Within a multimedia conference the media tools utilize the IP multicast mechanism by having every instance of a tool join the same multicast address. For each media there is a unique address and port pair, on which the tool is started. For example, the addresses used by MICE conferences are shown in the table 9.1 below.
Note that here the address is the same but the port changes. In other conferences both the address and port change. Furthermore, each tool may use more than one port, which is not always advertised. The tools send data to one port and non-core data to another port. The non-core data usually consists of session information such as the user name of the sender or some statistics regarding the quality of the received data. These session messages are sent at low bandwidth as they are secondary to the real data. It is most common for the session message port to be one higher than the advertised port number. So, for the previous examples we would see session messages for audio on port for video on port and for whiteboard on port
To record data from a multicast conference, the recorder need not be an active part of the conference by executing any of the conference tools, but it must attach to that conference to enable the multicast data to reach the recorder and hence be collected. As data for each media within a conference will be seen on a different address and port pair, the recorder must accommodate this. The collected data for each media can then be saved in an archive for later use. To playback the recorded data to a multicast conference, a player must join the multicast address for each media in the conference to enable the data from the archive to be sent into the conference. Again, each media's data will go to a different address and port pair. In figure 9.1 we see that before a recording or playback, the recorder or player respectively, is independent of the conference, and that the data packets and session messages are on different addresses.
In figure 9.2 we see that the recorder or player has attached to the conference in order to collect the data from each media.
Next: Current Media Tools and Up: Recording Previous: Recording Jon CROWCROFT