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A representation of video flows through the CMMC is shown in Figure 5. It should be noted that although the diagram implies video always travels through the CMMC, this is not the case for video traveling between two multicast based sites, which will travel directly from source to sink, as well as reaching the CMMC for multiplexing to non-multicast sites.
Broadly speaking, a video flow originates in a remote site codec (either hardware or software). It may then undergo a protocol conversion, before being transmitted over a network (either packet based or ISDN). Depending on the circumstances of the conference, it may or may not then require multiplexing or format conversion. Assuming it requires multiplexing, this can be done in one of three ways:
Clearly, only option 3 can be done without any loss of quality, although this option places certain restrictions on the remote site codecs.
Some protocol conversion may also be required before transmission to the receiving sites, which may not all be on the same type of network (for example, some may be on ISDN links, some on packet networks), and may not all have the same restrictions (some may be using software codecs, some may have hardware codecs on high speed networks), and so a single incoming video flow may be coded several times for distribution. Transmission to remote sites over packet networks will usually be via multicast, even if there is only one receiving site, as the Mbone [dee] should be configured to use the paths best suited to real time traffic, whereas the usual service traffic routes may not be.
The function of the video multiplexor is to combine the video into quadrants. The connection with the UCL Conference Rooms can be direct analogue via the video switch. This can also connect on to the other sites of the University of London over LIVEnet [kir], and also to direct broadcast satellite if required.
Which set of the codec/mux/codec paths is chosen for a particular conference through the CMMC will depend on a number of factors. Some examples are: