MOS: how rtcStats rates the quality of a stream

MOS (Mean Opinion Score) is the number rtcStats uses to tell you how good a stream sounded or looked to the user. You find it next to every audio and video stream, and it feeds the Audio, Video and Experience scores of the session.

What MOS is

MOS comes from telephony, where people listened to calls and rated them from 1 (bad) to 5 (excellent). The average of their ratings was the Mean Opinion Score.

Nobody rates your calls by hand. rtcStats estimates the score from what the WebRTC statistics say about each stream: the codec, the bitrate, packet loss, jitter and round-trip time. The estimate runs every time a measurement arrives, so a stream gets a MOS value per point in time and an average for the whole stream.

In rtcStats, MOS goes from 1 to 4.5, and a stream falls in one of three bands:

• Excellent: 4.0 and above.
• Average: from 3.6 up to 4.0.
• Poor: below 3.6.

A MOS of 1 is the minimum: the user could not hear the audio, or the network was unusable at that moment.

The models behind the estimate are described in the MOS metric reference.

When a stream has no MOS

A stream shows a dash instead of a MOS when:

• less than 5 seconds of media flowed through it, which is too little to rate
• its codec is not one rtcStats can rate

A stream without a MOS is left out of the Audio, Video and Experience scores. Within a stream, the moments when no media flowed have no MOS value either, and they are left out of the average.

A muted microphone still sends packets, so a muted stream is still rated.

What lowers MOS

MOS drops when the network damages the media or when there is too little bitrate to carry it well. To dig into a low MOS, look at the stream's packet loss, jitter and round-trip time around the time of the drop:

• WebRTC packet loss
• WebRTC jitter
• Choppy audio
• Video freezing

For how MOS rolls up into the session scores, see Experience score and other scores.