Voice over internet protocol (VoIP) as a new telephony method promises to revolutionize communication significantly as it boasts of several desirable features and advantages over the previously used technologies. A crucial determinant of efficient communication is the quality of videos and calls attainable while using a particular device or technology. Other essential ingredients for successful telephone communication include; network coverage, call rates, reliability and assured privacy levels. Call, and video quality as the principal measure of how effective communication is primarily for the VoIP telephony is gauged using a technique called the Mean Opinion Score (MOS).

What is MOS in VoIP?

Mean opinion score is a quantitative measure of the quality of voice in telephony explicitly VoIP. It is a numerical quality measure that has been used for an extended period to assess users’ opinion about the call quality. MOS has for decades been useful in VoIP networks to ascertain quality performance, quality voice testing and to avail a metric system for voice degradation and performance. It is a crucial measure of customers’ satisfaction and informs the service providers on areas that require improvements. As per IT U-T recommendations, MOS is commonly used to measure video, audio, and audiovisual quality evaluation among other communication modalities.

Determination of MOS   

Mean opinion score is a subjective measure that should reflect user experience and contention. Historically, this measure is derived from conducting polls to establish users’ acceptance and satisfaction levels about the telecommunication services that they receive. It is distinctively useful in static image compression, audio codecs, video codecs and ultimately voice calls. MOS is a metascore value averaged from some listening sessions that provide views on the quality of calls. Initially, experts were selected to act as a representation of the general population; this was reported to be biased and prone to manipulation.

Since call quality is a highly subjective parameter, a MOS calculation algorithm is usually applied to harmonize the variations and predict mean opinion score. However, such techniques can be deceptive and human trials, and testing is thus the goal standard in measuring call quality.  MOS Mean opinion score is estimated based on a ranking system referred to as Absolute Category Ranking (ACR) scale which varies between 1 and 5 and takes into account three significant considerations; listening quality, transmission quality, and conversation quality. 5 represents excellent voice quality while 1 represents the poorest call quality. After collecting opinions from several participants as designed in the test plan, averaging is done to come up with a MOS that will influence the plans of VoIP service providers. So what is MOS score? MOS score is a numerical value that indicates the discerned quality of received upon being transmitted and compressed using codecs.

The more comfortable and cheaper alternative of using MOS calculation algorithm is prompt and efficient but may not necessarily correlate to reality in the communication field. Proper call quality testing informs advancement in the VoIP telephony and has been responsible for the rapid influx and drastic retention of VoIP users. Typically, VoIP call quality range from 3.4 to 4.5 and this is attributable to several factors exclusive of the device and network providers. Some of the influential factors include; bandwidth, hardware, jitters, latency, packet loss, and codec version. Codec version is specifically critical in determining call quality since its essential in comprehending the voice projected.  A non-compressed codec gives the best voice quality, with the lack of compression and decompression making the signal less prone to audio quality loss. Depending on the condensation ratio, compression can degenerate voice quality significantly or remarkably improve it.

How to improve MOS score

With the understanding of MOS score, you can be able to make adjustments that will ensure you get a better MOS score. Some of the possible manipulations that will help improve your MOS score are listed below.

  • Minimize the propagation delays; the amount of time required to relay a digital signal over a network. The greater the distance and number of barriers the higher the distortion and thus poor sound quality. It is possible to decrease the impact of the propagation delays by setting up a smooth network with a lower number of firewalls and routers.
  • Using the latest version of the codec is essential to reduce the packetization delays. Packetization delay is the amount of duration necessary for the signal to be made digital and transmitted over the internet to the terminal end for decoding. Using newer codec versions promises efficient compression and decompression thus an enhanced MOS score.
  • Integrating a jitter buffer to compensate for voice wrongful voice coding is crucial for good MOS score. Jitter buffers will collect, store, and then send calls to the processor in more evenly spaced intervals. Jitter buffers located at the receiving end intentionally delay the packets, so that received voice is presented correctly with less distortion.


Owing to the shortcomings of the manual techniques of MOS determination, MOS score software are now being employed in assessing call qualities as they are thought to exhibit better dependency and more reliable results. The previously used manual methods are extensively subjective and extremely prone to biases and emotional well-being. Some examples of MOS score software are AppareNet Voice, Brix VoIP Measurement Suite, NetAlly, PsyVoIP, and VQmon/EP. They are further advantageous in that they factor in the network attributed factor that influences voice quality which is often never accounted for in the human method.

MOS score cisco elaborates how the call quality metric works and give recommendations on how to improve the deficient call quality scores. Some of the vital aspects underscored are the operation of the MOS score, monitoring procedure, usage and incorporation as well as the downsides. Other VoIP call quality metrics include; Perceptual speech quality measurement (PSQM) and Perceptual evaluation of speech quality (PESQ).

MOS as a VoIP call quality metric has been exceptionally useful in transforming telecommunication and bettering telephony as it shifts the focus to ensure user satisfaction. However, there are a few shortcomings that require improvement.