Coming to Grips With MOS (VoIP Deployment)

Most programs that allow you to test VoIP on your LAN don’t tell you the exact quantity of packets lost, or the amount of latency and jitter incurred during the life of the call. The programs usually give you the results of testing based on the MOS score (which I define in topic 3).
The MOS score established on the subjective quality rating of calls by a panel of listeners. I’m not saying that a battery of people independently rate your VoIP test-call quality and then average their individual scores. The VLM software programs employ testing algorithms that use specific criteria (such as latency, jitter, perceived increases or decreases in the volume of the audio, and packet loss) to arrive at a realistic MOS score for your test calls.
VLM software can determine the MOS scores in two ways:
✓ Summary metrics: This method plays a recorded message from a location on your LAN to an end destination and samples the quality at both ends. The server running the VLM software then compares the data from both sites to see how the call changed or degraded during transmission.
Use this option, if you can. The VLM software can rate the recorded message at both its point of origin and its termination. The VLM software then compares these two readings to provide an accurate representation of any degradation in voice quality experienced while passing through your LAN. If the call quality is the same at the receiving end as it was at the point of origin, then your network is solid — the path through it doesn’t affect the call in any negative way.
✓ Real-time MOS score: This method listens to live VoIP calls and identifies a MOS score by using mathematical algorithms.
Identifying a MOS score on a VoIP call is like taking your temperature when you feel sick. It gives you a number on a scale, and although that number doesn’t identify exactly what’s wrong with you, it indicates your general health. If you have a fever of 103 degrees, or a MOS score of 1.5, you know that you’re not doing well. Either score alerts you so that you can focus your attention on resolving whatever’s causing the condition. In the case of VoIP, your condition probably involves latency, jitter, and packet loss.
MOS isn’t only a rough approximation of the health of your LAN, it’s also a rough approximation of the quality of the call. Each call gets only one MOS score. Unfortunately, the quality of a call doesn’t always remain consistent throughout the duration of the call. If a call has low audio at the beginning and excessive audio at the end, that call gets a favorable MOS score because the two issues cancel each other out when averaged. Take MOS as a general indicator, telling you when you need to do some additional research.

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