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MOS(1) (Metal Oxide Semiconductor) See MOSFET.
(2) (Mean Opinion Score) The quality of a digitized voice line. It is a subjective measurement that is derived entirely by people listening to the calls and scoring the results from 1 to 5. Toll quality audio is generally considered to have a MOS rating of 4 and above.
The MOS is an average of the numbers for a particular codec. Because MOS testing requires carefully prepared and controlled test conditions, the best way to get a MOS test done is to have it outsourced. See codec.
An Objective Model
The E-Model is another rating system that provides an objective measurement of quality based on packet loss, jitter and delay. The E-Model reports results as R-values. Following is an approximate relationship between MOS and E-Model quality ratings.
Subjective Interpretation MOS R Perfect 5.0 100 Excellent - almost all satisfied 4.5 90 Very good - most are satisfied 4.0 80 Good - some are dissatisfied 3.6 70 Fair - many are dissatisfied 3.1 60 Poor - most are dissatisfied 2.6 50 Bad - not recommended 1.0 0