Maybe you haven't considered your network's Mean Opinion Score yet. If you already have a Voice over IP PBX, you should keep an eye on the MOS score of your network.
The Mean Opinion Score (MOS) has been introduced to provide quantitative assessment of the quality of VoIP communications. Wikipedia defines the MOS score as follows:
Mean opinion score (MOS) is a measure used in the domain of Quality of Experience and telecommunications engineering, representing overall quality of a stimulus or system. It is the arithmetic mean over all individual “values on a predefined scale that a subject assigns to his opinion of the performance of a system quality”.[1] Such ratings are usually gathered in a subjective quality evaluation test, but they can also be algorithmically estimated.
MOS is a commonly used measure for video, audio, and audiovisual quality evaluation, but not restricted to those modalities. ITU-T has defined several ways of referring to a MOS in Recommendation P.800.1, depending on whether the score was obtained from audiovisual, conversational, listening, talking, or video quality tests. (Wikipedia)
Basically, the MOS score is somewhere between the values of 1 and 5. A MOS score of 5.0 certifies excellent voice quality. A a comparison: Public Switched Telephone Network (PSTN) has a MOS score between 4 and 5, where 5 is the absolute optimum case.
The following table provides an initial overview of the different quality levels of the Mean Opinion Score.
| MOS | Quality | Impairment |
| 5 | Excellent | No effort is required to understand the language |
| 4 | Good | Through attentive listening, speech can be heard without effort |
| 3 | Fair | The language can be heard with a slight effort |
| 2 | Poor | It takes a lot of concentration and effort to understand the transmitted language |
| 1 | Bad | Despite great efforts, it is impossible to communicate |
The gradation of the score depends on the measurement. While (very subjective) measurements carried out by humans can usually only be defined to one decimal place, automated measurements provide more precise results, and can be interpreted to up to 2 or 3 decimal places.
In the field of Voice over IP, the following four metrics are mainly used for validation.
Listening MOS is a forecast of the wideband Listening Quality (MOS-LQ) of the played audio stream. This value takes into consideration the audio fidelity and distortion, and predicts how a large group of users would rate the quality of the audio they hear.
Sending MOS is a forecast of the wideband MOS-LQ of the audio stream that is being sent from the user. This value takes into consideration the speech and noise levels of the user, along with any distortion, and predicts how a large group of users would rate the audio quality they hear.
Network MOS is a forecast of the wideband MOS-LQ of audio that is played. This value takes into consideration only network factors such as the codec used, packet loss, and packet errors.
Conversational MOS is a forecast of the narrowband Conversational Quality (MOS-CQ) of the audio stream that is played. This value takes into consideration the listening quality of the audio played and sent across the network, the speech and noise levels for both audio streams, and echoes. It represents how a large group of people would rate the quality of the connection for holding a conversation.
It is important to understand that the MOS score is measured on a relative scale, built on a variety of factors that can affect voice quality. Because of this, there are many factors that can affect the MOS score on VoIP systems that would not be a factor on a regular phone line. These include, among others:
| Codec | Name/Description | Bandwidth | MOS |
| G.711 | Pulse Code Modulation (PCM) | 56 oder 64 kbit/s | 4,10 |
| G.722 | Adaptive Multi-Rate (AMR) | 48 bis 64 kBit/s | 4,50 |
| G.723.1 | Multiple Maximum Likelihood Quantization (MP-MLQ) | 5,6 - 6,3 kbit/s | 3,90 |
| G.723 | Algebraic Code Excited Linear Prediction (A-CELP) | 5,3 kbit/s | 3,65 |
| G.726 | Adaptive Differential Pulse Code Modulation (AD-PCM) | 16 - 40 kbit/s | 3,85 |
| G.728 | Low Delay Code Excited Linear Prediction (LD-CELP) | 16 kbit/s | 3,61 |
| G.729 | Conjugate Structure Algebraic Code Excited Linear Prediction (CSA-CELP) | 8 kbit/s | 3,92 |
| G.729A | Conjugate Structure Algebraic Code Excited Linear Prediction (CSA-CELP) | 8 kbit/s | 3,70 |
Monitoring the MOS Score in Your Network Environment
With the information gained so far, you learned that the highest possible MOS score is important for high-quality VoIP voice transmission, and that the MOS score can vary depending on the network section within the company infrastructure. It is therefore important to keep an eye on the MOS score in every corner of your network.
To prevent two admins from making test calls in the network all day long and recording the determined voice quality, there is special measuring software that carries out this process automatically. The results provided are either stored in a software database or retrieved and processed by a central network monitoring system. This allows a Quality of Service to be proven and maintained on a permanent basis.
Do you measure the MOS score in your network? Does VoIP negatively affect your network bandwidth? Share your experiences in the comments!