This is an instance of a multicast session where it is possible to think of a tree-based hierarchy for the whole session, with the students at the leaves and moderators at various levels of the tree. On the other hand, a web based multicast radio station which provides interactive services similar to traditional radio stations, would be an example of a much more asymmetric multicast session where the various listeners hardly ever communicate with each other. The floor control issues for this class of applications are much more challenging than those in the former because of the lack of a similar inherent structure in the multicast session. VAT was used for the audio multicast communication. The DJ maintains a song selection from which songs are played and a list of questions that he needs to answer or refuse.
Listeners vote on their favorite songs, which are played out in decreasing number of votes.
The interface displays the currently playing song and the song selection that can be sorted by song title, artist or number of votes. New questions get queued into the list of questions. The DJ can accept the question and then must hang up at the end of the conversation, or he can just refuse the question. Hanging up or refusing a question removes the question from the queue. Listener interface and complementing VAT window Figure 2 shows the listener interface.