Finisar Surveyor manual VQMon Metrics

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User’s Guide

VQMon Metrics

There are a variety of objective factors that contribute to call quality. Some of these factors, such as packet loss or packet delay variation (jitter), are reported in other Multi-QoS graph summaries. However, these individual measurements do not tell a complete story and do not attempt to quantify user perceptions of voice quality. The VQmon metrics in Multi-QoS, called R-factors, use a formula to take into account both user perceptions and the cumulative effect of equipment impairments to arrive at a numeric expression of voice quality.

Multi-QoS calculates two equipment impairment values to report as voice-quality metrics: the Network R-factor and the User R-factor. The Network R-factor is generated based on the physical equipment impairments. The User R-factor adds perceptual effects to the equipment impairment, such as recency and delay. The user R-factor attempts to add the “perceived” annoyance that a user may experience during a call based on a perceptual effect called recency. Recency is an auditory phenomenon where distracting events that have occurred more recently appear to have a greater impact on perceived quality. The User R-factor has been found to match well with users’ purely subjective ratings of voice quality.

These metrics are calculated by a formula that balances all equipment impairments and perception factors. Each metric is reported as a single number on a per-call basis, typically in the range of 15 to 94. Lower numbers indicate greater equipment impairment or perceived poor voice quality. In Multi-QoS, calls are broken down into a set of ranges for the Network R-factor and User R-factor values calculated for each call. The actual R-factor numbers associated with a single call can be viewed in the Channel Details Table for the call.

It takes some experience to map the exact meaning of the R-factor metrics to your particular network. In general, the R-factors should map well to a sliding scale of how voice quality is perceived. At the extremes, calls with values greater than 80 will have few quality problems and those with values less than 50 will have significant problems. The Network R-factor can be compared to the User R-factor to help determine which factors predominate in any voice quality degradation -- equipment impairments such as packet loss, or, more subjective factors such as recency and delay. Table 11-5 shows ranges of voice quality for the R-factors. The R-factor is also converted to a Mean Opinion Score (MOS), which corresponds to purely subjective rating by users of speech quality on a numeric scale of 1 to 5.

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