.4. The same network with 2% packet loss can fall below 3.6, which listeners describe as choppy.

MOS (mean opinion score): common questions

What is a good MOS score?
Above 4.3 is toll quality and indistinguishable from a traditional call. Between 3.6 and 4.0 is usable but noticeably imperfect; below 3.6 people complain.
Why is my MOS low when bandwidth is fine?
Because bandwidth is rarely the cause. Jitter, packet loss and latency degrade voice long before capacity does, and a saturated uplink produces all three even on a fast connection.
Can MOS be compared across providers?
Only if the codec and the measurement method match. A calculated MOS on a compressed codec is not comparable to one measured on an uncompressed codec, whatever the numbers suggest.

Sources

  1. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

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