IP Telephony: How to quantify the quality of VoIP using the MOS score

It is often said that quality is subjective. It is true, in many fields. As for IP telephony, there exists a scientific method enabling us to quantify the quality of VoIP. This measure is called the MOS score.

The International Telecommunication Union (ITU) has defined in its “Recommendation P.800.1 – Methods for objective and subjective assessment of speech and video quality” a mechanism to obtain a mean opinion score, or MOS, to measure the quality of VoIP calls. The MOS is expressed as a single rational number, typically in the range 1–5, where 1 is lowest perceived quality, and 5 is the highest perceived quality, or its original source. A score of 4 is considered to be very good.

The initial objective of the MOS score is to score the quality of various coding and decoding compression algorithms (codec) used in transporting voice over various networks. For example, most LTE networks use the G.722.2 codec while a number of VoIP carriers will use the highly compressed G.729 protocol. As for IP4B, it has chosen to use the non-compressed G.711 codec which has a maximum MOS score of 4.41 compared to 4.14 for G.722.2 (LTE) and 3.92 for G.729 (most other VoIP carriers).

The MOS score also measures the real-time effects of the underlying IP network on the quality of voice calls. An IP network experiencing a high level of packet loss, for example, will have a lower MOS score. There are a number of factors affecting the quality of a VoIP network including latency and jitter.

In order to survey its network in real time, IP4B uses Oracle Communications Operations Monitor as the tool to sample its IP network. The following screen shot, taken from this tool, illustrates the MOS score over time (horizontal axis) across the network density, or the number of simultaneous calls on its network (vertical axis).

As you can see, despite the significant increase in network traffic between 7:30 a.m. and 10:00 a.m., 98.5% of calls have a MOS score of 4.34 or more (dark green area). Below, in light green, are calls that receive a score between 4.03 and 4.33. Lastly, around 0.5% of calls (thin red area at bottom) receive a score of less than 3.09. This last group of calls are network internal calls which, for technical reasons, are not properly scored by the monitoring tool.

What powers this high IP4B MOS score is its solid network, which gets a lot of coverage in this blog. At the heart of this solid network are private IP connections and telco grade equipment.

We can proudly state that virtually 100% of calls on the IP4B network have a MOS score of 99%. That’s VoIP quality!