The State of Video Streaming Before DASH
To understand how DASH improves video streaming, let's look at the technology preceding it.
HTTP is the primary protocol used for Web content on the Internet. HTTP is also stateless, which means that when you request data from your computer or mobile device, it connects to the Web server on the Internet just long enough to get the data it needs, then terminates that connection. You may have seen this when you're using a Web browser to view a page and you lose your Internet connection: The Web page remains loaded in your browser, even though you're not actively connected to the server it came from.
As of this writing, the state-of-the-art in Internet video streaming was the Real-Time Streaming Protocol (RTSP). Unlike HTTP, RTSP is stateful. That means it relies on a persistent connection between your computer or mobile device and the Web server that's sending the video. This means that once the connection's established, the server sends a constant stream of data to the device until it sends all of the video or until you stop the video, whichever occurs first.
Because it's stateful, RTSP is less efficient than HTTP. That's because there's a chance you could download more data than actually you need. For example, your device might download an entire video even if you stop watching it halfway through.
If RTSP could adjust for differences in bitrate, this wouldn't be an issue. The bitrate is how many bits of data your device can download per second. This is affected by the speed and traffic volume on your current Internet connection. If you have a high bitrate, the RTSP simply downloads more video content over the same amount of time, whether or not you actually watch all of that content.
Internet speeds are getting faster, particularly on mobile networks. Faster download speeds have led to more video viewing, and consequently have pushed bandwidth usage to unexpected highs. This puts pressure on broadband providers to save money and resources. As a result, providers continue to raise their prices and to throttle or cap bandwidth for their users.
DASH is an attractive alternative to RTSP because it makes video streaming more efficient. Next, we'll see how DASH accomplishes this.