Mean Opinion Score (MOS): A Measure of Voice Quality

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In voice and video communication, quality usually dictates whether the experience is a good or bad one. Besides the qualitative description we hear, like "quite good" or "very bad," there is a numerical method of expressing voice and video quality. It is called Mean Opinion Score (MOS). MOS gives a numerical indication of the perceived quality of the media received after being transmitted and eventually compressed using codecs.

MOS is expressed in one number, from 1 to 5, 1 being the worst and 5 the best. MOS is quite subjective, as it is based on figures that result from what is perceived by people during tests. However, there are software applications that measure MOS on networks, as we see below.

The Mean Opinion Score Values

Taken in whole numbers, the numbers are quite easy to grade.

  • 5: Perfect. Like face-to-face conversation or radio reception.
  • 4: Fair. Imperfections can be perceived, but sound still clear. This is (supposedly) the range for cell phones.
  • 3: Annoying.
  • 2: Very annoying. Nearly impossible to communicate.
  • 1: Impossible to communicate

The values do not need to be whole numbers. Certain thresholds and limits are often expressed in decimal values from this MOS spectrum. For instance, a value of 4.0 to 4.5 is referred to as toll-quality and causes complete satisfaction. This is the normal value of PSTN and many VoIP services aim at it, often with success. Values dropping below 3.5 are termed unacceptable by many users.

How Are MOS Tests Conducted?

A certain number of people are sat and are made to hear some audio. Each one of them gives a rating from within 1 to 5. Then an arithmetic mean (average) is calculated, giving the Mean Opinion Score. When conducting MOS test, there are certain phrases that are recommended to be used by the ITU-T. They are:

  • You will have to be very quiet.
  • There was nothing to be seen.
  • They worshiped wooden idols.
  • I want a minute with the inspector.
  • Did he need any money?

Factors Affecting Mean Opinion Score

MOS can simply be used to compare VoIP services and providers. But more importantly, they are used to assess the work of codecs, which compress audio and video to save on bandwidth utilization but with a certain amount of drop in quality. MOS tests are then made for codecs in a certain environment.

There are however certain other factors that affect the quality of audio and video transferred. These factors are not supposed to be accounted for in MOS values, so when determining the MOS for a certain codec, service or network, it is important that all the other factors are favorable to the maximum for good quality, for MOS values are assumed to be obtained under ideal conditions.

Software Automated Mean Opinion Score Tests

Since the manual/human MOS tests are quite subjective and less than productive in many ways, there are nowadays a number of software tools that carry out automated MOS testing in a VoIP deployment. Although they lack the human touch, the good thing with these tests is that they take into account all the network dependency conditions that could influence voice quality. Some examples are AppareNet Voice, Brix VoIP Measurement Suite, NetAlly, PsyVoIP, and VQmon/EP.