Insight: What is MOS and Why Does it Matter?

Posted by Mae Kowalke on Thursday, May 07, 2015 with No comments

Mean Opinion Score, or MOS, was originally developed to assess the quality of voice over IP (VoIP) calls using a scale of 1-5 (5 being the highest quality). That ranking system was developed by having thousands of people listen to the same playback and rate their quality subjectively. 

Over time, scientific MOS models have evolved, and it is now a quantifiable calculation. Packet loss, delay, jitter, and other basic metrics are examined to estimate--with a high degree of accuracy--what the sound quality will be like for any individual user. MOS calculated this way correlates very well with human experience. 

In the real world of mobile telecom today, a typical call probably has a score of around 3.5 HD or voice over LTE (VoLTE) calls can get close to 4.5. 

Even though more people now use smartphones for data than voice, call quality (and MOS) remains the foundational metric for Quality of Service (QoS) and Quality of Experience (QoE). That's because voice calls, even over IP or high bandwidth connections, are the first thing to reveal flaws with latency, packet loss, etc. If you have a bad Skype call, you can be pretty sure you'll also have a bad experience with other data-intensive applications like gaming. 

Put another way: call quality/MOS is the canary in the coal mine.  

Added reading: Overview of MOS & R-Factor

Accedian Resource: Measuring & Monitoring VoLTE QoE
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