Call Quality - MOS or Mean Opinion Scores

Measuring VoIP Call Quality

Mean opinion score (MOS) is a test that has been used for decades in telephony networks to obtain the human user's view of the quality of the network.

The MOS is the arithmetic mean of all the individual scores, and can range from 1 (worst) to 5 (best).

Measuring Voice over IP (VoIP) is more objective, and is instead a calculation based on performance of the IP network over which it is carried. The calculation is defined in the ITU-T PESQ P.862 standard. Like most standards, the implementation is somewhat open to interpretation by the equipment or software manufacturer. Moreover, due to technological progress of phone manufacturers, a calculated MOS of 3.9 in a VoIP network may actually sound better than the formerly subjective score of > 4.0.

Mean opinion score (MOS)
MOSQualityImpairment
5ExcellentImperceptible
4GoodPerceptible but not annoying
3FairSlightly annoying
2PoorAnnoying
1BadVery annoying

Compressor/decompressor (codec) systems and digital signal processing (DSP) are commonly used in voice communications, and can be configured to conserve bandwidth, but there is a trade-off between voice quality and bandwidth conservation. The best codecs provide the most bandwidth conservation while producing the least degradation of voice quality. Bandwidth can be measured quantitatively, but voice quality requires human interpretation, although estimates of voice quality can be made by automatic test systems.

A similar process can be used to evaluate subjective video quality.

As an example, the following are mean opinion scores for one implementation of different codecs:

Platform Codecs and MOS ratings

Codec Bit Rate (kbps)MOS (from R)
G.711644.4
GSM FR    12.2 3.5 
G.722644.5*
OPUSVariableOptimal

One consideration when planning a VoIP deployment is the bandwidth usage for a particular codec versus the potential MOS. For example, G.711, with a sample size of 64kbit/s, achieves a maximum MOS of 4.1, whereas G.729, with a much smaller sample size of 8kbit/s, can achieve a MOS of 3.9. G.729 is "compressed eight times smaller than G.711 while sounding almost as good."

Calculations and Mapping R-Value to MOS ratings

Latency and jitter are summed up and a defined value for computation time is added (e.g., 10ms). For the estimation of MOS, the impact of latency by a certain factor is i.e., doubled. The resulting number is called “effective latency”.
Subtracting the effective latency from the defined R—the higher the effective latency, the lower R will get. Also, R gets a much more aggressive deduction if latency exceeds a certain time. The lower latency, the more is deducted from effective latency, and the higher stays R.
Then the percentage of packet loss from R. Depending on what impact packet loss should have, it is multiplied with a certain factor, for example, 2.5 times packet loss.
Finally, the reduced R is converted into an MOS value by applying a widely used formula for this purpose.

effectiveLatency = latency + jitter * latencyImpact + compTime
R = 93 – (effectiveLatency / factorLatencyBased)
R = R – (lostPackets * impact)
MOS = ( (R - 60) * (100 – R) * 0.000007R) + 0.035R + 1)

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