Latency

: The measure of time delay in moving packets from the transmitting agent to the receiving agent; the maximum duration of latency that a VoIP system can sustain without deterioration of service is 150 ms in any one direction.

Jitter

: A variation in packet transit delay caused by queuing, congestion, timing drifts, route changes and serialization effects on the path through the network; the maximum allowable duration of jitter is 40 ms before deterioration occurs.

Packet Loss

: The failure of one or more packets to reach their destination across the network; the maximum allowable packet loss is less than 1% for WANs and less than 0.05% for LANs.

MOS

: Mean opinion score is a subjective measure of voice quality that gives a numerical indication of the perceived quality of the media received; MOS is expressed as a number from 1 to 5, with 1 being the worst and 5 being the best.

R-value

: A quantitative expression of the subjective quality of speech in communication systems for digital networks that carry VoIP or for which VoIP is under consideration; R-values range from 1 (worst) to 100 (best), and the metric is often used in conjunction with MOS, although the R-value is considered a more accurate portrayal of the effects of packet loss and latency.