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(IDG) -- The combination of IP multicast over DSL lets network managers broadcast news, radio and television feeds, stock updates, and voice or video conferences with exceptional cost and bandwidth efficiency.

The architecture of the underlying protocol is the key to those efficiencies. Traditional systems would have to maintain multiple unicast streaming sessions - one for each user. This requires a lot of bandwidth and does not scale well.

Imagine a building with dozens of tenants, all subscribing to the same videoconference via the same ISP. Without IP multicast, a content server would have to send the same feed to each tenant. With a typical bandwidth requirement of 300K bit/sec for a television feed, multiplying this for each user dramatically illustrates the unicast bandwidth problem.

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New local loop broadband technologies, including cable and digital subscriber line, make multicast services possible. Both cable and DSL technologies have the bandwidth to support at least one live videostream. Most routers offer several protocols that enable IP multicast.

There are two components to the IP multicast support. One consists of the communication between the routers in the network, and the other is the last-leg connection between the access router and the client using the service. The result is a network tree, with the server residing at the top, routers forming branches among themselves and leaves in the direction of service users. As new users join, routers form new branches to connect the server and the client. As users go away, the routers prune, deleting those leaves.

In order to implement IP multicast fully, network hosts must be able to support a multicast protocol to enable them to connect to the local router or to the multicast server itself. This protocol, called Internet Gateway Multicast Protocol (IGMP), is a standard piece of the IP stack found on all PCs and most routers. It is mainly a registration protocol where PCs register to join a particular multicast group or service.

In addition to the registration and deregistration component, there is a "keep-alive" component. Routers can connect to each other all the way to the multicast server via interrouter protocols such as Protocol Independent Multicast, Distance Vector Multicast Routing Protocol and Multicast Open Shortest Path First. The multicast server may be either directly connected or routed to the clients, which dictates whether there is a need for inter-router multicast protocols.

If the customer is connected via DSL, there usually will be a Digital Subscriber Line Access Multiplexer (DSLAM) and customer premises equipment (CPE) pair between the point of presence router and each user's PC or each user's access router. The points of interest in that chain are the connection between the POP router and the DSLAM, followed by the connection between the DSLAM and the DSL CPE.

The behavior of the link between the DSL CPE and the user will be physical-network-dependent. In most cases, it will be Ethernet, which has built-in support for multicast. If Ethernet framing and Ethernet bridging are used to go through the DSL infrastructure all the way to the POP router, the POP router will be aware of all the IGMP join requests and will be able to service them by forwarding traffic with a multicast media access control address toward the DSLAM.

The DSLAM may support some aggregation techniques, such as those offered by an Ethernet link between the DSLAM and the router. In that case, the router would only send one packet from each server to the DSLAM. The job of the DSLAM would be to send only that packet to the appropriate DSL port. In order for that to happen, the DSLAM needs to be able to support IP multicast protocols.

As Internet backbone pipes get fatter and last-mile connections keep pace with new broadband technologies, IP multicast presents tremendous, new opportunities. IP multicast over DSL enables IT managers to offer scalable, broadband-enabled technologies at a quality level never before obtained.