A numerical measure of the perceived human quality of a telecommunications network or media stream. While technical metrics like packet loss or jitter tell you how the network is performing, MOS tells you how a human being actually experiences the call.
MOS Rating Scale
MOS is measured on a standard scale from 1 to 5, as defined by the ITU-T (International Telecommunication Union – Telecommunication Standardization Sector).
| Score | Quality | Description |
| 5 | Excellent | Perfect; like talking to someone in the same room. |
| 4 | Good | Clear and intelligible; minor imperfections that don’t distract. |
| 3 | Fair | Distortions are audible; requires some effort to follow the conversation. |
| 2 | Poor | Very distorted; conversation is difficult to maintain. |
| 1 | Bad | Impossible to communicate; call is unintelligible. |
How MOS is Calculated
Historically, MOS was determined through Subjective Testing, where groups of people would sit in a controlled room, listen to audio samples, and vote. Today, it is largely determined through Objective Modeling using algorithms:
- Perceptual Objective Listening Quality Analysis (POLQA): The current industry standard for testing wideband and 5G networks.
- Perceptual Evaluation of Speech Quality (PESQ): An older standard used primarily for legacy VoIP and public phone networks.
- The E-Model: A computational model that predicts user satisfaction based on network parameters like latency, jitter, and codec choice without needing to listen to the audio.
Factors That Lower MOS
Several technical variables directly impact the final score:
- Latency: A delay of more than 150ms (one-way) makes conversations feel unnatural, leading people to talk over one another.
- Jitter: Variation in the arrival time of data packets. High jitter causes choppy or robotic-sounding audio.
- Packet Loss: If data packets are lost in transit, parts of words or entire sentences will disappear.
- Codecs: Different compression methods have different ceilings. For example, the G.711 codec (high bandwidth) can reach a MOS of ~4.4, while the G.729 codec (high compression) typically tops out around 3.9.
Why Perfect 5 is Rare
In digital telecommunications, achieving a score of 5.0 is technically impossible due to the inherent nature of digitization and compression.
- 4.0 to 4.5 is considered Toll Quality (standard professional grade).
- 3.5 or below is usually the threshold where users will begin to complain to IT or service providers.
The R-Factor Connection
Engineers often use the R-Factor (Rating Factor) to calculate MOS. The R-Factor is a more granular scale (0 to 100). The mathematical relationship converts the technical R value into the user-friendly MOS value.
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