Let’s DASH!

In the last century, access to video delivered over networks was almost exclusively dominated by scheduled consumption on dedicated devices – broadcasters distributed premium content at a specific time to TV sets. Broadband internet, both fixed and mobile, as well as highly capable devices such as smartphones and tablets have changed video consumption patterns dramatically in recent years. Video is now consumed on-demand on a multiplicity of devices according to the schedule of the user.

Recent studies conclude that mobile data traffic will grow by a factor of 26 between 2011 and 2016 and that by 2016 video traffic will account for at least two-thirds of the total. The popularity of video also leads to dramatic data needs on the fixed internet. In North America, real-time entertainment traffic (excluding p2p video) today contributes more than 50% of the downstream traffic at peak periods, with notably 30% from Netflix and 11% from YouTube.

HTTP Delivers
The astonishing thing is that these data needs are not driven by traditional broadcast, IP multicast or managed walled-garden services, but by over-the-top video providers. One of the cornerstones of this success is the use of HTTP as the delivery protocol. HTTP enables reach, universal access, connectivity to any device, fixed-mobile convergence, reliability, robustness, and the reuse of existing delivery infrastructure for scalable distribution.

One of the few downsides of HTTP-based delivery is the lack of bitrate guarantees. This can be addressed by enabling the video client to dynamically switch between different quality/bitrate versions of the same content and therefore to adapt to changing network conditions. The provider offers the same media content in different versions and the client can itself select and switch to the appropriate version to ensure continuous playback. The figure below shows a typical distribution architecture for dynamic adaptive streaming over HTTP. HTTP-based Content Delivery Networks (CDNs) have been proven to provide an easy, cost-efficient and scalable means for large-scale video streaming services.

Click to enlarge

Setting a Standard
MPEG has taken the lead on defining a unified format for enabling Dynamic Adaptive Streaming over HTTP (DASH). MPEG-DASH was ratified in 2011 and published as a standard (ISO/IEC 23009-1) in April 2012. It is an evolution of existing proprietary technologies that also addresses new requirements and use cases. DASH enables convergence by addressing mobile, wireless and fixed access networks, different devices such as smartphones, tablets, PCs, laptops, gaming consoles and televisions, as well as different content sources such as on-demand providers, broadcasters, or usergenerated content offerings.

The standard defines two basic formats: the Media Presentation Description (MPD) uses XML to provide a manifest of the available content, its various alternatives, their URL addresses, and other characteristics; and Segments, which contain the actual media streams in the form of chunks, in single or multiple files. In the context of part 1 of MPEG-DASH the focus is on media formats based on the ISO Base Media File Format and the MPEG-2 Transport Stream.

With these basic formats MPEG-DASH provides for a very wide range of use cases and features, including support for server and client-side component synchronization and for efficient trick modes, as well as simple splicing and (targeted) ad insertion and session metrics. DASH can support multiple Digital Rights Management systems, content metadata, and support for advanced codecs including 3D video and multi-channel audio.

Towards Deployment
With the completion of the standard the focus has shifted towards deployment and commercialization of DASH. In this context MPEG will later this year publish Conformance Software and Implementation Guidelines and continues to work on client implementations and optimizations. This is especially relevant for a stable and consistent user experience under varying network conditions.

On the distribution side, coming optimizations include DASH for CDNs – to improve efficiency, scalability and user experience – along with integration into mobile networks and transition between unicast and multicast distribution.

The creation of the DASH Promotors’ Group will help to address interoperability and promotional activities. The EBU is among the 50 major industry players that make up this group. Support is also provided for other standards planning to include MPEG-DASH to enable over-the-top video, including HbbTV, DLNA, the Open IPTV Forum and 3GPP. Furthermore, the W3C consortium is considering extensions to the HTML5 video tag that would aid the integration of DASH into web browsers.

The significant efforts currently under way to deploy DASH in a wide range of contexts raise the expectation that MPEG-DASH will become the format for dynamic adaptive streaming over HTTP.

By Thomas Stockhammer, EBU Technical Magazine