Mean Opinion Score (MOS)

MOS is a measurement of quality of the audio heard by the listener on a phone. Most scores break down into the following categories:

  1. Bad
  2. Poor
  3. Fair
  4. Good
  5. Excellent

"Toll quality" sound is generally associated with a MOS score of at least 4. G.711 starts with an intrinsic MOS value of 4.4, while G.729, which performs significant compression, has an intrinsic MOS value of 4.1. Of course, the MOS score on a VoIP network is further reduced when there is packet loss, excessive delays, etc.

There are a number of qualifiers frequently appended to MOS, including MOS-CQS, MOS-LQS, etc. Refer to ITU-T Recommendation P.800.1 for a description of those.

Resources: ITU-T Recommendations P.10, P.800, and P.800.1

See Also: Voice over IP (VoIP), CODEC