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Next: Inter-media Synch Up: Synchronisation Previous: Intra-stream Synch

Inter-Stream Synch

The easiest way of synchronising between streams at different sites is based on providing a globally synchronised clock. There are two ways this might be done:

Have the network provide a clock. This is used in H.261/ISDN based systems. A single clock is propagated around a set of CODECs and MCUs.
Have a clock synchronisation protocol, such as NTP (the network Time Protocol) or DTS (Digital Time Service). This operates between all the computers in a data network, and continually exchanges messages between the computers to monitor:

Alternatively, the media streams between sites could carry clock offset information, and the media timestamps with arrival times could be used to measure network delays, and clocks adjusted accordingly, and then used to insert a baseline delay into the adaptive playout algorithms so that the different streams are all synchronised.