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4.3.2 Impact of Packet Loss
For loss distribution we have used a Bernoulli model. All the packet loss intro-
duced in the videos were made assuming random losses distributed over an uni-
form probability density function which means that all the packets have the same
probability to be dropped.
In the real world, packet dropping used to appear in the form of bursts of ran-
dom length. A burst is a period with a high density of losses with independent
probability which produces a larger distortion than isolated losses. Other models
like Gilbert and Markov describe state models that transition between gap (good)
states and bad (burst) states which have high or low density of independent losses.
For this reason when we calculate the quality in short intervals, the packet loss
density distribution can be considered uniform even if we are inside a burst.
The first step to process all the gathered data was to remove the atypical quali-
ties, i.e., those values that are too different from the majority. These atypical data
can be explained due to mistakes made during the input of the data, the videos
were watched in non-optimum conditions, the user had a bad day, etc. We con-
sider atypical (outliers values) all the data that were deviated from the mean more
than 3/2 of the standard deviation. Then we found the equations that fit the curves
described by the data clouds as shown in Fig. 3.
The equation that minimizes the mean error among all the opinions (once we
discard all the atypical data) for the different values of the analyzed bandwidth is
shown in Equation (1).
() = 0.06914 l 2 +1.545 l +1.719
f 150
l + 0.447
f 300 () = 0.1343 l 2 + 2.061 l + 2.041
l + 0.4721
() = 0.1591 l 2 + 2.333 l + 2.342
f 600
(1)
l + 0.5274
() = 0.1757 l 2 + 2.496 l + 2.5
f 900
l + 0.5391
f 1500 () = 0.21 l 2 + 2.73 l + 3.012
l + 0.6336
4.3.3 The Impact of Bandwidth
Another factor that can influence in the video stream quality is the bandwidth, for
this reason, many bandwidth rates has been studied. In order to obtain a model that
fits all the curves generated (one for each bandwidth) we calculated the equations
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