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Mean opinion score (MOS)


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Acceptable packet loss


Packet loss is closely associated with quality of service considerations, and is related to the erlang unit of measure.


The amount of packet loss that is acceptable depends on the type of data being sent. For example, for Voice over IP traffic, one commentator reckoned that "[m]issing one or two packets every now and then will not affect the quality of the conversation. Losses between 5% and 10% of the total packet stream will affect the quality significantly."[9] Another described less than 1% packet loss as "good" for streaming audio or video, and 1-2.5% as "acceptable".[10] On the other hand, when transmitting a text document or web page, a single dropped packet could result in losing part of the file, which is why a reliable delivery protocol would be used for this purpose (to retransmit dropped packets).


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  • 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.
  • Lost packets - 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 (bad) being the worst and 5 (excellent) being the best. (Mathematically, 4.41 is the maximum MOS for G711, while the maximum is 4.07 for G729.)
  • 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.

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