Glossary · VoIP and call quality
MOS (mean opinion score)
In one sentence
MOS (mean opinion score) rates perceived call quality on a scale of 1 to 5. In VoIP it is estimated from latency, jitter, packet loss and the codec in use, without anyone listening.
Also known as : mean opinion score · MOS score · MOS scores
What is MOS?
MOS, the mean opinion score, started life in telephone laboratories. Panels of listeners heard recorded calls and rated each one from 1 (bad) to 5 (excellent); the average was the MOS. The method is set out in ITU-T Recommendation P.800.
Nobody can put every business call in front of a listening panel, so VoIP uses an estimated MOS instead. Phones, phone systems and session border controllers measure network conditions during the call and work out the score an average listener would probably have given. It has become the simplest single figure for tracking call quality across an organisation.
How it works
The most widely used calculation is the E-model from ITU-T Recommendation G.107. It starts from a theoretical best quality and deducts points for:
- the codec: heavily compressed codecs start with a handicap;
- end-to-end latency;
- jitter and the packets the jitter buffer has to discard;
- packet loss on the network.
The result, called the R-factor, is then mapped onto the 1-to-5 MOS scale. As a rule of thumb: above 4.0 is good, 3.6 to 4.0 is acceptable, and below 3.5 a share of callers will be bothered. A perfect narrowband G.711 call rarely exceeds 4.4.
MOS tells you that a call was poor, not why. A low score is the signal to look at the detailed statistics, and the fix is often in the local network or in QoS settings rather than with the provider.
Example
A chain of five dental practices in the West Midlands reviews average MOS per site every month. One practice consistently shows 3.3 on Monday mornings while the others sit above 4.1. Cross-checking the detailed reports, the IT contractor finds heavy packet loss when the weekly backup of patient records runs at 9am. Moving the backup to the evening and adding a voice priority rule on the router brings the practice in line with the others.
MOS at Universal Telecom
The call-quality statistics on our virtual PBX and SIP trunk help you spot degraded calls. If MOS drops at one of your sites, our technical support team can help track down the cause. Contact us.
Frequently asked questions
What is a good MOS score for VoIP?
Above 4.0 most people rate the call as good. Between 3.6 and 4.0 it is acceptable; below 3.5 complaints start. Even a flawless G.711 call tops out around 4.4 because of its narrowband audio.
Can MOS be higher than 4.5?
With wideband codecs such as G.722 or Opus, estimated scores can go a little higher than with G.711, but in practice no real call reaches a perfect 5.
Related terms
- Latency
Latency is the time your voice takes to travel from your mouth to the other person's ear. In VoIP, once one-way delay passes about 150 ms, conversations start to feel stilted.
- Jitter
Jitter is the variation in the arrival time of voice packets. When it exceeds what the phone's jitter buffer can absorb, packets are discarded and the voice sounds choppy or robotic.
- Codec
A codec is the algorithm that digitises and compresses your voice during a VoIP call and turns it back into sound at the other end. It sets both the audio quality and the bandwidth used.
- QoS (quality of service)
QoS (quality of service) is the set of network settings that give time-sensitive traffic, such as voice, priority over other data, so calls avoid delay, jitter and packet loss when the link is busy.
- VoIP
VoIP (voice over internet protocol) carries phone calls as packets of data over an IP network, such as the internet or an office LAN, instead of over the copper lines of the traditional phone network.
Updated on