wiki:WiMAX/WiMAX-Tutorial/Dash/00

Background

In this tutorial we are going to experiment with Dash and some basic tweaks in its rate adaptation policy. Dash (Dynamic Adaptive Streaming HTTP) is a protocol adopted by 3GPP and based on technologies such as Adobe HTTP Dynamic Streaming and Apple Live Streaming and Microsoft Smooth Streaming, for streaming video over HTTP. It is embraced by important content providers such as Apple, Microsoft, Netflix and other important players such as Adobe and Qualcomm.

The Idea

Dash running over HTTP, is a server-client system. A HTTP Server runs on the server side and provides the media content to the client upon request. The media (MPEG-4 or MPEG-2 video) is divided into several chunks based on both timing and bit-rate. A Media Presentation Description file in XML format serves as a map of the media, enabling the server to send the chunks requested based on the rate adaptation, affected by channel conditions, at a given time.

  • A media player client, such as VLC, connects to the server through the url of the MDP.
  • The server sends the the media chunks based on the implemented policies and using the mdp file.

The Scenario

We will run a series of experiments trying to evaluate 3 primitive dash rate adaptation policies. In all of these policies, the client selects the highest available bitrate for a segment that is lower than some perceived available bitrate. The perceived available bitrate is first estimated as the actual bitrate observed over the download of the last segment. Then, it is adjusted according to one of the following policies:

  • Policy 1: When the buffer is less than 30% full, the perceived available bitrate is zero, and the client requests the lowest available bitrate.
  • Policy 2: The perceived available bitrate is not adjusted, i.e. it is always equal to the actual bitrate observed over the download of the last segment.
  • Policy 3: When the buffer is less than 30% full, the perceived available bitrate is equal to half of the actual bitrate observed over the download of the last segment.

These policies involve a tradeoff between video rate and buffer status. Obviously, we want to download segments of a high video rate, because these offer better quality. However, we also want to avoid freezes (the state where there is zero video in the buffer) and we want the video playback to be smooth, avoiding big jumps in video rate. We will use the WiMAX testbed to see how this tradeoff works to deliver DASH video over a wireless network.

After the end of the experiment the measurements collected will be archived in iRODS, where you can plot and compare the three rate adaptation policies.

DASH References

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Last modified 6 years ago Last modified on 07/22/13 20:27:58