What is the Formula for MOS Score in VoIP?
When it comes to measuring the quality of a VoIP (Voice over IP) call, one of the most commonly used metrics is the MOS (Mean Opinion Score). If you’ve been exploring how VoIP works and the various factors that influence call quality, you’ve likely encountered MOS. In this article, I’ll explain the formula for calculating the MOS score and how it reflects the perceived quality of a voice call over the internet.
First, let’s break it down: MOS is a score given to a voice call that ranges from 1 to 5, with 5 being excellent quality and 1 being terrible quality. MOS helps us understand how users perceive the voice quality of their calls, taking into account factors like latency, jitter, packet loss, and other network issues that can affect audio clarity.
Formula for MOS Score: There isn’t a simple, one-size-fits-all formula for MOS because it involves various parameters, but in the context of VoIP, the MOS score is typically calculated using the following:
- R-Factor (R): The R-Factor is the main parameter used to estimate the MOS score. The R-Factor itself is based on factors like codec type, packet loss, delay, and jitter.
- Equation: MOS = 1 + (R – 1) / 20. Here, the MOS score is determined by using the R-Factor.
To understand this better, let’s look at the steps involved in the process:
1. Calculate the R-Factor (R): The R-Factor represents the quality of a voice call and is derived from a combination of factors such as:
- Latency: The time it takes for a voice packet to travel from the sender to the receiver. High latency can reduce the call quality.
- Packet Loss: The percentage of packets lost during transmission. This can cause distortion or gaps in the voice stream.
- Jitter: Variability in packet arrival times, which can lead to uneven audio.
- Codec Used: Different codecs (such as G.711, G.729, or Opus) have varying levels of efficiency and impact on call quality.
2. Plug the R-Factor into the MOS Formula: Once you calculate the R-Factor, it’s used in the MOS equation to determine the MOS score. Typically, the R-Factor will range from 50 to 100. A higher R-Factor means better call quality, and the MOS score will reflect that.
Example: If the R-Factor is calculated to be 80, you would use the equation:
MOS = 1 + (80 – 1) / 20 = 1 + 79 / 20 = 1 + 3.95 = 4.95
This means the call quality is rated as excellent, close to 5, which is the highest possible MOS score.
3. Interpret the MOS Score: Once you have the MOS score, it helps you understand the quality of the call:
- 5: Excellent Quality
- 4.5 – 4.0: Good Quality
- 3.9 – 3.0: Fair Quality
- 2.9 – 1.0: Poor to Bad Quality
As you can see, the higher the MOS score, the better the user experience will be. A high MOS score means the VoIP call has minimal issues like latency, jitter, and packet loss, providing clear and consistent voice quality.
In the previous articles, we discussed how factors like network performance and VoIP protocols can impact the quality of a call. The MOS score serves as an aggregate measure of those influences, giving both service providers and users a benchmark for evaluating VoIP quality.