Triple-Play Service Deployment

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Chapter 8: Troubleshooting VoIP
Measuring VoIP service quality
Many ways exist to measure voice quality on a VoIP network.
Active methods involve sending known voice samples across the
network from one endpoint to a receiving endpoint.The receiving
endpoint does a comparison analysis of the degraded sample to
the original. Due to the complexity of the signal comparisons,
intrusive testing algorithms are computationally intensive and not
viable for real-time quality measurements.
Passive methods passively calculate voice quality without a
reference voice sample. These methods are most commonly used
to turn-up and test actual networks. The most popular passive
method, mean opinion score (MOS), assigns a value to the overall
quality of the delivered voice through a network. MOS scores
range from 1 (bad) to 5 (excellent).
A true MOS score is determined by people listening to the same
call and rating it from 1 to 5.Test devices can measure a MOS score
through complicated algorithms that are based on the data from
large groups of listeners rating calls. The test devices can then
provide overall and per call MOS scoring to give a good view of
network performance.
The E-model produces a single value called an R value. This value
is derived from a variety of factors including delay and other
network impairments. Originally the E-Model was intended to be
used for network planning and design. R values range from
0 (extremely poor) to 100 (high quality). Any R value below 50 is
unacceptable.TDM based phone calls have a maximum R value of 94.