This is done all the way through the network until the signal reaches the clients that want to receive the broadcast. Unfortunately, there is no one protocol or standard that defines how IP Multicast should work. Since its introduction in 1988, there have been a number of proposals and protocols developed to handle multicast traffic on the network. Dense mode floods the network tree with a broadcast so that every branch receives the signal. Branches that do not have clients requesting the signal are then pruned from the broadcast on the fly.
Sparse mode works in the opposite fashion.
Only those branches with clients requesting the broadcast actually receive the stream. The request from the client goes up the network tree until a multicast connection is made between client and server. Multicast broadcasts are announced typically announced like anything else, through e-mail or on a Web site. Dense mode is good for networks with plenty of bandwidth and users who are closely distributed on the network. Sparse mode is better suited for networks with users that are more dispersed.