TamoSoft: Network Analysis Tools & Security Software

Monitoring and Troubleshooting VoIP Networks with a Network Analyzer

MOS and R-Factor

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


Maximum using G.711


Very satisfied




Some users satisfied


Many users dissatisfied


Nearly all users dissatisfied


Not recommended


Less than 50

Some users believe R-Factor to be a more objective measure of the quality of a VoIP system than MOS. Still, a network analyzer should be able to calculate both scores and produce the two assessments for better judgment of the call quality.

The following screen shot displays the details about two RTP streams (one stream in each direction) within a SIP call with a measured MOS of 4.0 in one direction and 2.7 in the other direction. According to the quality standard, MOS of 4.0 means overall good sound quality, and demonstrates that the one direction of the call has a perceptible level of distortion that will be noticed by some users but will only disturb a few. At the same time, the RTP stream in the other direction demonstrates poor quality (MOS of 2.7), with varying MOS over time. Analyzing network parameters and making certain modifications to the network or choosing a different codec may still improve the quality of this particular VoIP network.