BBC Trialing Multicasting for the Olympics

24th May 2004

As the MediaGuardian article linked-to above says, the BBC fear that if very large numbers of people simultaneously access the BBC broadband video coverage of the Olympics this Summer then the UK internet could ground to a halt, and to overcome this the BBC are trialing multicasting.

Currently, when people stream audio or video over the net they do so using a method called unicasting, which means that each user has their own stream, so if 1,000 people are listening to, say, Radio 1 via the web at any one time, then there will be 1,000 individual streams leaving the BBC's server. Unicasting is a very inefficient method of broadcasting streams over the net. Multicasting on the other hand is the term for when one stream leaves the broadcaster's server and individual user streams branch of the main stream in a tree-like fashion. The BBC's trial system consists of them sending their streams to the ISPs, and the ISPs will then forward the streams to their users that are requesting them. Multicasting is a far more efficient method of broadcasting streams over the net.

The significance of this move by the BBC for digital TV and radio is that the BBC could do the same for all of their TV and radio channels, and because it is such an efficient means of broadcasting video or audio streams then they could deliver, for instance, their radio stations with a high audio quality, and they'd still only have to deliver a single stream of each of their channels to each ISP, rather than the thousands of unicast streams at low bit rates like they do with their BBC Radio/Listen Again Player at the moment.

The best thing about streaming over the net is that the BBC are not -- or shouldn't be -- restricted to using the codec that everybody has in their set-top box or DAB radio, because people can download the decoders for free over the net. This means that although the BBC may choose to use MP3 early on if they decided to multicast their radio stations, they could also multicast using AAC, which can deliver audio quality that is virtually indistinguishable from CD-quality at bit rates of 128kbps (compare this with MP2 that is used for Freeview, DSat and DAB, which would require 256kbps to deliver this level of audio quality).

Even if the BBC don't decide to multicast their TV and radio channels in the near-future, they will eventually, because the next generation IP protocol, IPv6, will include multicasting capability as standard. It was thought that we'd have to wait for multicasting of the BBC channels until IPv6 is rolled out later this decade, but this move by the BBC will hopefully bring this forward, if not to this Summer, then at least not into the too distant future.