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In order to provide quantitative assessment of the quality of VoIP communications, the Mean Opinion Score (MOS) has been introduced. The MOS indicates the perceived voice quality of a VoIP conversation, ranking the call quality as a number in the range 1 to 5.
Originally, the MOS was meant to represent the arithmetic mean average of all the individual quality assessments given by people who listened to a test phone call and ranked the quality of that call. Today, human participation is no longer required to determine the quality of the audio stream. Modern VoIP quality assessment tools employ artificial software models to calculate the MOS.
The MOS is highly subjective. One should not make decisions on a VoIP system based on the MOS alone. Other measurable parameters should be analyzed such as network delay, packet loss, jitter, and so on. As an alternative to the MOS, a different, less subjective rating has been introduced.
R-Factor is an alternative method of assessing call quality. Scaling from 0 to 120 as opposed to the limited scale of 1 to 5 makes R-Factor a somewhat more precise tool for measuring voice quality. R-Factor is calculated by evaluating user perceptions as well as the objective factors that affect the overall quality of a VoIP system, accounting for the Network R-factor and the User R-factor separately.
The following table demonstrates the effect of the MOS and R-Factor on the perceived call quality.
| User Satisfaction Level |
MOS |
R-Factor |
| Maximum using G.711 |
4.4 |
93 |
| Very satisfied |
4.3-5.0 |
90-100 |
| Satisfied |
4.0-4.3 |
80-90 |
| Some users satisfied |
3.6-4.0 |
70-80 |
| Many users dissatisfied |
3.1-3.6 |
60-70 |
| Nearly all users dissatisfied |
2.6-3.1 |
50-60 |
| Not recommended |
1.0-2.6 |
Less than 50 |
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