The relay receives the request and joins the multicast content using standard multicast routing protocols. The multicast stream is forwarded through the multicast-enabled network to the relay which forwards the stream over the tunnel to the gateway. The AMT gateway can be software run on the actual host PC or can be run on a home router which acts as gateway for all hosts at the home site (for example, if there are multiple computers in the home).
The result is that users on unicast-only networks receive multicast content. No action is needed by the end user's provider - the end user simply hops right over that uncooperative network to join the party on the multicast island. As a side benefit, the unicast-only network provider will begin to notice more of these AMT streams being tunneled over their network. This will be motivation to add multicast support to the network, as it will eliminate duplicate streams of tunneled (unicast) data and utilize the network more efficiently.
In this way, AMT can be viewed as a vital interim solution in the transition from unicast-only to multicast-enabled Internet networks. Of course, those providers are free to remain obstinately unicast-only and carry duplicate traffic across their networks. Most importantly, the end users are able to receive content from the sources.
AMT is not a new solution. In fact the AMT specification was originally drafted in 2001. What is new is that router vendors have recently added support for AMT in their large carrier routers, allowing service providers to offer AMT service in a scalable, manageable and profitable way.
The promise of Internet television was once a hallmark of the early Internet boom. But the multicast delivery mechanism required to make this vision possible ran into technical and economic real world obstacles. The recent availability of AMT helps hurdle those obstacles, restoring the possibility and promise of multicast. With multicast in place, the network is now ready to handle NextGenTV.
What NextGenTV would look like
If you wanted to start up a new television channel that was available on all the major cable and satellite systems and viewable to most Americans, a ballpark estimate of the cost would be in the hundreds of millions of dollars. Such high costs tend to keep out the riff raff. The number of television channels in existence is in the thousands, while the number available on most cable or satellite systems is typically in the hundreds. The number of American television viewers is roughly 300 million.
By contrast, there are more than 200 million Web sites on the Internet, which includes almost 2 billion users. Cost is the principle reason for this disparity. The cost of publishing Web content that could potentially reach a third of the planet's population can be done for tens of dollars per month. But the same cost model doesn't apply to video currently.