Managing VoIP Quality
You can manage only what you can measure. Managing a VoIP deployment therefore requires some hard numbers beyond subjective user assessments of quality (although these are obviously important as well).
Beyond monitoring the network parameters discussed in this paper, having an overall quality score such as a Mean Opinion Score (MOS) or R-factor score can also be a useful VoIP network health index.
VoIP monitoring tools calculate the MOS and R-factor scores using a formula known as the E-model. Using the statistics it has collected from the network, the analyzer calculates how much the various impairment factors (such as codec compression, jitter, delay, and packet loss) would affect the typical user’s perception of call quality.
MOS and R-factor are used to gauge user satisfaction with call quality. MOS levels under 3.5 and R-factor below 80 mean trouble.
 
Figure 57: How Call Quality Relates to User Satisfaction