VoIP R-Factor
The E-model uses cumulative measurements of impairments to estimate what their effect would be on users' perception of quality. In its simplest form, the E-model subtracts the impairments from the R-factor to come up with a score of 0 to 100:
R = Ro - Is - Id - Ie + A
Where Ro is the undistorted original signal (a constant of 100), Is measures the distortion of the speech signal from the telephone hardware, Id measures network delay and jitter, Ie measures the signal degradation due to speech encoding. A adds the users' typical expectations for the type of connection being used, accounting for the fact that people are willing to accept lower voice quality in exchange for some other benefit, such as the cost savings of DSL, or the mobility of wireless.
Observer constantly monitors network conditions to calculate the R-factor, and allows you to customize the non-network impairment factors such as room noise, loudness rating of the telephone, etc.