Content area
Full text
Delivering Real-Time Video Reliably over IP Networks
Today the video delivery landscape presents us with numerous delivery platforms: broadcast, cable, cellular, satellite, and just plain IP. Numerous web sites provide content inexpensively, e.g. YouTube, Hulu, Amazon, Netflix, and many others. This includes the major U.S. networks, as well as international ones like BBC, Canal+ and Deutsche Welle, not to mention free content from almost every country on earth.
The primary enabling technology is the internet protocol. While very capable, it is not suitable for streaming video since it is susceptible to jitter, packet loss, and other errors, unless complemented by some recovery mechanism.
One such mechanism is called ARQ: Automatic Repeat reQuest (also known as Automatic Repeat Query). ARQ is an ancient error correction mechanism that was first utilized in the days of Morse Code, when the receiver signaled the sender to re-send parts of a corrupted message. Here we are over 100 years later and this mechanism is still the best way to fix packet loss.
a brief overview of arq technology and principles
ARQ is the collective name for a set of "backward error correction" protocols that aim to guarantee the delivery of all packets across a data link. These protocols use positive or negative acknowledgements ("ACK" messages sent by the receiver confirming that it has correctly received a packet, or negative ACKs ["NACK"] sent only to signal that it is missing one or more packets).
The protocols also implement timeouts, defining a period of time that may elapse before an ACK must be received, to enable reliable data delivery over an otherwise unreliable channel. In case of...