What is MOS?

Mean opinion score or MOS is type of measurement that is utilized in various forms of voice communications. The main purpose of MOS is to evaluate the quality of human speech at the point of termination on any type of phone connection. MOS is considered to be especially helpful with accurately accessing the voice quality that is achieved through Internet telephony, but is also employed to evaluate the sound quality on bridging equipment that is hosting an audio conference call.

The structure for MOS is relatively straightforward. A simple sliding scale is utilized to measure the overall quality of sound reception within a given telephony environment. The scale is somewhat subjective, in that a selected group of listeners will simultaneously hear a reader recite a series of test sentences. Generally, the sentences are presented by a male voice, then repeated using a female voice. Some systems make use of computer generated voices for the recitation of the test sentences, but it is not unusual for human voices to be utilized.

The scale used for MOS is essentially configured to offer five different grading points. The lowest score on a MOS scale is one. One indicates bad sound quality that is considered unacceptable. At the other end of the scale, a score of five is considered to be excellent sound quality, and thus highly desirable.

Once a test group has logged their evaluations, the scores are averaged by the number of persons included in the test group. This final figure presents the MOS for the current session. Anything below a three generally indicates there are some issues with the signal processing or the bandwidth applied to the signal. Scores of four or five are usually considered stable and within industry standards, and thus acceptable.

Today, MOS is utilized to evaluate various forms of Voice over Internet Protocol signals, inbound signals received on conferencing bridges, and signaling on a cellular or wireless network. Most providers of voice communication services routinely test sound quality on different trunk groups at the point of termination to ensure service delivery is maintained at an acceptable level.