subject: Voip Call Quality Can Be Evaluated With Mean Opinion Score [print this page] When calling using VoIP, among the frequently asked question are inquiries on the call quality of the voice call - if there are noticeable gaps in the voice transmission, if the calls are suddenly cut-off, and if the voice is clear enough to be understood on the other end. These questions are understandable, given the bad quality of calls in older VoIP systems.
Fortunately, VoIP is way past its period of doubts and uncertainties due to the ascent of faster and more reliable broadband technologies, the development of new hardware and software that minimizes latency and jitter, and introduction of methods that improve and streamline the sending and receiving of VoIP data packets. The quality of calls is also assured by using metrics and testing methods that measure the strength and clarity of sound signals after they have been compressed, transmitted and converted back into voice.
Definition of MOS
One such metric used extensively in the telecommunication industry is the Mean Opinion Score, or MOS, which provides a numerical rating on the quality of received media after compression or transmission as perceived by the listener.
In simple terms, the standard MOS requires a listener to rate the audio quality of a set of sentences read aloud by male and female readers over the medium being tested, typically over telephone lines. The listener rates individually each sentence from 1 (lowest quality) to 5 (highest quality), judging together both the quality and perceived impairments in the transmission. The individual scores' arithmetic mean is the Mean Opinion Score.
Use of MOS in VoIP Systems
The different codecs used in VoIP (G.711, G.722, etc.) have all been tested by experts and the estimated MOS for each at given bit rates are handy in planning a VoIP infrastructure. For example, choosing G.711 as a core component of a VoIP system is a good idea, as it gives top-notch voice quality and it can also provide fax transmission capability.
In addition, MOS is also used to rate the call quality in existing traditional or VoIP telephones. The MOS score of 4.0 is adopted by the industry as standard, which is equivalent to the toll quality of regular telephone lines. The scoring is as follows:
1 - Bad call quality with so much static or noise that makes conversation impossible, short of both parties shouting at each other
2 - Poor call quality with constant static and line noise but with the parties able to talk with just a little more effort
3 - Fair call quality has occasional instances of static or line noise that are not quite disruptive to the conversation
4 - Good call quality has no static, line noise or echo of any kind, except for barely perceptible telephony-induced background noise
5 - Excellent call quality has no sound distortion and both parties can hear each other as if they are in the same room
In implementing a VoIP, it takes a lot of planning, testing and configuring to ensure a fail-safe and reliable system. Knowing about MOS and other metrics surely helps VoIP system engineers quickly identify possible degradation of call quality.
