Recent years have seen a great deal of research and development
in the area of multimedia conferencing.
Crowley[#crowley##1#] describes <#1833#> MMConf<#1833#>, a system for building
shared multimedia applications. Here, the actual protocols that
control and coordinate the applications are based around extensions
to the Diamond/Slate Multimedia mail systems.
Arango[#ara##1#] describes the <#1835#> Touring Machine<#1835#>, a system for moving
the media around. The conference control architecture is not
expounded in detail here.
Schooler[#schooler##1#] describes <#1837#> mmcc<#1837#>, a system for
controlling largely tightly bound multimedia conferences, based
around RPC.
The ITU standard, H.320[#h320##1#], is a classical design for the signaling of
messages between components in a telephony style conference control
system.
A different approach can be seen in the work at LBL on a Session Directory
and Visual Audio Conferencing tool [#VAT##1#] make use of a
distributed algorithm for conference identification and for session
and conference announcements based on periodic multicasting of
information in a similar way to beaconing in Cell-Based phone systems
or station-identification protocols. This can replace a
directory service, including session membership, and a fair portion
of a conference management system, including floor control.
The n-to-n use of multicast can lead to synchronized traffic unless care
is taken. We have named this behaviour of distributed applications
a ``systolic tendency'' from the original meaning of the word systolic,
meaning heartbeat. It has been reported in distributed routing
algorithms by Floyd. [#floyd##1#]
If engineered properly, we believe that the approach using
loosely coupled messages and events as espoused by these researchers
forms a more robust scalable starting point. Work at UCL on the
Conference Control Channel Protocol[#wakeman##1#] is attempting to
bring together all aspects of this approach.
The notion of <#1842#> tightly coupled<#1842#> conferences, with strict
membership and floor control, has led some researchers to re-examine
the more RPC-like approach. However, the CCCP work shows that the
semantics of conferencing are orthogonal to the way they are
constructed from IPC mechanisms.