Mean Opinion Score and How it Sounds

The Mean Opinion Score, or MOS (sometimes redundantly called the MOS score), is one way of ranking the quality of a phone call. This score is set on a five-point scale, according to the following ranking:

• 5. Excellent
• 4. Good
• 3. Fair
• 2. Poor
• 1. Bad

MOS never goes below 1, or above 5.

There is quite a science to establishing how to measure MOS based on real-world human studies, and the depth they go into is astounding. ITU P.800 lays out procedures for measuring MOS. Annex B of P.800 defines listening tests to determine quality in an absolute manner. The test requirements are spelt out in detail. The room to be used should be between 30 and 120 cubic meters, to ensure the echo remains within known values. The phone under test is used to record a series of phrases. The listeners are brought in, having been selected from a group that has never heard the recorded sentence lists, in order to avoid bias. The listeners are asked to mark the quality of the played-back speech, distorted as it may be by the phone system. The listeners' scores, on the one-to-five scale, are averaged, and this becomes the MOS for the system. The goal of all of this is to attempt to increase the repeatability of such experiments.

Clearly, performing MOS tests is not something that one would imagine can be done for most voice mobility networks. However, the MOS scale is so well known that the 1 to 5 scale is used as the standard yardstick for all voice quality metrics. The most important rule of thumb for the MOS scale is this: a MOS of 4.0 or better is toll-quality. This is the quality that voice mobility networks have to achieve, because this is the quality that nonmobility voice networks provide every day. Forgiveness will likely offered by users when the problem is well known and entirely relatable, such as for bad-quality calls when in a poor cellular coverage area. But, once inside the building, enterprise voice mobility users expect the same quality wirelessly as they do when using their desk phone.

Thus, when a device reports the MOS for a call, the number you are seeing has been generated electronically, based on formulas that are thought to be reasonable facsimiles of the human experience.