In this chapter, we look at two starkly contrasting approaches to multimedia conference control.
The ITU has devised a sophisticated system for controlling tightly coupled conferences based around its H.261 video codec systems and ISDN networks, incorporating the T.120 protocols, using Multipoint Control Units (MCUs) and a Multicast Communication Service. In its original design, the scheme depended on a centralised model of control, and on the reliable and constant bit rate nature of ISDN networks. More recently, it has eveolved to include packet networks, and a more loosely coupled and distributed approach to conference control that owes a lot to input from researchers working in the ITU and IETF working groups.
The IETF has evolved an altogether more loosely coupled approach, based around the use of IP multicast. This is used for local system control (for example multicasting packets with a TTL of 0, to control a constellation of processes associated with the same session within a single system) through to wide area multicast use for session formation (the session directory and invitation protocols, the RTP RTCP quality reports and so on) and Conference Control, for example using a system like CCCP to build various types of control applications such as floor management. This is usually claimed to scale to large numbers of participants rather better than the MCU based approaches. At the end of the chapter we also take a look at Interworking between the Internet and ISDN approaches.
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