Star/Mesh Duality and Rings

The most important property of the n-way conference to maintain is that of global sequencing. There are essentially two classes of distribution mechanism for this kind of application: The main problem with the first mechanism is that extra protocol is required to maintain global sequencing of the input and output to the conference, otherwise the appearance of separate interleaved conversations may become re-ordered on some (or even all) of the conference displays. A Common optimisation is to organize the conferees in a logical ring and pass a token round for sequence control. The second mechanism does not have the same problem, since the central conference server can act as a global sequencer. Simply by blocking input from all subsequent users until the input from the last user has been successfully output on all the displays, we ensure ordering. However, this mechanism does have two related problems. First, there is a large load on the central server. Second, the central server is a single point of failure. Either scheme would benefit from a reliable multicast protocol such as that described in [BiJo87][CrPa88]: In contrast to either of these, a distributed shared memory model for distributed programs could be used (albeit, in a distributed system, this must be on top of some message passing mechanism which would then require all the global sequence that the mesh approach needs). We have then just exchanged the sequence problem for that of controlling concurrent access to shared memory. Our pilot implementation used the central server model for reasons of simplicity.