Mean Opinion Score (MOS) is a numeric representation of the audio quality of a call, taking a value between 1.0 (worst) through 5.0 (best). It is a subjective measurement; a good MOS depends on the codec. For example, for G711 and G722 the top score is 4.5. For G729, the top score is 3.9.

The EdgeMarc MOS value is assigned to every call and for each direction of the call. This value is calculated by a mathematical formula that approximates real MOS scores. It is calculated using 10-second snippets of audio. The worst-case 10-second period is logged at the end of the call.

Every time a call is started and terminated (and sometimes mid-call) a logging record is added to the VOS log file. The log file can be viewed from the Admin > System Information page. Refer to Viewing System Information.

MOS scores are measured at the point that the audio (RTP) stream reaches the EdgeMarc. The scores calculated by the EdgeMarc make certain approximations:

  • The same call (RTP stream) can sound different on different VoIP phones. Some phones do a better job than others at hiding errors in a stream. The MOS score, however, will be the same regardless of the endpoint. So a given score can sound better or worse depending on the endpoint in use.
  • A momentary packet loss in a stream may not be noticed by the user, but such a loss might result in a low MOS score. A single low MOS score should not be assumed to mean poor quality for the user. A pattern of low scores across multiple calls is more significant.

What to look for in a low score:

  • For outbound scores (VOIP phone to Network), this is a measurement of the handset and intervening local area network. A low score in this direction can mean a handset or network issue.
  • For inbound scores (Network endpoint to VoIP phone), this is a measurement of the originating endpoint and full WAN path.
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