General TV, major and minor sports plus all manner of events can easily be broadcast over the internet using Multicast.

Multicast services over the NBN are largely absent from the debate put in front of the general public - and that's a pity, because they're an important part of the value of the network.

So what is multicast?

It's a capability that's analogous to an ordinary TV broadcast - not exactly the same, but it will do for a starting point. TV broadcasts are one-signal-per-channel: that signal gets sent to a broadcast tower, transmitted using radio waves, and received by the TV.

"Well, I can do this over the Internet! Just look at YouTube!" you might answer - perfectly correctly, if you ignore the amount of time you spend making snide remarks towards your computer while it buffers the stream.

Anyhow, multicast isn't there for YouTube, but to enable new distribution models using the fibre.

The idea behind multicast is breathtakingly simple: instead of addressing a packet to one user, it - and all the other packets associated with a stream - are addressed to many users. The reason for doing so is a simple matter of network efficiency: if we have a stream that demands 20 Mb/s, it can be injected into the network once, and taken to a distribution point somewhere close to the users.



Caption: A simplified view of multicast

While the world is full of stories about users wanting to break free from television schedules, there is one class of content in which the user is tied to "real time": the live event. Whether you're watching the stream of a concert, a sports fixture, breaking news, (heaven help you) parliamentary question time, or Senate estimates, some things remain ideal for a distribution model that assumes many people are watching the same content at the same time.

That's what makes multicast desirable.

Can't we multicast on today's broadband networks?

The answer is "yes and no".

ADSL technologies do support multicast. Whether it's available to end users depends on whether or not the company that owns the ADSL infrastructure has enabled it. iiNet and TPG, to name two, use multicast for their IPTV products. Optus does not seem to, and nor does Telstra.

However, the use of multicast on Australia's fixed broadband networks is subject to a couple of issues - one technical, the other structural.

The structural issue is this: there is, as far as I can discover, no wholesale multicast product on today's ADSL networks. This protects the wholesalers' multicast services from competition from downstream retailers: if I established a new retail ISP, I would be able to offer ADSL services, but not multicast services.

From a technical point of view, although multicast-over-ADSL exists, it's constrained by what ADSL can deliver. If uncompressed, an MPEG-2 video broadcast needs around 8 Mb/s, and in fast-moving sequences (such as a sports broadcast) it can demand peak bandwidth of around 15 Mb/s. Compression will deal with the bandwidth hunger, but that's more applicable to pre-recorded content than for live streams.

The notional 20 Mb/s-plus of ADSL2+ services is a pipe dream for most of us. According to Akamai's State of the Internet report, Australia's average download speeds right now run at around 4 Mb/s. This, however, doesn't rule out multicast, because Akamai measures end-to-end speed, whereas multicast only cares about the speed of the final leg of the journey - from the exchange to the user.

You have to be close to the exchange - within 1.5 KM - for your line speed to be high enough to support a high-quality live video stream and anything else you want to do on the internet. As this screen-grab from TPG's heat maps shows, most users in a place like Quaker's Hill in Sydney would struggle, because their connection will be below 10 Mb/s.



TPG's map shows that few people achieve 10 Mb/s download speeds.

Multicast on the NBN

The NBN changes both of these multicast dynamics: it's a wholesale product in its own right, offered on a high-bandwidth network.

With 100 Mb/s of download capacity available, there's plenty of space for 20 Mb/s to be devoted to streaming a sports fixture without disrupting any other traffic on the link. And anywhere that the fibre goes - which admittedly isn't all of Australia - performance will be equal. Multicast performance won't be defined by distance to exchange, as it is with ADSL.

On the NBN, multicast is a wholesale product, accessible to all retail service providers. While the RSP remains the conduit that gets content on the network, under the NBN structure, all retailers are on a level playing field when it comes to multicasting content.

That "level playing field" has interesting possibilities out in the world of content, because it brings two dynamics into play that haven't existed in Australia before.

1. A more competitive search for content - as the NBN reaches more households and signs on more retailers, we can expect the entry barriers for content providers to fall.

2. Network ubiquity - one of the barriers to content production is the prohibitive price of getting from venue to viewer.

The ubiquity of the NBN has the potential to transform where content comes from. Right now, only a minority of sports grounds in the country are connected to fibre - the big, iconic venues whose client sports are tied up in long-term contracts to major broadcasters.

When the National Broadband Network is rolled out, there will be fibre within usable distance of pretty much every sports venue in Australia. That eliminates one part of the cost of trying to cover sports at smaller stadiums - the need to use point-to-point microwave links or broadcast-style satellite services - to capture the content for distribution.

If (say) club-level cricket, netball, or other popular but media-ignored sports aren't giving thought to this today, they will be soon. After all: Andrew Demetriou of the Australian Football League wants to look at how the NBN can connect AFL to its fans - and he's savvy enough to extract multi-billion-dollar deals out of Australia's television broadcasters.

If you don't think this would ever happen, consider this: the Group Nine rugby league grand final each year gets radio coverage courtesy of the ABC - and a great many phone calls and SMS messages. Can a sceptic guarantee that there's not enough interest in such competitions to justify a small outside broadcast and a pay-per-view service over the NBN?

Richard Chirgwin is a Sydney-based telecommunications analyst and freelance journalist, and is Australian correspondent to The Register.