Measuring VoIP Quality with MOS Score (Mean Opinion Score)

Alyssa Lamberti
Alyssa Lamberti Last updated on Sep. 29, 2020

Measuring VoIP Quality with MOS Score (Mean Opinion Score)

VoIP Quality is highly reliant on network performance, which means that many network problems like packet loss, latency, and jitter can cause high levels of VoIP degradation. To avoid embarrassing choppy voice calls, or lagginess during your next client meeting, we’re running you through how to measure VoIP Quality with MOS Score (Mean Opinion Score).

Figuring out why VoIP calls are making you sound like a robot can be a challenge. Luckily Obkio’s VoIP monitoring solution gives you full visibility into your LAN, WAN, Internet and Service Provider Network, while keeping track of the metrics that affect VoIP Quality the most.

Measuring with MOS (Mean Opinion Score)
Measuring with MOS (Mean Opinion Score)

The Mean Opinion Score, usually referred to as MOS Score, has been the most commonly-used metric to measure the overall voice call quality for decades. Standardized by the International Telecommunications Union (ITU-T), MOS Score refers to a numerical measure of the human-judged overall quality of voice and video sessions.

Voice Quality MOS Score is most often judged on a scale from 1 (bad) to 5 (excellent) of the perceived quality of a voice call. Although originally Mean Opinion Scores were derived from surveys of expert observers, today a MOS Score is often produced by an Objective Measurement Method approximating a human ranking.

The MOS Score for Voice Quality was originally developed for traditional voice calls but has been adapted to Voice over IP (VoIP) in the ITU-T PESQ P.862.

The standard defines how to calculate MOS Score for VoIP Quality based on multiple factors such as the specific codec used for the VoIP call. Each VoIP codec (ex: G.711, G.722, G.723.1, G.729) behaves differently. Some codecs such as G.711 are uncompressed for higher quality but use more bandwidth than compressed codecs such as the G.729.

The MOS Score we measure is for the G.711 codec, which is by far the mostly used codec for VoIP calls. The maximum MOS Score for a G.711 call is 4.4.

MOS Score vs VoIP Call Quality
MOS Score vs VoIP Call Quality

When deciding how we would be measuring VoIP Quality in the Obkio app, we decided to measure it based on the MOS Score. Here are the details for how we measure VoIP MOS Score. The table below lists the different qualities and the lowest MOS Score limit for each of them. The limit values are from the ITU-T G.107 and ITU-T G.109 standards.

VoIP Quality Table - Measuring VoIP Quality with MOS Score

We also realized that most of our customers were not very familiar with the MOS Score and the interpretation of MOS in VoIP Quality. To help all our users understand more easily, we redesigned the MOS Score graph to create the VoIP Quality graph.

Obkio’s MOS VoIP Quality graph categorizes, for every minute, the MOS Call Quality as

  • Best
  • High
  • Medium
  • Low
  • or Poor

The exact MOS Voice Quality Score is always available in the graph tooltip.

Obkio App Screen Capture - Measuring VoIP Quality with MOS Score

This feature is a great measure of the Quality of Experience (QoE) for users using VoIP applications over their network and helps IT Pros evaluate the impact of Network Performance on VoIP Applications.

MOS Score Calculation from Network Performance Measurements
MOS Score Calculation from Network Performance Measurements

For geeks that want to understand how we calculate the MOS Score from the Network Performance metrics, the standard ITU-T G.107 defines the algorithm we use to calculate the G.711 MOS Score based on the Network Performance metrics (latency, jitter and packet loss). The exact algorithm is available below in pseudo-code.

# VoIP Codec Processing Delay in milliseconds (ms)

codec_delay = 10.0

# Effective Latency is the Average Latency plus the doubled
# Average Jitter plus the VoIP Codec Delays

effective_latency = average_latency + 2* average_jitter + codec_delay

# Calculate the R-Value (Transmission Rating Factor R) based on
# Effective Latency. The voice quality drops more significantly
# over 160ms so the R-Value is penalized more.

if effective_latency < 160:
    r = 93.2 - (effective_latency / 40)

else:
    r = 93.2 - ((effective_latency - 120)/ 10)


# Reduce R-Value 2.5 times the Packet Loss percentage

r = r - 2.5 * packet_loss

# Based on ITU-T G.107

if r < 0:
    mos = 1.0

else:
    mos = 1 + 0.035*r + 0.000007*r*(r-60)*(100-r)

Measuring MOS Score with Network & Device Monitoring
Measuring MOS Score with Network & Device Monitoring

When looking at how to calculate MOS Score, network metrics give you the most value and detail when they’re combined to give you a full overview. Combining VoIP MOS Score with Obkio’s Network Performance Monitoring solution and Network Device Monitoring solution allows you to perform a network assessment to identify exactly:

  • What network problems are affecting VoIP Quality.
  • Where the VoIP issues are located anywhere along your network.
  • Who: the owner of the problem is (user, application, network, or ISP), and who is responsible for fixing it.
  • How to solve problems for a quick and smooth fix.

You can check out the Network Performance Monitoring case study with Station 22, an international beverage manufacturer and distributor, to find out how they pinpointed undetected network problems on their firewall using MOS Score, Network Performance Monitoring, and Network Device Monitoring.

Network Monitoring Case Study Station 22 - Measuring VoIP Quality with MOS Score

He’s a little summary about their problem and their solution.

Station 22 began experiencing VPN network application slowness and instability for their Microsoft Teams and GoToMeeting application.

Using Obkio’s Network Performance Monitoring software, they identified that their network latency was constant but the packet loss kept increasing. Specifically, there was a lot of packet loss occurring over the Internet as the bandwidth usage increased during the day, likely due to increased activity from users. They pinpointed the exact percentage of packet loss in the Obkio app with the graph tooltip.

They looked into the MOS Score chart to find that it reported thresholds well above what is normally acceptable.

Finally, they moved to Obkio's Network Device Monitoring feature on their firewall and saw that the CPU usage was well above the 40% threshold shown on their Firewall's GUI.

Network Monitoring Case Study Station 22 Dashboard - Measuring VoIP Quality with MOS Score

By using all three features, Station 22 was able to get a complete overview of all of the metrics affecting their network performance, and realized that the type of problem they were facing was known and could be fixed with a firmware update of the firewall. They immediately performed emergency maintenance and the problem was resolved in no time.

Network performance, specifically VoIP Quality, can be affected by a variety of different factors, which is why it’s important to get a complete, end-to-end overview of all these factors to truly understand what is wrong, and how to fix it.

Start Measuring VoIP Quality
Start Measuring VoIP Quality

VoIP issues make us 50% of IT problems!

VoIP travels a long distance from your network, through the Internet, and up to your Service provider. That means that, when a problem occurs, no matter where it is, you need to manage it.

Try Obkio’s VoIP Monitoring solution for free today to measure VoIP Quality with MOS Score and start fixing network problems before your users even know they exist!

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