A high DF value can indicate that the data is being received
by the player at rate greater than the player can drain the
data from the receive buffer. Should the receive buffer be
exceeded, the excess packets will be discarded, resulting in
degraded video quality. This can be an indication that the
network latency between the server and the player is too low.
The MDI DF can be used to determine the depth of the
player s receive buffer. Below are two examples of DF values
from two different video streams.
In the first example we captured a video stream that was
encoded at 100 kilobits per second, sent over a Wide Area
Network. We can see that the Mean DF is 938 milliseconds.
This tells us that on an average for the stream a receive buf-
fer depth of 1 second would be enough to buffer most of the
traffic. However, to ensure all of the traffic is buffered with-
out overflow, the buffer should be no less than 2.6 seconds.
In the next example, a video stream was captured that was
encoded at 220 kilobits per second on a Local Area Network.
In this case we see a larger buffer would be required on the
player. The higher Delay Factor indicates that the data is
being delivered to player at a rate greater than the player is
able to drain the data from the receive buffer.
In this case, the receive buffer on the player should be
able to hold up to 5.7 seconds of video to prevent buffer
A low DF would indicate that the player is draining the buffer
at a rate equal to the rate at which the data is arriving. In
a network with varying rates of Jitter, a underflow condition
may occur where the player must pause while waiting for
data to arrive.
The Media Loss Rate (MLR) is a measurement of the number
of packets lost or out of sequence over the measurement
interval. It is important to include the out of sequence
packets in this calculation, since not all devices will attempt
to reorder the packets as they arrive. The DSL Form s WT-126
recommends the following limits on the MLR value:
There are many factors that will determine the quality of the
video displayed by the player. Some of these factors such as
Jitter can be compensated for by the proper sizing of the
receive buffer. Others, such as packet loss will have a signifi-
cant impact on the quality of the video.
By carefully monitoring the video streams both prior to de-
ployment in a production environment and after deployment
thresholds can be determined to ensure the video quality
meets or exceeds to expectations of the viewer. By monitor-
ing the video stream at various locations along the path
between the server and the player, sources of impairment
can be identified and resolved.
As with VoIP troubleshooting, the playback feature of the
ClearSight analyzer can be used to determine the quality of
the video at each of these points within the network.
Maximum Average MLR
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