TotalView uses a proprietary method of calculating MOS is based on the latency, jitter, and loss seen between endpoints in a conversation.
- Latency is the time in milliseconds that it takes for packets to be communicated between endpoints, averaged over a 1-second sample period.
- Jitter is the peak variance seen during the 1-second sample period.
- Loss is the percentage of packets that were lost during the 1-second sample period.
- Out of Order Packets are the number of packets that were not in sequential order during the 1-second sample period.
When troubleshooting VoIP issues, Loss should be investigated first as it has the largest effect on call quality. Missing/lost packets means missing voice information and can lead to whole words of the conversation being lost as well as the clipping of words/phrases.
Jitter is the second element to be investigated, as it has the second largest effect on call quality. If packets don’t arrive on a regular interval, it means that some packets may be late to arrive and have to be discarded because it cannot be sequenced into the voice stream.
VoIP conversations can have high latencies, but weird things start to happen with higher latencies: Echo is the biggest problem that occurs with high latency connections. RTP (Real Time Protocol) sessions can also be disconnected (call drop) if the latency gets too high.