Tech-X Flex™
Base Unit User Guide Issue 1
5: IP and Video Testing
5-29
Intro
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MOS scoring is frequently produced by software algorithms that monitor video streams and attempt to
“emulate” a subjective viewer experience. Such software is intended to produce results that are similar to
MOS scores that would be recorded by actual human participants watching and evaluating the media.
About gap and burst states
The software models the distribution of packet loss over the measurement duration, which allows for a
more detailed characterization of the packet loss experienced by the audio/video stream. This is a four-
state model in which two periods of loss exist, gap and burst periods, each of which has two states.
The stream is considered to be in a gap condition of loss when consecutive packet loss is less than or
equal to one packet. If two or more consecutive packets are lost, the stream is considered to be in a burst
condition. Following the entry into a burst period, 128 consecutive packets must be received in order to
return to the gap condition, a number determined though research of quality measurements. Note that
the successfully received packets will be considered to have arrived during a gap period.
Other test results
About packet delay variation (PDV)
Packet delay variation is a calculation based on the variation of a packet’s expected arrival time versus
its actual arrival time. Each packet has its own PDV, which is determined by:
| Expected time - Arrival time |
...noting the use of absolute values. So, if a packet is expected to arrive at time
1
but actually arrives at
time
2
, it has a PDV of | time
1
- time
2
|. Typically, individual PDVs are used for calculating an average for
multiple packets in a stream, or reporting the maximum PDV experienced during a measurement period.
NOTE:
Packet delay variation is sometimes referred to as
jitter
. However, the use of PDV terminology
is preferred in this documentation due to its more specific definition.
About packet-to-packet delay variation (PPDV)
Packet-to-packet delay variation (PPDV) is a statistical calculation of delay variation, based on the
method described by the IETF RFC 3550. It differs from basic packet delay variation (PDV) which looks
at variations in arrival time overall, not necessarily variations between adjacent, sequential packets.
As an example, consider four sequential packets, whose delays in arrival are 40, 42, 38, and 39 msec
respectively. The delay variation of the second packet is 2 msec ( | 40 - 42 | ), the delay variation of the
third packet is 4 msec, and so forth. The measurements continue for all selected packets in the
measurement stream, with all measurements considered in the end for a calculation of statistical
variance.