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Wednesday
Feb162011

Paper: An Experimental Investigation of the Akamai Adaptive Video Streaming

Video is hot on the Internet and people are really interested in knowing how to make it work. Dan Rayburn has a post pointing to a fascinating paper: An Experimental Investigation of the Akamai Adaptive Video Streaming, which talks in some detail about the protocols big players like YouTube, Skype and Akamai use to serve video over on an inherently video unfriendly medium like the Internet. For Akamai they found:

  1. Each video is encoded in five versions at different bit rates and stored in separate files.
  2. The client sends commands to the server with an average inter departure time of about 2 s, i.e. the control algorithm is executed on average each 2 seconds. 
  3. Akamai uses only the video level to adapt the video source to the available bandwidth, whereas the frame rate of the video is kept constant.
  4. When a sudden drop in the available bandwidth occurs, short interruptions of the video playback can occur due to the a large actuation delay.
  5. For a sudden increase of the available bandwidth, the transient time to match the new bandwidth is roughly 150 seconds.

Abstract:

Akamai offers the largest Content Delivery Network (CDN) service in the world. Building upon its CDN, it recently started to offer High Denition (HD) video distribution using HTTP-based adaptive video streaming. In this paper we experimentally investigate the performance of this new Akamai service aiming at measuring how fast the video quality tracks the Internet available bandwidth and to what extent the service is able to ensure continuous video distribution in the presence of abrupt changes of available bandwidth. Moreover, we provide details on the client-server protocol employed by Akamai to implement the quality adaptation algorithm. Main results are: 1) any video is encoded at five different bit rates and each level is stored at the server; 2) the video client computes the available bandwidth and sends a feedback signal to the server that selects the video at the bit rate that matches the available bandwidth; 3) the video bit rate matches the available bandwidth in roughly 150 seconds; 4) a feedback control law is employed to ensure that the player buffer length tracks a desired buffer length; 5) when an abrupt variation of the available bandwidth occurs, the suitable video level is selected after roughly 14 seconds and the video reproduction is affected by short interruptions.

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