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VoIP test's Glossary


Jitter a measure for the difference in time that each packet takes to reach the destination. In an ideal world it would be nice if each packet sent took exactly the same time to travel between the client and the server (0% jitter) but in reality this is seldom the case and packets vary in the length of time (Latency) it takes to reach the destination which on a bad connection can be very larger. Jitter is an expression of the variance.

Packet Loss is a measure of how many packets did not reach the destination for one reason or another, expressed as a percentage of the total number of packets. Any packet loss is bad and affects the quality of applications.

Packet Loss Distribution is a measure of the packet loss distribution across the timeline. If a test of 1000 packets lost 1% (10 packets) then if that one packet in every 100 the distribution would be 1%. However if the loss was 10 packets in just one window of 100 packets. The loss will remain 1% (10 packets) but the distribution would be 10%. A high distribution percentage means that all the lost packets are in a small window of time causing a bigger quality problem for the application.

Packet Order is a measure in percentage of how many packets arrived in order. Packet do not necessarily take the same route or the same time to reach the destination. This results in packets arriving out of order which causes other packets to be delayed or even in very bad cases discarded. Delayed or discarded packets cause a quality problem for the application.

Packet Discards is a measure of packets that arrive too late to be used by the application. Packets are very time dependant when it comes to media based applications. There is a time window when packets can be used after which it is too late and the packet has to be intentionally discarded when it arrives. A bit like missing a connecting flight because the first flight was delayed and arrived after the second flight had taken-off.

MOS score is a measure from 1 (being the worst) to 5 (being the best). MOS is quite subjective, as it originated from the phone companies and used human input from related quality tests. Software applications have adopted the MOS score and scale, namely 5 – Clear as if in a real face to face conversation; 4 – Fair, small interference but sound still clear. Cell phones are a good example of an everyday; 3 – Not fair, enough interference to start to annoy; 2 – Poor, very annoying and almost unusable; 1 – Not fit for purpose.

David Mozer at 613 828-6611
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Updated Friday 22 December, 2017 12:14 PM
Webmaster: David Mozer