Application Note

10

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

overflow.

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.

MDI MLR

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:

Conclusion:

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.

Codec

Maximum Average MLR

SDTV

0.004

VOD

0.004

HDTV

0.0005

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