Measuring VoIP Call Quality with MOS

    What you are going to learn:

  • What is the VoIP Quality MOS Score
  • How to calcule MOS Score using network metrics
  • How network metrics influence MOS Score

VoIP Quality Mean Opinion Score (MOS)
VoIP Quality Mean Opinion Score (MOS)

The Mean Opinion Score (MOS) has been a commonly-used metric to measure the overall voice call quality for decades. MOS score is a rating from 1 to 5 of the perceived quality of a voice call, 1 being the lowest score and 5 the highest for excellent quality. It has been standardized by the International Telecommunications Union ITU-T.

MOS 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 calls 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 the G.711 codec, which is by far the most commonly used codec for VoIP calls. The maximum MOS in VoIP for a G.711 call is 4.4.

MOS Score vs Call Quality
MOS Score vs Call Quality

We select the Call Quality in the App based on the MOS Score. The following table lists the different qualities and the lower MOS limit for each of them. The limit values are from the ITU-T G.107 and ITU-T G.109 standards.

voip quality scores

VoIP Quality and Network Monitoring Session
VoIP Quality and Network Monitoring Session

On each Network Monitoring Session page, located below the standard Network Monitoring Graph, there a is a graph that details the VoIP Quality for the session. This is where we display results related to MOS Score VoIP metrics. The y axis on the graph is the VoIP Call Quality (from Best to Poor) and the exact VoIP MOS Score test is available in the mouse tooltip.

voip quality graph

In the graph, the yellow area indicates a voip call quality score of Medium and the red area a voip call quality of Bad or Poor. As for the network response time, if there is aggregation, the worst value is displayed (i.e. the smallest MOS score).

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

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)

    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

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

Blog Post
Blog Post

A more detailed article about measuring VoIP MOS quality of a call is available on our Blog: Measuring VoIP Quality with MOS Score (Mean Opinion Score).