ITU Model H.320/T.Gcc

This design is based around the starting point of person to person video telephony, across the POTS (Plain Old Telephone System) or its digital successor, ISDN (Integrated Services Digital Networking). The Public Network Operators (PNOs, or telcos or PTTs), have a network already, and its based on a circuit model - you place a call using a signaling protocol with several stages - call request, call indication, call proceeding, call complete and so on. Once the call has been made, the resources are in place for the duration of the call. You are guaranteed (through expensive engineering, and you pay!), that your bits will get to the destination with constant rate and constant delay, plus or minus a few bit times in a million.

To achieve this, the provider has a complex arrangement of global clocks and an over resourced backbone network. To match video traffic to such a service, the output from a video compression algorithm has to be padded out to a constant bit rate (i.e. its constant rate, not constant quality). The assumption is that you have a special purpose box that you plug cameras and mikes into, (a CODEC) and it plugs into the phone or ISDN line or leased line, and you conference with your equivalent at the far end of the call.

In such a system, we can ask: How is multisite conferencing achieved?