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Understanding MOS for voice quality

By Irwin Lazar

User satisfaction with the dialing experience and perception of voice quality are the foremost evaluation factors used to judge quality of phone calls. Users assign descriptive adjectives -- good, OK, poor or terrible -- to voice quality. The user's judgment of voice quality is subjective. If you know the caller well and understand his speech patterns, even a poor connection can carry an understandable conversation.

Now, imagine that you have the same poor-quality connection with someone who has an unfamiliar accent or speaks rapidly. Understanding the conversation becomes much less likely.

Factors that affect mean opinion score

Good clarity is the primary description of an acceptable voice call. Clarity is the speech clearness, fidelity, intelligibility and lack of distortion. The following five components define the elements of sound quality for one direction of a call:

A variety of factors can negatively influence clarity. These include the following:

Technologies that can improve voice clarity can include the following:

Evaluating the mean opinion score

Mean opinion score (MOS) is a standard numeric value, defined by the International Telecommunication Union (ITU) in recommendation P.10 used to measure and report on voice quality. MOS has a range from a maximum score of 5, which is considered to be the same as speaking directly into the person's ear, to a minimum score of 1, which is an unacceptable voice quality to all users. MOS does not include what has been defined as the call experience, only the sound or voice quality.

An MOS of 4.4 to 4.5 is considered equivalent to a toll-quality call as experienced on the PSTN. Users who experience an MOS of 4.5 will be very satisfied. An MOS of 4.0 is still considered acceptable to the vast majority of users. When the MOS decreases to 3.5, some users may find the voice quality unacceptable. Most non-HD voice cellular calls have an MOS rating of 3.8 to 4.0, where speaker and word recognition may be impaired.

When the MOS falls below 3.5, users will be dissatisfied and will either retry the call or potentially contact the help desk and open a trouble ticket. An MOS below 2.6 is considered to be an awful call. The user with an MOS of 2.6 will need to find an alternative network for this call -- for example, when a wireless call is terminated and the speaker moves to the PSTN.

The ITU P.800 standard for MOS defines scoring for narrowband calls. The P.800 methodology is based on having approximately 30 or more people, sitting in a quiet location, listen to eight to 10 seconds of speech under controlled conditions. The listeners are asked to rate their opinions of the calls from very satisfied to awful, scoring the calls from 5 to 1.

A newer standard, called E-model, defined in ITU-T Rec. G.107 uses measurements of transmission parameters to calculate a quality score, called the Transmission Rating Factor, or R-factor. R-factor scores may be converted to MOS. R-factor is especially useful for measuring improvements in voice quality when using wideband audio codecs.

Today, VoIP management platforms use an automated approach, based on ITU standard P.862, to calculate MOS and R-factor based on measured latency, jitter and use of wideband audio for both live calls, as well as synthetic transactions that simulate call performance. This approach provides those responsible for VoIP quality management not only historical call quality performance data, but real-time alerts when performance scores drop below an acceptable range. VoIP management platforms are typically able to identify the culprit causing a poor-performing call, giving IT support teams the ability to quickly address and rectify performance-related issues.

21 Dec 2018

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