I’ve been talking about MOS scores recently and answering a lot of questions. So I thought it might be worth a quick note. I’ll write a more detailed article about IP voice quality a little later.
MOS Scores are used to quantify the quality of a phone conversation. In the past this was based on the “Mean Opinion” of several call testers. On modern IP telephony systems, there are mathematical alogrithms that do this for us. They take into account the various inconsistencies that plague VoIP calls: loss, jitter and latency.
Those algorithms then give us a number to rate the results. The number is from 1 to 5. 5 being a theoretical maximum. I’m not aware of any system that claims 5.0 MOS.
|
MOS Score |
Quality |
Example |
|
5.0 |
Perfect Audio |
Face to face conversation |
|
4.4 |
Maximum digital* |
Crystal clear phone call |
|
4.0 |
Very good |
Normal phone call |
|
3.8 |
Good |
Average cell phone call |
|
3.5 |
Minimum for Faxing |
A few skips, but otherwise okay |
|
3.0 |
Minimum phone call |
A few missed words, but otherwise okay |
|
2.0 |
Poor but usable |
Callers need to repeat messages often |
|
1.0 |
Very Poor quality |
Zombies have taken over the planet and this is the only way to communicate with other outposts. It sucks but it’s all we got. |