Since each client independently requests streams to adapt to changing network conditions, adaptive streaming servers can serve multiple clients simultaneously.
Fig. 1 shows an adaptive streaming server in operation. Each content stream is rendered in multiple versions that correspond to different delivery bit rates. Each version is stored as a sequence of files that can be delivered to a client upon demand.
High bit rate file sequences can offer things such as larger image sizes, faster frame rates, improved sound quality and more detailed images than lower bit rate file sequences. The different versions are synchronized, so that each file (or chunk) represents a fixed amount of time (typically 2, 5, 10 or 30 seconds) in the content sequence.
Each time a client device needs to download the next chunk in a sequence, it can choose one that uses a bit rate that is most suitable for the current condition of the network between the server and the client.
For example, Client A in Fig. 1 was able to request three high-bandwidth chunks in succession, but due to a deteriorating network connection, Client A decided to select a lower-bandwidth chunk for the fourth segment of the media file. Client B has selected smaller chunks since it has a lower-speed connection.
Even though HTTP is less efficient for delivering multimedia content than other protocols such as Real-time Transport Protocol (RTP) over User Datagram Protocol (UDP), it is much less likely to be blocked by browsers and firewalls. Hence, many video delivery websites such as Netflix and YouTube have adopted HTTP adaptive streaming.
The three most widespread HTTP streaming systems on the Web today are Adobe's HTTP Dynamic Streaming, Apple's HTTP Live Streaming, and Microsoft Smooth Streaming, which all use similar, but incompatible technologies. This forces each user to have three different streaming clients on their device to view these different stream formats.
Since DASH is being developed as an open standard, it will become possible for browsers and servers from different manufacturers to work together, just as H.264 signals can work across many platforms. Of course, intellectual property claims and licensing terms will need to be resolved before DASH achieves widespread usage, but in balance, this development should be good for the IP video delivery industry. Expect to see a lot more about DASH in the future.
Wes Simpson is an independent consultant and author who is looking forward to an open standard for HTTP streaming. He can be reached at firstname.lastname@example.org.