What is jitter?
Jitter is a key measure of VoIP quality. Jitter refers to is the variation in time between packets sent and packets arriving caused by network difficulties such as route changes, congestion, packet loss, traffic regulators etc. VoIP works by sending voice data as a stream of packets from source to destination. These packets can take a varying amount of time to reach the destination and invariably do not arrive in the order in which they were sent.

For a VoIP telephone call to work well the packets sent from the source must arrive within a certain time window (or 'buffer') in order for the receiving end to reassemble the packets in the correct order and reproduce the spoken words. When there is excessive jitter the time delay is too long (high latency) and packets arrive outside the time window and get lost from the call (discarded). As a result, the recomposed sound no longer reflects exactly what was sent, and depending of the extent of the delay may not be understandable by the recipient. PrimeVoice Call tester measures the level of jitter on a connection and reports the associated level of VoIP sound quality.

What is packet loss?
Packet loss plays a key role in the quality of VoIP connections, as high packet loss causes some of the voice data not to arrive to the recipient. Packet loss occurs when voice packets are discarded by the jitter buffer (see above), or dropped by network routers/switches due to high congestion. PrimeVoice Call tester measures the percentage of packet loss and reports the associated level of sound quality.

What is MOS?
The MOS, or Mean Opinion Score, is a numeric measure used for VoIP, indicating the sound quality at the receiving end of a communication circuit. Although the score is subjective it provides a widely-used method to rate the quality of voice communication in a simple way that meaningful to end users. The score is normally between 1 and 4.5 with 4.5 being the best.

PrimeVoice Call tester tests the line quality and measures the highest MOS that a connection can achieve - this is independent of the Codec selected for the VoIP test. The MOS value is reported in the PrimeVoice Call tester summary tab once a connection test completes, a VoIP simulation that drops below 3.5 is considered poor quality, a measure of 4.2-4.5 is considered excellent quality.

MOS Quality Impairment

4.5 Excellent Imperceptible

4 GoodPerceptible but not annoying

3 FairSlightly annoying

2 PoorAnnoying

1 BadVery annoying